Multiregionalism v. Out-of-Africa:

Two Mechanically Different Explanations to the Current State of Homo sapiens

The origin of anatomically modern humans, the hominid species Homo sapiens, is a hotly debated argument among two principle origin stories. The two competing hypotheses of Multiregionalism and Out-of-Africa explanations for the origin of our species take very different stances on the mechanisms and order of evolutionary processes that have come to lead us to our existing state.

As a socially and racially diverse species, Multiregionalists policy derives its explanation of the current state of Homo sapiens as a single specie unit as a result of parallel evolution of archaic hominid population-pockets. The multiregional hypothesis diagrams the interplay of archaic Homo erectus populations whose development in isolated units' maintained localized traits, while with the mechanism of peripheral genetic interplay between these pockets, the genetic identity of Homo sapiens became unilateral. This methodology would provide explanation of the racially diverse species with almost identical genealogy.

In regards to the genetically homogenous species that we are, the Out-of-Africa hypothesis derives explanation of the current state of Homo sapiens as a result of a single diaspera of an evolutionary dominant species coming directly out of Africa. The model provides the evidence of mtDNA as a window into the genesis of our species from a single "Eve" incarnation that resulted in biologically successful species that spread across the face of the planet coming to out compete its other hominid competitors.

Both models acknowledge an outcome in the species of Homo sapiens however they are diametrically opposite in the means by which this outcome was achieved. We as a group will examine the differing arguments for and against the Out-of-Africa and Multiregional models as well as respond to the criticisms of each. We will provide an overall analysis of the foundational characteristics of the hypothesis and then provide a discussion as to the merits of each culminating in a determination of the more plausible model that best explains modern Homo sapiens origins.

The Arguments in Favor of Multiregionalism

The Multiregional Hypothesis, also known as regional continuity, is a competing theory for the origin of modern humans (Homo sapiens). This theory attempts to explain not only the origin of Homo sapiens, but also the existence of anatomical diversity in the world today among humans (Thorne and Wolpoff, 28). Arguments supporting the Multiregional Hypothesis will be discussed below.

An important argument for the Multiregional Hypothesis is the concept of gene flow. Gene flow is an element of the evolutionary process and is a possible cause of evolutionary change. Because of this gene flow, new traits evolving in one region would have been carried inevitably to all other regions (internet source #6). From this concept, people who believe in this theory are able to explain how modern humans have evolved from the single species, Homo erectus, and helps explain a similar rate and "direction" through similar successive stages within the evolution of all modern humans (Cameron and Groves, 14). Moreover, it is a way to explain the genetic exchange between species constantly migrating to different regions of the Old World by allowing us to gain insight on reproductive patterns.

From this one can assume that interbreeding of various populations of Homo erectus must have occurred. This leads to another argument for the Multiregional Hypothesis which is continuity traits. Continuity traits provide an evolutionary link between archaic populations and modern humans. It is a notion that illustrates the idea that in each region certain anatomical patterns of traits can show distinct and recognizable features. "Some anatomical features are said to have developed in a particular region as a result of the need to cope with new and unique environmental conditions encountered within that region and to have been maintained through time(to the present day) within those regions" (Cameron and Groves, ). According to supporters of the Multiregional Hypothesis, these anatomical features or morphological traits that have been persevered regardless of additional global trends support the claim that the human lineage evolved multiregionally. These distinct and recognizable features among archaic populations can be evident and as distinct in the evolution of modern humans. The physical difference among modern humans is therefore a result of hundreds and thousands of years of regional evolution. These continuity traits include large suborbital torus, flat frontal bone, a developed occipital torus, facial prognathism, and shovel-shaped incisors. An example of continuity traits between modern Australian populations and the later specimens of Homo erectus, the Ngangdong sample from Java, show a number of cranial traits (Foley, 340). This can be proven through comparisons of modern hominid skulls including the cast of Ngangdong , Lake Mungo 3, and Keilor.

