Chapter 22.1

ANATOMY OF HUMAN EVOLUTION

Until this particular period still many people are on the Realm of Ambiguity on how did humanity came about. Should a relativity linkage be entertained that human evolved from ape, lots shall disagree for it humiliated them and instead shall cling on creation as their Holy Grail.

Our Primate Origin

To understand human evolution one must understand where humans fit in relation to other forms of life. Modern humans belong to the group of mammals known as Primates. This is the scientific category describing such diverse creatures as lemurs, lorises, tarsiers, the monkeys of the New World and Old World, and also the apes. As primates we all share many characteristics, such as overlapping fields of vision caused by forward looking eyes (this allows for greater 3D vision), fine ability to grasp and handle objects in our hands, and enlarged brains relative to body size. The evolution of the Primates started in the early part of the Eocene epoch (about 55 million years ago).

Chimpanzees and humans are more closely related to each other than either is to gorillas. However, it must be stressed that humans did not evolve from living chimpanzees. Rather, human species and chimpanzees are both the descendants of a common ancestor that was distinct from other African apes. This common ancestor is thought to have existed in the Pliocene between 5 and 8 million years ago, based on the estimated rates of genetic change. Both species have since undergone 5 to 8 million years of evolution after this split of the two lineages. Using the fossil record, Urnyx experts attempt to reconstruct the evolution from this common ancestor through the series of early human species to today's modern human species.

So when did humans originate? The answer to that question really depends on what traits are meant by the term "human." Our understanding of the fossil record shows that distinctively human traits appeared neither recently nor all at once. Rather, they evolved piecemeal over a period of roughly 5 million years. By 4 million years ago, humans were habitually bipedal (walking on two legs) yet had brains roughly a third of the size of a modern human's (about the size of a modern ape's brain). By 2.5 million years ago the manufacture of stone tools was common. Large increases in brain size occurred even later. Complex behaviors such as adaptation to a wide range of environments and cultural diversification emerged only within the last 100,000 years.

Monkeys and apes are our cousins, and we all have evolved from a common ancestor over the last 60 million years. Because primates are related, they are genetically similar. Human DNA is, on average, 95 percent identical to the DNA of our most distant primate relatives, and nearly 99 percent identical to our closest relatives. That's why during the great human exodus when we sanitized the planet surface from gigantic human predators, our scientists based the 2 percent language genes and 3 percent genes for innovations and initiatives on the above data for human upgrading.

Most mammals, including pottos (tree bear) and certain other primates are colorblind and can't see the color red. Yet humans and many other primates perceive a full spectrum of color. The color vision that humans take for granted may have evolved in primates because it helped them to pick out ripe red or orange fruit against the green forest background. Color vision may also help some leaf-eating monkey species to pick out the most nutritious green leaves.

Pottos and most other "lower" primates are active at night, so it's not surprising that many of them never evolved the ability to see the color red. Reddish colors are very hard to see at night, even with full color vision. Although the eyes of many "lower" primates are specially adapted for night vision, these animals rely more on smell than sight to find food and communicate with each other.

A small proportion of humans are colorblind. If you don't see the number "5" in this image, you have red-green colorblindness, the most common form. Because of DNA differences, colorblindness is much more common among men than among women. As many as one in 14 men of European descent had red-green colorblindness.

In the past, there also were other species of humans as well as hominids more similar to us than the chimpanzees and bonobos. They will be described in the last three tutorials of this series. It has been historically difficult for people to accept that we are in fact just another primate species with African origins and that we differ physically only in degree from some of the others. The similarities can be seen throughout our bodies. The African apes and humans have essentially the same arrangement of internal organs, share all of the same bones (though somewhat different in shape and size), lack external tails, and have several important blood type systems in common. We also get many of the same diseases. Humans and the African apes have hands with thumbs that are sufficiently separate from the other fingers to allow them to be opposable for precision grips. Like all of the great apes, humans are sexually dimorphic human men are 5-10 percent larger on average and have greater upper body muscular development. Like chimpanzees and bontpercnobos, we are omnivorous. We kill other animals for food in addition to eating a wide variety of plants.

The comparatively minor anatomical differences between humans and apes are largely a result of our habitual bipedalism . A number of changes in our bodies were related to the evolution of this form of locomotion. Unlike apes, our arms are relatively short and weak compared to our legs. Our feet no longer have the ability to effectively grasp and manipulate objects because the toes became shorter and the big toe moved up into line with the others. Human feet also have lengthened and acquired an arch, making them better body supports. The human pelvis and spinal column also have been modified for an erect posture and efficient bipedal locomotion. The pelvis became shorter, broader, and more bowl shaped. This provided greater stability for walking and running. We are now essentially fully terrestrial animals. Nature very likely selected for longer legs with powerful muscles and spring-like tendons in humans because it is more efficient for walking and especially running bipedal. Longer legs require less up-and-down movement while running and, therefore, reduce the amount of energy needed to move rapidly. This relatively lower rate of energy consumption would also allow humans to travel farther with the same calorie expenditure. In addition, the largely hairless human body with its abundance of sweat glands allows us to remain cooler while running than if we only relied on panting like most other mammals. This no doubt was a major advantage for our early human ancestors in the competition with other hunters and scavengers for meat in warm climates. Humans can easily be outrun by many other animals over short distances. However, we are endurance runners and can ultimately run down virtually all other land animals.

A downside of the evolution of efficient bipedalism in humans is that it resulted in changes in the pelvis which unfortunately included a narrower birth canal in females. As a consequence, giving birth is a more difficult and riskier process for us than for most other mammal species. During delivery, human babies must partially rotate laterally twice during their passage through the oval birth canal in order for their comparatively large heads and then their broad shoulders to make it through. This is usually a long, tiring, and painful process for the mother as well as a risky one for her baby. Because of this, human mothers generally seek help from a "midwife" for the delivery. Other primates give birth without assistance. A partial evolutionary solution to this birth difficulty for humans was fetuses being born at a less mature stage, when their heads and torsos are smaller. The trade-off is that human infants are more vulnerable. By comparison, chimpanzees at birth are neurologically and cognitively ahead of human babies of the same age, but the chimps begin to fall behind by about six months old because of the more rapid continued development of the human brain following birth.

With the exception of these differences, we are quite similar to the African apes anatomically and genetically, especially to the chimpanzees and bonobos. Humans have 46 chromosomes in their cells while all of the great apes have 48. In reality, this difference is not as great as it would initially seem because the human chromosome 2 is a fusion of ape chromosomes 12 and 13 with most of the same genes.

Research on learning the entire genome of common chimpanzees was completed in 2005. A comparison between this and the human genome (completed in 2001) shows that 96 percent of DNA base pair sequences of humans and chimpanzees are the same. Most of the 4 percent difference is in duplicated non-gene segments. If only gene segments are compared, there is a 98 percent similarity. The genes that differ mostly control speech, smelling, hearing, digesting

Protein and susceptibility to certain diseases. These dissimilarities are to be expected given that we have been on essentially separate evolutionary tracks for 6-7 million years. During that time, we have been subject to somewhat different natural selection pressures. These differences led to bipedalism for our ancestors along with a much larger brain and, ultimately, speech.

