<P dir=ltr align=left><STRONG>Small jaws and big brains in human evolution</STRONG></P>
Small jaws and big brains in human evolution.
We know that modern humans have small weak jaws and big impressive brains compared with our closest living relatives, the chimps. Fossil evidence puts the transition between 2.5 Ma and 1.7 Ma, in the transition from one or other species of Australopithecus to Homo erectus, with two or three species events in that million-year span. The authors of this new study have found a genetic difference between chimps and living humans: humans lack a gene which turns on powerful growth of the masseter and temporalis muscles that work the lower jaw in chewing. But chimp ancestors and our ancestors diverged maybe 7 Ma, and you can't do genetics on 2-million-year-old hominids. The authors magic their data into giving a date of 2.5 Ma for the mutation that allegedly gave our ancestors suddenly weak jaws, and they call on weak jaw muscles to allow growth of a big brain. Now geneticists don't do evolution very well, any more than designers of microchip circuits do Internet search software. It is nutty to call on a mutation that cripples chewing to generate modern humans. So I'll tell you what really happened‹for free‹and it's worth every penny you paid for it. The suite of discovery, innovation, and increasing intelligence that gave some australopithecine a better ability to hunt, prepare food, and so on, happened perhaps around 2.5 Ma at a brain size that was typically australopithecine (evidence: A. garhi and/or A. africanus). That suite changed diet toward meat, higher protein, less chewing, etc., and at some point the evolving jaw reached a morphology where the Big-Muscle gene wasn't being turned on much at all, so it could be lost without penalty. The larger brain came later, but it perhaps came more easily. So you can see that this story is not far from the one the authors put forward. The difference is that mine is truly evolutionary (it happens over a long time); it does not demand a Hopeful Mutant; it does not call for an Event; and it is not tied to statistical magic that gives a one-time miracle. The paper is in Nature, so is not freely available on the Web. Stedman, H. H. et al. 2004. Nature 428: 415-418. News stories: