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Current evidence suggests that many of the major events in hominin evolution occurred in East Africa. Hence, over the past two decades, there has been intensive work undertaken to understand African palaeoclimate and tectonics in order to put together a coherent picture of how the environment of Africa has varied over the past 10 Myr.

A new consensus is emerging that suggests the unusual geology and climate of East Africa created a complex, environmentally very variable setting. This new understanding of East African climate has led to the pulsed climate variability hypothesis that suggests the long-term drying trend in East Africa was punctuated by episodes of short alternating periods of extreme humidity and aridity which may have driven hominin speciation, encephalization and dispersals out of Africa.

This hypothesis is unique as it provides a conceptual framework within which other evolutionary theories can be examined: It is proposed that each one of these mechanisms may have been acting on hominins during these short periods of climate variability, which then produce a range of different traits that led to the emergence of new species.

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In the case of Homo erectus sensu lato , it is not just brain size that changes but life history shortened inter-birth intervals, delayed development , body size and dimorphism, shoulder morphology to allow thrown projectiles, adaptation to long-distance running, ecological flexibility and social behaviour. The future of evolutionary research should be to create evidence-based meta-narratives, which encompass multiple mechanisms that select for different traits leading ultimately to speciation.

Many theories have been proposed to link environmental changes to these human evolution events [ 8 — 11 ]. This synthesis paper presents each of these theories in the context of the pulsed climate variability conceptual framework [ 11 , 12 ], which has been developed from the latest tectonic and palaeoclimate data from East Africa. This greater understanding of the past climate of East Africa suggests that different hominin species or, at the very least, different emerging traits within a species could have evolved through various different mechanisms that are described by the turnover pulse hypothesis , aridity hypothesis , variability selection hypothesis, Red Queen hypothesis , allopatric or sympatric speciation.

The recent expansion of the hominin fossil record has been dramatic, with 11 new species and four new genera named since This richer fossil record has provided two major improvements. Second, extensive use of new dating techniques has provided chronological precision to link those phenotypes to the environments in which they evolved.

However, the fossil record is still very limited with many gaps figure 1 ; the most significant for this study is the lack of cranial capacity data between 2 and 2. There is also considerable discussion about defining the new species and genera [ 13 , 14 ], which has an influence on understanding changes in overall hominin diversity.

However, conflating or expanding the defined species has little overall influence on the diversity pattern, the pattern of species first appearance dates suggests contemporary speciation events [ 11 ]. First appearance dates are dependent on taphonomy and sampling biases; however, the consistency of hominin first appearance dates FAD in East Africa supports this region as the primary location of speciation events.

The other key debate is where all the new hominin species evolved. The fossil record at the moment suggests that the majority of the new species evolved in East Africa and then dispersed outwards. This is supported by the current brain capacity evidence, which suggests brain expansion occurs first in East Africa and only appears elsewhere once there has been a dispersal event [ 15 ]. However, it should be noted that other authors suggest the possibility of South Africa, European and Asian origins for hominin speciation e.

Hominin specimen dates and brain size estimates were taken from Shultz et al. Homo erectus and H. Online version in colour. The fossil record suggests four main stages in hominin evolution: The taxonomic classification of many specimens, as well as their role in the evolution of modern humans, is continually discussed e. What is not disputed is that, apart from Sahelanthropus remains from Chad, all the earliest specimens for each of the main genera were found in the East African Rift System [ 18 ].

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The earliest disputed hominin is Sahelanthropus tchadensis , dated to approximately 7 Ma [ 19 ]. The remains are limited to cranial fragments that suggest a mosaic of hominin and non-hominin features and a brain size equivalent to modern chimpanzees [ 20 ]. The lack of post-cranial remains makes it extremely difficult to reconstruct its lifestyle and whether it was bipedal or whether it was truly a hominin.

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The next putative hominin is Orrorin tungenesis from Western Kenyan deposits aged around 6 Ma [ 21 ], but its taxonomic position, lifestyle and locomotion are all disputed owing to the fragmentary nature of the specimens. Both Sahelanthropus and Orrorin have been suggested to be members of a clade that includes Ardipithecus [ 20 ].