Furthermore, the discovery of mitochondrial DNA (mtDNA) has provided support for the Multiregional Hypothesis as well. Mitochondrial DNA, similar to gene flow, allows us to trace a common ancestor between archaic populations and modern humans. Similar to the continuance of a family name, mitochondrial DNA is passed on from generation to generation through the mother. Between the exchanges from mother to daughter of mitochondrial DNA, mutations can occur and is the only source of new variation since recombination does not occur. Additional advantages of mitochondrial DNA are rapid evolution and the small amount of genetic information found in this type of DNA.

The rapid evolution of mitochondrial DNA helps support the Multiregional Hypothesis by dating a "mitochondrial Eve", a woman which all modern humans have descended from, to have lived some time ago. The dates published for the age of mitochondrial Eve are astonishingly variable (Caspari and Wolpoff, 309). The emergence of the mitochondrial Eve does not fit the timeline given by fossils found in the field. For supporters of the Multiregional Hypothesis, this was a positive discovery because instead of showing that interbreeding did not occur (Out-of-Africa theory), the mitochondrial DNA show the continuity of genetics between the species Homo erectus and Homo sapiens. Without the Multiregional Hypothesis, there would not be a way to describe how modern humans show continuous anatomical features of older fossils found around the world.

In accordance, Mitochondrial DNA supports the Multiregional Hypothesis by verifying the validity of some of the assumptions the model requires (Caspari and Wolpoff, 302). As listed in the book Race and Human Evolution Wolpoff and Caspari list the three key elements of the Multiregional Hypothesis and they are as follows: 1) Long-standing intermixture 2) Prehistoric population size differences and 3) Genetic bottlenecks (302).

The importance of long-standing intermixture to the Multiregional Hypothesis is to show that there is a reoccurring pattern of differences and similarities persistent over a long period of time. Mitochondrial DNA helps support this assumption by offering genetic evidence for the same nuclear and mitochondrial variations found over and over again, in the most widely separated populations. Supporters of the Multiregional Hypothesis can explain this by saying that the divided populations were not really separated, and the distribution of these genetic variations could be the result of persistent genic exchanges in the past (Caspari and Wolpoff, 302).

Prehistoric population size differences are largely due to how large or small populations were in the past and how much they fluctuated (Caspari and Wolpoff, 305). This fluctuation in population size can affect mitochondrial DNA greatly and is a contributing factor on the debate over African origin, as the Out-of-Africa theory suggests, versus ancient population size differences. This debate is over the genetic variation of Africans being greater than any other population. With the help of population size history, supporters of the Multiregional Hypothesis can explain the pattern of mitochondrial DNA variation and shows that there were more people living in Africa and those human populations outside of Africa were smaller and fluctuated more because of the changing ice-age environments (Caspari and Wolpoff, 306). The authors, Caspari and Wolpoff make it clear that this explanation does not intend to suggest that mitochondrial DNA is population history but rather a means of explaining the pattern of variation of mitochondrial DNA among ancient populations.

Genetic bottlenecks, as described by Caspari and Wolpoff, is when a brief period of intense selection or of a very small population size through which only certain genes survive and come to characterize the population (301). The importance of genetic bottlenecks for the Multiregional Hypothesis revolves around when mitochondrial DNA finds a last common ancestor. The different bottlenecks in various gene systems, and the absence of any bottlenecks in some, show that while Eve was the mother of all mitochondria, she could hardly have been the mother of humanity (Caspari and Wolpoff, 309).

The Arguments in Favor of the Out-of-Africa Model

Proponents of the "Out Of Africa" model argue that modern humans evolved exclusively in Africa, and the Asian and European hominids were evolutionary "dead ends," side branches that did not leave any offspring that survive today. Neanderthals were our "distant cousins" but not our ancestors and only African Hominids like Home ergaster are direct ancestral relatives to modern humans.

For the purpose of scientists studying when lineages diverged, mitochondrial DNA has two advantages over nuclear DNA. First, the sequences in mitochondrial DNA that interest us accumulate mutations rapidly and steadily, according to empirical observations. Because many mutations do not alter the mitochondrion's function, they are effectively neutral, and natural selection does not eliminate them. This mitochondrial DNA therefore behaves like a fast-ticking clock, which is essential for identifying recent genetic changes. Accepting that the mutation rates are constant, we can examine the number of shared and unique bases along any strand of mtDNA within a given population and then calculate the molecular distance between populations. "The molecular distance between species, therefore, should also be proportional to their separation in time, that is, the time when they last shared a common ancestor" (Cameron and Groves 13). Secondly, unlike nuclear DNA, mitochondrial DNA is inherited from the mother alone, unchanged except for chance mutations. For the studies of modern human origins, we focus on the mitochondrial, maternal lineages.

