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Family tree. There may be more than one way to evolve six legs.

Six Legs Evolved Twice

The insect body plan--the most common animal architecture on Earth--evolved more than once, according to a new DNA analysis. The study overturns a widespread assumption of entomology, but some scientists want more proof.

Recent debate over insect ancestry has centered on whether they are most closely related to crustaceans, such as crabs and lobsters, or myriapods, such as centipedes and millipedes. Most researchers assumed that a common ancestor, most likely resembling a crustacean, gave rise to all hexapods--including insects and six-legged creatures such as collembola, a wingless soil dweller considered by some scientists to be an insect.

To find out if genes could confirm the evidence of anatomy, evolutionary biologist Francesco Nardi of the University of Siena in Italy and colleagues examined the mitochondrial DNA of more than 30 insects, collembola, and relatives. The researchers found that certain crustaceans are more closely related to insects than collembola are, suggesting that collembola are not insects at all, but a group that branched off from the main crustacean lineage even earlier. They report the results in the 21 March issue of Science.

This revision in the insect family tree will dramatically alter scientists' understanding of how and why natural selection shaped physiological and biochemical processes to create a six-legged body plan, says evolutionary biologist Chris Simon of the University of Connecticut, Storrs. But evolutionary biologist Frederick Schram of the University of Amsterdam says there are too few species in the analysis. And family trees assembled this way are sensitive to which species are analyzed, he says. "Given other taxa and other characters, the tree could be different."

Related sites
Simon's site
Schram's site