The oldest member of the Ardipithecus genus is A. A much more extensive fossil record exists for the second member of the genus, A. Ardipithecus had brain and body sizes roughly equivalent to modern chimpanzees, their teeth indicate a highly omnivorous diet and their post-crania suggest a lifestyle of arboreality coupled with primitive bipedality [ 23 ].

The fauna and vegetation associated with the A. This appearance of bipedality in closed woodland environments undermines theories of bipedality evolving exclusively as an adaptation to open habitats. The first members of the Australopithecus genus, attributed to A. These individuals show strong evidence of bipedality combined with primitive cranial features. They are followed by A. Afarensis still retains a small brain size, yet the post-cranial morphology is more similar to modern humans than to apes and suggests a lifestyle strongly adapted to long-distance walking [ 27 ].

Australopithecus africanus , the first hominin found in South Africa, is similar to A. The longer femur in A. The final gracile australopith is A. In a separate development, a group of hominins with robust dentition and jaw muscles appeared around 2. These hominins, generally attributed to the Paranthropus genus, include the East African P.

These species have been attributed to more open habitats [ 25 ], though the evidence to support this inference has been questioned [ 30 ]. The earliest fossil evidence of Homo comes from 1. Inferences from fossil demography are that development slowed down, coupled with decreased inter-birth intervals.

The final stages in the evolution of modern humans were the appearance of H. Arguably, the most important episode in hominin evolution occurred in East Africa around 1. In addition to speciation, a second major process that began during this period was the episodic migration of hominins out of the Rift Valley and into Eurasia.

This period also witnessed the most dramatic increases in hominin brain size; early representatives of the H. By contrast, from the appearance of the early australopithecines until the appearance of the first member of the genus Homo, there was remarkably little change in hominin brain size. The emergence of the H. The dramatic increase in brain size was also accompanied by changes in life history shortened inter-birth intervals, delayed development , pelvic morphology see [ 32 , 33 ] in this issue , body size and dimorphism, shoulder morphology allowing throwing of projectiles [ 34 ], adaptation to long-distance running [ 35 ], ecological flexibility [ 36 ] and social behaviour [ 37 ].

Some of these changes are consistent with a change in strategy towards flexibility and the ability to colonize novel environments. By contrast, the robust Australopithecus sp. Thus, two strategies arose during this period, one of increased flexibility and one of increased specialization. With the appearance of H. These final stages of increased brain capacity were due to the appearance of H.

Environmental pressures have long been assumed to play a key role in hominin speciation and adaptation [ 40 ] and a number of iconic theories have been developed to frame and develop the discussion of hominin evolution. Table 1 tries to put these key theories into the context of overarching evolutionary theory. Although the split between phylogenetic gradualism and punctuated equilibrium is artificial, it does provide a starting point with which to discuss theories of early human evolution.

In table 1 , gradualism has been split into constant and variable evolution rates to reflect the full range of current opinions. Early human evolutionary theories placed in the context of overall evolutionary theory and modes of climatic change. The first key environmental theory to explain bipedalism was the savannah hypothesis , which suggested that hominins were forced to descend from the trees and adapted to life on the savannah facilitated by walking erect on two feet.

This theory was refined as the aridity hypothesis , which suggested that the long-term trend towards increased aridity and the expansion of the savannah was a major driver of hominin evolution [ 1 , 38 , 45 ].

Environmental determinism

A key addition to this theory was the suggestion that during periods when aridification accelerated, owing to thresholds in the global climate system, then thresholds in evolution were reached and major hominin speciation events occurred [ 1 ]. Isotopic studies done by Dr. Margaret Schoeninger and her colleagues indicate that most of the Allia Bay vegetation consisted of woody plants such as trees and shrubs known as C 3 vegetation. Australopithecus anamensis at Allia Bay was thus associated with a mosaic environment, including woodlands near the ancestral Omo River and open savanna further away.

