A worm-like animal that was half the size of a rice grain and lived more than 550 million years ago is likely to be humanity's oldest known ancestor.
Ikaria wariootia lived during the Ediacaran period, when much of central Australia was under water.
Fossils of it were found at Nilpena in South Australia's Flinders Ranges and studies published recently in the Proceedings of the National Academy of Sciences revealed it was the earliest known bilaterian.
Humans and most animals are bilaterians because they are symmetrical, have a mouth, an anus and a through-gut.
Mary Droser, a geologist from the University of California-Riverside, said the discovery shed new light on the earliest period of history.
"It's another piece in the puzzle of understanding how animal life evolved on Earth," she said.
"It's your oldest relative in Australia, which is very cool — it's the ultimate Ancestory.com.
"This is the organism that evolutionary biologists said the first bilaterian should look like, and then we find the fossil and it does look like that.
"It paints this picture of lots of weird things at the time, the beginning of bilaterians which later give rise to animal life as we know it."
The world 550 million years ago
Professor Droser, who has done extensive research in the Flinders Ranges, said the world would have looked very different half a billion years ago.
She said most of South Australia, from the Flinders Ranges to Adelaide, would have been under water.
"It would have been a shallow sea, and the seafloor would have been covered in this pond scum, really gross to our view — you wouldn't have wanted to put your foot in it.
"This guy, Ikaria, would have been one of the very few things that moved and it ate this green scum on the seafloor — it would have been like a massive buffet.
"There weren't a lot of things that mixed and ate stuff on the seafloor; most things were sedentary and didn't move."
Footprint before the animal
Scientists knew an animal lived during the Ediacaran period and grazed on pond scum long before the Ikaria wariootia fossil was discovered.
That's because the marks it made in the ground while moving and eating the scum had left a trail, which after millions of years became a trace fossil.
"Palaeontologists and evolutionary biologists had long accepted that whatever made this trail was a bilaterian," Professor Droser said.
"But we've never known what that organism was, we just had its burrow.
"The discovery of this fossil is very important, because this is the fossil of the organism that made that burrow."
When the discovery was made, Professor Droser said there were celebrations.
"We weren't positive we had it at first," she said.
"We were able to get a laser scanner which scanned the rocks and so we got a 3D laser image of the fossil.
"Once we did that, we were really able to get a feel for the organism, so that was when we decided it was something real."
Region important to early history
Flinders University lecturer in palaeontology Aaron Camens said the Flinders Ranges held a wealth of information about early history.
"This is one of the few places in the world that preserves evidence of the first complex life on this planet," he said.
"The Flinders Ranges is the internationally recognised type section between the geological periods when there isn't complex life and when first complex life evolves.
"The Ediacaran fossils show us some of the first complex life and potentially the first animals.
"We see things like bilateral symmetry, animals with a gut that can consume food, process it and eject it as waste, and that's really the first step in the evolution of animal life on this planet."