August 17, 2012

A simple genetic cause for larger ape and human brains?

The copy number of a protein-making gene, DUF1220, which has been related in humans with microcephaly, may underly the extra size of our brains and therefore that of our rather unique intelligence.

Laura J. Dumas, DUF1220-Domain Copy Number Implicated in Human Brain-Size Pathology and Evolution. AJHG 2012. Pay per view (freely accessible after 6-month embargo) ··> LINK [doi:10.1016/j.ajhg.2012.07.016]

Humans have more than 270 copies of DUF1220 encoded in the genome, far more than other species. The closer a species is to humans, the more copies of DUF1220 show up. Chimpanzees have the next highest number, 125. Gorillas have 99, marmosets 30 and mice just one. "The one over-riding theme that we saw repeatedly was that the more copies of DUF1220 in the genome, the bigger the brain. And this held true whether we looked at different species or within the human population."

Source: Science Daily.

5 comments:

  1. Interesting. There are obvious follow up questions. What is the copy number of an Elephant or an Orca? Degraded ancient DNA is far from ideal for determining the copy number of short DNA sequences, yet one would wish to know the copy number in Denisovans, Neanderthals, and Cro Mangons. Is there any copy number variation in normal modern humans?

    Still, brain size is not everything. African Grey Parrots and New Caledonian Crows are very clever creatures... domestic sheep - not so much.

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    1. I have not yet access to this paper but for what I have seen online, Neanderthal sequence was slightly above the H. sapiens level, consistent with their slightly larger brains.

      But haven't read anything for animals outside of euarchontoglires (i.e. rodents, lagomorphs and the primate-leading branch, euarchonta). It may work the same way or not - I do not know but I imagine that they will check soon.

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    2. A bit off topic, but the phylogeny of eutherians is still far from settled. Unpublished micro RNA data is rumored to severely disrupt mammalian clades again. Maybe euarchontoglires is not monophyletic? I am skeptical, but await the evidence:
      http://www.nature.com/news/phylogeny-rewriting-evolution-1.10885

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    3. First time I stumble on this issue (thanks for mentioning) but I can't but feel that such short sequences as are microRNAs (22 nucleotides on average) should not be reliable to establish any phylogeny - because statistically the same mutations will happen over and over.

      The fact that they are functional suggests to me that they can re-evolve once and again to perform similar functions, so for example humans and elephants might have co-evolved those miRNAs just because both have big brains, pectoral breasts, rich social lives, lots of melanin (the basic type) or lack of fur.

      To me it looks rather fringe and I can't but help reading that serious geneticists have thought exactly the same I did before even reaching that paragraph.

      A key matter is that miRNAs are essentially coded in the regular genome, so they are derived from it and not really independent. A change in a single SNP in the nuclear DNA my make a miRNA disappear.

      Or reappear. Although this is less probable, I guess, if it happens and does work it will stick around.

      It's an interesting matter of research but I would not build a phylogeny on such feeble bricks.

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  2. My thoughts also. FWIW, I believe that Afrotheria and Xenarthra are valid sister clades in the superclade of Atlantogenata which is the sister of the superclade Boreoeutheria. This makes sense due to plate tectonics and is pretty well supported by genetic phylogeny.
    The relationships within Boreoeutheria are less well resolved, probably because a lot of divergence happened in a relatively short period of time.

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