From Humans to Apes

[DBAHO]

A SINGLE mutation in one jaw muscle gene is the first key genetic difference to be found between between humans and apes.

 

What's more, the mutation - estimated to have occurred 2.4 million years ago - may have been the critical change that sent ancestral humans down a different evolutionary path than their primitive primate cousins, claim researchers at the University of Pennsylvania in Philadelphia.

 

"This is the first genetic distinction between humans and chimpanzees that can be linked to the fossil record," the team's leader, Hansell Stedman, said yesterday.

 

In Sydney, geneticist and developmental biologist Pete Currie agreed that the nature and timing of the mutation "linked exquisitely" with the timing of evolutionary changes identified by paleontologists.

 

"This is the first window for looking into and seeing what it is about our genes that makes us human," said Professor Currie, of The Victor Chang Cardiac Research Institute and University of NSW.

 

Generally, fossil experts conclude that hominids and chimps diverged about 2.4 million years ago. Hominids like Australopithecus afarensis and, later, Homo erectus appeared, leading to modern humans.

 

Writing today in the journal Nature, Professor Stedman and his colleagues claim that a tiny change to the so-called myosin heavy chain (MYH16) gene was enough to weaken our ancestral jaw muscles.

 

As a consequence, the hominid skull had room to shift shape, enabling it to accommodate an ever-enlarging brain. In contrast, chimpanzees were stuck with big, powerful jaws and, by necessity, much smaller brains.

 

According to Professor Currie, a "simple change" to muscle anatomy could indeed affect the skull.

 

"Altering the size of different muscles can produce dramatic alterations in the bones to which they attach," he said.

[FlaSoxxJim]

I saw this and I am anxiously awaiting my institution's copy of this week's nature so I can read the entire report.

 

Thus far the biggest criticism of this report is that it hangs so much on this single mutation, which is really not how modern evolutionary biologists would think a 3x increase in hominid brain size would come about. In conjunction with the reduction in jaw musculature (the gene the researchers found), there would need to be concurrent reductions in jaw and tooth structural robustness or else the single mutation would more likely be a detriment instead of a benefit.

 

Not to say the findings are not wicked cool and quite intriguing, given that the jaw musculature reduction coincides with the rise of the first tool using hominid, Homo habilis, 2.5 million years ago. The suggestion by these researchers that a tool-capable animal could do much of its food processing (cooking, chopping, cutting...) before consuming the food and therefore would not need the robust jaw musculature is certainly sensible enough. Still, the explanation of what happened over the next million years of Homo evolution is lacking thus far.

 

It's a common quandry for evolutionary biologists - how do small-scale mutational changes that by themselves are possibly detrimental and at best only slightly advantageous remain in the genome of the species and accumulate to the point that a major new system or function arises? The envenomation apparatus of snakes and the vertebrate eye are the textbook examples highlighting the issue, each of these systems arising only through the accumulation of several mutational changes. The neutralist arguments of Kimura and the arguments against absolute all-or-nothing benefits of mutations by Dawkins most notably provide some help, but it's still hard to wrap your brain around how these elegant and highly integrated multi-gene systems and structures arise.