Shelly fossils, found beneath a 635 million-year-old glacial deposit in South Australia, represent the earliest evidence of animal body forms in the current fossil record, predating other evidence by at least 70 million years.
This means that animals may have appeared on Earth 90 million years earlier than previously known.
The scientists who made the findings may have discovered in Australia the oldest fossils of animal bodies.
Turning the clock
These findings push back the clock on the scientific world's thinking regarding when animal life appeared on Earth. The results suggest that primitive sponge-like creatures lived in ocean reefs about 650 million years ago.
Previously, the oldest known fossils of hard-bodied animals were from two reef-dwelling organisms that lived around 550 million years ago, dating to the Cambrian Period.
There are also controversial fossils of soft-bodied animals that date to the latter part of the Ediacaran period between 577 and 542 million years ago.
Princeton University geoscientists Adam Maloof and Catherine Rose happened upon the new fossils while working on a project focused on the severe ice age that marked the end of the Cryogenian period 635 million years ago.
Their findings, published online on August 17 in the journal Nature Geoscience, provide the first direct evidence that animal life existed before — and probably survived — the severe “snowball Earth” event known as the Marinoan glaciation that left much of the globe covered in ice at the end of the Cryogenian.
“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 realized we had stumbled upon some sort of organism, and we decided to analyze 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.'”
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 3D model of the creature.
The technique is similar to the way CAT scan technology combines a series of two-dimensional X-rays to create a three-dimensional image of the inside of the body.
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 specialized body channels.
Previously, the oldest known undisputed fossilized sponges were around 520 million years old, dating to the Cambrian Period.