In the May 10th issue of the weekly magazine Science, an Australian-Swedish team of scientists report fossil evidence that animal-like organisms were around more than 1200 million years ago.
This is more than twice as old as any animal fossils generally accepted by palaeontologists. It is more in agreement, however, with some calculations made by molecular biologists of when animals appeared on Earth.
The fossils are from the Stirling Range in southwestern Australia, from sandstones representing an ancient shallow sea floor. Sandstones are notoriously difficult to date, but Birger Rasmussen and his colleagues at The University of Western Australia in Perth measured the decay products of radioactive elements in minerals formed before and after the sand was deposited.
They found that the youngest mineral grains of the original sand are about 2000 million years old, whereas minerals formed when the sandstone hardened to dense quartzite are about 1200 million years old. Therefore the organisms lived some time between these two dates.
The fossils consist of double ridges a couple of millimeters apart, following a winding course on the surfaces of sandstone beds. Stefan Bengtson at the Swedish Museum of Natural History in Stockholm interprets them as traces of organisms moving along a slime tract, much as many animals do today, both on land and in water.
Though the occurrence of animal-like traces in rocks of this age is remarkable, it does not by itself resolve the controversial issue of when the first animals appeared on Earth. Traces may tell us a lot about the behaviour of an organism, but they say little about what it looked like.
Similar slime traces can today be made by a number of animals, such as molluscs, corals, and various sorts of worms, complex or simple. The Stirling traces could have been made by these kinds of animals, but it is more likely that they were made by extinct multicellular organisms that may or may not be closely related to animals.
The Precambrian, spanning four billion years (4550 to 540 million years ago) and comprising close to 90 percent of geological time, has long been viewed as the almost exclusive realm of simple, microscopic organisms.
If the interpretation of the Stirling traces is correct, the history of life was more complex than previously supposed. The results open up exciting new possibilities in the search for ancient multicellular organisms in vast stretches of rock long thought to be barren.
University of Western Australia
Swedish Museum of Natural History
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