Two different types of environment — dense woodlands and open bushland — occurred in the same areas of East Africa during the period of human evolution. Climate fluctuation altered the proportion of these habitats, and thus led to repeated changes only in population density and variable conditions of natural selection.

Australopithecus afarensis, "Lucy", reconstructed skeleton. By about 4 million years ago, the genus Australopithecus had evolved a skeletal form that enabled adjustment to changes in moisture and vegetation. The best current example of adaptability in Australopithecus is apparent in the skeleton known as Lucy, which represents Au. This combination of features, which appears to have characterized Australopithecus for nearly 2 million years and possibly older hominins, afforded an ability to move around in diverse habitats by changing the degree of reliance on terrestrial walking and arboreal climbing.

This flexibility may also have characterized earlier hominins such as Ardipithecus ramidus.

Homo sapiens | Meaning & Stages of Human Evolution |

The first known stone tools date to around 2. Making and using stone tools also conferred versatility in how hominin toolmakers interacted with and adjusted to their surroundings. Simple toolmaking by stone-on-stone fracturing of rock conferred a selective advantage in that these hominin toolmakers possessed sharp flakes for cutting and hammerstones that were useful in pounding and crushing foods. Basic stone tools thus greatly enhanced the functions of teeth in a way that allowed access to an enormous variety of foods.

These foods included meat from large animals, which was sliced from carcasses using sharp edges of flakes. Bones were broken open using stones to access the marrow inside. Other tools could be used to grind plants or to sharpen sticks to dig for tubers. Tool use would have made it easier for hominins to obtain food from a variety of different sources.

Tool use would have widened the diet of hominins. Meat, in particular, is a food that was obtainable in equivalent ways, with similar nutritional value, in virtually any type of habitat that early humans encountered. Although making simple toolmaking may have developed originally in one type of environment, the carrying of stone tools over considerable distances — and becoming reliant on stone technology — may have arisen due to the benefits of altering the diet as environments changed.

The oldest known stone technology — called Oldowan toolmaking — involved carrying rock over several kilometers and is found associated with a variety of ancient habitats. Redistributing stone and other resources, such as parts of animal carcasses, by transporting them may have helped hominins cope with variable habitats. As predicted by the variability selection hypothesis, hominins were not found solely in one kind of habitat, but rather in a variety.

A major signal of the ability to tolerate different environments was the dispersal of the genus early Homo beyond Africa into Asian environments. Early evidence of the diversity of Homo erectus environments in Asia includes the following sites:. In these locations, hominin groups encountered distinctly different environments, different plants and animals and foods, and different climatic conditions — a very wide range of temperature and strong variations in aridity and monsoonal rains.

Environmental instability may have been a factor not only in shaping adaptations but also in contributing to the extinction of some lineages. Environmental variability associated with the extinction of large mammal species has been proposed for the southern Kenya region. Sediments, stone artifacts, and animal faunal at the site of Olorgesailie span most of the past 1. Numerous environmental shifts are recorded in the Olorgesailie deposits.

The ancient lake level and its chemistry, for example, changed frequently, and sometimes the lake dried up, leaving small wetlands and streams as the main source of water in the basin. Volcanic eruptions also blanketed the landscape in ash, killing off grass and reshaping the properties of the ecosystem.

An example of a hillside of sediments in the Olorgesailie region. The hillside, which represents about 10, years of time with a volcanic ash at its base dated around 1 million years ago, shows evidence of strong environmental shifts. Layers of sediments show the fluctuation between dry and wet environments and a time when volcanic ash covered the ancient landscape. Rick Potts studied the pattern of climatic turnover in the fauna and the occurrence of archeological sites at Olorgesailie and another site in southern Kenya, and found that several large mammal species that had previously dominated the fauna of this region went extinct between about , and , years ago, during a period of repeated environmental instability.