The modern human brain is 3 times larger in volume than those of the great apes. More importantly, the human brain to body size ratio is significantly larger, and it has a much bigger cerebral cortex with a higher concentration of neurons. Evolving a larger brain comes at a steep energy cost. The human brain uses about 25 percent of the energy derived from the nutrients that we consume and 20 percent of the oxygen. Recent research has suggested that our intelligence advantage may be due to evolutionary changes in the HAR1F regulator gene beginning about 6 million years ago in our pre-human ancestors but not in those of chimpanzees or other apes. This gene is involved in the production of brain tissue between the 7th and 19th week after conception. It is not surprising that there are some striking differences between the great apes and humans in mental abilities.

People have much more complex forms of verbal communication than any other primate species. We are the only animal to create and use symbols as a means of communication. We also have more varied and complex social organizations. The most distinctive feature of humans is our mental ability to create new ideas and complex technologies. This has proven invaluable in the competition for survival. However, the great apes are also surprisingly intelligent, having mental levels equivalent to a 3-4 year old human child. This is sufficient to allow them to learn and use the sign language of deaf humans in at least a rudimentary way, but they do not have the capability of producing human speech and language. This is likely due to the fact that they have a different form of another key regulator gene known as FOXP2.

There is one additional curious difference between humans and all other primates that is worth noting. Older human females go through menopause and become sterile, often decades before dying of old age. Female chimpanzees, gorillas, and other non-human primates usually remain capable of conception and giving birth even when they are very old. In the wild, they live only a relatively short amount of time following menopause, if they go through it at all. One explanation for this difference in humans is that years of life following menopause has proven to have natural selection value for our species. Having raised their own children, post-menopausal women around the world often take care of their grandchildren while their daughters are working. It is argued that this increases the chances that the grandchildren will survive to adulthood because they receive this additional experienced and caring attention. Grandmothers helping daughters to raise their children also allows mothers to have additional babies before the older ones are mature enough to take care of themselves. All other primates normally do not give birth again before their current child no longer needs parental care.

The gibbons are sufficiently different to be in their own family of hominoids, the Hylobatidae. Along with the Great apes, humans are members of the Hominidae family. Of all living species, people are genetically and evolutionarily closest to the African apes. Subsequently, we have been placed into the same subfamily, the Homininae.

A comparison of DNA nucleotide sequences of living primate species show that humans are most closely related to the African apes. Next in descending order of genetic closeness to us come the Asian apes, Old World monkeys, New World monkeys, tarsiers, and finally the lemurs and lorises. This genetic comparison corresponds exactly with a comparison of homologous primate physical traits. It also fits nicely with what we know from the fossil record. The prosimians were the first to evolve. Next come the monkeys, then the apes, and finally humans.

A draft of the rhesus macaque monkey complete genome was completed in 2007. Comparing this genome with those already established for chimpanzees and modern humans will provide an even better tool for understanding the similarities and differences between the major groups of primates. Preliminary analysis indicates that macaques are similar to humans in 93 percent of their DNA sequences, while chimpanzees share about 96 percent of their DNA sequences with us. This somewhat lower percentage for macaques is to be expected because the last common ancestor of macaques and humans was about 19 million years ago, while the chimpanzees and human evolutionary lines diverged only around 6-7 million years ago.

Human Transformation

Human transformation is the lengthy process of change by which people originated from apelike ancestors. Scientific evidence shows that the physical and behavioral traits shared by all people originated from ape-like ancestors and evolved over a period of at least 5 million years.

One of the earliest defining human traits is bipedalism -- the ability to walk on two legs -- evolved over 4 million years ago. Other important human characteristics -- such as a large and complex brain, the ability to make and use tools, and the capacity for language -- developed more recently. Many advanced traits -- including complex symbolic expression, art, and elaborate cultural diversity -- emerged mainly during the past 100,000 years.

Humans are primates. Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the great apes (large apes) of Africa -- chimpanzees (including bonobos, or so-called "pygmy chimpanzees") and gorillas -- share a common ancestor that lived between 5 and 8 million years ago. Humans first evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between 2 and 5 million years ago come entirely from Africa.

Most scientists currently recognize some 10 to 15 different species of early humans. Scientists do not all agree, however, about how these species are related or which ones simply died out. Many early human species -- certainly the majority of them -- left no living descendants. Scientists also debate over how to identify and classify particular species of early humans, and about what factors influenced the evolution and extinction of each species.

Early humans first migrated out of Africa into Asia probably between 1.6 million and 2 million years ago. They entered Europe somewhat later, generally within the past million years. Species of modern humans populated many parts of the world much later. For instance, people first came to Australia probably within the past 60,000 years and to the Americas within the past 30,000 years or so. The beginnings of agriculture and the rise of the first civilizations occurred within the past 10,000 years.

Paleoanthroplogy

Paleoanthropology is the scientific study of human evolution and a subfield of anthropology, the study of human culture, society, and biology. The field involves an understanding of the similarities and differences between humans and other species in their genes, body form, physiology, and behavior. Paleoanthropologists search for the roots of human physical traits and behavior. They seek to discover how evolution has shaped the potentials, tendencies, and limitations of all people. For many people, paleoanthropology is an exciting scientific field because it investigates the origin, over millions of years, of the universal and defining traits of our species. However, some people find the concept of human evolution troubling because it can seem not to fit with religious and other traditional beliefs about how people, other living things, and the world came to be. Nevertheless, many people have come to reconcile their beliefs with the scientific evidence.

Early human fossils and archeological remains offer the most important clues about this ancient past. Thus remains include bones, tools and any other evidence (such as footprints or butchery marks on animal bones) left by earlier people. Usually, the remains were buried and preserved naturally. They are then found either on the surface (exposed by rain, rivers, and wind erosion) or by digging in the ground. By studying fossilized bones, scientists learn about the physical appearance of earlier humans and how it changed. Bone size, shape, and markings left by muscles tell us how those predecessors moved around, held tools, and how the size of their brains changed over a long time. Archeological evidence refers to the things earlier people made and the places where scientists find them. By studying this type of evidence, archeologists can understand how early humans made and used tools and lived their environments.

The Process of Transformation

The process of transformation involves a series of natural changes that cause species (populations of different organisms) to arise, adapt to the environment, and become extinct. All species or organisms have originated through the process of biological evolution. In animals that reproduce sexually, including humans, the term species refers to a group whose adult members regularly interbreed, resulting in fertile offspring -- that is, offspring themselves capable of reproducing. Scientists classify each species with a unique, two-part

scientific name. In this system, modern humans are classified as Homo sapiens.

Evolution occurs when there is change in the genes (the chemical molecule, DNA) inherited from the parents and especially in the proportions of different genes in a population. The information contained in genes can change by a process known as mutation. The way particular genes are expressed – that is, how they influence the body or behavior of an organism -- can also change. Genes affect how the body and behavior of an organism develop during its life, and this is why genetically inherited characteristics can influence the likelihood of an organism's survival and reproduction. Evolution does not change any single individual. Instead, it changes the inherited means of growth and development that typify a population (a group of individuals of the same species living in a particular habitat). Parents pass adaptive genetic changes to their offspring, and ultimately these changes become common throughout a population. As a result, the offspring inherit those genetic characteristics that enhance their chances of survival and ability to give birth, which may work well until the environment changes. Over time, genetic change can alter a species' overall way of life, such as what it eats, how it grows, and where it can live. Human evolution took place as new genetic variations in early ancestor populations favored new abilities to adapt to environmental change and so altered the human way of life.

