The search for the oldest animals.

The discovery of small grooves in a 1.1-billion-year-old rock from central India may dramatically push back the record of animal life on Earth.

"This is the oldest animal. I would say it is a worm burrow," says paleobiologist Adolf Seilacher of Tubingen University in Germany and Yale University. He presented his find last month in Salt Lake City at a meeting of the Geological Society of America Seilacher proposes that worms made the lines as they slid across a carpet of bacteria on the seafloor.

The oldest currently accepted animal fossils come from rocks 600 million years old. Throughout this century, many scientists have reported finding older animal fossils, but none of those specimens has convinced the paleontological community.

The Indian rocks will also face considerable scrutiny. "I'm cautious," comments Guy M. Narbonne of Queen's University in Kingston. Ontario. "I don't think we can rule out the possibility that it could be a modern artifact." He wonders whether termites or other tunneling insects could have made the grooves quite recently.

Friedrich Pflueger, also at Yale, has come up with another possible explanation. He has found in experiments that gas bubbles can make horizontal tracks in sand when trapped beneath an impermeable layer. A billion years ago, bacterial films may have provided such a layer.

The Indian specimens, if accepted as worm traces, would resolve an ongoing debate about the timing of animal evolution. The fossil evidence suggests that animals appeared 600 million years ago, quite late in the history of life. Information gleaned from animal genes, however, points to a much more ancient origin, perhaps as early as 1.2 billion to 1 billion years ago (SN: 11/23/96, p. 335).

Pushing back the record of animal life would raise new problems, though. Geologists calculate from the chemicals in rocks that oxygen concentrations in the ocean 1 billion years ago hovered far below present values. Because animals require oxygen for growth and respiration, it seems unlikely that the oceans could have supported worms big enough to make the Indian traces, says Narbonne. According to the chemical evidence, oxygen finally reached adequate concentrations roughly 600 million years ago--about the time that generally accepted animal traces appeared in rocks for the first time.