April 19, 2011

Global demographic expansions followed agricultural spread

The authors make a good argument that if hunter-gatherers had adopted agriculture early on, then there would be a signal of population growth in hunter-gatherer derived haplogroups at the same time as the onset of the farming economy. I don't follow mtDNA age estimation as closely as that of the Y-chromosome, but the fact that their method estimates demographic growth for Neolithic haplogroups that coincide with the onset of the Neolithic, while it estimates earlier growth for Paleolithic haplogroups, does suggest to me that they're onto something.

They write:
Our results are consistent with recent publications using ancient
DNA to assign the maternal affinities of early agriculturalists
and hunter-gatherers.
Our LGP European sample includes
the U5a and U5b1 haplogroups, associated with Mesolithic hunter gatherers
at the majority of archaeological sites in Bramanti
et al. (3) dated to older than 5,000 ya. The presence of similar
mtDNA haplogroups in Mesolithic hunter-gatherers and contemporary
Europeans supports a model involving (maternal)
population continuity, even if this Mesolithic ancestry makes up
only a fraction of contemporary European genomes. U5a, U5b1,
V, and 3H combined account for ≈15% of western Europeans
mtDNA haplogroups (7) (Table S1).
As I've argued in this blog over the last few years, the mtDNA evidence speaks of near total replacement of forager mtDNA in Europe. In a sense there is continuity, as the forager mtDNA has not gone extinct, but I guess that is a matter of words. We could just as easily say that there is population continuity in parts of South America where a predominant Caucasoid population has absorbed a small native population.

What's more important is to see that modern and ancient mtDNA evidence are in agreement. This is slightly ironic, as it was modern mtDNA evidence -as interpreted in the late 90s- that largely launched the once successful and now semi-retired "Europeans are largely Paleolithic" meme.

PNAS doi: 10.1073/pnas.0914274108

Rapid, global demographic expansions after the origins of agriculture

Christopher R. Gignoux et al.

The invention of agriculture is widely assumed to have driven recent human population growth. However, direct genetic evidence for population growth after independent agricultural origins has been elusive. We estimated population sizes through time from a set of globally distributed whole mitochondrial genomes, after separating lineages associated with agricultural populations from those associated with hunter-gatherers. The coalescent-based analysis revealed strong evidence for distinct demographic expansions in Europe, southeastern Asia, and sub-Saharan Africa within the past 10,000 y. Estimates of the timing of population growth based on genetic data correspond neatly to dates for the initial origins of agriculture derived from archaeological evidence. Comparisons of rates of population growth through time reveal that the invention of agriculture facilitated a fivefold increase in population growth relative to more ancient expansions of hunter-gatherers.

Link

5 comments:

McG said...

Would you say this paper supports the Myres study or refutes it?

Annie Mouse said...

I can't get to this paper. But I dont buy it.

Western Europe is almost 50% H mito-haplogroups. The root of H is about 40k years old or older, I have seen 50k estimates. The H1/H3explansion that dominates Europe is about 22k years old. It appears to have rolled over into North Africa before the neolithic.

In no way does this match a neolithic expansion.

Average Joe said...

Does this mean that R1b arrived in Europe during the Neolithic or the Paleolithic?

eurologist said...

I am not impressed by this paper, at all. I voiced my severe critique of the Haak paper in that thread. This paper has almost as many circular arguments. I can't tell to what extent they used Haak's haplotypes, but the source they cite for the segregation of Holocene vs. last glacial/ earlier mtDNA groups is completely outdated (it is from 2000) and at any rate puts the carriage before the horse. Another circular argument involving dates and timing:

The paper relies on extremely poorly known TMRCA estimates to start their segregation and timings, e.g., "the coalescence of multiple Out-of-Africa haplogroups within a set is generally 40,000–60,000 y ago" --- that could be off by a factor of three, easily.

Finally, in Fig. 1 there is less than 30% growth for the Holocene component versus a factor of 2 for the ancient one in Europe, in the past 4000 years; also, only the latter is exponential throughout.

What this indicates is that the author's segregation is not between Holocene newcomers from the near East and indigenous, but rather between locals in the center of early expansion vs. locals at the fringe, who profited from the new methods and had more land in more climate zones to work with (thus a slower, but steadier exponential growth).

pconroy said...

Annie Mouse,

As most people know, agrarian demographic expansions are predominantly driven by males in search of new land... so they encroach on Hunter-Gatherer (HG) lands and drive them off and take their women. Or the HG women come over to the farming areas, as they're doing better.

So naturally there is an expansion of preexisting HG mtDNA lineages, concommitently with demographic expansion. So you need to look at the pre-Neolithic mtDNA and NRY distributions, and compare to the Neolithic ones.

For evidence of this, just look at the Bantu expansion, which shows evidence of Pygmy mtDNA expanding with them south...

Now most pre-Neolithic HG mtDNA in Northern and Central Europe was U4 or U5, but there was much mtDNA H in the Iberian pre-Neolithic HG population, to name just one.