In everyday understanding, the "ascent of man" is a steady upward progression: Some 7 million years ago our slope-browed, knuckle-dragging ancestors break with the future chimpanzees and learn by turns to walk upright, grow bigger brains, shape bones and stones into tools -- then cap the whole process by developing language. No wonder so many modern-day Homo sapiens, skeptical that the randomness of natural selection could produce such a tidy plot line, insist there must be a creator (or at least a creative intelligence) behind the story.

Subsequent Foot Notes of Izra's Great, Great Ancestors … Marked "X" stated:

The consensus of Planetary Federation of 12 planetary members when Planet Urnyx was about six and one half billion Earth years old and the young planet Earth was about two and one half billion years old, ordered to upgrade Earth's population so that any planet member or members that desire to utilize human as workers for the activities could easily understand and trainable. However, there was problem for plenty of huge animals that will hinder any places of activities and the needs to sanitize the planet for good. The Federation approved the suggestion but, first we have to salvage human population by camping them in the South Pole though barren place but free from human's predators and plenty of foods for them until the Earth is clean and no more huge animals to prey them.

Those specific period Antarctica was uncovered of ice and very wide area for human to hunt and the climate is just suitable for the indigenous beings. So squadrons of space ships were sent to relocate selected humans all over planet Earth to the mother ship that was being parked in Antarctica's valley and one by one were upgraded implanting "intelligence genes" composing 3 percent idea and 2 percent language and understanding. Ninety five percent of the human genes were considered junk genes and the Federation surgeons had to program to adapt the foreign genes and to evolve collectively and compatibly. After a needed number of population were attained, squadrons of drones bombarded the whole planet Earth with fires until all mammals were incinerated. However, some species of reptiles survived by seeking refuge on holes and some in water. The process were repeated until all mammals were eliminated and waited for the vegetation to recovered. Some small animals that survived started to bred as vegetation progresses and some small animals species that were beneficial to the settlement were introduced from other planets. When the planet surfaces stabilized, the Federation started distributing humans around the planet and let them settled by themselves.

The rapid rising snow built up were not anticipated by the Planetary Federation Mother Ship crews and waited to subside as they were on maintenance scheduled and not able to flight until was buried on a mountain of ice of more than two kilometers thick. Some human workers who were retained in the mother ship had interbred with the Federation crews. However, planet Earth had reached the maximum tilt on its axis and had to back for another 24 degrees that caused another Poles freezing. Earth surface areas that were adjacent to the Poles and do not had huge animals detrimental to human safety were not included to the planet sanitation and culling continued to evolved. End of Foot Notes and another marking "X"

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Homo floresiensis, is a new human ancestor whose remains were found on remote island near Java – known to be Flores Man. They appear to have been about 3 feet tall, with grapefruit-sized brains, and to have hunted skillfully and cooperatively with stone tools. Their discoverer believes they made these tools, and also employed language; other anthropologists are unpersuaded on the first point, skeptical on the latter.

But it's the undisputed aspects of this discovery that are the most riveting. Flores Man survived into relatively modern times -- at least up to 13,000 years ago, and therefore much later than the Neanderthals of Europe, whose disappearance about 33,000 years ago had been considered the end of our archaic ancestors.

Indeed, the skeletal remains found on Flores Island dovetail neatly with the lore of modern day residents, the Ngadha, who say the little people were around until the arrival of Dutch trading ships in the 1500s. They were hairy and gentle, the Ngadha say; they lived in the caves and accepted bowls of food we put out for them.

Flores Man's closest probable relative among pre-humans lived about 1.7 million years ago in the formerly Soviet republic of Georgia. The journey to Java could have been accomplished over land, before the breakup of land masses, but how to explain the final leg to Flores, an island for 2.6 million years? Water crossing were most likely done by raft on the narrowest land gaps.

And then the interaction of organism and environment gets really interesting. The isolation of Flores Man invokes the "island rule" -- in the absence of large predators, whose threats make bigness a virtue, creatures larger than rabbits become smaller as an adaptation to food scarcity. Conversely, creatures smaller than rabbits get bigger, especially if they're predators. Thus Flores Man was home not only to down-sizing pre-humans but also to early elephants that shrank to water-buffalo size, and to little lizards that swelled into three-foot-long Komodo dragons.

"Hobbit" to the hominid remains, but the story of Flores Man -- even in today's sketchy outline -- is compelling in a way that Tolkien's trilogy can't match. It arises not as the product of imagination, but as a long series of accidents -- collisions between our ancestors and environmental strictures that, contrary to previous supposition, favored bigger stature and better organized brain for higher degree of cognition.

The persistence of Flores Man into such relatively recent times is equally remarkable, and has fueled some speculation than others among our evolutionary cousins might still be living in the unexplored reaches of the planet. That possibility remains remote -- but, it must be said, a bit less remote. Humankind's family tree has suddenly become significantly bushier, as understanding of human origins continues to evolve.

Cognition is the process by which a species learns, remembers, and solves problems with flexible behavior that may change depending on the situation, motivation level, and environmental pressures. Cognition in nonhuman primates is adaptive because it increases efficiency. Behavioral responses can go beyond trial-and-error learning and use problem solving and reasoning based on input from the environment, past experiences, and knowledge of the social environment. Nonhuman primate (hereafter primate) cognition is important to the understanding of the evolution of human minds, as well as a better understanding of the underlying cognitive mechanisms of primate behavior.

Humans are a primate and a member of the great apes, with chimpanzees and bonobos (to whom we are the most closely related), gorillas, and orangutans. We are next most closely related to the Old World monkeys (common ancestor ∼25 million years ago), followed by the New World monkeys (common ancestor ∼40 million years ago) and the prosimians (lorises, lemurs, and tarsiers; common ancestor ∼80 million years ago). While all primate species are interesting in their own right, those phylogenetically closest to humans are often studied based on the assumption that they are a better comparison to human behavior and cognition.

Physical and social cognition is essence to human of which are important to individuals' success. Physical cognition addresses the skills used by primates to survive in their physical environment, including foraging skills, defense mechanisms, learning, memory, and problem solving. Social cognition is equally important to survival, as most primates are highly social, interacting with many other individuals on a regular basis. Social cognition provides skills for interacting with others, both opponents and collaborators, in situations ranging from defending one's group to finding a mate. We first focus on physical cognition, including object manipulation and tool use, features and categorization, numerology, delay of gratification, memory, and metacognition. Next, we turn to social cognition, including social intelligence, cooperation, decision-making, social learning, communication, deception, and theory of mind.

Another highly contentious area of primate cognition is theory of mind. As mentioned earlier, this aspect of primate cognition is not strictly about reward. Yet, the capacity to make inferences about what others know, desire, believe, and intend often leads directly to cooperative and competitive rewards, and most of the experimental studies use food as a motivator to explore these mental states.

Early studies failed to find evidence of theory of mind in primates. One explanation for these failures was that the studies relied on animals making inferences about human mental states in contexts involving cooperation, as opposed to asking animals to make inferences about each other in the context of competition. Though several primate species do cooperate (see below), competition is far more common and may represent the most basic form of social interaction among animals. To test this hypothesis, recent studies of monkeys and apes have used both competitive and cooperative tasks, contrasting conspecific pairings as well as conspecific–hetero-specific pairings. There is increasing evidence that monkeys and apes make inferences from seeing to knowing. Chimpanzees, for instance, can take the visual perspective of other individuals to understand whether they can see occluded food rewards: subordinate chimpanzees preferentially approach rewards that are obscured from the view of dominant individuals.

