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July 17, 2018

The new African “multi-regionalism” & pan-Neanderthalism

Filed under: Human Evolution,Multi-regionalism — Razib Khan @ 7:13 am

We live in times when our understanding of the origin and diversification of modern humans is undergoing great change. More concretely, our understanding of what it means to be human is transforming. The terms are overused, but perhaps it could be called a “revolution” or “paradigm shift” between the year 2000 and today.

At the end of 2010 ancient DNA made it highly likely that people outside of Sub-Saharan Africa had non-trivial Neanderthal ancestry. That is, enough ancestry that it is detectable genomically. I should also add that I think it is highly probable that the good majority of people within Sub-Saharan Africa have Neanderthal ancestry. Some of this is due to recent attenuated Eurasian back-migration (e.g., many West Africans, Nilotic people, and KhoeSan have Holocene gene-flow signals which derive from the agricultural expansions of the past 10,000 years). But, I think once deep Pleistocene genomes of African humans are sequenced we will see evidence of some Eurasian back-migration at a very ancient date (there is already some suggestive inferential evidence of this).*

Talking with a few friends this week, I realized that the famous “We are all Africans” t-shirts, which have turned into recognizable memes, should be supplemented with “We are all Neanderthals” t-shirts. So yeah, now selling them on DNA Geeks. If the Richard Dawkins Foundation can make quid on it, why not the Razib Khan et al. Foundation?

This has all been on my mind due to a review paper in Trends in Ecology and Evolution, Did Our Species Evolve in Subdivided Populations across Africa, and Why Does It Matter? (OA). If you read this blog closely you’ll see there’s not much new in it. But, it is a signpost, a marker, of the times we live in. Here’s the important bit:

Together with recent archaeological and genetic lines of evidence, these data are consistent with the view that our species originated and diversified within strongly subdivided (i.e., structured) populations, probably living across Africa, that were connected by sporadic gene flow…This concept of ‘African multiregionalism’…may also include hybridization between H. sapiens and more divergent hominins (see Glossary) living in different regions…Crucially, such population subdivisions may have been shaped and sustained by shifts in ecological boundaries…challenging the view that our species was endemic to a single region or habitat, and implying an often underacknowledged complexity to our African origins.

The first person who explicitly used the term “African multi-regionalism” that I recall was Alwyn Scally, though the general framework was shaping up years before. Frankly, I was waiting for someone to use that word. If Richard Klein’s The Dawn of Human Culture, published in 2002, was the apogee of the old model, often inchoate and more crisp in popularization than within the scientific community that we are all descended from a single East African tribe, this review paper heralds the emergence of a more complex and pluralistic framework. The emergence of modern humans within Africa then may have been a polycentric gradual and interactive process; not a singular explosion against the firmament of the antique savanna landscape.

By the late 2000s, even before the 2010 Neanderthal draft genome paper, it was starting to be evident due to genome-wide analyses of contemporary populations, that the extreme bottleneck clear in non-African populations was much more modest within Africa. That opened the possibility for the existence of deep structure within the continent that pre-dated the “Out of Africa” event. A deeper look at African hunter-gatherers indicated to many researchers that these groups diverged from other modern humans in the range of ~200,000 years before the well. Recent paleontological work has confirmed this genetic insight.

Where we are today is that some people are now arguing for the overthrow of the “Out-of-Africa” idea, whether by replacing it with an “Into-Africa” model of some sort, or resurrecting a more polycentric classical multi-regionalism (“some people” as evident in the increased frequency of emails and Twitter messages I get in this vein). I don’t think we’re there yet, not by any measure. But, it is now in the realm of very unlikely, not extremely unlikely (at least the “Into-Africa” model; it is clear that strong overwhelming demographic pulses from somewhere singular dominate the genome of most modern humans).

* I don’t think it is all that implausible that some Neanderthal back-migration into Africa occurred at some point in the last ~500,000.

July 6, 2011

Is Chris Stringer a multi-regionalist?

In an interesting piece in The Guardian on possible proto-gorilla/proto-human hybridization, the journalist lobs this grenade:

But now that the once popular “single-origin model” of the evolution of Homo sapiens has been disproved, and the previously controversial “multiregional hypothesis” has been proven by DNA evidence, perhaps we need a rethink. According to the multiregional hypothesis all modern people, including modern Africans, are the descendents of breeding and hybridising between separate ancestral groups, all at various stages of evolutionary development.

Evolutionary lice research has helped palaeoanthropologists, including Stringer, to embrace the multi-regional hypothesis. “I’m sure there is plenty more to come from the lice research,” he told me. We know that it took 4m years, 5-9m years ago, for our ancestors to completely split from archaic chimps. During that time hybrids would have been born that mated both with our ancestors and ancestral chimps.

The issue here is semantics. I think regular readers of this weblog will know to be more cautious than to contend that the “single-origin model” of our species has been “disproved,” while its inverse has been “proven.” Those are strong words in science. Additionally, I seriously doubt that Chris Stringer would identify as a multi-regionalist. Some wires ...

January 28, 2011

A ‘leaky’ model

John Farrell pointed me to this Anne Gibbons’ piece, A New View Of the Birth of Homo sapiens. Here’s some interesting passages:

The new picture most resembles so-called assimilation models, which got relatively little attention over the years. “This means so much,” says Fred Smith of Illinois State University in Normal, who proposed such a model. “I just thought ‘Hallelujah! No matter what anybody else says, I was as close to correct as anybody.’ ”

But the genomic data don’t prove the classic multiregionalism model correct either. They suggest only a small amount of interbreeding, presumably at the margins where invading moderns met archaic groups that were the worldwide descendants of H. erectus, the human ancestor that left Africa 1.8 million years ago. “I have lately taken to talking about the best model as replacement with hybridization, … [or] ‘leaky replacement,’ ” says paleogeneticist Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, lead author of the two nuclear genome studies.

I suppose ‘assimilation’ sounds too generic, but ‘leaky replacement’ seems more fitting for a building ‘super’. But it isn’t as if paleoanthropology has a Don Draper, a rogue with a way with words.

Here’s the ...

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