Scientists chew over vital mutation to the jawbones

[dArKeR]

AP , DENVER, COLORADO
Thursday, Mar 25, 2004,Page 7

Igniting a scientific furor, scientists say they may have found the genetic mutation that first separated the earliest humans from their apelike ancestors.

The provocative discovery suggests that this genetic twist -- toward smaller, weaker jaws -- unleashed a cascade of profound biological changes. The smaller jaws would allow for dramatic brain growth necessary for tool-making, language and other hallmarks of human evolution on the plains of East Africa.

The mutation is reported in the latest issue of the journal Nature, not by anthropologists, but by a team of biologists and plastic surgeons at the University of Pennsylvania and the Children's Hospital of Philadelphia.

The report provoked strong reactions throughout the hotly contested field of human origins with one scientist declaring it "counter to the fundamentals of evolution" and another saying it was "super."

http://www.taipeitimes.com/News/world/archives/2004/03/25/2003107697

[Piltdown13]

1. Color me skeptical....

Interesting, but I find it highly unlikely that *one* mutation in *one* gene is responsible for the differentiation of the genus Homo from the australopithecines. And there's so much yet to be examined regarding genetic data anyhow -- although the basics of the molecular clock are fairly straightforward, there is a certain circularity in some of the arguments; it will be interesting to see if our current expectation that mutation rates are constant will ultimately borne out (which is a major underpinning of the molecular clock).

Even if this checks out, I'm inclined to think they've got the logic backwards -- a genetic mutation leading toward a smaller, weaker chewing apparatus would be quite deleterious, all else being equal. In other words, it's all well and good to argue that the change "enabled" brains to expand, but that doesn't help out the first generation with that mutation -- their brains couldn't have yet expanded (unless you want to aruge a simultaneous brain-size-enlarging mutation) -- so it's hard to see how such a mutation could have allowed Homo to branch off in the absence of other, compensatory factors. And of course there are issues with which fossils get placed into which species...there are some early Homo specimens with very large teeth, for example, and not all australopithecines have giant teeth and jaws.

Hmmm...interesting quotes. Not sure who said the first one, but I'll bet Milford Wolpoff is the one who thought this finding was "super." (...checks article...yep!).

Oh, and one other thing. There's a longer version of the article on Yahoo (http://story.news.yahoo.com/news?tmpl=story&cid=1894&e=1&u=/ap/20040324/ap_on_sc/the_human_gene), which tells a bit about what the gene does. Apparently it codes for a protein that is associated with muscle strength (or at least the gene is often found/expressed in strong muscles). However, there's no mechanism mentioned by which strong jaw muscles somehow constrain brain growth mechanically -- after all, the (presumably) large jaw muscles of early hominids resulted in the development of crests and other fortifications on the neurocranium, *not* in thicker skull walls or any other characteristic that might keep the size of brains small. On edit: And this mutation presumably results in weaker muscles...I want to see the argument for how it reduces the size of those muscles as well, because there are features of those australopithecine skulls that indicate that the muscles were physically larage, however strong they were. In other words, I want to know if this mutation has to do with strength only or with size as well. Guess I'll have to snag this week's Nature before my office-mates!

I could go on even more excessively about this...for me, the bottom line is that the increase in brain size in our lineage is such a complex question that I'm immediately skeptical of any theory that suggests a single cause.

[treepig]

2. it's a bit simplistic to suggest the ONE gene is, by itself, responsible

after all, there are other candidate genes out there that claim to be the "key" difference between humans and their closest relatives, for example see a SCIENCE article from the not too distant past:

http://www.sciencemag.org/cgi/content/full/291/5512/2340a?ck=nck

Science, Vol 291, Issue 5512, 2340 , 23 March 2001

Sugar Separates Humans From Apes

Joseph Alper

Humans and chimps differ at the genomic level by 1% to 2%. Yet so far, the only identified gene that differs between humans and chimpanzees codes for an enzyme that makes a particular form of a sugar called sialic acid: Chimps, and all other mammals for that matter, have the gene, while humans do not.

To Ajit Varki and his wife and colleague Nissi Varki--both professors at the University of California, San Diego (UCSD)--this fact may provide a clue to how evolutionary pressure and molecular biology interact to produce changes that have multiple consequences. "Since many pathogens bind to sialic acids on cell surfaces, changing those sialic acids is one way for an organism to evade a particular kind of pathogen," says Ajit Varki. "Such a change could give a big selective advantage to an individual with such a mutation." But the Varkis, together with UCSD colleague Elaine Muchmore, are now exploring an even more intriguing possibility: Perhaps the change may also make the brain work better.

"Ajit and Nissi are asking a very important question, which is what are the consequences of a single gene change on the physiology, and therefore the evolution, of humans," says Bernard Wood, the Henry R. Luce Professor of Human Origins at George Washington University in Washington, D.C. "In terms of trying to understand how humans fit into the rest of the world, this work has a very important place. It's one of those bits of biology that will become a citation classic."

The gene in question codes for the enzyme CMP-sialic acid hydroxylase, which adds an oxygen atom to a sialic acid variant known as N-acetylneuraminic acid, creating N-glycolylneuraminic acid (Neu5Gc). All mammals except humans have this enzyme, so all mammals except humans have both forms of sialic acid in their cells. It turns out, however, that the distribution of this enzyme is always skewed: plentiful throughout the body, but present in only small amounts in the brain and central nervous system. "For unknown reasons, the expression of this sugar is selectively down-regulated in the brains of all mammals, while being widely expressed everywhere else in the body," says Ajit Varki. "We wonder if its total elimination in the human brain might then have prompted a further improvement in the brain."

[phantom power]

3. excuse me for the tangent...

But does every science-news headline, ever, have to have a cheesy pun in it? It was bad enough to see it in the entertainment news. It's becoming painful for me to read science news sites.

[TX-RAT]

4. Isn't evolution just a ongoing process of mutation