When they were first discovered, the wiggly grooves on slabs of ancient sandstone from central India were dramatic enough: They appeared to some to be 1.1-billion-year-old worm tracks. That date would push the earliest known record of complex animals back a startling half-billion years (ScienceNOW, 1 October 1998). But, it turns out that the first report greatly underestimated the rocks' age.
Researchers now report in the February issue of Geology that the rock marked by the putative tracks is a whopping 1.6 billion years old. That predates the earliest generally accepted trace fossil of a complex animal by a billion years. To some researchers, such a long gap strains credulity. Instead of traces of life, they are now seeing meaningless marks in ancient, squishy mud.
The new, solid age for the Indian grooves comes from radiometric dating by two independent groups. They measured the clocklike radioactive decay of uranium to lead in tiny crystals of zircon deposited with volcanic ash just before and just after the grooves formed. Both groups--one led by paleontologist Birger Rasmussen of the University of Western Australia in Crawley, the other by geochemist Jyotiranjan Ray of the University of Hawaii, Honolulu--got ages of just over 1.6 billion years.
With doubts about the appropriate age resolved, the biological origins of the grooves become "even more exciting or more improbable," says paleontologist Adolph Seilacher of Yale University, who with colleagues proposed that the grooves were formed by evolutionarily advanced worms burrowing just beneath the sea floor. "This age makes it unlikely these are animal trace fossils," Seilacher concedes. "At the same time, I have to go with the evidence. I have not found or heard of any other explanation. Do we have any nonbiological interpretation of these things?"
Well, yes, says paleontologist Mary Droser of the University of California, Riverside. To her eye, the grooves "look much more like cracks than trace fossils." The debate over the earliest traces of animal life "is a great dress rehearsal for when we get samples from Mars," says geochemist Samuel Bowring of the Massachusetts Institute of Technology, who collaborated with Ray. "How do you decide when something is biogenic? Paleontologists haven't completely come to grips with that." Perhaps squiggly grooves from India can help prepare us for that encounter.
1998 Science paper describing the rocks