Sunday, 10 May 2015

Evolution for Brainy People

Gene that makes human brain unique identified by scientists | Science | The Guardian

A friend on Facebook has just drawn my attention to this scientific discovery from last February. I can't think how I missed it with such a catchy title to the paper (Human-specific gene ARHGAP11B promotes basal progenitor amplification and neocortex expansion). Hmm...

It appears then that a single gene might be responsible the the most profound difference between us and the other apes and between us and our immediate hominid ancestors.

Geneticists at the Max Planck Institute of Molecular Cell Biology and Genetics in Germany have been searching for the unique genes that make us human rather than something else. At this point I need to point out to any creationists who may be tempted to begin celebrating that scientists have conceded that humans are unique and have some special genes that make us different to the 'animals' that this is true for every species - that's why they are distinct species. Species are all unique because they have a small number of unique genes.

So, what the team led by Marta Florio has found is a gene, named ARHGAP11B, unique to humans, that has been shown in embryo mice to grow bigger brains by increasing the number of neurons. This might seem obvious retrospectively. After all, what really distinguishes humans from even our closest relatives is our amazing ability to learn, retrieve and process information and acquire new information by deductive reasoning, and we can only do that better than even our close relatives because we have far more processing power in our brain because we have far more brain cells than they have.

The interesting, though not really surprising thing, is that the gene which causes this increased brain size was also present in Neanderthals and Denisovans. It's not surprising because we know that Neanderthals had a brain on a par with ours. We don't have any Denisovan skulls yet but it's probably safe to assume they had brains comparable to ours too. This means that this gene arose some time after we split from the chimpanzee line about 7 million years ago since when hominid brain size has tripled. It probably evolved it's present form some time in the last 2 million years ago since when our brain size has doubled from that of Homo erectus.

Abstract

Evolutionary expansion of the human neocortex reflects increased amplification of basal progenitors in the subventricular zone, producing more neurons during fetal corticogenesis. In this work, we analyze the transcriptomes of distinct progenitor subpopulations isolated by a cell polarity–based approach from developing mouse and human neocortex. We identify 56 genes preferentially expressed in human apical and basal radial glia that lack mouse orthologs. Among these, ARHGAP11B has the highest degree of radial glia–specific expression. ARHGAP11B arose from partial duplication of ARHGAP11A (which encodes a Rho guanosine triphosphatase–activating protein) on the human lineage after separation from the chimpanzee lineage. Expression of ARHGAP11B in embryonic mouse neocortex promotes basal progenitor generation and self-renewal and can increase cortical plate area and induce gyrification. Hence, ARHGAP11B may have contributed to evolutionary expansion of human neocortex.


But why hasn't the human brain continued to get bigger? Why did it seemingly get very much larger very recently and then stop expanding? Quite simply because there is a limit to the size of head a human female can give birth to. Any tendency to 'improve' the gene so it made even bigger brains would have been deleted from the human gene pool by natural selection.

Now, creationist tend to latch onto thing like this and make all sorts of daft claims. Number one on their list is normally that all mutations are harmful so this new gene can't be a mutation because something else would have stopped working - whatever it was this new gene used to do. They imagine this proves something must have made it specially, with a plan in mind. The second is that even if this new gene really was a mutation, it can't contain any new information because the Second Law of Thermodynamics (sic) forbids that. (You need to not understand basic physics or scientific words like 'Law', as well as basic biology, to be a creationist).

What creationists will now need to ignore though is that the team that discovered this new gene have also shown that it arose by a partial duplication of an existing gene (ARHGAP11A). So, not only was there no loss of function because the original gene is still there and still working, but the same information, with a few mutations, acquired an entirely new meaning and so an entirely new function in an example of exaptation of an existing gene.

An accident in the duplication of a common gene found in almost all living things from yeasts to mice, probably triggered the evolution of the human brain.

If any creationist can explain to me how the Second Law of Thermodynamics means strands of DNA can't get accidentally duplicated then please fell free to do so in the comments below. Better still, present a paper on it to a journal of molecular evolution and let the reviewers explain where you went wrong and why you should go back to school or stick to stories about magic.

Credit to Facebook member Jane Thomas for the tipoff.

'via Blog this'

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