Why humans have big brains but don’t have whiskers or penises with spines
Why are we different from other creatures? After all, our genomes are 95% or more identical to that of chimps. For all the genome-busting that’s gone on in recent years -- human genomes, chimp genomes, mouse genomes, platypus genomes -- scientists in large part don’t know the answer to these kinds of questions.
This week in the journal Nature are clues about three human characteristics -- big brains, lack of sensory whiskers and penises without spines -- that appear to be caused by chunks of DNA that we lost but other mammals still have.
In one case, our species seems to have lost a chunk of DNA close to a gene for making an androgen receptor. The androgen receptor is needed for formation of “sensory vibrissae” -- that’s whiskers to you and me -- in chimps and macaques and mice. It’s also needed for the formation of a bony spine in the penises of these creatures.
In another case, our species has also lost a DNA chunk right next to a gene called GADD45G. GADD45G carries instructions for a tumor-suppressing protein, is active during development and seems to stop certain brain cells from proliferating too much. The authors speculate that loss of the DNA chunk right near it may allow parts of the developing human forebrain to grow bigger -- might, in other words, help explain our hulking brains.
In both cases, the deleted DNA chunks aren’t parts of genes -- they just sit near genes and control when, where and how much those genes are turned on or off. Much of the trick to why mice, people and chimps, etc., turn out so differently even though they mostly share the same genes may be rooted in evolutionary changes in such regions, geneticists say. Changes in how genes are regulated -- instead of what those genes are -- would produce widely differing species even if the genes and proteins they possess are very, very similar.