Recent single origin hypothesis

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Map of early human migrations according to mitochondrial population genetics

In paleoanthropology, the recent single-origin hypothesis (RSOH, or Out-of-Africa model, or Replacement Hypothesis) is one of two accounts of the origin of anatomically modern humans, Homo sapiens. According to the RSOH, anatomically modern humans evolved in Africa between 200,000 and 100,000 years ago, with members of one branch leaving Africa between 55,000 to 60,000 years ago. These emigrants spread to the rest of the world, replacing (and not interbreeding with) other Homo species already there, such as Neanderthals and Homo erectus. The hypothesis is derived from research in several disciplines, chiefly genetics, archeology and linguistics.

Currently available genetic and archaeological evidence is generally interpreted as supportive of a recent single origin of modern humans in East Africa. The alternative theory is the multiregional hypothesis, including the Hybrid-origin theory archaeogenetic.

History

Charles Darwin was one of the first to suggest that all humans had a common ancestor who lived in Africa. In the Descent of Man he writes:

“

In each great region of the world the living mammals are closely related to the extinct species of the same region. It is, therefore, probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as these two species are now man's nearest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere.

”

The prediction was highly insightful because at the time, in 1871, there were hardly any human fossils of ancient hominids available. Almost fifty years later Darwin was vindicated, as anthropologists began finding numerous fossils of ancient hominids all over Africa (List of hominina fossils).

Ancient hominids

Homo habilis evolved in Africa 2 million years ago and is considered the first species of the genus homo. A descendent of Homo habilis known as Homo erectus or "upright man" is thought to be the first hominid to migrate out of Africa at least 1.5 million years ago. Erectus is believed to have left Africa during the warm periods between ice ages. An occasional phenomenon known as the Sahara pump theory, during which the Sahara desert receives significant rainfall, allows African flora and African fauna to penetrate the otherwise arid Middle East. It is believed that during one such period some Homo erectus migrated out to ultimately spread all over Europe and Asia and dominate the world for the next 1 million years. Fossils of homo erectus include Peking man from China and Java man from Indonesia.

Then, following a severe ice age 350,000 years ago, another large brained hominid, Homo heidelbergensis appeared on the African stage. Some of these hominids migrated to Europe and evolved into the Neanderthals. The Neanderthals occupied Europe until approximately 30,000 years ago when they became extinct. Some scientists propose that the Neanderthals were displaced and possibly wiped out by encroaching modern humans. While some researchers have found evidence which suggests that neanderthalensis had vocal capabilities similar to, or possibly exceeding that of, modern humans, others conclude that although they may have possessed some form of speech, their phonetic abilities were limited relative to anatomically modern Homo sapiens from the same time period. Furthermore, evidence that Neanderthal tool culture was simple and relatively static suggests that Neanderthal language was less developed than that of modern humans and that this might have played a role in their demise. Stone technology remained relatively unchanged and unsophisticated for millions of years during the periods of erectus and the Neanderthals.

Anatomically modern humans

Scientists believe modern humans first appeared in Africa less than 200,000 years ago. One of the reasons they believe this is that the oldest known remains of modern humans have been found in Africa and nowhere else. The Omo remains found near the Omo river in Ethiopia have been dated to 130,000 - 195,000 years ago and are the oldest fossil evidence of anatomically modern humans.

Humans did attempt on one occasion to leave Africa through the Middle East. Fossils of modern humans were found in a cave in Israel at Qafzeh and have been dated to 100,000 years ago. However these humans seem to have either gone extinct or retreated back to Africa 80,000 - 70,000 years ago, possibly replaced by south bound Neanderthals escaping the colder regions of ice age Europe.

All other fossils of fully modern humans outside of Africa have been dated to more recent times. The next oldest fossil of modern humans outside of Africa are those of Mungo Man found in Australia and have been dated to about 42,000 ago.

