Geographic isolation and reproductive barriers
As we have seen, speciation (the events that lead to reproductive isolation between populations of organisms) can be a prolonged and complex process. Populations can become isolated geographically (e.g. through migration) and begin to accumulate genetic differences that may raise a barrier to reproduction between them. This barrier may only be a partial barrier, however. The stickleback populations we discussed previously are an example: the first event leading to speciation was physical separation when some marine fish colonized new freshwater habitats. Even after significant differences accumulated between the marine and freshwater forms, a second wave of colonization of fresh water by the marine form brought the two groups into contact again, leading to some genetic exchange even as the two groups remained largely distinct. At the point of the second colonization, whether one or two species is/are present is a point of discussion: the case can be made for either. A scientist arguing for one species would point out that the two groups can still produce fertile offspring, whereas a colleague might argue for two based the distinct characteristics and ecological niches of the two populations, as well as the observation that the hybrids resulting from interbreeding are not as well adapted to either niche. The point is clear: speciation, as a slow process, is a gradient, and a clear line of demarcation cannot be drawn on a gradient. To return to our flip-book analogy, every adjacent page is only slightly different from the pages on either side. If we compare widely separated pages, the differences are clear. The point is that there is no single page in between them that we can identify as the point where the images “became different.”
While this discussion might seem a little academic and uninteresting (perhaps because one might discount such events as mere ‘microevolution’ of sticklebacks), we have recently learned that similar events shaped human speciation. As far as we can tell, sticklebacks are not aware of, nor concerned about, the theological implications of how they came to be, but we certainly are for our own species (and perhaps even for sticklebacks). What was once an area of interest mainly for specialists is about to become a topic of intense discussion among evangelicals: we have only recently learned that a portion of the lineage leading to modern humans interbred with other hominid species they encountered as they migrated out of Africa ~50,000 years ago. In order to explain what happened, let’s pick up the tale at an earlier point, around 450,000 years earlier.
Out of Africa, twice over
Somewhere between 500,000 and 300,000 years ago, the ancestors of the Neanderthals (Homo neanderthalensis) left Africa and migrated into the Middle East region, and from there on to Europe and parts of Asia. (Recall that human ancestors, at this point, are all still in Africa, and will stay put until around 50,000 years ago). Neanderthals persisted in the Middle East and Europe until ~30,000 years ago, meaning there was a time where the humans leaving Africa about 50,000 years ago could have interbred with them before they went extinct. This remained an open question until techniques improved to recover and sequence ancient DNA. It is now possible to obtain and sequence DNA from Neanderthal remains, and the complete genome sequence of Neanderthals was published in early 2010. The results were fascinating: DNA sequence comparisons between the two species indicates that modern, non-African humans have about 1-4% Neanderthal DNA in their genomes. This variation, however, is not present in sub-Saharan Africans, since they are descended from humans that did not leave Africa and and thereby, because of geographical separation, never had the opportunity to interbreed with Neanderthals. We also know that the group that left Africa went through a reduction in population size to about 1200 individuals (a genetic bottleneck), whereas those that stayed behind maintained a larger population size (about 6000) over the same period.
In addition to this information, we have recently discovered a new hominid species from Asia, as Darrel Falk recently highlighted here on BioLogos. This species, named the “Denisovans” is known to us only from a few bone fragments and one molar, but - wonder of wonders in this age of paleogenomics - this was enough for us to determine its complete genome sequence. The results were, again, fascinating: the Denisovans are relative of Neanderthals that split off from them after their common ancestor left Africa. The Neanderthals went west to Europe, and the Denisovans colonized Asia (and evidence suggests they were quite widespread). Even more interesting is that comparing Denisovan and human DNA indicates that some humans (modern Melanesians) have about 5% Denisovan DNA in their genomes. This variation is not found in Europeans or Africans.
Putting the story together
Assembling all of this information reveals the following tale: the common ancestor of Neanderthals and Denisovans migrated from Africa to the Middle East between 500,000 and 300,000 years ago, leaving a population behind that would eventually become modern humans (at around 200,000 years ago). In the Middle East, the populations destined to become Neanderthals and Denisovans part ways, with their differences accumulating over the next several hundred thousand years to make them distinct species. When a population of modern humans leave Africa around 50,000 years ago, they encounter, and breed with, Neanderthals shortly after. This genetic exchange is small, since there are partial reproductive barriers in place, but a small fraction of Neanderthal DNA becomes established in this lineage. Groups from this population then part ways, with some migrating into Europe and others into Asia. This latter group then encounters the Denisovan hominids, interbreeds with them, and a fraction of Denisovan DNA takes hold as a result. This population goes on to colonize southeast Asia, Oceania and Australia, where we see this variation today in Melanesians. Modern humans thus have different evolutionary trajectories: Melanesians have both Neanderthals and Denisovans in their lineage, Europeans have Neanderthals, and Africans have neither.
New data, new questions
Even as I stand amazed in what God has revealed to us about our origins through science, I know that this new information will be difficult for some within the evangelical community to accept. Moreover, it is almost certain that some Christian groups, unfortunately, will misrepresent this data to their constituents (whether intentionally or not), and thus spread confusion that hinders the needed theological conversation. Still, I have reason for hope: God has seen it fit to reveal this information to us, and that suggests that He believes the evangelical Christian community is ready for this conversation to happen. As Darrel mentioned at the end of his recent piece, we at BioLogos want to assist our evangelical sisters and brothers in this conversation in any way we can, in full confidence that it can be done in an edifying way:
BioLogos exists to help Christians think carefully about the ramifications of these new data in light of long-standing traditional ways of viewing human creation. We have some re-thinking to do, but it can be done and will be done within the context of a Christian faith that is fully orthodox and thoroughly evangelical. Any time we draw closer to truth, to God’s truth, we have nothing to fear. There is still much to learn, but we can look back at what we have learned with awe—absolute awe.