As a follow-up to the post below, I thought I would make certain expectations and assumptions more explicit on my part. The new methods to infer our species’ population history are quite complicated and require a lot of analytical and computational firepower. They’re predicated on big datasets (e.g., whole genomes, and lots of them) and high-powered computational methods (not just in inference and analysis, but also simulation). All models are wrong, but some give more insight than others. From talking to people who work on this field, no one even working on these models assumes that they’re extremely high fidelity to the past. Rather, they’re pulling out insightful fragments of the truth. We’ll need to bring together both genetics and paleoanthropology to really get what’s going on.
In any case, there is a simpler and more old-fashioned framework that I always keep in mind to which I think is important. The past few million years of hominin evolution are strongly shaped by biogeographic parameters. There are two areas of the world where I see that researchers are digging up a fair amount of complexity for the origins of modern humans. One of them is Africa. But the other is Southeast Asia. For example, last year’s Multiple Deeply Divergent Denisovan Ancestries in Papuans (this paper is an illustration, if you keep track of this field you know it’s not an outlier for this region). Why is this?
I think the answer is simple, and it has to do with geography and climate. During the Pleistocene Africa and Southeast Asia had the greatest area of tropical woodland in the Old World. This is optimal hominin habit in many ways, though clearly hominins can occupy other habits (e.g., the Dminasi hominins). Though Eurasian hominins such as Neanderthals and Denisovans were quite successful as measured by persistence for long periods of time, the extant genomic evidence indicates that at northern latitudes hominins tended to be able to maintain themselves only at low population densities (at least before agriculture). The genetic data from Mesolithic European hunter-gatherers tend to support this proposition as well; they were characterized by low diversity. Similarly, Amerindian populations seem to have gone through a striking bottleneck during their high latitude sojourn.
For various reasons, a lot of genetics, genomics, and ancient DNA, has focused on high latitude hominins. Modern genetics is skewed toward Europeans, while ancient DNA began in the north due to better preservation. But I think high and mid-latitude hominins give a skewed and simple view of the human past due to small effective population sizes and high levels of regional turnover. In contrast, both Africa and Southeast Asia have been characterized by high population sizes of hominins and high speciosity. As we dig deeper into the genomics of these regions for our lineage, we’ll stumble upon “mysteries” which reflect the reality that these regions were home to many different and large numbers of hominins, and we can detect these imprints in the genomes…