Some friends and associates of mine have just had their paper (finally!) come out in Molecular Biology and Evolution: mtDNA Variation Predicts Population Size in Humans and Reveals a Major Southern Asian Chapter in Human Prehistory.
In this paper, they estimate ancestral human population sizes from 357 human mtDNA sequences. They use a method developed by Alexei Drummond (one of the co-authors), Bayesian Skyline Plots, which takes a bayesian approach to coalescent theory to infer the effective population sizes by simultaneously estimating and accounting for the ancestral genealogy, branch lengths, substitution model of DNA evolution, and population parameters.
This method allows them to make a number of exciting inferences about human prehistory:
- 143,000-193,000 years ago – a slow, exponential population growth in sub-Saharan Africa.
- 50,000-70,000 years ago – a rapid period of population expansion in Eurasia.
- ~52,000 years ago – South Asia shows a five-fold increase in population size. This strongly supports the “Southern Route” hypothesis where humans are thought to have migrated along the coast from Africa into Southern Asia. Their results suggest that for the period between 45,000-20,000 years ago, over half the global human population lived in this region, peaking at over 60% around 38,000 years ago.
- This is followed by slower population growth in the following regions:
- ~49,000 years ago – North/Central Asia
- ~42,000 years ago – Europe
- ~40,000years ago – Middle East & North Africa.
- ~48,000 years ago – population in Australia expands
- ~39,000 years ago – population in New Guinea expands
- Finally, at around 18,000 years ago, they show evidence for a rapid population expansion in the Americas. This is consistent theories arguing that the Americas were colonized after the last glacial maximum (~20,000 years ago) dropped the sea levels making a land-bridge across the Bering straits.
To double-check their results, they compared the population sizes that their algorithm proposed for current human populations (i.e. time=now) to independent anthropological/historical estimates. This showed that their estimates were strikingly accurate in most cases, which suggests that these results are quite realistic.
Alexei has more information about this paper on his blog, and we’ll hopefully see some news stories about it soon!