Tiny tunnels preserved in rock may be the oldest trace fossils ever found and proof that complex animals evolved a billion years ago, far earlier in Earth's history than previously believed.
Researchers said Wednesday that the tunnels, about the diameter of a soda straw, may have been burrowed by wormlike creatures that lived beneath the floor of an ancient sea in what is now India.Adolf Seilacher of Yale University, lead author in a study to be published Friday in the journal Science, said the tunnels mean "the birth of multicellular animals was at least twice as long ago as we thought."
"It means that animals have a much longer history than we once believed," Seilacher added in a telephone interview from Germany.
Multicellular animals made a dramatic appearance in the fossil record about 540 million years ago at the beginning of what is called the Cambrian period. Animals had then developed skeletons, shells and mineralized bodies that were preserved in the fossils.
Before that, it has been believed, life consisted of primitive, soft-bodied organisms that left no trace in the fossil record. Scientists generally believed that life started some 4 billion years ago with simple, single-celled creatures that crept slowly up the evolutionary ladder until an explosion of new, complex life forms during the Cambrian period.
But Seilacher, a professor emeritus at the University of Tubingen in Germany, said that discovery of the worm tunnels in India shows that multicellular animals, with complicated and intricate lifestyles, lived more than a half-billion years before the Cambrian period.
His announcement, made at a German news conference, met with immediate skepticism among some paleontologists.
"It this were true, it would be very important," said Bruce Runnegar, a UCLA paleontologist. "I would like to see evidence for animals 1 billion years old. But I don't think this discovery represents the final, unequivocal proof."
Seilacher and his colleagues found the tunnels, now eroded to mere meandering grooves, in sandstone in northern India.
The rock was formed from sand that once was the floor of a shallow sea. Seilacher said he believes the wormlike creatures lived in the sand and fed on a mat of decaying organic matter that coated the sea floor. The organic matter, he said, probably was the bodies of micro-organisms and algae that lived in the water, died and sank to the bottom.
Seilacher said the paths of the tunnels seem to purposely follow the contours of the sea floor, as if the animals were feeding from below on the organic mat. Some of the tunnels have branches, he said, suggesting that the animals sometimes dug forward and then backed out to take a new burrowing path.
This, he said, suggests a complex life form that had nerves, instincts and senses.
The shape of the tunnels, said Seilacher, suggests the animals moved by a wave-like action and could have been coated with a mucous that eased the passage through the sand. He said no other natural process could have created such tunnels.