Modern behaviour

See also: Behavioral modernity

Though fossil remains of modern humans appear about 200,000 years ago, significant changes in technology do not appear until much later. Early humans apparently continued to use the same technology of the Neanderthals. Beginning about 100,000 years ago evidence of more sophisticated technology and artwork begins to emerge and by 50,000 years ago fully modern behaviour becomes more prominent. By this time the ritual burying of the dead is noted. Stone tools show regular patterns that are reproduced or duplicated with more precision. Tools made of bone and antler appear for the first time. These new changes are suggestive of more advanced behaviour and scientists attribute these changes to the development of language. The new stone tool types have been described as being distinctly differentiated from each other as if each tool had a specific name. This period is referred to as the Upper paleolithic.

For the first time in the fossil record evidence of fishing indisputably appears in Africa at 50,000 years ago. Homo erectus and the Neanderthals lived alongside oceans, rivers and lakes but never ate fish. Archaeological coastal sites that are dated to before 50,000 years contain no fish bones whereas those dated to after 50,000 do contain fish bones. This also serves as evidence for significant change in human behaviour at the 50,000 year mark.

Cultural universals are the key elements shared by all groups of people throughout the history of man. Examples of elements that may be considered cultural universals are language, religion, art, music, marriage, gender roles, the incest taboo, myth, cooking, games, and jokes. These traits distinguish homo sapiens from other species. Since cultural universals are found in all cultures including the most isolated indigenous groups, scientists believe that these traits must have evolved or have been invented in Africa prior to the exodus.

The journey of humans to Australia would have required sailing across considerable distances of ocean that separated the land masses, even at the lowest sea levels of the Ice Age. There was never a land bridge across the Bab al-Mandeb at the southern end of the Red Sea (although this strait was much more narrow at the height of the Ice Age), nor across the Wallace Line which separates Borneo and Bali on the west from Sulawesi and Lombok on the east. This ability is also another indicator that modern human behavior began in Africa before the dispersal of humans.

Origin of language

Main article: Origin of language

There is considerable debate regarding when modern human language first came into existence. Much of the debate centers on whether modern language arose suddenly with anatomically modern humans or whether language developed gradually over millions of years with all archaic hominids. Those in favor of the "sudden occurrence" of language argue that the first indisputable signs of symbolism such as art, which are associated with language, occur in the fossil record 50,000 BP, and become significantly more abundant thereafter. They contend that language was a necessary prerequisite for modern humans to leave Africa and reach continents such as Australia, that had never before been populated by Archaic hominids. Since all these major historic events appear to take place around the 50,000 year mark, scholars believe this is when language suddenly arose, with some suggesting that it may have required some biological change such as a mutation affecting the brain.

Other schools of thought disagree with the sudden emergence of language. They argue that since only a few materials such as bone and stone fossilize, the lives of archaic hominids may have involved the use of several materials that do not fossilize, such as wood or bark. Hence it would be impossible to concretely ascribe a date to the first symbolism. In addition a few fossils that appear to be symbolic have been controversially dated to much earlier than 50,000 BP. These include the Katanda bone points from the Congo, dated to 100,000 years ago and engravings found on red ochre dated to 75,000 from Blombos cave in South Africa. This would indicate that language may have arisen much earlier. However these findings are disputed with some arguing that they are simply anomalies in the fossil record.

Since the human line branched off from the common ancestor shared with chimpanzees six million years ago, the human vocal tract has been evolving. Hence some scholars argue that it must have been evolving for a reason. If the Neanderthals possessed a near modern if not fully modern vocal tract, then it would only make sense that it must have evolved for them to use some sort of speech. However critics once again point to the Neanderthal stone tool kit, that remained relatively unchanged and unsophisticated from millions of years before.

Exodus

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Red sea

According to this hypothesis a small group of humans living in East Africa migrated north east, possibly searching for food or escaping climate changes, crossing the Red Sea and in the process going on to populate the rest of the world. Around 50,000 years ago the world was entering the last ice age and sea levels were much lower as water was trapped in the polar ice caps. Today at the Gate of Grief the Red Sea is about 12 miles wide but 50,000 years ago it was much narrower and sea levels were 70 meters lower. Though the straits were never completely closed, there may have been islands in between which could be reached using simple rafts. Shell middens 125,000 years old have been found in Eritrea indicating the diet of early humans was sea food obtained by beachcombing. This is perceived to be evidence that humans may have crossed the Red Sea in search of new food sources available on uninhabited beaches.

