In the eye of the beholder. Some say the regular pattern on bits of rock like these make them look like fossils, but others say they are only artifacts.
Richard A. Kerr
*Science 1998; 282: 1020.
TORONTO--In the past month, the apparent age of the first known animals nearly doubled to a startling 1.1 billion years, then swung back to the conventional figure of 600 million years. And last week at the annual meeting here of the Geological Society of America, the pendulum swung one more time, back toward the extraordinarily early dates claimed a month ago. Paleontologists may have to reckon after all with signs of animals 500 million years earlier than the first known animal fossils.
The first dramatic claim came in the 2 October issue of Science (pp. 19 and 80), when researchers said they had found tracks of multicellular animals in 1.1-billion-year-old Indian rocks. Then, paleontologist Rafat Jamal Azmi of the Wadia Institute of Himalayan Geology in Dehra Dun, India, claimed in the Journal of the Geological Society of India that he had found tiny fossils, known to be from about 540 million years ago, in rocks just above the purported trace fossils. If so, the tracks might actually be only about 600 million years old (Science, 23 October, p. 627). Paleontologist Anshu K. Sinha, director of the Birbal Sahni Institute of Paleobotany in Lucknow, noted that Azmi's finds might be confused with certain kinds of sedimentary structure and that his work had not been replicated. But Sinha and other paleontologists who read Azmi's paper and studied scanning electron microscope (SEM) images of the finds concurred that they were indeed small, shelly fossils (Science, 23 October, p. 601).
In the eye of the beholder. Some say the regular pattern on bits of rock
like these make them look like fossils, but others say they are only artifacts.
CREDIT: R. J. AZMI/WADIA INSTITUTE OF HIMALAYAN GEOLOGY
In a question-and-answer session at the meeting, however, paleontologist Nicholas Butterfield of the University of Cambridge reported that after Azmi visited and gave him a look at actual samples, he believes they are not fossils at all but artifacts. "They're very convincing in black-and-white" SEM images, says Butterfield, "but they're absolutely not biogenic when seen in Technicolor" under a light microscope. Once he could view the objects from any angle and under varied lighting, Butterfield concluded that their ribbed structure was simply a reflection of fine layers in the rock itself. The texture of the rock plus the acid treatment Azmi used to extract any fossils apparently created the oddly shaped bits, Butterfield says.
Others also have doubts. Two other Cambridge experts in Cambrian fossils, Simon Conway Morris and Soren Jensen, studied the samples with Butterfield when Azmi visited Cambridge 2 weeks ago, and they agree that the bits are not fossils. Even one-time supporters, such as paleontologist Martin Brasier of the University of Oxford, who found the photographs persuasive but hasn't seen the samples themselves, now agrees that, based on the Cambridge experts' views, "it looks doubtful that they are convincing."
Azmi, however, stands by his find, saying that Conway Morris studied unpublished fossils rather than the examples cited in his recent paper. He says that Butterfield's "generalized statement" is "very confusing," because it does not address the issue "specimen by specimen." Azmi concludes: "There cannot be any doubt that these are fossils, for they are not artifacts."
Even if this particular challenge to the claim of billion-year-old animal tracks may be fading, paleontologists at the meeting weren't quite ready to embrace such a startlingly ancient origin of animals. Some critics still aren't sure the tracks are those of living creatures. Confirming the age of the rocks may require new radiometric dates, which will take a few years to complete. The age of the first animals is--still--a question mark.