Forrest O. Gulden
The Origins of Anatomically Modern Humans
Deciding on a hypothesis
There are two primary models as to the origin of modern humans, the "Out of Africa" model and the "Multiregional Evolution" model. Each of these models has different points of support and each model also has a number of variations (including the Strong and the Weak Garden of Eden theories and the Multiple Dispersals Model). The Multiregional Evolution theory (MRE) holds that multiple H. erectus populations across the globe evolved simultaneously into Anatomically Modern Humans (AMH). The most extreme theory in this set believes that there was sufficient gene flow to prevent speciation and that there was never a period of replacement of an older population by a newer population, and that instead there was a very high level of admixture and interbreeding. The Out of Africa model is centered on three key elements. These are that AMH evolved only once in one specific location in Africa, that this species spread across the globe, and that this species did so with a pattern of total replacement without admixture. It is the position of this essay that Anatomically Modern Humans originated once only in Africa and through a series of dispersals through different routes populated the rest of the world (as held by the Weak Garden of Eden theory and the Multiple Dispersal model). The evidence for this can be analyzed through several methods, both genetic and non-genetic, and all yield roughly equivalent results. Through the evidence and a comparison of features of opposing and contrasting views, this thesis is supported.
The first recorded widespread hypothesis as to the origin of modern man was a multiregional one. This was based on morphological characteristics of H. erectus skulls found at sites across the globe. There appeared to be a rough correlation between H. erectus features and the features of the current inhabitants of that same area, which made it appear as though there was an evolutionary connection between the two populations. However, the foundation of this concept is eroded by a number of details.
Among other things, there are questions as to why H. erectus didn't evolve into a number of different species instead of just one. The answer to that question of course must be gene flow, the process by which genetic information is shared by different populations resulting in less diversity and a lack of speciation. If there is evidence of adequate gene flow between H. erectus populations, MRE can not be ruled out. However, it appears that the H. erectus population was not large enough to support adequate gene flow (Harpending et al).
Also, definite quantitative data is needed as to the exact level of similarity of prehistoric and historic populations. How closely do modern people resemble the H. erectus of the past in the same area? Data in this area is hardly conclusive. Three major studies have been done, one of which was inconclusive (some H. erectus traits matched the appropriate population but other traits matched a remote population), one supportive, and one opposed (Lahr).
Then, as a final check, multiregional evolution should be seen in other organisms. Human kind is, after all, nothing but a modified African Ape (according to this paper, at the least). Why would humans undergo evolution differently from other species? As it turns out, no other species undergoes MRE (with the possible exception of the Java Rhinoceros) (Lahr).
The other condemning proof against MRE is that which supports the Out of Africa hypothesis. MRE predicts that there should be great continuity between cultural changes, that there be a generally homogeneous population of hominoids throughout the inhabited world, and that any replacements that do occur as a result of warfare be very hard to distinguish through archaeological/anthropological methods due to the extreme similarity between the groups. This is clearly not the case. An example of this can be seen in prehistoric Europe. Europe demonstrates a consistent and repeating strategy of replacement of older populations (H. erectus by archaics by H. neanderthalensis by AMH) by newer anatomically modern ones. In Europe is found clear-cut evidence of isolated populations of different species pushing back and forth against each other seemingly without any admixture (although cross-culturation did occur). This evidence appears as a strong blow against MRE (Kurland).
Also hampering the argument for MRE is the fossil record. Although incomplete, this record tells two tales that severely cripple MRE. The fossil record tells us clearly that anatomically modern humans, Neanderthals, archaics, and erectus populations overlapped in both time and space (as will be seen in the following pages) with at least one other species. There are H. sapiens dated to nearly 200 kya, Neanderthals present 35 kya (), erectus present from 2 mya to 27 kya (Kluger), and archaics 500 kya to 200 kya. It is quite clear that not all erectus evolved into sapiens, regardless of any views otherwise. There were still erectus present when AMH began to flourish. Secondly, the fossil record tells us that AMH first appeared in Africa almost sixty thousand years before it appeared elsewhere. Dates of approximately 120 kya are available for the Middle East and S. Africa (Kurland), and nothing is seen outside of this area until approximately 60 kya. Although the fossil record is notoriously incomplete, it is becoming more and more unlikely that any evidence refuting this finding exists.
It can now be noted that none of the evidence against MRE conflicts with the Out of Africa hypothesis. Out of Africa predicts the replacement strategies seen in Europe and that older AMH fossils will be found in Africa than elsewhere. Out of Africa does not preclude that species overlap in time and space as MRE does. And finally, the Out of Africa hypothesis is supported by further genetic and non-genetic data, as will be discussed below.
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