Asia and Australia

See also: Prehistory of Australia

Genetic evidence points to a single exodus of a small group of people with some estimating as few as 150 people. From this small group descended all non-African people. Once in Asia they spread generation by generation around the coast of Arabia and Iran until they reached India which appears to be the first important settling point. Once in India the populations split, One group ventured inland northwest towards Europe and would eventually go on to displace the Neanderthals. The other group headed along the southeast coast of Asia reaching Australia about 46,000 years ago.

File:Sunda shelf.jpg

The map shows the probable extent of land and water at the time of the last glacial maximum a when the sea level was probably more than 150m lower than today

During that time the sea levels were much lower and most of Maritime Southeast Asia was one land mass known as the lost continent of Sunda. The settlers would have continued on the coastal route southeast until they reached the channel between Sunda and Sahul, the continental land mass that comprised present day Australia and New Guinea. This channel is also known as the Wallace Line. The Channel was 90 km wide, indicating that settlers must have had knowledge of seafaring skills in order to reach Australia. Archaic humans such as Homo erectus never reached Australia.

If these dates are correct it would mean that Australia was populated before Europe by up to 10,000 years. This is possibly because humans avoided the colder regions of the North favoring the warmer tropical regions, possibly lacking technology to survive the cold. Another piece of evidence favoring human occupation in Australia is that by 46,000 years ago all large mammals weighing more than 100 kg had suddenly become extinct. The new settlers are the likely suspects of this extinction. Many of the animals may have been accustomed to living without predators and become docile and vulnerable to attack.

While some settlers crossed into Australia others may have continued eastwards along the coast of Sunda eventually turning northeast to China and finally reaching Japan, leaving a trail of coastal settlements. This coastal migration leaves its trail in the mitochondrial haplogroups descended from haplogroup M, and in Y-chromosome haplogroup C. Thereafter it may have become necessary to venture inland possibly bringing modern humans into contact with archaics such as erectus. Recent genetic studies suggest that Australia and New Guinea were populated by one single migration from Asia as opposed to several waves. The land bridge separating New Guinea and Australia became submerged approximately 8,000 years ago, thus isolating the respective populations of the two land masses.

Europe

Europe is thought to have been colonized by northwest bound migrants from India and the Middle East. The expansion from India is thought to have begun 45,000 years ago and may have taken up to 15,000 years for Europe to be fully colonized. During this time the Neanderthals were slowly being displaced. Because it took so long for Europe to be overrun, it appears that humans and Neanderthals may have been constantly competing for territory. The Neanderthals were larger and had a more robust or heavy built frame which may suggest that they were physically stronger than modern homo sapiens. Having lived in Europe for 200,000 years they would have been better adapted to the cold weather. The Anatomically Modern Humans, known as the Cro-Magnons, however, with superior technology and language would eventually completely displace the Neanderthals whose last refuge was in the Iberian peninsula. After about 30,000 years ago the fossil record of the Neanderthals ends, indicating that they had become extinct. The last known population lived around a cave system on the remote south facing coast of Gibraltar from 30,000 to 24,000 years ago.

Multiregionalists have long believed that Europeans were descended from Neanderthals and not from humans from Africa. Others believed the Neanderthals had interbred with modern humans. In 1997 researchers managed to extract mitochondrial DNA from a 40,000 year old specimen of a Neanderthal. On comparison with human DNA, its sequences differed significantly, indicating that based on the mitochondrial DNA, modern Europeans are not descended from the Neanderthals and that no interbreeding took place. Some scientists continue to search autosomal DNA for traces of Neanderthal admixture. A few alleles of some autosomal genes such as the H2 allele of the MAPT gene have been suggested, since they were only found among Europeans. However in the absence of autosomal DNA from a Neanderthal, the scientists conclude that this hypothesis is entirely speculative.

Some archeologists doubt that Neanderthals and homo sapiens were interfertile. This is because Neanderthals and Europeans shared the same habitat for up to 20,000 years yet no undisputed skeletal fossils have been found so far that show intermediate properties between the two hominids.

The Americas

Further information: [[:Models of migration to the New World]]

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Beringia

The Americas were occupied by Asian people who crossed from Siberia into Alaska. At the time sea levels were lower and a land bridge of the lost continent of Beringia connected North America to Eurasia. It is likely they used the southern route that may have been much warmer.

