One Common Ancestor?Edit

The Catechism teaches that "Because of its common origin the human race forms a unity, for 'from one ancestor [God] made all nations to inhabit the whole earth'" (#360, emphasis original). To the extent that our common origin is God, this claim is relatively insulated from scientific examination, but the historical claim that all humanity descends from one common ancestor is outside of the current scientific mainstream - it is difficult to even operationalize this concept. A 2008 EMBO Reports article, entitled Human Origins: The molecular perspective, summarizes the state of the scientific debate:

[Scientists agree] that all human evolution took place in Africa until approximately 1.5–2 million years ago, and that between 1.5 million and 50,000 years ago, various waves of migration spread our ancestors, or their relatives, from Africa across the world...
...the replacement model argues that the transformation to modern humans occurred in a single population in Africa roughly 200,000–300,000 years ago, which then spread across and out of Africa between 50,000 and 100,000 years ago and replaced completely, without any interbreeding, the archaic populations from earlier migrations from Africa...
The replacement model is the most extreme version of the out-of-Africa models; others acknowledge that the transformation to modern humans occurred in Africa, but hold that the spread of modern humans was not a complete replacement event, but rather was accompanied by some amount of interbreeding with non-African, archaic humans.
...all of the mtDNA types in the entire population of humans today must trace back to a single common female ancestor—the so-called ‘mitochondrial Eve’.
This concept of a single woman as the maternal ancestor of everyone alive today has caused much confusion, not only among the public but also among some biologists who ought to know better. Yet the concept is relatively straightforward: given that there was a single origin of life on this planet that all living things today are derived from, then it has to be the case that all of the variation in any DNA sequence, not just mtDNA, must trace back to a single ancestor at some point in the past. The fact that in the case of mtDNA the ancestor was a woman follows from the maternal inheritance of mtDNA.
However, mitochondrial Eve differs from the biblical Eve in one important aspect: she was not the only woman alive on the planet at the time that she lived; instead, she was a member of a population that included many other women, but they did not contribute mtDNA types to the people living today. If one could follow the descendants of all women who lived at the same time as mitochondrial Eve, generation after generation, sooner or later all of the female descendants of each woman would either have no offspring or only male offspring, resulting in the extinction of that mtDNA lineage.
There are some additional important characteristics of mitochondrial Eve. First, she was probably not the ancestor of any of our other genes—to be sure, all of our genes have common ancestors, but they were undoubtedly different individuals, living at different times and in different places. Second, she was not necessarily the first member of anatomically modern humans, even though one often reads in the popular press that she was the first modern human...
Therefore, the fact that all of the variation in our mtDNA types traces back to a single common ancestor is a straightforward consequence of evolutionary theory and is not even particularly interesting...
In conclusion, the genetic data do not currently allow us to distinguish between the replacement model and assimilation models. It might well be that some small fraction of our 3 billion nucleotides of DNA comes from Neanderthals and/or some other archaic, non-African population. However, further analyses of DNA variation in contemporary human populations, as well as exciting new developments in ancient DNA analyses—such as the Neanderthal Genome Project—should provide an answer to this question.

So in the sense that all modern people descended from an African species that lived 2 million years ago, the scientific consensus is compatible with the doctrinal teaching of a common human ancestor. On the other hand, the scientific study of human origins shows how difficult it is to even speak coherently about who the first humans were, much less to show that all humans share that single first couple as ancestors. Surely we should not be identifying the oldest female to have the same mitochondria as us as the biblical Eve.

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