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What our tails tell us

Atavistic incidents, such as humans born with tails, actually bolster the case for Darwinism.

February 15, 2007

HUMAN BABIES BORN with tails? That may sound like a headline from the Weekly World News, but it was the respected New Scientist magazine that recently published a cover story about the phenomenon of evolution "running backward." Entertainment value aside, the article represents a new twist in the politically charged debate about evolution.

The author of "The Ancestor Within," Michael Le Page, cited the babies with tails as a likely example of atavism, a phenomenon in which ancestral traits suddenly reappear after thousands or even millions of years. Another example, one remarked on by Charles Darwin, is the appearance in some human mouths of large, ape-like canine teeth. (Stephen King, call your office.)

It's not just humans who experience these altered states. Le Page also cites the less cringe-making example of a humpback whale with leg-like appendages that was caught off Canada's Vancouver Island in 1919.

These and other throwbacks might seem to call into question the validity of evolution, which has been on something of a roll lately, with a federal judge's decision last year against a Pennsylvania school district that wanted to teach "intelligent design" and, only this week, the decision by the Kansas State Board of Education to repeal guidelines that said there was "considerable scientific and public controversy" over human origins.

In fact, Le Page suggests, atavisms like tails on humans are the exceptions that prove the evolutionary rule. Atavisms are possible, he says, because genes for primitive traits haven't disappeared from the genome; they simply have been switched off. In rare cases, they are switched back on (and then the tails are promptly snipped off). Sometimes, Le Page adds, so-called reverse evolution serves the cause of improving a species by allowing it to adapt to a changed environment.

Atavism isn't the only explanation for the reappearance of a seemingly extinct trait or body part. It's also possible that some life forms that have lost a certain trait evolve it again "from scratch," through the same mutations that produced it in the first place. After all, different species sometimes develop similar features separately -- a process known as "convergent evolution."

But the existence of atavism complicates the case against evolution. Far from undermining Darwinism, throwbacks challenge the creationist idea that every living species emerged full-blown in its present form. Atavism is impossible unless there's something to throw back to -- like an ancestor with a tail that nobody wanted to snip off.

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