These species were replaced by modern relatives, which tended to be smaller in body size and not as specialized in diet or habitat. For example, the zebra Equus oldowayensis had large and tall teeth specialized for eating grass. Its last known appearance in the fossil record of southern Kenya is between , and , years ago; it was replaced by Equus grevyi , which can graze feed on grass as well as browse feed on leaves and other high-growing vegetation. The fossil baboon Theropithecus oswaldi , which weighed over 58 kg over It also went extinct between , and , years ago.

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Its extant relative, Papio anubis , is omnivorous and moves easily on the ground and in trees. Two other large-bodied animals that specialized in eating grass, the elephant Elephas recki and the ancient pig Metridiochoerus , were also replaced by related species that were smaller and had more versatile diets Loxodonta africana and Phacochoerus aethiopicus. The aquatic specialist Hippopotamus gorgops was replaced by the living hippopotamus, which is capable of traversing long distances between water bodies.

The replacement of the specialized species by closely related animals that possessed more flexible adaptations during a time of wide fluctuation in climate was a key piece of initial evidence that led to the variability selection hypothesis. Although Acheulean toolmaking hominins were able to cope with changing habitats throughout much of the Olorgesailie record, the Acheulean way of life disappeared from the region sometime between , and , years ago, perhaps also a casualty of strong environmental uncertainty and changing circumstances.

Brain enlargement during human evolution has been dramatic. During the first four million years of human evolution, brain size increased very slowly. Encephalization, or the evolutionary enlargement of the brain relative to body size, was especially pronounced over the past , years, coinciding with the period of strongest climate fluctuation worldwide. Larger brains allowed hominins to process and store information, to plan ahead, and to solve abstract problems. A large brain able to produce versatile solutions to new and diverse survival challenges was, according to the variability selection hypothesis, favored with an increase in the range of environments hominins confronted over time and space.

After , years ago, hominins found new ways of coping with the environment by creating a variety of different tools. In some parts of Africa, a shift occurred in which a technology dominated by large cutting tools was replaced by smaller, more diverse toolkits. Technological innovations began to appear in the Middle Stone Age in Africa, with some early examples dating prior to , years ago.

Some of the new tools provided ways for hominins to access food in new ways. Points were hafted, or attached to handles such as spear or arrow shafts, and were later used as part of projectile weapons, which allowed hominins to hunt fast and dangerous prey without approaching as closely. Barbed points were used to spear fish. Barbed points made from bone were found at the site of Katanda, in the Democratic Republic of the Congo, along with the remains of huge catfish.

Grindstones were used to process plant foods. Other tools were used to make clothing which would have been important for hominins in cold environments. Over the past , years or so, the direct ancestors of living humans developed the capacity to create new and diverse tools. Archeological discoveries show that wider social networks began to arise, enabling the transfer of stone material over long distances.

Symbolic artifacts connoting complex language and the ability to plan are also evident in the archeological record of the Middle Stone Age of Africa. These findings indicate an improved capacity to adjust to new environments. Most of the past , years in East Africa was a time of strong climate oscillation. Darwin's second book on evolution, The Descent of Man, features extensive argumentation addressing the single origin of the races, at times explicitly opposing Agassiz's theories.

Arthur de Gobineau — was a successful diplomat for the French Second Empire. Initially he was posted to Persia, before working in Brazil and other countries. He came to believe that race created culture, arguing that distinctions between the three "black", "white", and "yellow" races were natural barriers, and that "race-mixing" breaks those barriers and leads to chaos.

Gobineau believed the white race was superior to the others. He thought it corresponded to the ancient Indo-European culture, also known as " Aryan ". Gobineau originally wrote that white race miscegenation was inevitable. He attributed much of the economic turmoils in France to pollution of races. Later on in his life, he altered his opinion to believe that the white race could be saved. To Gobineau, the development of empires was ultimately destructive to the "superior races" that created them, since they led to the mixing of distinct races.