There is considerable controversy over when the Americas were first colonized and how many migrations there were. Controversial findings in Chile at Monte Verde may indicate a human presence in the Americas by up to 33,000 years ago. The oldest indisputable evidence of human presence in the Americas are, however, findings related to the Clovis culture, which have been dated to about 11,000 years ago. The findings of Clovis points indicate the early settlers hunted large animals. About the same time as the arrival of the clovis culture many large animals such as Mammoths became extinct (as in Australia, possibly due to hunting).

Linguist Joseph Greenberg controversially classified American languages into three major families. The Eskimo-Aleut spoken by the Inuit peoples. The Na-Dené are 32 languages spoken only in North America by the Apache, Navajo and tribes in Alaska and Canada. Finally Amerind languages comprise more than 500 languages spoken in North and South America. Greenberg suggested that these three languages families represented three separate migrations that filled the Americas in the order they arrived.

Genetics

Up until recently the only way of learning about ancient ancestors was through old fossils and stone tools. As we travel further back in time fossils become more rare. Of the billions of people who lived before the invention of agriculture only the fossilized remains of a few hundred have been found. In the absence of fossils, human DNA that transmits genetic information from one generation to the next has proved to be a valuable tool in recording the evolution of the human species.

Two pieces of the human genome are particularly useful in deciphering human history. One is the Mitochondrial DNA and the other is the Y chromosome. These are the only two parts of the genome that are not shuffled about by the evolutionary mechanisms designed to generate diversity with each generation. Hence the Mitochondrial DNA and the Y chromosome are passed down generation to generation intact. All 6.5 billion people alive today have inherited the same Mitochondria from one woman who lived in Africa about 150,000 years ago; she has been named Mitochondrial Eve. All men today have inherited their Y chromosomes from a man who lived 60,000 years ago, probably in Africa. He has been named Y-chromosomal Adam.

Genetic Recombination

Main article: Genetic recombination

File:Morgan crossover 2.jpg

chromosomal swap

The Human Genome is comprised of 3.1 billion base DNA base pairs packed in the chromosomes located in the Nucleus. Each person has 23 pairs of chromosomes. In each pair one chromosome is descended from the sperm cell of the father and the other chromosome is descended from the mother's egg. When an adult organism begins to produce sperm or egg cells these two chromosome line up against each other and exchange pieces of DNA with each other in a process known as chromosomal crossover.

Mitochondrial DNA

Main article: Mitochondrial DNA

Mitochondria are components found in the cell outside the nucleus that provide energy to the cell. They are believed to have originally been free bacteria that became incorporated into the cells of organisms billions of years ago. This is because the mitochondria have their own strand of DNA in a loop similar to those of bacteria. As the mitochondria are located outside the nucleus they do not participate in the shuffling of DNA that occurs during reproduction. When the sperm fuses with an egg cell during fertilization, the sperm's mitochondria are destroyed, leaving only the egg cell's mitochondria in the new fertilized zygote. This process only passes down the mother's mitochondrial DNA (mtDNA) to the next generation. All people have their mother's mtDNA but not their father's. Thus, a family tree of humanity based on the maternal line can be drawn, whereby the tree eventually coalesces on one female, and that person is Mitochondrial Eve.

The DNA in mitochondria is relatively short with only 16,569 base pairs. Thus it is much easier to study than the rest of the genome. When a woman's mitochondria are copied and packed into an egg cell, the DNA is almost always the same. However, every once in a while a mutation takes place which alters the sequence of the DNA strand. If Mitochondrial Eve had two daughters, one of whom happened to have a mutation in her Mitochondrial DNA, then all the women alive today descended from that daughter would share that mutation. All the women descended from the other daughter would not. Thus mutations in mtDNA are useful in determining lineages and migratory patterns.

Y chromosome

Further information: [[:Y chromosome]]

The pair of sex chromosomes are unlike the other 22 pairs of chromosomes. The X and Y chromosomes do not exchange segments of their DNA (except at the very tips). All other pairs of chromosomes are virtually identical in size and number of genes and hence compatible to exchange segments. The Y chromosome is significantly smaller than the X and thus incompatible for large scale shuffling. This process is necessary to ensure that the Y's most important gene, the one responsible for making a person male, is not transferred to the X chromosome, which is responsible for making a person female.