Molecular studies of mitochondrial DNA (mtDNA) have yielded an estimate of the divergence time of all human populations at around 200,000 years. "This means that all contemporary human mtDNA originated approximately 200,000 years ago; this supports the out of Africa hypothesis that humans originated in Africa." These studies have also shown that contemporary African populations have a greater diversity in their mtDNA than do other geographical populations. This is expected if humans originated in Africa and then spread gradually: as new populations were founded, they were likely subject to population jam, and during these events they would lose some genetic variation (through the process of genetic drift). Today, then, we would see less variation within each of these populations than we would within the ancestors of the original population.

In another study, mtDNA was extracted from a Neanderthal fossil and compared with modern human mtDNA. "The extracted Neanderthal mtDNA comes from specimens dated to around 35,000 years ago and is distinct not only from modern humans but also from Mungo 3; that is, the older Australian mtDNA is closer to that of modern humans than is the later Neanderthal mtDNA" (Cameron and Groves 27). Overall, the genetic evidence tends to support the "Out of Africa Hypothesis."

Fossil evidence also tends to favor the out of Africa hypothesis. "The oldest anatomically modern human fossils have been found in Africa, and are ~130,000 years old. The next oldest come from the Near East (~90,000 years old)." Modern humans are not seen in Europe until about 40,000 years ago. This chronological pattern would be expected with a migration pattern predicted by the Out of Africa hypothesis.

Homo ergaster probably first evolved in Africa around 2 million years ago. "This species had a rounded cranium with a brain size of between 700 and 850 cu cm (49 to 52 cu in), a prominent brow ridge, small teeth, and many other features that it shared with the later H. erectus." (Potts). Many paleoanthropologists consider H. ergaster a good candidate for an ancestor of modern humans because it had several modern skull features, including relatively thin cranial bones. Most H. ergaster fossils come from the time range of 1.8 million to 1.5 million years ago.

The most important fossil of this species yet found is a nearly complete skeleton of a young male from West Turkana, Kenya, which dates from about 1.55 million years ago. Scientists determined the sex of the skeleton from the shape of its pelvis. They also determined from patterns of tooth eruption and bone growth that the boy had died when he was between 9 and 12 years old. The Turkana boy, as the skeleton is known, had elongated leg bones and arm, leg, and trunk proportions that essentially match those of a modern human, in sharp contrast with the apelike proportions of H. habilis and Australopithecus afarensis. He appears to have been quite tall and slender. Scientists estimate that, had he grown into adulthood, the boy would have reached a height of 1.8 m (6 ft) and a weight of 68 kg (150 lb). The anatomy of the Turkana boy indicates that H. ergaster was particularly well adapted for walking and perhaps for running long distances in a hot environment. This skeleton is believed to be the most human-like fossil.

The Argument Against the Multiregionalism

The multiregional hypothesis has been widely disputed amongst anthropologists. The book, Bones, Stones and Molecules, raises some criticisms to the revised multiregional hypothesis by Curnoe and Thorne (2003). I will discuss three main issues with this hypothesis that they raise as a counter-argument to the multiregional hypothesis. The first problem with this argument is that it confuses species and genera. They also don't look at the fossil record. The last issue I will discuss is the implications that the Eve hypothesis has on the multiregional hypothesis.