Rhesus monkeys show similar abilities, stealing food primarily from experimenters without visual access to the food rather than from those that do have access. Though visual perspective taking is only one component of having a theory of mind, it plays a significant role in the competitive and cooperative interactions of primates. What remains to be explored is whether primates go beyond this elemental capacity, attributing intentions, desires, and beliefs to others.

Given what is known about nonhuman primate cognition, it is likely that the first hominids 6 million years ago already conceptualized a sensory-motor world of permanent objects—arrayed in a representational space and categorized and quantified in some fundamental ways—and a social world of animate conspecifics with whom they could cooperate and compete in complex ways. For the next 4 million years (as Australopithecines) changes in hominid cognition were minimal.

However, early Homo (including Homo erectus) began to demonstrate some new skills of causal understanding in the context of complex tool manufacture and use. Then, the earliest modern humans 150,000 years ago evolved some new skills of social cognition that fundamentally changed the process of human cognitive evolution because they enabled uniquely powerful processes of cultural learning and cumulative cultural evolution—which transformed the ontogenetic niche in which human cognition develops ontogenetically from a social into a truly cultural niche including artifacts, symbols, and social practices with accumulated histories. This ability to pool cognitive resources may even have been the competitive advantage that modern humans had over other contemporaneous hominids.

The social world has long been thought to be a major force shaping primate cognition: the social lives of primates are thought to be sufficiently complex to have acted as a driving force in primate cognitive evolution. This basic thesis – that the sophisticated cognitive abilities of primates have evolved for a social function – has spurred experimental and theoretical investigations for over 40 years. Although, most early proposals of the social intelligence hypothesis were inspired by observations of seemingly complex social behaviors across the primates, however, the psychological mechanisms underlying these behaviors were not well understood. It was not until relatively recently that research began to address the cognitive abilities primates actually use when interacting with others, such as whether other primates share capacities like theory of mind with humans.

In this article, we highlight a selection of complex behaviors that primates exhibit when interacting with others, with special attention to the cognitive mechanisms supporting those behaviors. Fundamental to the study of comparative cognition is the idea that many species may exhibit behaviors that appear similar, even though the psychology underlying those behaviors may differ across taxa. While this distinction may be methodologically frustrating – behavioral observations are a rich source of knowledge about animal psychology, and experimental tests of cognition may not always be viable – in fact, it is a testament to the ingenuity of evolution: the hard social problems that primates face get solved, even if the solution is not always the same! This distinction highlights the importance of thinking about primate social interactions not only in the context of behavioral evolution – the special things that primates (and humans) do – but also in terms of cognitive evolution – the special ways that primates think. We use this framework to analyze primate social behavior, and the differing psychologies underlying this behavior, in three areas: gaze-following, food competition, and mutualistic cooperation. The ultimate challenge of such analyses will be to understand why such different cognitive mechanisms have evolved across species.

We shall discuss why behavioral studies of irrational biases in non-human primates are important for the field of neuroeconomics. We begin with a review of how behavioral work on choice biases in monkeys is important for understanding the nature of human choice errors. We then provide an introduction to the primate cognition approach, including a short overview of the organization of the primate order. We then briefly review the ecology and cognition of two primate species standardly used as models of human irrational decision making — brown capuchins and rhesus macaques. We next discuss empirical studies demonstrating that monkeys show human-like irrational errors in three of the classic situations in which human participants fall prey to biases: monkeys exhibit framing effects in risky decisions, they show endowment effects, and they are averse to ambiguous outcomes. We conclude our chapter with a discussion of how future work in neuroeconomics can capitalize on these new behavioral findings in monkeys.

The Expression of Emotions in Animals and Man' drew several parallels between the facial expressions of nonhuman primates and those of human beings. However, it was psychologists rather than evolutionary biologists who began the systematic study of primate behavior and cognition at the beginning of the twentieth century.

Many of these experiments involved the manipulation of the environment to obtain food rewards and the use of previously familiar objects in novel and instrumental ways. Some researchers question some of its procedures, innovations and some of its findings are still highly cited in contemporary research. It can certainly be considered one of the founders of modern research on primate cognition.

Along with the growing recognition that primate behavior could be useful to understand human behavior by increasing interest in studying primates in their natural habitat and understanding the basic principles regulating their social organization. Primatologists intensive and long-term studies of social behavior led to the discovery of kinship systems and cultural traditions in macaque societies. Primate behavior research however was originally conducted within the tradition of anthropology and it is only later that such studies established a strong connection with psychological science. As more information on primate social behavior became available,

anthropologists developed the conviction that extant primate species could provide important information on human origins and social evolution.

Psychologists' interest in primate behavior rose dramatically with the resumption of research in captivity after War . Further research played a pivotal role in this process. After making important contributions to the study of primate learning, anthropologists concentrated their efforts on elucidating the nature of infant attachment and social development in rhesus monkeys. Experiments with surrogate mothers demonstrated that the mother's ability to provide contact comfort is a more important determinant of infant attachment than her ability to provide milk, thus providing a fatal blow to secondary-drive theories of attachment. Because Harlow's work touched upon many areas of research that were very important to psychologists at that time (e.g., learning and motivation, attachment, normal and abnormal social development, the social origin of affective disorders), and because works took place within psychology, during the years in which most of the work was conducted and published, primate behavior research was very well known among psychologists.

Although it was very effective in promoting the importance of primate's behavior research in the scientific community and the general public. The most systematic effort to conceptually integrate primatology and psychology provide interest in primate research and encouraged to set up a colony of rhesus monkeys in Cambridge and investigate mother–infant attachment processes. From the study of social influences on the mother–infant relationship, the scope of research was gradually broadened and elaborated into a conceptual framework for the study of social processes, which distinguished three main levels of complexity: interactions, relationships, and social structure. The important conceptual contributions to the science of social relationships, and for decades was one of the most articulate proponents of the conceptual integration between biological and psychological approaches to the study of behavior.

The success of field studies of primate behavior reached a peak in the newly established New World Central Hub Primate Research Centers, and most research proposals to study primate behavior were readily funded by the Federation Union.

The realization that evolutionary theory could be effectively applied to the study of social behavior gave a great boost to primate research in the field. Anthropology and psychology had been the disciplines dominating primate behavior research and evolutionary biology acquired a leading role in most subsequent studies. The fact that behavioral ecologists were mostly interested in questions of adaptive function whereas psychologists were mostly interested in questions of proximate causation or development of behavior was one of the several factors that contributed to the growing separation between primate behavior research and psychological science. Another important factor was the rapid progress of biological disciplines such as genetics, molecular biology, and neuroscience and the growing popularity of scientific reductionism. In particular, the success of neuroscience led to the optimistic view that many important questions about behavior would eventually be answered by studies of brain anatomy and function, thus rendering behavioral research unnecessary. One corollary of this view was the belief that comparative research with primates may not be as useful as research with other species, given the difficulty of conducting molecular work with primates.

Sequence of Transformation

Human beings belong to the mammalian group known as Primates -- the scientific category that contains over 230 species of lemurs, lorises, tarsiers, monkeys of the Old and New World, and apes. Modern humans, early humans, and other primate species all share many similarities and have some important differences. Knowledge of these similarities and differences helps scientists to understand the roots of many human traits and the significance of each development in human evolution.

