BIG BRAINS
What makes human beings human? Our big brains. And where did they come from? Apparently a genetic defect that proved advantageous under just the right circumstances. Here's an excerpt from the Discover.com report:
Scientists have long suspected that humans evolved large brains because our hominid ancestors had to outwit and elude predators, learn to use fire, and develop complex social structures. The smart hominids survived, while the stupid ones were more likely to get eaten or freeze to death. Over millions of years, the result of this game of survival of the fittest was the appearance of big-brained, peculiarly intelligent modern humans. Now Bruce Lahn, a biomedical researcher at the University of Chicago, has found the first clear indication of the genetic changes that led to the rapid expansion of our brain.
Lahn and his colleagues looked at the abnormal spindle-like microcephaly associated (ASPM) gene, which scientists had previously identified as a key player in brain development. He grew intrigued by ASPM after other researchers discovered that serious defects in the gene cause microcephaly--a drastic reduction in the size of the brain's cerebral cortex, the region responsible for such higher brain functions as abstract thought and planning. Lahn wondered: Could changes in this gene, favored by the pressures of natural selection, have directed the development of the big, modern human brain?
To find out, Lahn compared the sequence of the human ASPM gene with the equivalent gene sequences of various primates--including chimpanzees, gorillas, and gibbons--and with the sequences of nonprimate species such as mice, cows, and dogs. He isolated genetic mutations that altered the structure of the ASPM protein and thus could have affected brain size, while weeding out the random mutations that had no structural effect and hence would have been unaffected by evolutionary pressures. Lahn found that the ASPM gene in humans has undergone 15 important mutations since we last shared a common ancestor with chimpanzees, about 5 million years ago. Significantly, compared with the other animals studied, humans have experienced the fastest overall rate of change in the gene since our evolutionary line parted ways with chimpanzees and other primates. Evidently, ASPM responded to natural selection, and the resulting changes contributed to our large brains.
This sort of indirect genetic evidence for human evolution is going to pile up until it resembles the overwhelming geological case against believing the earth is a few thousand years old. More important, and more interesting, will be how understanding the genetic origins of brain functions lets us affect how our minds work. With the Kass Commission hot to talk about brains, can proposals for new criminal laws against neuroscience be far behind? After all, that research might threaten classical conceptions of the mind. And if they were good enough for Plato, they're good enough for us.