March 07, 2008

Genetic traces of east-to-west human expansion waves in Eurasia

American Journal of Physical Anthropology (early view)


Raphaëlle Chaix et al.

In this study, we describe the landscape of human demographic expansions in Eurasia using a large continental Y chromosome and mitochondrial DNA dataset. Variation at these two uniparentally-inherited genetic systems retraces expansions that occurred in the past 60 ky, and shows a clear decrease of expansion ages from east to west Eurasia. To investigate the demographic events at the origin of this westward decrease of expansion ages, the estimated divergence ages between Eurasian populations are compared with the estimated expansion ages within each population. Both markers suggest that the demographic expansion diffused from east to west in Eurasia in a demic way, i.e., through migrations of individuals (and not just through diffusion of new technologies), highlighting the prominent role of eastern regions within Eurasia during Palaeolithic times.



arborist said...

I am surprised that this did not draw more interest. They are saying that people moved from China to Central Asia and then to the Middle East and Europe.

n/a said...

No, they aren't (or at least I highly doubt it). Have you read the paper?

arborist said...

Here is a passage from the discussion section of the paper.

"If we extend these results to other parts of Eurasia, which always show proportions of LIC equal to 39% or less, our mtDNA results support a scenario in which
the expansion diffused westward from the Far East (China and Mongolia) to Europe through recurrent gene flows and/or massive and sudden moves of people."

n/a said...

There is ample evidence (archaeological and genetic) of ancient expansions from Central Asia into Europe. There is none I'm aware for expansion from China (besides, some would claim, Y haplogroup N).

Did the authors completely ignore phylogeny and focus purely on diversity? That's the impression I get from the abstract and supplementary material.

I have no access to AJPA at the moment, but send me a copy of the pdf and I'll comment further.

n/a said...

OK, thanks for sending the paper.

I don't follow the authors' logic here:

Indeed, our data point to Central Asia as having played
first a role of receiver of genetic diversity coming from
the Far East, and subsequently, a role of starting point
of expansion waves toward western regions. Such scenario
could explain the current distribution of mtDNA
haplogroups in Eurasia, showing a clear distinction
between eastern and western haplogroups and a mixture
of these geographically-distinct haplogroups in Central
Asia (Comas et al., 1998, 2004; Quintana-Murci et al.,
2004; Derenko et al., 2007).

No, the "clear distinction between eastern and western haplogroups" is strong evidence against the scenario propounded here. There is simply no way to interpret the mtDNA phylogeographic data as supporting an east-west demic diffusion starting in China.

The basic idea behind the study (looking at "LIC") seems reasonable to me, at least on the surface. But I haven't reviewed in detail all the assumptions underlying it.

The results on the Y chromosome are broadly in line with the phylogeographic evidence:

Regarding Y chromosome data, the proportions of LIC
observed between China and western Eurasia were high
(76% and above) and such high percentages of LIC are
compatible with the existence of independent expansion
events in China on one hand and in western Eurasia on
the other hand
, although the percentage of LIC between
China and Central Asia (76%) could also correspond to
an extreme case of a diffusion of expansion through
migrations with an old divergence age between the two
regions (about 1,700 generations), a migration rate not
higher than 0.0005 and a size before expansion N0 equal
to at least 500.

Exactly where they go wrong on the mtDNA side, I can't immediately say. The authors themselves mentions one possible confounding factor:

However, given that
the two genetic systems investigated here represent only
two realizations of evolution and are both putative candidates
for the action of natural selection
et al., 2000; Mishmar et al., 2003; Bazin et al., 2006),
sequence-based data from independent noncoding regions
of the human genome would be useful to obtain a
more robust view of Eurasian demographic history.

Admixture may be the biggest issue, though the authors downplay the possibility:

This approach has been criticized for being sensitive to a
couple of phenomena: (i) the heterogeneity of mutation
rates (Aris-Brosou and Excoffier, 1996) and (ii) past
admixture events, which can both affect the shape of the
mismatch distributions
. However, there is no objective
reason why these processes should affect the eastern
and western populations differentially. Moreover, our
simulation study comforts us in the validity of our
approach, in particular since we took into account the
heterogeneity of mutations rate in these simulations,
this heterogeneity having little impact of the LIC proportions.
Finally, the high proportion of observed unimodal
mismatch distributions suggests that admixture events
between genetically differentiated populations have been
rare enough not to affect substantially the genetic landscape
of Eurasian populations.

In fact, both the Chinese and Central Asians are most likely products of "admixture events between genetically differentiated populations".