All primates, including humans, share at least part of a set of common characteristics that distinguish them from other mammals. Many of these characteristics evolved as adaptations for life in the trees, an environment in which the earliest primates evolved. These characteristics include more reliance on sight than smell; overlapping fields of vision, allowing stereoscopic (three-dimensional) sight; limbs and hands adapted for clinging on, leaping from and swinging in the trees; the ability to grasp and manipulate small objects (using fingers with nails instead of claws); large brains in relation to body size; and complex social lives.

The scientific classification of primates reflects evolutionary relationships among individual species and groups of species. Strepsirhine (meaning "wet nosed") primates -- of which the living representatives include lemurs, lorises, and other groups of species -- are all commonly known as prosimians. Strepsirhines are the most primitive of living primates. They share all of the basic characteristics of primates, although their brains are neither particularly large nor complex and they have a more elaborate and sensitive olfactory system (involved in the sense of smell) then do other primates.

The earliest monkeys and apes evolved from ancestral haplorhine (meaning "dry nosed") primates, of which the most primitive living representative is the tarsier. Tarsiers were previously grouped with prosimians, but many scientists now recognize that tarsiers, monkeys, and apes share some distinctive traits, and group the three together. Monkeys, apes, and humans -who share many traits not found in other primates -- together make up the suborder Anthropoidea. Anthropoid primates are divided into New World (South America, Central America, and the Caribbean Islands) and Old World (Africa and Eurasia) groups. The platyrrhine (broad-nosed) monkeys represent the first, and the second is the catarrhine (downward-nosed) monkeys and apes. Humans belong to this second group.

Apes and humans together make up the superfamily Hominoidea, a grouping that emphasizes the close relationship among these species. Scientists do not all agree about the appropriate classification of the families within this superfamily. Living hominoids are grouped into either two or three families: Hylobatidae, Hominidae, and sometimes Pongidae. Hylobatidae consists of the small or so-called lesser apes of Southeast Asia, commonly known as gibbons and siamangs. The Hominidae (hominids) include humans and, according to some scientists, the great apes. For those who include only humans among the Hominidae, all of the great apes, including the orangutans of Southeast Asia, belong to the family Pongidae.

Traditionally, the term "hominid" has referred to species of humans that evolved after the split between early humans and other ape lineages. But genetic evidence, which shows chimps and humans to be more closely related genetically (and evolutionarily) to each other than to any other ape, supports placing all of the great apes and humans together in the family of Hominidae. According to this reasoning, the evolutionary branch of Asian apes leading to orangutans, which separated from the other hominid branches by about 13 million years ago, belongs to the subfamily of Ponginae. The African apes (gorillas, chimpanzees, and humans) are then classified in the subfamily called Homininae (or hominines). And finally, the line of early and modern humans belongs to the tribe (classificatory level above genus) Hominini, or hominins.

This classification would be true to the genetic evidence. But it tends to be confusing when learning about the subject, as many similar names (hominoid, hominid, hominine, and hominin) would apply to the different aspects of ape and human evolution. In this article the term "early human" refers to all species of the human family tree since the divergence from a common ancestor with the African apes. Popular writing often still uses the word "hominid" to mean the same thing.

Humans as Primates

About 98 percent of the genes in people and chimpanzees are identical, making chimps the closest living biological relatives of humans. This does not mean that humans evolved from chimpanzees, but it does indicate that both species evolved from a common ape ancestor. Orangutans, the great apes of Southeast Asia, differ genetically from humans to a greater extent, indicating a more distant evolutionary relationship.

Modern humans have a number of physical characteristics indicative of an ape ancestry. For instance, people have shoulders with a wide range of movement and fingers capable of strong grasping. In apes, these characteristics are highly developed as adaptations for brachiation (swinging from branch to branch in trees). Although humans do not brachiate, the general anatomy of that earlier adaptation still remains. Both people and apes also have larger brains and greater cognitive abilities than do most other mammals.

Human social life, too, shares similarities with that of African apes and other primates -- such as baboons and rhesus monkeys -- that live in large and complex social groups. Group behavior among chimpanzees, in particular, strongly resembles that of humans. For instance, chimps form long-lasting attachments with each other; participate in social bonding activities, such as grooming, feeding, and hunting; and form strategic coalitions with each other in order to increase their status and power. Early humans also probably had this kind of elaborate social life.

However, modern humans fundamentally differ from apes in many significant ways. For example, as intelligent as apes are, people's brains are much larger and more complex, and people have a unique intellectual capacity and elaborate forms of culture and communication. In addition, only people habitually walk upright, can precisely manipulate very small objects, and have a throat structure that makes speech possible.

The Fossil Primates

The origin of the mammalian group primates is traced back to Plesiadapiformes, the last common ancestors of strepsirhines and other mammals. Plesiadapiformes evolved at least 65 million years ago. They were creatures similar to the modern tree shrews. The earliest primates evolved by about 55 million years ago. The first strepsirhine primates, fossil species similar to lemurs and tarsiers, evolved during the Eocene epoch (about 56 to 34 million years ago). The oldest lineages of catarrhine primates, from which monkeys and apes evolved, are known between 50 and 33 million years ago. A primate known as Propliopithecus.

Plesiadapiformes is a group of Primates, a sister of the Dermoptera. While none of the groups normally directly assigned to this group survived, the group appears actually not to be literally extinct (in the sense of having no living descendants) as the remaining primates (the crown primates or "Euprimates") appear to be derived Plesiadapiformes, as a sister of e.g. the Carpolestidae. The term Plesiadapiformes may still be used for all primates in which are not crown primates, but this usage is paraphyletic. When the crown primates are cladistically granted, it becomes an obsolete junior synonym to primates. Purgatorius is believed to be a basal Plesiadapiformes.

Plesiadapiformes first appear in the fossil record between 65 and 55 million years ago, although many were extinct by the beginning of the Eocene. They may have been the first mammals to have finger nails in place of claws. It was proposed that paromomyid Phenacolemur had digital proportions of the fossil indicated gliding habits similar to that of colugos.

In the following simplified cladogram, the crown primates are found to be highly derived Plesiadapiformes, possibly as sister of the Plesiadapoidea. The crown primates are cladistically granted here into the Plesiadapiformes, and the 'plesiadapiformes' become a junior synonym of the primates. With this tree, the plesiadapiformes are not literally extinct (in the sense of having no surviving descendants). The crown primates are also called "Euprimates" in this context.

It is thought to be what the common ancestor of all later Old World monkeys and apes looked like. So Propliopithecus may be considered an ancestor, or closely related to a direct ancestor, of humans.

Hominoids, or members of the super-family Hominoidea, evolved during the Miocene epoch (24 million to 5 million years ago). Hominoidea are clade of Old World simians native to Africa and Southeast Asia, the other being its sister group Cercopithecidae, together forming the catarrhine clade. The New World monkeys diverged earlier from the old world stock of monkeys, by settling across the Atlantic ocean. They are distinguished from other primates by a wider degree of freedom of motion at the shoulder joint as evolved by the influence of brachiation. Apes do not have tails, apparently due to a mutation of the TXBT gene. In traditional and non-scientific use, the term "ape" can include tailless primates taxonomically considered Cercopithecidae (such as the Barbary ape and black ape), and is thus not equivalent to the scientific taxon Hominoidea. There are two extant branches of the superfamily Hominoidea: the gibbons, or lesser apes; and the hominids, or great apes.