This he saw as a degenerative process. According to his definitions, the people of Spain, most of France, most of Germany, southern and western Iran as well as Switzerland, Austria, northern Italy, and a large part of Britain, consisted of a degenerative race arising from miscegenation. Also according to him, the whole of north India consisted of a yellow race. Thomas Huxley — wrote one paper, "On the Geographical Distribution of the Chief Modifications of Mankind" , in which he proposed a distinction within the human species 'races' , and their distribution across the earth.

He also acknowledged that certain geographical areas with more complex ethnic compositions, including much of the Horn of Africa and the India subcontinent, did not fit into his racial paradigm. As such, he noted that: His Melanochroi thus eventually also comprised various other dark Caucasoid populations, including the Hamites e. Berbers, Somalis, northern Sudanese, ancient Egyptians and Moors. Huxley's paper was rejected by the Royal Society , and this became one of the many theories to be advanced and dropped by the early exponents of evolution. Despite rejection by Huxley and the science community, the paper is sometimes cited in support of racialism.

This view contrasts polygenism, the theory that each race is actually a separate species with separate sites of origin. Despite Huxley's monogenism and his abolitionism on ethical grounds, Huxley assumed a hierarchy of innate abilities, a stance evinced in papers such as "Emancipation Black and White" and his most famous paper, "Evolution and Ethics". In the former, he writes that the "highest places in the hierarchy of civilization will assuredly not be within the reach of our dusky cousins, though it is by no means necessary that they should be restricted to the lowest".

This application by Darwin would not become explicit until with the publication of his second great book on evolution, The Descent of Man, and Selection in Relation to Sex. Darwin's publication of this book occurred within the heated debates between advocates of monogeny, who held that all races came from a common ancestor, and advocates of polygeny, who held that the races were separately created. Darwin, who had come from a family with strong abolitionist ties, had experienced and was disturbed by cultures of slavery during his voyage on the Beagle years earlier. Noted Darwin biographers Adrian Desmond and James Moore argue that Darwin's writings on evolution were not only influenced by his abolitionist tendencies, but also his belief that non-white races were equal in regard to their intellectual capacity as white races, a belief which had been strongly disputed by scientists such as Morton, Agassiz and Broca, all noted polygenists.

By the late s, however, Darwin's theory of evolution had been thought to be compatible with the polygenist thesis Stepan Darwin thus used Descent of Man to disprove the polygenist thesis and end the debate between polygeny and monogeny once and for all. Darwin also used it to disprove other hypotheses about racial difference that had persisted since the time of ancient Greece, for example, that differences in skin color and body constitution occurred because of differences of geography and climate. Darwin concluded, for example, that the biological similarities between the different races were "too great" for the polygenist thesis to be plausible.

He also used the idea of races to argue for the continuity between humans and animals, noting that it would be highly implausible that man should, by mere accident acquire characteristics shared by many apes.

Darwin sought to demonstrate that the physical characteristics that were being used to define race for centuries i. Because, according to Darwin, any characteristic that did not have survival value could not have been naturally selected, he devised another hypothesis for the development and persistence of these characteristics. The mechanism Darwin developed is known as sexual selection. Though the idea of sexual selection had appeared in earlier works by Darwin, it was not until the late s when it received full consideration Stepan Furthermore, it was not until that sexual selection received serious consideration as a racial theory by naturalist thinkers.

Darwin defined sexual selection as the "struggle between individuals of one sex, generally the males, for the possession of the other sex". Sexual selection consisted of two types for Darwin: The physical struggle for a mate, and 2. The preference for some color or another, typically by females of a given species. Darwin asserted that the differing human races insofar as race was conceived phenotypically had arbitrary standards of ideal beauty, and that these standards reflected important physical characteristics sought in mates. Broadly speaking, Darwin's attitudes of what race was and how it developed in the human species are attributable to two assertions, 1.

That all human beings, regardless of race share a single, common ancestor and 2. Phenotypic racial differences are superficially selected, and have no survival value. Al, as well as notions that there existed a natural racial hierarchy that reflected inborn differences and measures of value between the different human races.