It follows that the Y chromosome is passed down largely intact from fathers to sons through the generations. Just as with mtDNA, mutations in the Y-chromosome make a fork in the family tree and can be used to study lineages. The genome of the Y chromosome is much larger (58 million base pairs) than that of mtDNA and initially mutations in it were much harder to find. More recently the mapping of the Y has been completed. All men today have inherted the Y-chromosome of Y-chromosomal Adam who may have lived 60,000 years ago in Africa. Comparing the profile of the Y chromosome with that of the mtDNA of a population may give useful hints about differences in ancient migratory patterns of men and women.

Molecular Clock

Main article: molecular clock

The molecular clock is a technique in genetics used to estimate when populations diverged. When comparing the mtDNA sequences of two populations, researchers can target mutations that are present in one population and absent in the other. The more mutations that differentiate the sequences, the longer the populations have been separated. The assumption is that mutations are random events that occur at a steady rate; for example, in humans it is sometimes estimated that a mutation takes place every 10,000 years. Thus, if mtDNA sequences of two populations differ by 5 nucleotides, it can be inferred that the two populations split from a common ancestral population 50,000 years ago. The clock can be calibrated by using a references pair of groups of living species whose date of speciation was already known from the fossil record.

The molecular clock is a statistical analysis based on many assumptions and hence its accuracy is sometimes questioned. However, studies do show some consistency with fossil records. For example, Mitochondrial Eve is calculated to have lived about 150,000 years ago. This is consistent with the emergence of anatomically modern humans based on fossil evidence. Furthermore, the dates calculated based on the Y-chromosome are in general agreement with those for mitochondrial DNA. This is a useful comparison because Y and mtDNA are inherited independently and they have different mutation rates.

Indigenous peoples

In formulating the single origin hypothesis, scientists focussed their research on many indigenous groups. This is because these groups have lived in the same location for thousands of years. Since many indigenous groups have remained relatively isolated from the later immigrant populations they live with, their genetics may be the least influenced by the effects of long distance migration. Their genetic makeup would thus be the best available representation of the early settlers in a region.

Language isolates are languages that have no known historical or linguistic relationship to any other languages. They are useful to anthropologists in identifying groups that may have a more unique history. Basque spoken in Spain and France has confounded linguists because it is one of the only languages in Europe that doesn't belong to the Indo-European language family. The language of the Ainu in Japan is also another isolate. So is Burushaski, the language spoken near the Pamirs in the Himalayas. The Khoisan languages are also of interest because of the use of the click consonant. The Hadza language of Tanzania also uses the click consonant but appears to be unrelated to the Khoisan languages. There are several isolates among the indigenous languages of New Guinea and Australia.

Of particular interest are the so called Negritos who are the indigenous people of the Andaman Islands. It is believed that they were isolated on the islands perhaps thousands or even tens of thousands of years ago due to rising sea levels. They were once thought to be related to African pygmies because of their short stature, however they are genetically more linked to the surrounding Asian populations . Their language is also unique and linguists have all but failed to find a connection to any of the world's major language families. A related group, the Sentinelese, have resisted all attempts to be contacted and almost nothing is known about them. This suggests that these groups have been genetically isolated for long periods and may have been part of the first group of people to settle in Asia.

African Eve

File:Major mtdna haplogroups.png

phylogenetic tree of Major mtDNA lineages

When researchers first began studying the mtDNA peoples around the world in 1987 they found that the greatest diversity of lineages was found in Africa. Of the 33 maternal clans of the world, 13 were in Africa. Though Africa had 13% of the worlds population it had 40% of the world's deeper mitochondrial lineages. This indicated that Africa had more time to accumulate mutations than the rest of the world. As a rule of thumb for any species, the region of greatest diversity is very likely the region of origin.

Scientists were then able to construct the genetic relationships between the various mitochondrial haplogroups and build a family tree. One by one they found the African and also all non-African mitochondrial lineages converged on a single root, and this mitochondrial ancestor was named Mitochondrial Eve. They then identified a single African mitochondrial lineage, haplogroup L3e, that was the single root for all the mitochondrial lineages found outside Africa. This evidence indicated that the human family arose as one single genetic line in Africa within the last 200,000 years and not as multiple lineages in separate locations.