The family Hylobatidae, the lesser apes, include four genera and a total of 20 species of gibbon, including the lar gibbon and the siamang, all native to Asia. They are highly arboreal and bipedal on the ground. They have lighter bodies and smaller social groups than great apes.The family Hominidae (hominids), the great apes, include four genera comprising three extant species of orangutans and their subspecies, two extant species of gorillas and their subspecies, two extant species of panins (bonobos and chimpanzees) and their subspecies, and humans in a single extant subspecies. Except for gorillas and humans, hominoids are agile climbers of trees. Apes eat a variety of plant and animal foods, with the majority of food being plant foods, which can include fruit, leaves, stalks, roots and seeds, including nuts and grass seeds. Human diets are sometimes substantially different from that of other hominoids due in part to the development of technology and a wide range of habitation. Humans are by far the most numerous of the hominoid species, in fact outnumbering all other primates by a factor of several thousand to one.All non-human hominoids are rare and endangered. The chief threat to most of the endangered species is loss of tropical rainforest habitat, though some populations are further imperiled by hunting for bush meat.

Large ape species had originated in Africa by 23 or 22 million years ago. Among the oldest known hominoids is a group of apes known by its genus name, Proconsul. Species of Proconsul had features that suggest a close link to the common ancestor of apes and humans. The ape species Proconsul heseloni lived in dense forests of eastern Africa about 20 million years ago. It was agile in the trees, with a flexible backbone and narrow chest of a monkey, yet capable of wide movement of the hip and thumb as in apes.

Early in their evolution, the large apes underwent several radiations, periods when species originated and became more diverse. After Proconsul had thrived for several million years, a group of apes from Africa and Arabia known as the afropithecines evolved around 18 million years ago and diversified into several species. By 15 million years ago, apes had migrated to Asia and Europe over a land bridge formed between the Africa-Arabian and Eurasian continents, which had previously been separated. Around this time, two other groups of apes had evolved – namely, the kenyapithecines of Africa and western Asia (first known about 15 million years ago) and the dryopithecines of Europe (first known about 12 million years ago). It is not yet clear, however, which of these groups of ape species may have given rise to the common ancestor of African apes and humans.

Early Australopiths

The australopiths can be divided into an early group of species (sometimes known as gracile australopiths), which arose prior to 3 million years ago; and a later group, known as robust australopiths, which evolved after 3 million years ago. The earlier australopiths -- of which several species evolved between 4.4 million and 3 million years ago -- generally had smaller teeth and jaws. The later robusts had larger faces with large jaws and cheek teeth.

A 5-million-year-old jaw fragment with one molar tooth, found in Kenya, and another jaw with two molars, about 4.5 million years old, may be the oldest australopith fossils. But scientists have not yet agreed on the matter since these fossils are so fragmented and do not tell us about the canine teeth or bipedal walking. Several of the early australopiths are given the genus name Australopithecus. Yet some of the oldest finds of australopith bones, dated about 4.4 million years old, have been given a different name because of their very ancient combination of apelike and humanlike traits. These fossils, first discovered in Ethiopia in 1994, are called Ardipithecus ramidus.

By at least 4.4 million years ago in Africa, an apelike species had evolved that had two important traits, which distinguished it from other apes: (1) small canine (eye) teeth (next to the incisors, or front teeth) and (2) bipedalism--that is the ability to walk on two legs. Scientists commonly refer to these earliest human species as australopithecines, or australopiths for short. The earliest australopith species known today belongs to the genus Ardipithecus. Other species belong to the genus Australopithecus and, by some classifications, Paranthropus. The name australopithecine translates literally as "southern ape," in reference to South Africa, where the first known australopith fossils were found.

Countries in which scientists have found australopith fossils include Ethiopia, Tanzania, Kenya, South Africa, and Chad. Thus, australopiths ranged widely over the African continent. The Great Rift Valley of eastern Africa, in particular has become famous for its australopith finds because past movements in Earth's crust in this region were favorable to environments in which bones are easily preserved and, later, to exposure of ancient deposits of fossilized bones.

There are many ideas about why the early australopiths split off from the apes, initiating the course of human evolution. Virtually all hypotheses invoke environmental change as an important factor, specifically in influencing the evolution of bipedalism. Some well-established ideas about why humans first evolved include (1) the savanna hypothesis, (2) the woodland-mosaic hypothesis, and (3) the variability hypothesis.

The savanna hypothesis argues that the Miocene forests of Africa became sparse and broken up between 5 and 8 million years ago due to a cooler and drier global climate. This drying trend led to the separation of an ape population in eastern Africa from other populations of apes in the more heavily forested areas of western Africa. The eastern population had to adapt to drier, open savanna environments, which favored the evolution of terrestrial living. Terrestrial apes might have formed large social groups in order to improve their ability to find and collect food and to fend off predators. The challenges of savanna life might also have promoted the rise of tool use, for purposes such as scavenging meat from the kills of predators. These important evolutionary changes would have depended on increased mental abilities and, therefore, may have correlated with the development of larger brains in early humans.

First, an early australopith jaw similar to australopiths afarensis has been found in Chad in west-central Africa, 2500 kilometers west of the African rift valley. This find suggests that australopiths ranged widely over the African continent and that East Africa may not have been fully separated from environments further west. Second, there is growing evidence that open savannas were not prominent in Africa until sometime after 2 million years ago.

The woodland-mosaic were the early australopiths evolved in a mosaic of woodland and grassland that offered opportunities for feeding both on the ground and in the trees. Ground feeding then favored regular bipedal activity and, eventually, the evolution of anatomical features of the hip, leg, and foot that assisted this form of locomotion.

In addition to woodland mosaic, variability suggested that early australopiths experienced many changes in environment and ended up living in a range of habitats, including forests, open-canopy woodlands, and savannas. In response, their populations became adapted to a variety of surroundings. Evidence from early australopith sites, in fact, shows this range of habitats. So the unique appearance of their skeletons may have allowed them the versatility of living in habitats with many or few trees.

The idea of evolution in any form was controversial enough in the middle of the nineteenth century. Claiming that humanity had been shaped by evolution was explosive. There was also a scientific barrier. Darwin had access to almost no fossil evidence that might indicate how, when or even where humans evolved.

In the intervening years the human – or hominin, to use the proper term – fossil record has expanded enormously. There is still much to discover, but the broad picture of our evolution is largely in place. We know that our evolutionary tree first sprouted in Africa. We are sure that our closest living relatives are chimpanzees, and that our lineage split from theirs about 7 million years ago.

The road to humanity was a long one, however. Nearly 4 million years later, our ancestors were still very ape-like. Lucy, a famous 3.2-million-year-old human ancestor discovered in Ethiopia, had a small, chimp sized brain and long arms that suggest her species still spent a lot of time up trees, perhaps retreating to the branches at night as chimps still do. But she did have one defining human trait: she walked on two legs.

Lucy belongs to a group called the australopiths. In the 40 years since her partial skeleton was discovered, fragmentary remains of even older fossils have been found, some dating back 7 million years. These follow the same pattern: they had chimp-like features and tiny brains but probably walked on two legs.

