How biogeography will be more important in understanding human evolutionary history


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…

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5 thoughts on “How biogeography will be more important in understanding human evolutionary history

  1. I think two aspects have to be considered speaking about this:
    1st: The climatic zones and habitats shifted. E.g. the Green Sahara might have been an important region – as long as it stayed green. Much more important than the whole of Western, Central and Southern Africa during its existence for human evolution.

    2nd is related to 1st, the tropics are no good habitat for humans, they probably never were and they only gave refuge to those forms of Homo which were pushed into this unfavourable places.
    The tropics only became more interesting and “human-friendly” with the introduction of novel techniques, with the Neolithic package – in some cases even just with the introduction of very advanced crops, animals and metals. This relates very directly to the Bantu expansion in Africa and the East Asian expansions into SEA. There was nothing comparable there before.

    High diversity doesn’t equal high population sizes and developmental pace. It might even mean the opposite – at least from a certain point on. I might relate this very directly to the fact that humans, as they became more advanced, moved directly to the higher climatic zones with a cold winter and a risky dependence from migrating herds.
    Why did they so?

    The reason is simple: Try to survive as a hunter-gatherer in the tropics and try the same in the Eurasian steppe megafauna habitat. At first the tropics seem easy, females can be more productive on their own, the climate is warmer etc. But concerning diseases and the level of productivity, they are not better.

    I recall various documentaries accompanying bands of hunters in the tropics – while their wives where gathering around the camp. The crop of the tour was meagre. Some small birds usually, sometimes if they were lucky a small ape and if they were very lucky a pig or something like that. On many days, without their women, they would have stayed hungry and couldn’t provide at all.

    Comparing that situation even with that of the inuits, which could hunt big mammals on a regular bases – even if they had bad days too, if they were lucky, the whole community could live from their kill for many days to come. Such a success practically never happened in the tropical forests.

    I know that e.g. Sahul too had different habitats to offer, but generally speaking, I don’t agree with the tropics to have been so favourable at all. This also relates to why Neandertals and “Denisovans” were largely eliminated so early, while you could still find archaic hominins elsewhere. Neandertal-Denisovans were in the way of modern humans which wanted their highly favourable hunting grounds.

    Some tropical regions of the world on the other hand were not as interesting and they had no such clear competitive situation. If two human groups follow the same mammoth herd, to break it down, conflict is much more likely than if you can simply march by in a forest zone or sail to the next island instead.

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  2. @Obs

    “The tropics only became more interesting and “human-friendly” with the introduction of novel techniques, with the Neolithic package.”

    Hunter-gatherers were already exploiting tropical environments with specialized hunting techniques in Sri Lanka 45,000 years ago. Though I do think you’re on the right track about the overall return on hunting small tropical mammals like monkeys vs much larger megafauna influencing the shape of human demography.

    https://www.nature.com/articles/s41467-019-08623-1

    Also, could you flesh out this comment a bit more: “High diversity doesn’t equal high population sizes and developmental pace. It might even mean the opposite – at least from a certain point on.”

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  3. @Mick: “Also, could you flesh out this comment a bit more:”

    This has two aspects to it. For one its the tropical flora and fauna with a lot of diversity, but in the end little of interest for humans and a lot of threats and nuisances.

    The second refers to human diversity in regions which show a lot of separated, small habitats in which small human subpopulations and populations live, largely isolated from each other and without a bigger meta-population.
    This kind of diversity in a macroregion is usually not competitive in comparison to the same species/genus living in an interconnected (mega-) metapopulation in which on a regular base innovations pop up on one end and spread to the other in the course of expansions, admixture and replacements. The smaller and more conservative a population is, the lower or more one sided is its evolutionary pace. And Homo, Homo sapiens in particular, is a specialised generalist. So I wouldn’t expect it to come from extreme and unfavourable environments, but something rather ideal for human foragers.

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  4. @Obs

    Pretty sure most of the tropics isn’t rainforest; savanna and grassland have loads of large herd animals. Disease, parasites, and nasty poisonous beasties, yes.

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  5. Savannah and grasslands yes, but like said above, the Green Sahara might have been as and more important than most of Africa South of it. The recent papers are just starting to prove that point with archaic Homo and archaic Homo sapiens in West Africa until quite recently and just one side branch of Homo sapiens in South Africa. The real action was somewhere else.

    Grasslands aren’t the same of course. What you need is large herds of big mammals. That’s ideal. The second best is sea food in coastal stretches and everything else comes after those two options.

    In North and West Africa, as well as the Near East, the borderline and the trend from ideal to hostile changed various times, as the other recent papers for the best conditions for a passage out of Africa prove.

    But what will really show up is that there were developmental, demographic centres, which too changed and moved over time. The tropics were probably never part of any such centre and metapopulation. But every region and time has to be looked at separately of course.

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