Eve was not the only woman alive at the time but only her line of descent remains unbroken today in all humans. Some scientists believe that the human family faced near extinction in the last 100,000 years due to some catastrophic event (see Toba catastrophe theory). The human population may have dwindled to as few as 2,000 people, causing the lineages of other women to die out leaving only those of Eve's to dominate. This process has been described as the founder effect.

Genetic reconstruction

Further information: [[:Human mitochondrial DNA haplogroup]]

File:Migration map4.png

One model of human migration based on Mitochondrial DNA

It is widely accepted that humans first arose in Africa and later colonized Eurasia and the rest of the world. However, the timing of the exit out of Africa and the routes taken remain controversial. Owing to the time frame, the patterns of migration are very complex and scientists are likely to continue making revisions and adjustments to existing theories as further studies yield new information.

The first lineage to branch off from Eve is L1. This haplogroup is found in high proportions among the San and the Mbuti people. These groups branched off early in human history and have remained relatively isolated genetically since. Haplogroups L2 and L3 are descendents of L1 and are largely confined to Africa. The macro haplogroups M and N, which are the lineages of the rest of the world outside Africa, descended from L3.

Some scientists believe that only a few people left Africa in a single migration that went on to populate the rest of the world. It has been estimated that from a population of 2,000 - 5,000 in Africa, only a small group of possibly 150 people crossed the Red Sea. This is because, of all the lineages present in Africa, only the daughters of one lineage, L3, are found outside Africa. Had there been several migrations one would expect more than one African lineage outside Africa. L3's daughters, the M and N lineages, are found in very low frequencies in Africa and appear to be recent arrivals. A possible explanation is that these mutations occurred in East Africa shortly before the exodus and by the founder effect became the dominant haplogroups after the exodus from Africa. Alternatively, the mutations may have arisen shortly after the exodus from Africa.

File:Single origin multiple dispersals.jpg

Single origin from Africa but with multiple dispersals out of Africa of haplogroup M and haplogroup N

Other scientists propose that there were two migrations out of Africa, one across the Red Sea travelling along the coastal regions to India, which would be represented by Haplogroup M. Another group of migrants with Haplogroup N followed the Nile from East Africa, heading northwards and crossing into Asia through the Sinai. This group then branched in several directions, some moving into Europe and others heading east into Asia. This hypothesis attempts to explain why Haplogroup N is predominant in Europe and why Haplogroup M is absent in Europe.

The group that crossed the Red Sea travelled along the coastal route around the coast of Arabia and Iran until reaching India, which appears to be the first major settling point. M is found in high frequencies along the southern coastal regions of Pakistan and India and it has the greatest diversity in India, indicating that it is here where the mutation may have occurred. 60% of the Indian population belong to Haplogroup M. The indigenous people of the Andaman Islands also belong to the M lineage. The Andamanese are thought to be offshoots of some of the earliest inhabitants in Asia because of their long isolation from mainland Asia. They are evidence of the coastal route of early settlers that extends from India along the coasts of Thailand and Indonesia all the way to Papua New Guinea. Since M is found in high frequencies in highlanders from New Guinea as well, and both the Andamanese and New Guineans have dark skin and frizzy hair typically found in Africa, some scientists believe they are all part of the same wave of migrants who departed across the Red Sea. Others suggest that their physical resemblance to Africans is more likely to be an example of convergent evolution.

From Saudi Arabia to India the proportion of haplogroup M increases eastwards: in eastern India, M outnumbers N by a ratio of 3:1. However, crossing over into East Asia, Haplogroup N reappears as the dominant lineage. M is predominant in South East Asia but amongst Indigenous Australians N reemerges as the more common lineage. This discontinuous distribution of Haplogroup N from Europe to Australia has confounded scientists attempting to trace migratory routes.

The descendents of Haplogroups M and N are both found in the Americas.

