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The Evolutionary Origins of Human Population Variation. As we have seen, there is a geographically based pattern to human biological variation. Later on, we will explore more of the varying biological features that have been studied.
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As we have seen, there is a geographically based pattern to human biological variation.
Later on, we will explore more of the varying biological features that have been studied.
Today, our task is to trace the evolutionary origins of modern human population variation:
How did it arise?
What are the evolutionary and other processes that have brought it about?
The examination of the gross anatomical features of humans and apes reveals their close similarity and evolutionary relatedness.
Comparisons of the genetic materials of the African apes and humans documents that these creatures are more closely related to each other than to any other living animal.
Although there is much debate about what terms to use when referring to our extinct ancestors, for our purposes, we will call them hominids, and speak of human evolution as the evolution of the hominids (Using this term places ourselves and our extinct bipedal ancestors in a separate family: Hominidae) .
Thus, we can say that hominid evolution seems to have begun in Africa, and it is only much later in our evolutionary history that we find signs of our ancestors outside the African continent.
We are,more specifically, members in the genus Homo, of which there are a number of now extinct species, and we are the only living member: Homo sapiens.
humans chimpanzees gorillas
5 - 8 myr
common ancestry of humans and
An evolutionary diagram of human and African ape
relationships, based on various genetic studies
The two competing models are known as:
1. The Multi Regional Evolutionary Model.
2. The Single Origins Model (usually called “Out of Africa”).
While in Europe these now isolated hominids evolve into a new species, the Neandertals, In Africa and Asia, other species of Homo were also evolving. Like the Neandertals in Europe, they also possess low sloping brain cases, and large projecting faces lacking a chin. They had large brains, often within the range of living humans.
1) Multi Regional Evolution
2) Single Origins : “Out of Africa”
Said one English anatomist of the first find in the Neander Valley:
“It may have been one of those wild men, half-crazed, half idiotic, cruel and strong, who are always more or less to be found living on the outskirts of barbarous tribes, and who now and then appear in civilized communities to be consigned perhaps to the penitentiary or the gallows, when their murderous propensities manifest themselves”.
The Skull of a Neandertal from France
Large brain case with a brain
often larger than those of living
1) Multi regional evolution relies primarily on fossil evidence from Asia.
2) Single Origins emphasizes fossil evidence from Africa and comparative genetic evidence from living human populations.
This theory would be acceptable if fossils were found that were modern human in form, but dated earlier in time than Neandertals. Clearly, it would be difficult to support multi-regional evolution if modern humans were around either before or at the same time as their presumed ancestors, the Neandertals.
1) southern Ethiopia
2) on the border between South Africa and Swaziland
3) on the very southern most coast of South Africa
1) Documents a greater amount of mtDNA variation in Africans in comparison to human populations in other parts of the world.
2) Discovered unique variations in Africa.
1) Modern humans originated in Africa.
2) There was a subsequent spread to other parts of the Old World, replacing earlier hominid populations.
Fossil and archaeological evidence from the Middle East would appear to be damaging to both theories.
1) Human fossil evidence argues against multi regional evolution.
2) Archaeological evidence argues against a Single Origins theory.
A neandertal from Iraq an early modern human from Israel
(probable date: 60,000 years) (date: 92,000 years)
This fossil evidence would seem to argue strongly against the multi regional evolution theory.
If Neandertals are known both before, as well as after, well dated occurrences of modern humans in the Middle East, it would appear unreasonable to suggest that there was regional evolutionary continuity between Neandertals and early modern humans.
If Neandertals and modern humans are different species, with a long evolutionary separation, how can the common use of the same stone tools be explained? The process of speciation requires reproductive isolation, which would have prevented separated populations from sharing knowledge of tools.
were they a separate species?
did neandertals contribute to the ancestry of living humans?
How did modern humans evolve? Quickly in one place or slowly over the Old World?