Animal Life's Ancient Origins
Animals appeared on Earth more than a billion years ago--twice as early as previously estimated, according to a provocative study published in the 25 October issue of Science.
Paleontologists had traced the origins of animal life to roughly 565 million years ago, during an explosion of diverse animal life-forms in fossils dated to the Cambrian Period. However, microbiologist Gregory Wray of the State University of New York, Stony Brook, and others question whether the fossil record tells the whole story. If more ancient animals were too small or too "squishy," Wray says, they would have left no trace in the fossil record.
Wray and his colleagues therefore attempted to trace animal life--defined as multicellular creatures whose cells are held together by collagen--back to its origins without heading to a paleontology dig. Instead, they used a "molecular clock." They assumed that nucleotide bases--the building blocks of DNA--change into other nucleotides at a constant rate. Wray's team counted nucleotide differences in DNA of living species from 16 phyla, which are groups of related species. They examined species sharing a known common ancestor, such as humans and carp. By plotting the number of nucleotide changes against the estimated time since the pair diverged, the researchers calculated "how fast the clock was ticking," Wray says. His team applied this rate to species pairs with no known common ancestor--such as humans and clams--to determine how much time had elapsed since the species had diverged. Winding their molecular clock backward through time, Wray's team estimated that the mother of all animals must have been born 1 billion to 1.2 billion years ago.
Not everyone is convinced that Wray's clock ticks at a constant rate, however. Although the study is interesting, says Harvard University paleontologist Andrew H. Knoll, "molecular clocks are not Timexes." University of California, Davis, geologist Geerat J. Vermeij agrees that it's better not to assume a constant rate. "What I am advocating is that people sit down and test this assumption," he says.