Curnoe and Thorne believe that the human lineage has consisted of one genus over the course of six million years and that only four or five species has existed in this genus. Homo sapiens being the last of these emerged around two million years ago. This "Single Species Hypothesis" was originally rejected because the fossil evidence showed that a number of different species needed to be recognized (Cameron and Groves 17). Curnoe and Thorne's version "ignores fossil evidence and is based on an abstract interpretation of the available molecular data" (Cameron and Groves 17). They recognize four species (Homo ramidus, Homo africanus, Homo habilis and Homo sapiens). Arsuaga, quoted by Cameron and Groves states:

In reality, a species' complete disappearance from the world does not necessarily have to coincide with the appearance of its descendant species in any given place. This would be a theoretical prerequisite only if one species evolved into another species throughout its entire geographical range, in a process that affected each and every one of its separate populations….If a descendant species extends its range to other areas still inhabited by its ancestral species…Eventually, if the two species occupy the same ecological niche, the compete with each other and the ancestral species could finally disappear (Cameron and Groves 17).

Cameron and Groves suggest that early hominids such as Homo neanderthalensis (Eurasia), Homo erectus (Indonesia) dispersed into regions out of Africa, later followed by Homo sapiens and eventually replaced by them. Homo sapiens are not from the same lineage as other hominids, as the multiregional hypothesis states. Curnoe and Thorne confuse species and genius. "A Species is a real biological unit, while a genus is merely a system of biological classification" (Cameron and Groves 18). The multiregional hypothesis groups these different hominids into one species.

Another problem with the multiregional hypothesis by Curnoe and Thorne is that they do not consider the fossil record in their model. "They say what the species ought to be, not what they actually are according to a detailed analysis of the evidence" (Cameron and Groves 21). There are too many dissimilarities within a "species according to this model." For instance Homo africanus, according to Curnoe and Thorne, would consist of five different species currently allocated to Australopithecus (Cameron and Groves 21). Their model would revise the whole human family tree yet they do not provide a solid anatomical definition of their species. The species that they group into one species have different evolutionary adaptive trends that are distinctive to that species separating it from the others. Furthermore we would not expect to see such "distinct trajectories in one species….refuting the idea that these taxa represent one species" (Cameron and Groves 22).

The third argument raised against the Multiregional Hypothesis is the Displacement from Africa or Eve hypothesis. The recent research in molecular biology suggests that the descendants of female Homo sapiens "left Africa between 140,000 and 290,000, migrated inter-continentally to Europe and Asia, and displaced the earlier inhabitants without much interbreeding" (Jackson and Lieberman 234). . This model has been assisted by interpretations of the fossil record by paleoanthropologists, Stringer and Andrews. The conclusions of the molecular data research proves the fossil record of Homo Erectus inhabiting Europe and Asia about one million years ago had become extraneous, this deemphasizes the likelihood of regional morphological continuity (Jackson and Lieberman 235).

The revised Multiregional Hypothesis by Curnoe and Thorne has some key elements to their model absent. As stated by Cameron and Groves, the hypothesis fails to take into account the fossil record and shows a lack of evidence for their claim of a "single species". This idea also goes against current research finding in Mitochondrial DNA; dating that Homo sapiens dispersed out of Africa roughly 200,000 years ago instead of 2 million years ago as believed in the multiregional hypothesis.

The Argument Against the Out-of-Africa Model

For years and years anthropologists have made a case of which of the two opposing theories is correct about the origin of our breed. The Out of Africa hypothesis is one of the two theories, which is known as the single-origin hypothesis upholds that modern humans have developed in Africa and then spread out to different places in the world. One of the main predictions from the anthropologists who support this hypothesis is that there should be remains of modern humans found in Africa, if modern human are recent and came from Africa.

The other hypothesis is the Multiregional hypothesis which argues that modern humans have evolved from a predecessor class, Homo erectus, evolving from man from different places around the world about one to two million years ago. Those in favor of this hypothesis argued that interbreeding has taken place. Due to genetic contributions from some of the earlier lineages that evolved a bit independently in different places of the world would explain the distinctiveness of modern humans. Also, to even differentiate races, since many of the populations according to this theory were isolated from one another giving them distinctive features. The prediction of this hypothesis is based on the remains of early modern humans can be found throughout in the Old World where they will date around the same time.

