Substack cometh, and lo it is good. (Pricing)

Caveman rules: prepare to be surprised!

250px-Caveman_1By now you may have heard that mitochondrial DNA, passed down the female lineage, has been extracted from a ~400,000 year old human fossil from Spain. If you haven’t heard, I recommend Ewen Callaway and Carl Zimmer’s takes. The paper is at Nature, so gated, A mitochondrial genome sequence of a hominin from Sima de los Huesos. The big surprise is that these proto-Neandertals carry a mtDNA lineage which is closer to that of the Siberian Denisovans than that of later Neandertals. That’s the specific finding, and if you read the reaction it is rather clear that this is confusing researchers who work in this area. But take a step back, imagine what a world without ancient DNA would be like. Yes, the broad conjectures would be supported (e.g., Out of Africa), but many specific details would be off. So praise the data! Sometimes complexity is closer to the truth, and this is one of those cases. These are good problems to have.

NeanderIn the primary figure of the paper you can see that these humans are closest on the mtDNA phylogenetic tree to the Denisovans. But, it is important to note that they’re also hundreds of thousands of years older than any other ancient human DNA. Because mtDNA is only passed down through females it tends to be more strongly subject to genetic drift, which might turn over lineages rather rapidly. It is not that unlikely that over hundreds of thousands of years some populations would lose ancestral mtDNA lines. This is what occurred with “mitochondrial Eve.” She wasn’t the only female alive in Africa at the time, but all the other direct maternal lineages went extinct. There are ‘ghost branches’ within the tree which terminated. All you see are the lines of descent back up to the single last common ancestor on the mitochondrial lineage. This doesn’t mean that the ancestry of these women who did not contribute mtDNA disappeared. It is just that their lines of descent may have passed through sons at a given point (my maternal grandmother’s specific mtDNA almost went extinct, she had six son and one daughter). And of course there are other explanations for this pattern, highlighted in the articles linked. Gene flow between lineages, or from a different lineage altogether. We have to remember that the mtDNA of the Denisovan human was more diverged from Neandertals than the whole genome was later found to be, perhaps indicating complex admixture scenarios. The mtDNA tree falsifies, but I do not think it allows us to draw any robust conclusions.

These results are going to get updated in the next year or so with autosomal DNA from the rest of the genome. Even if they can’t get the whole genome sequenced, even a few tens of thousands of markers should be sufficient to clarify issues. Though all of these findings need to be interpreted cautiously in light of the fact that this is a very old lineage, perhaps closer to the time period of diversification for many Eurasian ‘archaic’ H. sapiens than we may have thought.

Posted in Uncategorized

Comments are closed.