Physical appearance

There is considerable speculation on the physical appearance of ancient homo sapiens during the period of Mitochondrial Eve and prior to exodus from Africa. The reason for this is that all variation in human physical appearance visible in today's people around the world is theorized by some scientists to have come from this small population in Africa.

Hairiness is the default state of most mammals, though a few have lost much of their hair for a variety of reasons. They include many aquatic mammals such as dolphins and hippopotami, the naked mole rat and humans. Most non-human primates have lightly pigmented skin covered by fur. Scientists believe that early protohominids resembled our closest relative, the chimpanzee, with white skin covered by dark hair. The hominids began to walk upright and left the shade of the trees for the open savannah and therefore required a more efficient cooling system. The brain uses significant amounts of energy but is very sensitive to heat, so the increased brain power of the early hominids also required a finer thermoregulatory system. As a result humans evolved more sweat glands, especially on the face, which required the loss of hair for more effective evaporation. Sexual selection by a preference for naked skin may have played a secondary role as well. Though naked skin is advantageous for thermoregulation, it exposes the epidermis to destructive levels of UV radiation that can cause sunburn, skin cancer and birth defects resulting from the destruction of the essential vitamin B folate. To protect the epidermis natural selection favored increased levels of melanin in the skin.

The general consensus among scholars is therefore that the first modern humans would have been dark skinned. When humans migrated to less sun intensive regions in the north, the dark skin that was adapted to blocking out much of the UV radiation in the tropics would block even the minimum amount of radiation required for cells under the skin to produce Vitamin D. This is essential for bone growth, as deficiencies in vitamin D cause rickets. Thus skin color would revert back to its default form present before the process of hair loss began, but this time without the hair. Whilst the timing of this change from dark to light skin has not yet been established it is possible that the early settlers of Europe and Asia were dark skinned. Aside from skin color however, which despite the above could also arguably be included, the majority of apparent difference in human physical appearance around the world, or what may also be called racial features, can also be explained through the process of regional sexual selection of relatively recent evolutionary origin.

Multiregional Theory

Main article: multiregional hypothesis

The multi-regional hypothesis consists of several models of human evolution which all posit that the human races evolved from separate archaic humans over millions of years. The Multiregional theory is based largely on archaeological and fossil evidence. Proponents of the Multiregional theory argue that physical similarities between modern and archaic humans, such as the brow ridges and comparatively robust skeletons of some modern Europeans (as compared to Neanderthals), and the shovel-shaped incisors and other distinct craniofacial features noted in modern Chinese (as compared with Chinese homo erectus), could only be the result of genetic contributions from earlier lineages that evolved semi-independently. Opponents, however, cite the lack of DNA evidence supporting these theories.

There are several models of multiregionlism that depend largely on whether gene flow between the populations took place. Polygenism is a more extreme form multiregionalism in that it implies separate origins for the human races. Proponents include Carleton Coon who hypothesized that modern humans, Homo sapiens, arose five separate times from Homo erectus in five separate places. Their descendents are the the major races of today. Polygenists such as Arthur de Gobineau believed in the existence of pure races.

The hybrid-origin theory states that significant gene exchange did take place between widely divergent hominid species, or subspecies, that were geographically dispersed throughout Africa, Southeast Asia and the Indian subcontinent. According to this theory the resulting hybrid 'Homo sapiens sapiens', was superior to both its ancestors due to what is commonly termed hybrid vigour. They argue that very strong genetic similarities among all humans do not prove recent common ancestry, but rather reflect the interconnectedness of human populations around the world, resulting in relatively constant gene flow (Thorne and Wolpoff 1992).

Criticism of Multiregionalism

Aspects of multiregionalism has been criticized as not being based on objective scientific observation. Some critics even argue that multiregionalism may be motivated by ethnocentrism and is meant to instill beliefs of purity of lineage.

Multiregionalists have long claimed that modern Europeans are descended from the Neanderthals. In 1997, DNA testing performed on a Neanderthal skeleton showed modern humans and Neanderthals last shared a common ancestor between 500,000 and 800,000 years ago, and furthermore that all modern humans, from the ethnic Siberians to the !Kung people of Africa, are more closely related to each other than to the Neanderthals -- further evidence supporting the Out-of-Africa theory.

