All other photos in this story by Ian Connellan.
On a hillside just west of the Flinders Ranges, geologist Mary Droser lounges casually on a slab of 550-million-year-old sandstone. The sun has just hit its golden afternoon angle; it illuminates squiggled lines in the rocks, throwing into sharp contrast some of the earliest evidence for animal life on the planet.
“This guy here is a burrow, made by an animal moving through the sediment,” Droser tells me, tracing her finger over the lines.
This creature must have been moving with purpose and direction along the ancient sea floor, shifting thin layers of sand to form burrows that are now frozen in time. “Head, way in, way out, through-gut, able to move, muscles,” she muses. “A lot of things go into the organism that can do this.”
These burrows are of particular interest to Droser and her team. For a long time, they only had the burrows; the tiny creature that created them was elusive. Then Droser got her hands on a 3D laser scanner to investigate minuscule blobs found nearby, and discovered the impressions of a simple, worm-like animal half the size of a grain of rice.
This critter, Ikaria wariootia, marked a breakthrough in our understanding of life on Earth, becoming one of the oldest known animal fossils. “This had to have been an animal,” Droser says. “That was a champagne moment for us.”
This hillside we’re sitting on is part of the new Nilpena Ediacara National Park, on an old cattle station 600 dusty kilometres north of Adelaide. Today, these slopes are covered in loose rocks and scrubby vegetation, but half a billion years ago they were at the bottom of a calm, shallow, tropical sea.
The world had recently undergone a crucial transformation: as the planet emerged from an ice age that stretched from poles to equator, the atmosphere warmed and oxygen levels soared. Sometime in this period, single-celled life tumbled into multi-cellular creatures and gave rise to the very first complex lifeforms on Earth. These evolutionary experiments lived, thrived and died on microbial mats on the sea floor created by filaments of cyanobacteria – like the muck on the bottom of a pond.
From where I’m sitting with Droser, cross-legged on the burrow-marked rock, I can spot at least half a dozen more sandstone beds dotting the slope. Each one has been painstakingly excavated and pieced back together like a jigsaw puzzle, and each one captures a snapshot of life from the Ediacaran period, 635 to 541 million years ago. These slabs – some still rippled from waves – are like a painting of an entire marine community, formed when storms buried the ecosystem in sand.
“In terms of capturing communities, it’s one of the greatest fossil localities in the world,” she says.
Propped up on one elbow with a faded baseball cap tugged low over her eyes, Droser is completely at ease in this landscape. She’s an ocean away from her home in California, where she’s a professor of geology at the University of California, Riverside, but she has been making annual pilgrimages to this semi-arid, saltbush-speckled expanse of South Australia for 20 years.
She returns because it holds some of the best fossil evidence for early animals. But Ikaria is only one of the oldest we know about – the last common ancestor of all animals is much older still, and it is yet to be found.
Scientists have been searching for decades for our earliest ancestor, using different approaches to glean clues from rocks, the fossil record, the animals we see today, and even our DNA. It’s a complicated discussion that spans palaeontology, evolutionary biology, phylogeny, genetics, geology, geochemistry, chronology and more
“Of course, it’s a big debate because we are animals, and we are very much interested in ourselves,” says Professor Jochen Brocks, an organic chemist from the Australian National University (ANU). “There’s an inherent interest in ourselves and our ancestors’ tale.”
Today, we take it for granted that a diverse menagerie of animals swim, slither, walk, climb and fly in every corner of the globe. But for the vast majority of the Earth’s history, single-celled organisms dominated the planet, living in the stagnant soup of the primordial oceans. They did nothing much of note for eons, until finally, within the last billion years, life worked out how to combine cells into more complex creatures.
This sparked a period of great evolutionary experimentation, with life trying to go in all kinds of curious directions to find out what worked. While the vast majority of critters didn’t survive to present day, some went on to become the ancestors of everything faunal we know and love. So how do we search for an animal that lived more than half a billion years ago?
This excerpt is republished online from Cosmos Magazine issue 92, which went on sale on Thursday 2 September 2021.
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