"I yam what I yam"07 Jul 2010
This isn’t normally the kind of story – oh, who am I kidding? I love to snark on these kinds of stories!
“Popeye-like arms”. Hmmmm….
Neandertals had a low brachial index – that is, with a short forearm were relative to the humerus. Popeye, well, you can see that he has the brachial index of a giant ground sloth. Neandertals were not built like Popeye.
The article itself reports the ideas of a group of Russian scientists, who think that hormonal changes may explain the Neandertal pattern of muscle development and cortical bone strength.
Remains of an early Neanderthal with a super strong arm suggest that Neanderthal fellows were heavily pumped up on male hormones, possessing a hormonal status unlike anything that exists in humans today, according to a recent paper.
The mixture [of big muscles and highly mineralized bones] is puzzling, because "Neanderthals demonstrate a markedly androgenic constitution," meaning they seemed to have a lot of steroids, yet these same hormones can cause reduced mineralization.
As a result, the researchers say "Neanderthals were characterized not only by peculiar biomechanical adaptations, but also by a specific hormonal condition which has no close parallels among modern human hormonal conditions either normal or pathological."
There’s no mechanism being proposed here, the androgen system has effects all over the body. This is not a testable hypothesis, it’s really just a speculation.
Or is it? The cool thing about having a Neandertal genome is that in principle we can look for differences in systems like the androgen receptor pathway. Looking for coding changes in androgen-associated genes is really just a browser window away.
So I did some checking.
Now, let me put some caveats here. This is good blog material, but the Neandertal genome sequencing has not reached a point where we can be at all certain about mutations. There are many gaps with no coverage at all in any Neandertal individuals. Most of the sequence of human coding regions is covered by at least one read, and a good fraction of sites have multiple Neandertal reads. As I’ve been looking through sequence, I tend to think a site may be interesting if it has a change in the Neandertal relative to the human sequence, and if it’s not near the end of a read. If the same change is present in multiple Neandertal reads, that makes it a good candidate for a genuine change in Neandertals relative to the human sequence. A large fraction of those Neandertal-specific changes actually aren’t Neandertal at all. They’re shared with chimpanzees and represent new human-specific changes. Many of those are SNPs in humans where the genome draft has the derived version; there are also sites where the Neandertal shares a derived SNP allele with some other humans. Then there are ones not in chimpanzees or humans, which might be Neandertal-specific alleles or substitutions.
Looking at the androgen receptor gene and the 5-alpha-reductase gene, both central to the androgen pathway, there aren’t any interesting-looking sites in the Neandertal sequencing reads. I don’t think the data refute the hypothesis that the Neandertals were like humans for these genes. That’s just a little bit of looking, of course, and that particular fishing expedition wasn’t likely to turn up anything new. But that’s the point! We shouldn’t just go off speculating about fundamental changes in hormonal biology in Neandertals anymore. We can look.
That is just the beginning of answering a question like this. To test the hypothesis, we’d want to check many other genes that lie between the androgen receptor and its final effects on gene transcription. And of course, coding changes aren’t the whole story of evolution in Neandertals. Promoter and enhancer changes, or even alternative splicing changes, may be more important than coding changes, especially for a system so broadly represented in different tissues. They’re harder to look for by just firing up the genome browser.
But even these kinds of changes are potentially testable. It’s not quite as fast as an interview with a reporter, but it doesn’t take days to look.