But Theobald hoped to go beyond conventional wisdom.

"What I wanted to do was not make the assumption that similar traits imply a shared ancestry ... because we know that's not always true," Theobald said.

"For instance, you could get similarities that are not due to common ancestry but that are due to natural selection"—that is, when environmental forces, such as predators or climate, result in certain mutations taking hold, such as claws or thicker fur.

Biologists call the independent development of similar traits in different lineages "convergent evolution." The wings of bats, birds, and insects are prime examples: They perform similar functions but evolved independently of one another.

But it's highly unlikely that the protein groups would have independently evolved into such similar DNA sequences, according to the new study, to be published tomorrow in the journal Nature.

"I asked, What's the probability that I would see a human DNA polymerase [protein] sequence and another protein with an E. coli DNA polymerase sequence?" he explained.

"It turns out that probability is much higher if you use the hypothesis that [humans and E. coli] are actually related."

No Special Treatment for Evolutionary Theory?

David Penny, an evolutionary biologist at Massey University in New Zealand, called the grand scope of Theobald's study "bold."

Penny had been part of a similar, but more narrowly focused, study in the 1980s. His team had looked at shared proteins in mammals and concluded that different mammalian species are likely descended from a common ancestor.

Testing the theory of universal common ancestry is important, because biologists should question their major tenets just as scientists in other fields do, said Penny, who wasn't part of the new study.

"Evolution," he said, "should not be given any special status."

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