But it would be an endless task to specify the numerous points of difference. The races differ also in constitution, in acclimatization and in liability to certain diseases. Their mental characteristics are likewise very distinct; chiefly as it would appear in their emotion, but partly in their intellectual faculties. In The Descent of Man , Darwin noted the great difficulty naturalists had in trying to decide how many "races" there actually were:. Man has been studied more carefully than any other animal, and yet there is the greatest possible diversity amongst capable judges whether he should be classed as a single species or race, or as two Virey , as three Jacquinot , as four Kant , five Blumenbach , six Buffon , seven Hunter , eight Agassiz , eleven Pickering , fifteen Bory St.

Vincent , sixteen Desmoulins , twenty-two Morton , sixty Crawfurd , or as sixty-three, according to Burke. This diversity of judgment does not prove that the races ought not to be ranked as species, but it shews that they graduate into each other, and that it is hardly possible to discover clear distinctive characters between them. Several social and political developments that occurred at the end of the 19th century and into the 20th century led to the transformation in the discourse of race. Three movements that historians have considered are: Nazism made an argument for racial superiority based on a biological basis.

This led to the idea that people could be divided into discrete groups and based on the divisions, there would be severe, tortuous, and often fatal consequence. The exposition of racial theory beginning in the Third Reich , up to the Final Solution , created a popular moral revolution against racism. Consequently, studies of human variation focused more on actual patterns of variation and evolutionary patterns among populations and less about classification.

Some scientists point to three discoveries. Firstly, African populations exhibit greater genetic diversity and less linkage disequilibrium because of their long history. Secondly, genetic similarity is directly correlated with geographic proximity. Lastly, some loci reflect selection in response to environmental gradients. Therefore, some argue, human racial groups do not appear to be distinct ethnic groups. Boas made significant contributions within anthropology , more specifically, physical anthropology , linguistics , archaeology , and cultural anthropology.

His work put an emphasis on cultural and environmental effects on people to explain their development into adulthood and evaluated them in concert with human biology and evolution. This encouraged academics to break away from static taxonomical classifications of race. It is said that before Boas, anthropology was the study of race, and after Boas, anthropology was the study of culture.


Sir Julian Sorell Huxley — was an English evolutionary biologist, humanist and internationalist. After returning to England from a tour of the United States in , Huxley wrote a series of articles for the Spectator which he expressed his belief in the drastic differences between "negros" and "whites". He was a proponent of racial inequality and segregation. By , Huxley's ideas on race and inherited intellectual capacity of human groups became more liberal. By the mids, Huxley was considered one of the leading antiracist and committed much of his time and efforts into publicizing the fight against Nazism.

Alfred Cort Haddon — was a British anthropologist and ethnologist. In , Huxley and A. Haddon wrote, We Europeans , which greatly popularized the struggle against racial science and attacked the Nazis' abuse of science to promote their racial theories. Although they argued that 'any biological arrangement of the types of European man is still largely a subjective process', they proposed that humankind could be divided up into "major" and "minor subspecies". They believed that races were a classification based on hereditary traits but should not by nature be used to condemn or deem inferior to another group.

Like most of their peers, they continued to maintain a distinction between the social meaning of race and the scientific study of race. From a scientific stand point, they were willing to accept that concepts of superiority and inferiority did not exist, but from a social stand point, they continued to believe that racial differences were significant. For example, they argued that genetic differences between groups were functionally important for certain jobs or tasks.

Carleton Stevens Coon — was an American physical anthropologist , Professor of Anthropology at the University of Pennsylvania , lecturer and professor at Harvard , and president of the American Association of Physical Anthropologists. In , Coon published The Races of Europe , where he concluded: In , Coon also published The Origin of Races , wherein he offered a definitive statement of the polygenist view. He also argued that human fossils could be assigned a date, a race, and an evolutionary grade. Coon divided humanity into five races and believed that each race had ascended the ladder of human evolution at different rates.

In , he made a strong effort to have the word "race" replaced with " ethnic group " by publishing his book, Man's Most Dangerous Myth: The Fallacy of Race.