To the untrained eye, they're not much to look at--a smattering of tiny squiggles decorating slabs of sandstone. But don't let their modest appearance fool you: these may be the earliest traces of multicelluar animals, or metazoans, yet found--more than twice as old as the roughly 600-million-year-old remains considered by most paleontologists to be the first uncontroversial metazoans. Researchers writing today in the journal Science argue that the unassuming doodles are most likely trails left behind by mucus-producing, wormlike creatures that crossed the sands of southwestern Australia at least 1.2 billion years ago.
This is not the first time metazoan fossils considerably older than 600 million years have been reported. But earlier cases have generally been reinterpreted as nonmetazoan fossils or the results of inorganic activity. The newly discovered specimens, say Birger Rasmussen of the University of Western Australia and his colleagues, do not lend themselves to nonbiological explanation (although that possibility cannot be ruled out for disk-shaped imprints also visible on the rocks). Nor do the fossils resemble the forms produced by nonmetazoan organisms.
But if relatively large, multicellular animals capable of getting around existed by 1.2 billion years ago, why did 600 million years apparently then pass before life on earth attained the diversity associated with the famed Cambrian explosion? This is the million-dollar question scholars will face if the team's interpretation of the Australian material meets with approval. For their part, Rasmussen and his collaborators suggest that extreme environmental conditions just before the Cambrian "may have been the final bottleneck before which no diversification of organisms with metazoanlike modes of life could have had lasting success."