It's not that the trait has suddenly reappeared, it's more that the trait has been re-expressed. Everybody now alive has the genes for a heavy coat of fur. But in the vast majority of the population those genes are partially or completely suppressed. Every once in a great while those genes that evolved to suppress the early genes are either suppressed themselves, don't activate in the first place, or aren't passed on to the child.
Then there's environment. Toddlers learn how to walk upright by watching adults and older children walking upright. Without those examples to follow toddlers will never learn how to do it right, and as a result skeletal and joint structure will be damaged. Most often to the point the child will never be able to walk fully upright without some degree of pain.
The wonder is not that such atavisms show up so rarely, the wonder is that such atavisms show up at all; for we as humans have a remarkably stable genome. Entire species and genera have appear since Homo sapiens arose, and with less genetic encouragement. Remember, as a species the Virginia Opossum is younger than Man.Posted by Alan Kellogg at March 1, 2006 5:58 PM