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Scientists zero in on suspected common ancestor of all living things

WASHINGTON—Thanks to the tools of modern genetics, scientists are working to identify a little bug that they believe was the ancestor of every creature alive today.

They call it LUCA—shorthand for the "last universal common ancestor"—and they think it inhabited the Earth 3 billion to 4 billion years ago.

LUCA consisted of only a single cell, like a bacterium, scientists say, but its descendants comprise modern humans, animals, plants, fungi and invisible microbes.

"Amazingly, every living thing we see around us, and many more that we can only see with the aid of a microscope, is related," said Anthony Poole, a molecular biologist at Stockholm University in Sweden.

In effect, LUCA's genealogy makes us distant cousins of everything from whales to bumblebees and pond scum.

"All contemporary life is descended from a single last common ancestor that had a biochemistry closely related to contemporary biochemistry," said Max Bernstein, a biochemist at NASA's Ames Research Center in Moffett Field, Calif.

"If we go sufficiently far back, everybody's ancestors are shared," evolutionary biologist Richard Dawkins wrote in his latest book, "The Ancestor's Tale." "Go backward and no matter where you start, you end up celebrating the unity of life."

Researchers say the effort to understand LUCA can shed light on evolution and genetics, help medical science, and even improve the chances of finding primitive life on other planets.

Despite its great age, LUCA was "a sophisticated, essentially modern organism," said James Lake, a molecular biologist at the University of California, Los Angeles.

Lake calls it "the lucky bug" because its descendants survived while other ancient microbial lines died out. "It wasn't the first life," he pointed out. "Life had already been going on for a long time."

LUCA "set the stage for 4 billion years of evolution," said Blair Hedges, a biologist at Pennsylvania State University in University Park. Understanding the evolution of genes and their functions "is like having a blueprint for modern medicine," he added.

The quest for a universal common ancestor is made possible by the fact that all organisms from LUCA on down share a few hundred or thousand basic genes that enable them to eat, grow and reproduce. Those universally inherited genes provide the clues that evolutionary scientists are using to figure out what LUCA must have been like.

The nature of the last common ancestor is "one of the big questions" in evolution, David Penny, a molecular biologist at Massey University in New Zealand, said in an e-mail message.

Defining LUCA would be "a major step in determining what life was like on Earth and how life arose," Lake said. "If we get close to the right answer, then we can follow the history, geography, environment, everything about early life."

"We are now entering a very exciting period in uncovering the history of the LUCA," said Poole, who presented the results of his latest research at a conference last week in Hamilton, Ontario.

Nevertheless, the field is filled with uncertainty and doubt. Researchers disagree on many points.

"Ask any two researchers to give an overview of what they think the LUCA was like, and you will no doubt get different answers," Poole wrote in a paper published in 2002. "With such a tricky scientific endeavour as this—working out what an organism that lived billions of years ago was like—this is hardly surprising."

Some researchers, for example, think LUCA stored its genes in strings of DNA, like modern organisms. Others think it used a more primitive storage medium called RNA.

Some even doubt that there was a LUCA.

"I think the story is more complex," said Mitchell Sogin, an evolutionary biologist at the Marine Biological Laboratory in Woods Hole, Mass. "Rather than a single last common ancestor as implied by the concept of LUCA, there were probably populations of organisms that readily exchanged or assimilated genetic information from neighboring genomes.

"Currently there is no consensus and I don't think we should expect one in our lifetime."


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(c) 2005, Knight Ridder/Tribune Information Services.

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