A Critique of the out of Africa Model Michael Maystadt Introduction
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Introduction http://www.soa.ilstu.edu/anthropology/theses/maystadt/maystadt1.htm A Critique of the Out of Africa Model Michael Maystadt Illinois State University In the scientific community, there is a general consensus as to the origin and history of modern humans; most anthropologists agree that the earliest members of our species, Homo sapiens, emerged in Africa relatively recently and migrated to all parts of the earth. Stringer claimed that Homo sapiens only emerged in Africa. He based this on the fossil record, which showed that the oldest Homo sapiens fossils are from Africa. Mitochondrial DNA (mtDNA) lineage studies by Wilson, Cann and Stoneking in the late 1980’s and early 1990’s also reinforced this model. The Out of Africa model therefore states that all Homo sapiens evolved in Africa quite recently (circa 150,000 years ago) and replaced all other forms of the genus Homo with little or no interbreeding between the two. Wolpoff has been a strong critic of the Out of Africa model; he claims that modern humans evolved in separate parts of the world and have been exchanging genes for hundreds of thousands of years. He cites contradictions in Stringer’s argument such as the non-African appearance of early Homo sapiens fossils outside of Africa, and inconsistencies in the interpretation of mtDNA evidence. Templeton has also raised questions regarding the mtDNA evidence and what it actually shows. This paper demonstrates that there are significant inconsistencies between the Out of Africa model and all the available archaeological, fossil, and mtDNA evidence. The conclusion reached is that the Out of Africa model is not a legitimate explanation for the emergence and subsequent spread of Homo sapiens. Introduction Around 150,000 years ago, a small group of hunter-gatherers roamed the African savannah. Included in this hunter-gatherer group were a man and a woman who ultimately became the maternal and paternal ancestors of every human living today. Modern geneticists have dubbed this couple “Adam and Eve”. In the simplest terms, this is the ‘Out of Africa model’. I must admit that when I first heard everyone on earth was potentially descended from one male and one female, I was excited. I was only about 10 years old when my mother told me about the Out of Africa theory, and I really did not think about it again until I took a Paleoanthropology class last year. When I began to read and examine some of the evidence, as presented by Chris Stringer (supporting Out of Africa) and Millard Wolpoff (supporting Multi-regional evolution and disputing Out of Africa), I became skeptical. I can’t help that I am a very skeptical person by nature. So here is what the two theories claim. The Out of Africa model for the emergence of modern Homo sapiens claims that humans only evolved in Africa and that modern humans moved out of Africa within the last 120,000 years. When humans encountered other species, (such as Homo erectus or Homo neanderthalensis) they did not interbreed with them, but rather slowly displaced them and eventually replaced them. That means that all modern humans can trace their ancestry from a small population that lived in Africa over 150,000 years ago. The “multi-regional” model claims that modern humans can trace their ancestry back millions of years to local populations of Homo erectus that emigrated out of Africa almost two million years ago. Through 1 of 20 11/13/2007 7:51 AM Introduction http://www.soa.ilstu.edu/anthropology/theses/maystadt/maystadt1.htm evolution and gene flow, eventually these populations acquired modern traits and emerged as Homo sapiens. In this model, modern humans may have emerged in Africa first (and there were no doubt multiple migrations out of Africa) but there was not an all out replacement of earlier archaic populations; rat researchers argue that certain “regional distinctions” stay intact through hundreds of thousands of year different geographical populations. This indicates that local archaic populations may have made some contribution to modern human morphology. In this paper, I ask several questions: Does all the available evidence prove or disprove the Out of Afric model? Does the genetic evidence prove that all humans are descendants of one male and one female who lived almost 150,000 years ago? Does the fossil evidence support a complete replacement of earlier forms of the genus Homo? If the Out of Africa model is correct, what was the huge advantage that modern humans possessed that allowed them replace Homo erectus and the Neanderthals so quickly and completely? After these questions are answered, I will assess if the Out of Africa model is a valid model for the emergence of modern humans. The Out of Africa Model: The Fossil