WASHINGTON (AP) _ Millipedes and flies, spiders and lobsters _ they seem to have little in common except lots of exotic legs.

Yet scientists have argued for a century whether these arthropods, the largest and most diverse division of the animal kingdom, were really related. Now University of Wisconsin researchers have found evidence that they all evolved from a common ancestor millions of years ago.

``This was a missing piece of evolution,'' explained Wisconsin molecular biologist Grace Panganiban, chief author of a study appearing today in the journal Science.

Panganiban was investigating a gene called Distal-less that causes limbs to grow in flies and butterflies, when she decided to dip into the arthropod controversy by seeing if crustaceans had this gene too.

Each branch of each crustacean limb carries the same version of Distal-less as do insect legs, Panganiban discovered. That indicates a single gene is responsible for the formation of all arthropods' legs _ it just caused different-shaped legs in different species by interacting with their other unique genes, she explained.

That means arthropods' common ancestor probably was a wormlike creature, from whom Distal-less helped evolve successive generations of animals with continually different legs, Panganiban said.

But she acknowledged the controversy won't be settled by the finding, so she's searching possible arthropod precursors _ simpler animals like leeches and worms _ for further proof.