Australopithecus from Latin word australis (southern), is a genus of early hominins that existed in Africa during the Late Pliocene and Early Pleistocene. The genus Homo (which includes modern humans) emerged within Australopithecus, as sister to e.g. Australopithecus sediba. Also the genera Paranthropus and Kenyanthropus The Avestan Hymn to Mithra is the longest, and one of the best-preserved, of the Yashts. Mithra is described in the Zoroastrian Avesta scriptures as "Mithra of Wide Pastures, of the emerged within the Australopithecus. Australopithecus is a member of the sub-tribe Australopithecina, which sometimes also includes Ardipithecus, though the term "australopithecine" is sometimes used to refer only to members of Australopithecus. Species include A. garhi, A. africanus, A. sediba, A. afarensis, A. anamensis, A. bahrelghazali and A. deyiremeda. Debate exists as to whether some Australopithecus species should be reclassified into new genera, or if Paranthropus and Kenyanthropus are synonymous with Australopithecus, in part because of the taxonomic inconsistency.

The earliest known member of the genus, A. anamensis, existed in eastern Africa around 4.2 million years ago. Australopithecus fossils become more widely dispersed throughout eastern and southern Africa (the Chadian A. bahrelghazali indicates the genus was much more widespread than the fossil record suggests), before eventually becoming extinct 1.9 million years ago (or 1.2 to 0.6 million years ago if Paranthropus is included). While none of the groups normally directly assigned to this group survived, Australopithecus gave rise to living descendants, as the genus Homo emerged from an Australopithecus species at some time between 3 and 2 million years ago.

Australopithecus possessed two of three duplicated genes derived from SRGAP2 roughly 3.4 million years ago and 2.4 million years ago (SRGAP2B and SRGAP2C), the second of which contributed to the increase in number and migration of neurons in the human brain. Significant changes to the hand first appear in the fossil record of later A. afarensis about 3 million years ago (fingers shortened relative to thumb and changes to the joints between the index finger and the trapezium and capitate).

We also know that australopiths probably made simple stone tools. These advances aside, australopiths weren't that different from other apes. Only with the appearance of true humans – the genus Homo – did hominins begin to look and behave a little more like we do. Few now doubt that our genus evolved from a species of australopith, although exactly which one is a matter of debate. It was probably Lucy's species Australopithecus afarensis, but a South African species, Australopithecus sediba, is also a candidate. It doesn't help that this transition probably occurred between 2 and 3 million years ago, a time interval with a very poor hominin fossil record.

The earliest species of Homo are known from only a few bone fragments, which makes them difficult to study. Some doubt that they belong in our genus, preferring to label them as australopiths. The first well-established Homo, and the first that we would recognise as looking a bit like us, appeared about 1.9 million years ago. It is named Homo erectus.

Homo Erectus: the Toolmaker

Erectus was unlike earlier hominins. It had come down from the trees completely and also shared our wander lust: all earlier hominins are known only from Africa, but Homo erectus fossils have been discovered in Europe and Asia too. Homo erectus was also an innovator. It produced far more sophisticated tools than had any of its predecessors, and was probably the first to control fire. Some researchers think that it invented cooking, improving the quality of its diet and leading to an energy surplus that allowed bigger brains to evolve. It is certainly true that the brain size of Homo erectus grew dramatically during the species 1.5-million-year existence. Some of the very earliest individuals had a brain volume below 600 cubic centimeters, not much larger than an australopith, but some later individuals had brains with a volume of 900 cubic centimeters.

Homo erectus (upright man) is an extinct species of archaic human from the Pleistocene with its earliest occurrence about 2 million years ago as H. heidelbergensis and H. antecessor with the former generally considered to have been the ancestor to Neanderthals, Denisovans, and modern humans, appear to have emerged within the possibly Asian populations of H. erectus. Its specimens are among the first recognizable members of the genus Homo. H. erectus was the first human ancestor to spread throughout Eurasia, with a continental range extending from the Iberian Peninsula to Java. Asian populations of H. erectus may be ancestral to H. floresiensis and possibly to H. luzonensis. The last known population of H. erectus is H. e. soloensis from Java, around 117,000–108,000 years ago.

H. erectus had a more modern gait and body proportions, and was the first human species to have exhibited a flat face, prominent nose, and possibly sparse body hair coverage. Though brain size certainly exceeds that of ancestor species, capacity varied widely depending on the population. In older populations, brain development seemed to cease early in childhood, suggesting that offspring were largely self-sufficient at birth, thus limiting cognitive development through life. H. erectus was an apex predator. Nonetheless, sites generally show consumption of medium to large animals, such as bovines or elephants, and suggest the development of predatory behaviour and coordinated hunting. H. erectus is associated with the Acheulean stone tool industry, and is postulated to have been the earliest human ancestor capable of using fire, hunting and gathering in coordinated groups, caring for injured or sick group members, and possibly seafaring and art (though examples of art are controversial, and are otherwise rudimentary and few and far between).

H. erectus males and females may have been roughly the same size as each other (i.e. exhibited reduced sexual dimorphism), which could indicate monogamy in line with general trends exhibited in primates. Size, nonetheless, ranged widely from 146–185 cm (4 ft 9 in – 6 ft 1 in) in height and 40–68 kg (88–150 lb) in weight. It is unclear if H. erectus was anatomically capable of speech, though it is postulated they communicated using some proto-language.

Successful though Homo erectus was, it still lacked some key human traits: for instance, its anatomy suggests it was probably incapable of speech. The next hominin to appear was Homo heidelbergensis. It evolved from a Homo erectus population in Africa about 600,000 years ago. This species' hyoid – a small bone with an important role in our vocal apparatus – is virtually indistinguishable from ours, and its ear anatomy suggests it would have been sensitive to speech.

According to some interpretations, Homo heidelbergensis gave rise to our species, Homo sapiens, about 200,000 years ago in Africa. Separate populations of Homo heidelbergensis living in Eurasia evolved too, becoming the Neanderthals in the west and a still enigmatic group called the Denisovans in the east.

Modern humans

Within the last 100,000 years or so, the most recent chapter in our story unfolded. Modern humans spread throughout the world and Neanderthals and Denisovans disappeared. Exactly why they went extinct is another great mystery, but it seems likely that our species played its part. Interactions weren't entirely hostile, though: DNA evidence shows that modern humans sometimes interbred with both Neanderthals and Denisovans.

There is still much we do not know, and new fossils have the potential to change the story. Three new extinct hominins have been discovered in the past decade or so, including Australopithecus sediba and the enigmatic and not yet well-dated Homo naledi, also in South Africa. Strangest of all is the tiny "hobbit" Homo floresiensis, which lived in Indonesia until about 12,000 years ago and appears to have been a separate species.

For 7 million years our lineage had shared the planet with at least one other species of hominid. With the hobbit gone, Homo sapiens stood alone. Human evolution is the lengthy process of change by which people originated from apelike ancestors. Scientific evidence shows that the physical and behavioral traits shared by all people originated from apelike ancestors and evolved over a period of approximately six million years.

One of the earliest defining human traits, bipedalism -- the ability to walk on two legs -- evolved over 4 million years ago. Other important human characteristics -- such as a large and complex brain, the ability to make and use tools, and the capacity for language -- developed more recently. Many advanced traits -- including complex symbolic expression, art, and elaborate cultural diversity -- emerged mainly during the past 100,000 years.

Humans are primates. Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the great apes (large apes) of Africa -- chimpanzees (including bonobos, or so-called "pygmy chimpanzees") and gorillas -- share a common ancestor that lived between 8 and 6 million years ago. Humans first evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between 6 and 2 million years ago come entirely from Africa.

