AUSTRALIA: Fossil find pushes back biological clock
The fossils date from about 650 million years ago and are the remains of sponge-like creatures that lived in ocean reefs.
"These scientists have found that animals may have appeared on Earth 90 million years earlier than previously known," said H Richard Lane of the United States' National Science Foundation's Division of Earth Sciences, which funded the research.
"This is comparable to resetting modern times to begin during the late Cretaceous."
Previously, the oldest known fossils of hard-bodied animals were from two reef-dwelling organisms that lived around 550 million years ago.
There are also controversial fossils of soft-bodied animals that date to between 577 and 542 million years ago.
Princeton University geoscientists Adam Maloof and Catherine Rose found the new fossils while working on a project focused on the severe ice age that began 635 million years ago.
Their findings, published in the 17 August issue of the journal Nature Geoscience, provide the first direct evidence that animal life existed before - and probably survived - the severe 'snowball Earth' event that covered much of the globe in ice.
"We were accustomed to finding rocks with embedded mud chips, and at first this is what we thought we were seeing," 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, and we decided to analyse the fossils.
"No-one was expecting that we would find animals that lived before the ice age, and since animals probably did not evolve twice we are suddenly confronted with the question of how a relative of these reef-dwelling animals survived the 'snowball Earth'."
Analysing the fossils turned out to be easier said than done, as their composition and location made it such that they could not be removed from the surrounding rock using conventional techniques, nor could they be imaged using X-ray scanning techniques.
Maloof, Rose and their collaborators teamed up with professionals at Situ Studio, a Brooklyn-based design and digital fabrication studio, to create three-dimensional digital models of two individual fossils that were embedded in the surrounding rock.
As part of the process, team members shaved off 50 microns of sample at a time - about half the width of a human hair - and photographed the polished rock surface each time. The team ground and imaged nearly 500 slices of the rock.
Using specialised software techniques developed specifically for this project, the researchers then 'stacked' the outlines on top of one another to create a complete three-dimensional model of the creature.
The model revealed irregularly shaped, centimetre-scale animals with a network of internal canals.
After considering a variety of alternatives, the scientists decided that the fossil organisms most closely resembled sponges - simple filter-feeding animals that extract food from water as it flows through specialised body channels.
Previously, the oldest known undisputed fossilised sponges were around 520 million years old, dating to the Cambrian Period.