There is evidence that challenges the Out-of-Africa theory. The usage of DNA and fossil findings tests the Out-of-Africa theory. For instance, there has been fossil evidence against the Out-of-Africa theory coming from three skulls and several jaw fragments, which was found in Georgia. They are estimated to be about one point seven million years old. These skulls were found at a medieval caste at Dmanisi during an archaeological dig. One of the skulls was nearly complete, while only the skullcap survived from the remains of the other skull. There were found with bones of animals and a few stone tools. Due to the findings of these skulls, this suggested that humans have migrated from Africa hundreds of thousands of years then anthropologists would have expected. From the Republic of Georgia Sate Museum in Tbilisi, David Lordkipanidze believed that the skulls were "the first proof for the presence of humans outside Africa at this time." The Dmanisi skulls show that they must have lived at around the same time as the Homo ergaster, who lived in Eastern Africa, due to their age and skeletal characteristics. All the stone tools, skulls, and the other significant materials found from this site were discovered from the same layer of sediment. Scientists now have to come up with a new explanation why humans could leave Africa. Many scientists thought humans were able to migrate from Africa due to have big brains, but due to the Dmanisi skulls, are evidence showing that the new specimen from Dmanisi brains were actually pretty small at the time. The brain of this specimen would have only been half the size of the modern human's brain. The Manisi is located on a peninsula, formed in between by the convergence of two rivers, the Black Sea and the Caspian Sea. This excavation site gives scientists a chance to study materials that involve human evolution from a certain time period from one site. "Lordkipanidze and his colleagues have categorized all three skulls as belonging to Homo Erectus. They are a bit uncertain since the brains of the specimen are small and these skulls do bear a resemblance to the Homo habilis. Homo habilis were a bit apelike due to long, drooping arms, huge sharp teeth, and a thin brow. The stone tools that were discovered with the Dmanisi skulls were found were not as complex as the "pebble-chopper" or the "Oldowan" tools. There were more then a thousand stone tools recovered from the Dmanisi site. Lordkipanidze suggest since these simple tools were found with the skulls these human were able to expand from Africa with archaic technology.

There has also been some DNA evidence that helped support the Multiregional hypothesis. Found in Australia, known as the "Mungo Man." Mungo Man, which is sixty thousand years old, is considered to be the oldest skeletal remain where analysis of the oldest DNA was ever taken. These skeletal remains portrayed him to have a genetic lineage that is older and distinct from the African line. This shows how the Australian Aborigines must have arrived from members of two migrations then one. "Lake Mungo man known as LM3, who has been dated by three separate methods at more then 60,000 years old. LM3 is ‘the oldest individual dated accurately and possibly the oldest human from whom DNA has been recovered.' Thorne says." (Science, 12, January 2001) The major finding from the Mungo Man is in the mitochondrial DNA (mtDNA) which shows how a sequence is different from modern humans and from other fossils. This sequence is said to only exist as a remain or an insert on chromosome 11, even though it does not exist in modern human mtDNA. Researchers concluded that the sequence is a replica of old mtDNA, must have found its way to the cell nucleus, as many other sequences have been known to do. Researcher, Alan Thorne says "The data undermine studies that support the Out of Africa scenario with genetic evidence from living populations. By analyzing variations in modern DNA sequences and tracing their "roots" backward in time scientists have concluded that everybody now alive stems from African ancestors who replaced earlier types of humans without interbreeding with them. Now, the most divergent, deep-rooted mtDNA sequence of any anatomically modern human has turned up thousands of miles from Africa." (Science, 12, January 2001) This data helps to support the multiregional hypothesis since modern humans could have inherited DNA from ancestors as Homo erectus, who migrated more then two million years ago before supposedly the Out of African migrants did to Eurasia.

Scientists are still debating whether the Multiregional hypothesis or the Out of Africa hypothesis is the better theory about where humans originated. Using evidence such as fossil findings and DNA helps to support as evidence. The DNA evidence from the Mungo Man weakens the Out of Africa theory, showing that the skeleton's mtDNA did not bore similarity to that of Neanderthals, ancient Aborigines, and present Aborigines. The Dmanisi skulls and stone tools are evidence that humans have traveled from Africa hundreds of thousands of years then anthropologists would have predicted. The Mungo Man and the Dmanisi skulls is just a couple of evidence that some scientists use against the Out of Africa hypothesis.