Another example is the case of Peking man, a fossil skull of homo erectus found in China dating to possibly 400,000 years. Some Paleoanthropoligists in China have asserted that the modern Chinese are descendents of earlier forms of humans such as Peking Man. However, Chinese geneticists performed microsatellite analysis on the Chinese population in 1998 and discovered genetic similarities with Africans, yielding the first evidence that the Chinese descended from Africa. A recent study undertaken by Jin Li showed no inter-breeding between modern human immigrants to East Asia and Homo erectus, contradicting the Peking Man-origin hypothesis and affirming that the Chinese descended from Africans. In 2001, Chinese geneticists analyzed Y chromosomes in Chinese people and concluded that all Chinese samples contained a mutated gene M168G which is a marker believed to have appeared in the last 79,000 years on a number of Africans.

The fossil record also favors a single origin. This is because mainly in Africa is there a sensible progression of fossils over the last 3 million years that shows the various intermediate stages of evolution from the most archaic ancestors to modern man.

Proponents of the single-origin hypothesis

• James Burnett, Lord Monboddo, born 1714
• Luigi Luca Cavalli-Sforza, born 1922
• Donald Johanson, born 1943
• Chris Stringer, born 1947
• Spencer Wells, born 1969
• Tim Crow, contemporary
• Stephen Oppenheimer, contemporary
• Jin Li, contemporary
• Ian Tattersall, contemporary

See also

• Mitochondrial Eve
• Y-chromosomal Adam
• Human migration
• Multiregional hypothesis

Further reading

• Cavalli-Sforza, Luigi Luca and Francesco Cavalli-Sforza, The Great Human Diasporas – The History of Diversity and Evolution (Italian original Chi Siamo: La Storia della Diversit`a Umana), ISBN 0-201-44231-0 (paperback), 1993.
• Crow, Tim J, Editor The Speciation of Modern Homo Sapiens, ISBN 0-19-726311-9 (paperback) 2002.
• Foley, Robert, Humans Before Humanity, ISBN 0-631-20528-4 (paperback), 1995.
• Olsen, Steve, Mapping Human History: Discovering the past through our genes ISBN 0618352104 2002
• Oppenheimer, Stephen, The Real Eve: Modern Man's Journey Out of Africa, ISBN 0-7867-1192-2 (Hardcover), 2003.
• Stringer, Chris and Robin McKie, African Exodus, ISBN 0-7126-7307-5 (paperback), 1996.
• Sykes, Brian, The Seven Daughters of Eve: The Science That Reveals Our Genetic Ancestry (2002) ISBN 0552152188
• Wade, Nicholas, Before the Dawn: Recovering the Lost History of Our Ancestors (2006) ISBN 1594200793
• Wells, Spencer, The Journey of Man: A Genetic Odyssey (2003) ISBN 069111532X
• Wells, Spencer, Deep Ancestry: Inside the Genographic Project (2006) ISBN 0792262158
• "New Research Proves Single Origin Of Humans In Africa," Science Daily, July 19, 2007, retrieved July 19, 2007

References

• ^ Modern Men Trace Ancestry to African Migrants, Science, 11 May 2001
• Underhill et al. (2001). "The phylogeography of Y chromosome binary haplotypes and the origins of modern human populations" Ann. Hum. Genet. 65: 43-62. PDF. Retrieved 25 July 2007.
• ^ Neanderthals 'mated with modern humans', BBC News, 21 April 1999
• ^ New analysis shows three human migrations out of Africa - Replacement theory 'demolished', Washington University in St. Louis, 2 February 2006
• Long and Kittles (2003). "Human genetic variation and the nonexistence of human races" Human Biology, 75: 449-471. PDF. Retrieved 10 January 2007.
• Risch, N., Burchard, E., Ziv, E. and Tang, H. (2002). "Categorization of humans in biomedical research: genes, race and disease". Genome Biology. 3 (7): comment2007.2001 - comment2007.2012.CS1 maint: Multiple names: authors list (link)
• Tishkoff, S. and Kidd, K. Implications of biogeography of human populations for 'race' and medicine Nature Genetics 36: S21 - S27 (2004)

External links

• National Geographic: Atlas of the Human Journey
• Bradshaw Foundation: The Journey of Mankind
• Natural History Museum - Video