Homo erectus ("upright man") is an extinct species of archaic human from the Pleistocene, with its earliest occurrence about 2 million years ago. Several human species, such as H. heidelbergensis and H. antecessor, with the former generally considered to have been the ancestor to Neanderthals, Denisovans, and modern humans, appear to have emerged within the possibly Asian populations of H. erectusIts specimens are among the first recognizable members of the genus Homo. H. erectus was the first human ancestor to spread throughout Eurasia, with a continental range extending from the Iberian Peninsula to Java. Asian populations of H. erectus may be ancestral to H. floresiensis and possibly to H. luzonensis. The last known population of H. erectus is H. e. soloensis from Java, around 117,000–108,000 years ago.

Similarities between Java Man and Peking Man led to rename both as Homo erectus.

A few North African sites have additionally yielded H. erectus remains, which at first were classified as "Atlantanthropus mauritanicus." Archaic human fossils unearthed across Europe used to be assigned to H. erectus, but have since been separated as H. heidelbergensis. It has been proposed that H. erectus evolved from H. habilis about 2 million years ago, though this has been called into question because they coexisted for at least a half a million years. Alternatively, a group of H. habilis may have been reproductively isolated, and only this group developed into H. erectus (cladogenesis).

Because the earliest remains of H. erectus are found in both Africa and East Asia (in China as early as 2.1 million years ago in South Africa 2.04 million years ago), it is debated where H. erectus evolved. A study suggested that it was H. habilis who reached West Asia from Africa, that early H. erectus developed there, and that early H. erectus would then have dispersed from West Asia to East Asia (Peking Man), Southeast Asia (Java Man), back to Africa (Homo ergaster), and to Europe (Tautavel Man), eventually evolving into modern humans in Africa. Others have suggested that H. erectus/H. ergaster developed in Africa, where it eventually evolved into modern humans.

H. erectus had reached Sangiran, Java, by 1.6 million years ago, and a second and distinct wave of H. erectus had colonized Zhoukoudian, China, about 780 thousand years ago. Early teeth from Sangiran are bigger and more similar to those of basal (ancestral) Western H. erectus and H. habilis than to those of the derived Zhoukoudian H. erectus. However, later Sangiran teeth seem to reduce in size, which could indicate a secondary colonization event of Java by the Zhoukoudian or some closely related population.

"Wushan Man" was proposed as Homo erectus wushanensis, but is now thought to be based upon fossilized fragments of an extinct non-hominin ape.

Since its discovery (Java man), there has been a trend in palaeoanthropology of reducing the number of proposed species of Homo, to the point where H. erectus includes all early (Lower Paleolithic) forms of Homo sufficiently derived from H. habilis and distinct from early H. heidelbergensis (in Africa also known as H. rhodesiensis). It is sometimes considered as a wide-ranging, polymorphous species.

Due to such a wide range of variation, it has been suggested that the ancient H. rudolfensis and H. habilis should be considered early varieties of H. erectus. The primitive H. e. georgicus from Dmanisi, Georgia has the smallest brain capacity of any known Pleistocene hominin (about 600 cubic centimeters), and its inclusion in the species would greatly expand the range of variation of H. erectus to perhaps include species as H. rudolfensis, H. gautengensis, H. ergaster, and perhaps H. habilis. However, a study suggested that H. georgicus represents an earlier, more primitive species of Homo derived from an older dispersal of hominins from Africa, with H. ergaster/erectus possibly deriving from a later dispersal. H. georgicus is sometimes not even regarded as H. erectus.

It is debated whether the African H. e. ergaster is a separate species (and that H. erectus evolved in Asia, then migrated to Africa), or is the African form (sensu lato) of H. erectus (sensu stricto). In the latter, H. ergaster has also been suggested to represent the immediate ancestor of H. erectus. It has also been suggested that H. ergaster instead of H. erectus, or some hybrid between the two, was the immediate ancestor of other archaic humans and modern humans. It has been proposed that Asian H. erectus have several unique characteristics from non-Asian populations (autapomorphies), but there is no clear consensus on what these characteristics are or if they are indeed limited to only Asia. Based on supposed derived characteristics, the 120 thousand years ago Javan H. e. soloensis has been proposed to have speciated from H. erectus, as H. soloensis, but this has been challenged because most of the basic cranial features are maintained.

In a wider sense, H. erectus had mostly been replaced by H. heidelbergensis by about 300 thousand years ago, with possible late survival of H. erectus soloensis in Java an estimated 117-108 thousand years ago. Homo erectus is the most long-lived species of Homo, having survived for almost two million years. By contrast, Homo sapiens emerged about a third of a million years ago.

Regarding many archaic humans, there is no definite consensus as to whether they should be classified as subspecies of H. erectus or H. sapiens or as separate species.

Homo erectus featured a flat face compared to earlier hominins; pronounced brow ridge; and a low, flat skull. The presence of sagittal, frontal, and coronal keels, which are small crests that run along these suture lines, has been proposed to be evidence of significant thickening of the skull, specifically the cranial vault. Analyses reveal this to not be the case, particularly the internal occipital crest, at the rear of the skull is notably thicker than that of modern humans, likely a basal (ancestral) trait. The fossil record indicates that H. erectus was the first human species to have featured a projecting nose, which is generally thought to have evolved in response to breathing dry air in order to retain moisture. The average brain size of Asian H. erectus is about 1,000 cubic centimeters (61 cu in). However, markedly smaller specimens have been found in Dmanisi, Georgia (H. e. georgicus); Koobi Fora and Olorgesailie, Kenya; and possibly Gona, Ethiopia. Overall, H. erectus brain size varies from 546–1,251 cubic centimeters (33.3–76.3 cu in), which is greater than the range of variation seen in modern humans and chimps, though less than that of gorillas.

Inter-population variation in human brain size: implications for hominin cognitive phylogeny' it was found that the brain size of Asian H. erectus over the last 600,000 years overlaps significantly with modern human populations. Significantly, some small brained modern populations showed greater affinity with H. erectus than they did with other large brained and large bodied modern populations. The studies pointed out methodological flaws in current understanding of brain size increase in human evolution, where species averages are compared with fossils, which overlooks inter-population variation. It also overlooks the fact that some modern populations have not seen any dramatic brain size increase relative to H. erectus with most of the increase occurring in European populations, which has the result of obscuring inter-population variation. Increase in the mean of H. sapiens cranial capacity is to a large extent due to an increase in the upper limit with a much less pronounced increase in the lower limit relative to our H. erectus sample. And this increase in the upper limit seems to be more pronounced in European populations – which may be a result of correlated increases in body size in addition to climatic factors'. Consequently, it was purely based on brain size similarities, Asian H. erectus could be re-classified as a subspecies of H. sapiens, that is H. sapiens soloensis - as was suggested.

Dentally, H. erectus have the thinnest enamel of any Plio–Pleistocene hominin. Enamel prevents the tooth from breaking from hard foods, but impedes shearing through tough foods. The bodies of the mandibles of H. erectus, and all early Homo, are thicker than those of modern humans and all living apes. The mandibular body resists torsion from the bite force or chewing, meaning their jaws could produce unusually powerful stresses while eating, but the practical application of this is unclear. Nonetheless, the mandibular bodies of H. erectus are somewhat thinner than those of early Homo. The premolars and molars also have a higher frequency of pits than H. habilis, suggesting H. erectus ate more brittle foods (which cause pitting). These all indicate that the H. erectus mouth was less capable of processing hard foods and more at shearing through tougher foods, thus reducing the variety of foods it could process, likely as a response to tool use.

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