Evidence Two categories of evidence point to Africa as the birthplace of Homo sapiens: fossil and genetic. Chris Stringer believes the fossil evidence shows anatomically modern humans living in Africa forwards of 150,000 years ago. He also believes that humans became modern in Africa first (2003:692-694). Six archaeological sites have produced fossils between 200,000 and 90,000 years old, which is crucial in proving that Homo sapiens were living in Africa far longer than they were living anywhere else (see figure 1). From the Omo Basin in Ethiopia are two crania, labeled Omo I and Omo II. The Omo II skullcap represents a robust and archaic looking individual, but Omo I, though fragmentary, looks completely modern (see figure 2). The modern traits that the Omo I skull display include long and curved parietal bones that shape the brain case, where the cranium reaches its maximum breadth, coupled with a short, broad face and high forehead. The prominent browridge tapers at the sides instead of forming a consistently thick bar as in more archaic humans. The fragmentary face also lends clues to Omo I’s modernity. The upper jaw, when pieced together, revealed 2 of 20 11/13/2007 7:51 AM Introduction http://www.soa.ilstu.edu/anthropology/theses/maystadt/maystadt1.htm a modern looking, U shaped palate. The lower jaw possesses a chin, and the few surviving teeth appear modern in size and shape (Johanson 1996:236). Originally dated to around 130,000 years old, recent dating has revised this date to 195,000 years old. This is the oldest anatomically modern human cranium known (Bower 2005:141). The next oldest fossils come from Herto, also in Ethiopia. These fossils represent a “transition population,” one that is almost anatomically modern but retains some archaic attributes (see figure 3). Modern features this population possesses include less prominent browridges and high cranial vaults. These fossils have been dated to around 160,000 years old, making them some of the oldest “near modern” specimens discovered (Stringer 2003:692-694). The Klasies River Mouth, located near the southern tip of South Africa, produced fossils tentatively dated to 120,000 years old. Unfortunately, the remains are highly fragmentary, and have given rise to a great debate concerning their modernity (see figure 4). They are quite Homo sapiens-like, but whether any or all of these fossils represent our species still awaits definitive classification (Tattersall 2001:226). From the same area of South Africa comes the Border Cave site, which produced some unarguably anatomically modern specimens. Although there is the possibility that this specimen was an intrusive burial, the Border Cave I skull may be anywhere from 75,000 years old up to 100,000 years old (Johanson 1996:44; Tattersall 2001:226) (see figure 5). With the Klasies River Mouth fossils and the Border Cave fossils taken together, there is potential evidence for modern Homo sapiens living in South Africa between 100,000 and 120,000 years ago. Another very important fossil find comes from the extreme northern tip of Africa, from present day Morocco. The site named Jebel Irhoud produced two modern looking human craniums. The Jebel Irhoud I fossil still retained very robust and archaic features, such as a heavy browridge, but the skull was close to being modern in morphology (see figure 6). Although the date is contested, the Jebel Irhoud site may date anywhere between 100,000 and 200,000 years old (Johanson 1994:235-236; Tattersall 1995:182). The last important site is Jebel Qafzeh, which is located just north of Africa in present day Israel. Many anthropologists think that this site provides direct evidence of a migration ‘out of Africa’ around 90,000 years ago (Tattersall 2001:252) or 120,000 years ago (Oppenheimer 2003:54-55). Two near complete crania, Qafzeh 6 and 9, show very modern morphology in most or all respects (see figure 7 and 8). Qafzeh 6 3 of 20 11/13/2007 7:51 AM Introduction http://www.soa.ilstu.edu/anthropology/theses/maystadt/maystadt1.htm is archaic looking in some attributes, but Qafzeh 9 is a prototypical Homo sapiens skull (Johanson 1994:236; Tattersall 2001:252). With all the fossil evidence examined together, Stringer concludes that modern humans emerged in Africa and only in Africa. “Africa is not just the best source; it’s the only source for fossils exceeding 100,000 years old that show a blend of primitive and incipient modern human features” (Johanson 1994:243). The Genetic Evidence Other evidence also points to Africa as the original homeland of modern humans. In the late 1980’s new techniques of molecular analysis began to have a significant impact on answering the questions of human origins. Mitochondrial DNA (or mtDNA) was looked at as an excellent way to infer ancestral relationships. MtDNA has five attributes that make it particularly useful.