Take 90M years off the Proterozoic
THE oldest known animal life on Earth has been discovered in Australia, pushing back its origins by about 90 million years, research has suggested.
Tiny fossils measuring up to a centimetre across have been identified as sea sponges that lived about 650 million years ago, indicating that they survived a global ice catastrophe known as "Snowball Earth".
The remains, found by a team led by Adam Maloof, of Princeton University in New Jersey, are the oldest convincing fossils left behind by the bodies of primitive early animals.
As well as extending the prehistory of animal life, they indicate that it must have predated the last Snowball Earth event, in which most of the planet's surface iced over about 635 million years ago. This would overturn the prevailing idea that animals first emerged after this event.
"No one was expecting that we would find animals that lived before the (Snowball Earth) ice age," Dr Maloof said. "Since animals probably did not evolve twice, we are suddenly confronted with the question of how some relative of these reef-dwelling animals survived the Snowball Earth."
Before the new discovery, from the Trezona Formation of the Flinders Ranges in South Australia, the oldest firm evidence for fossilised animal life came from a creature called Namacalathus, which lived about 550 million years ago. There is more controversial evidence for older fossils that are up to 572 million years old.
DNA evidence from sponges, however, has suggested that their origins predate this, and fats that appear to have been secreted by sponges have been dated to rocks from Oman from the same period as the Australian fossils. "There are now three separate lines of evidence for sponges before this Snowball Earth event," said Marc Laflamme, of Yale University.
Dr Maloof's team made the discovery by chance while studying rocks from the Snowball Earth episode known as the Marinoan glaciation. The research is published in the journal Nature Geoscience.
"We were accustomed to finding rocks with embedded mud chips, and at first this is what we thought we were seeing," Dr Maloof said. "But then we noticed these repeated shapes that we were finding everywhere - wishbones, rings, perforated slabs and anvils. We realised we had stumbled upon some sort of organism."
The fossils were impossible to remove from their surrounding rock. To analyse them, digital models of them had to be created by a company in New York. The scientists shaved off 50 microns of rock at a time, and photographed each new face. The pictures were then digitally combined to make three-dimensional images.