6 million year old fossils may be oldest true animal

Contender for oldest true animal
Tiny fossils may have been alive 600 million years ago in China
David Perlman, San Francisco Chronicle Science Editor
Thursday, June 3, 2004

They are only 10 tiny fossils, found among 10,000 wafer-thin slices of ancient rocks, but scientists believe they could be the oldest true animals ever discovered.

The fossils, found recently in southwest China, are known as "bilaterians" because they are creatures with completely symmetrical, or bilateral, bodies, each with a front and rear, right and left sides, as all true animals have.

Scientists estimate the creatures were alive between 580 million and 600 million years ago -- more then 50 million years before the period known as the "Cambrian explosion," when within a few million years most of the major groups of animals first appear in the fossil record.

The microscopic oval-shaped creatures, looking like flattened turtle shells, are less than 180 microns long, barely the width of four human hairs.

Each has a mouth, an anus, and a pair of pits in its body that could have been sense organs capable of detecting changes in its environment, like light or darkness, said David Bottjer of the University of Southern California, a member of the Chinese and American team that made the discovery.

Their report is published today in Sciencexpress, the online version of the journal Science.

"It was a little button-shaped organism that probably scooted along the sea floor," Bottjer said in a statement. "It had a little mouth, sort of like a vacuum cleaner. It was tiny, but microbes are even smaller, so it probably sucked them up so it could eat them."

If the newfound fossil organisms are indeed Earth's earliest known animals, then current views of the winding path of evolution will surely undergo new revisions, the scientists say.

Six years ago, fossil hunters from the United States and Germany reported they had discovered "trace fossils" of long, sinuous burrows in the sandstones of central India. The burrows, they claimed, could have been made only by wormlike animals that lived at least 1.2 billion years ago -- hundreds of millions of years before the organisms that Bottjer and his Chinese colleagues are reporting.

Most scientists, however, have dismissed the "trace fossil" report as wishful thinking.

Kevin Peterson, a Dartmouth biologist and expert on the early evolution of animal body plans, recently reported that by using a "molecular clock" to track the average mutation rates of well-dated fossils and their modern descendants, he and his colleagues place the emergence of the first common ancestors of the bilaterian organisms at between 573 million and 656 million years ago.

Peterson, whose work was published in April in the Proceedings of the National Academy of Sciences, said in an interview Wednesday that his dates appear "right on target" with the dates that Bottjer and his colleagues have assigned to their fossils.

Bottjer and Jun-Yuan Chen of the Nanjing Institute of Geology and Paleontology, the report's lead author, note that the oldest common ancestors of their fossil creatures, perhaps millions of years earlier, must have already possessed a complex genetic "tool kit" to yield the complex structures of the animals they found.

The scientists have named their fossil animals Vernanimalcula guizhouena. The genus name means "small spring animal" in Latin; the species name refers to the area where the fossils were found, once a seabed among the rolling hills, rice farms and the phosphate quarries of China's Guizhou province.

"Spring" refers to the time some 600 million years ago when a major worldwide glaciation ended. Under the controversial "Snowball Earth" theory, this period gave way to a later era of warming that may have lasted for many millions of years.

"These were really true animals, with truly symmetrical body plans," Bottjer said of the team's fossils in a phone interview. "They were much different than the animals like sponges and jellyfish, corals and sea anemones whose bodies aren't bilateral.

"So we think we've got something real here, something that could change our ideas about the timing of evolution, although others may disagree that we've found what we think we've found. That's science, however; that's what research is all about."
---------------------------------

E-mail David Perlman at [email protected].

URL: http://sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/2004/06/03/MNG0N6VQ021.DTL