Animal? Vegetable? Now mineral.

If you climbed down through the branches of the evolutionary tree, from child to parent for countless generations, you would eventually meet the greatest-grandparent of us all. But when did that animal evolve? And would we know it when we saw it?

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?

By looking at how quickly the DNA of living species is mutating, researchers can wind back the “molecular clock” to estimate that the critter at the root of the animal evolutionary tree lived somewhere between 900 and 635 million years ago.

The same technique indicates that the common ancestors of all bilaterians – a major group of animals that includes us, defined by left-right body symmetry with two “openings” and a through-gut – emerged about 630 million years ago, which just about coincides with the beginning of the Ediacaran period.

But bilaterians aren’t the only kind of animal. Cnidarians (like corals and jellyfish), poriferans (sponges) and ctenophores (comb jellies) also fall within the animal kingdom, and so scientists have wondered whether these groups provide a better analogue of our earliest ancestor.

Perhaps, some think, our great^great grandparents resembled very simple sponges: mostly stationary critters without nervous, digestive or circulatory systems, and without much organisation of their tissues. Others back a different ancestral candidate: comb jellies. These delicate and translucent marine carnivores seem to be more “complex” than sponges, with nervous systems and many more types of cells.

Over the past decade, a wave of studies have used genome analysis to determine which group – sponges or comb jellies – belong to a more ancient branch of the family tree. But it seems that as soon as one research team declares sponges are closer to the earliest animal, another team will publish a paper with more evidence for comb jellies, and so on.

You’d be forgiven for thinking that palaeontologists could just trot off to a fossil site and settle this. However, the fossil record is patchy – several hundred million years is a lot of time for specimens to be deformed or destroyed by geological processes.

Besides, the earliest creatures were likely small, with soft and fragile bodies that don’t fossilise well – they didn’t have skeletons or any hard parts at all, and their muscles, nerves and guts disappeared quickly after death.

It isn’t until the Cambrian period, directly after the Ediacaran, that we begin to see a rich fossil record. This is when animals pulled away from their soft and squishy origins and built bones and skeletons, which could undergo bio-mineralisation and thus preserve the body of the animal.

Brocks says that the chance of “finding a fossil or any remains of something that is soft, not abundant and very small is virtually zero”.

Plus, it’s not easy to find something when we don’t exactly know what we’re looking for. “Our vision of what things look like today don’t apply back in time,” Brocks says. “The definitions even completely disappear.”

It was in the Cambrian that life began to explode in diversity and the first clear ancestors of today’s animals emerged – the fossil record has plenty of evidence of recognisable arthropods, molluscs, annelids, echinoderms and chordates (us).

Ediacarans, on the other hand, are weird.

“I don’t want to call them alien, but it’s very different from what we see today,” says Associate Professor Diego Garcia-Bellido, a palaeobiologist at the University of Adelaide and the South Australian Museum. Garcia-Bellido’s research focuses mainly on the Cambrian, particularly on fossils from Emu Bay on South Australia’s Kangaroo Island; sometimes, there are even difficulties drawing clear ties from the Ediacarans to the Cambrian fauna.

“These early organisms look very strange to us because they don’t share the shape or attributes of what we see around us today,” he explains.

“Evolution is a slow process and so things don’t turn from one thing to the next from one day to the other. Some of these complex organisms already have features or characteristics of animals, but others don’t… For instance, they don’t have mouths, they don’t have muscles, they don’t have shells.”

The Ediacarans are a recent addition to geological history. The first fossil specimens were uncovered in 1946 by South Australian geologist Reg Sprigg, just a few kilometres north of the spectacular hillside site where Droser works. Further discoveries have now been made at more than 30 sites across all continents except Antarctica, revealing over 100 species; the best sites are found in South Australia, Newfoundland, Russia and Namibia.

At first palaeontologists didn’t know what to make of these odd-looking fossils – did they belong to the kingdom of plants, fungi, animals, protists or something else entirely?

“Early on, everybody just shoehorned things into modern phyla,” Droser says. “Then there was a time when everybody said, ‘This is an alien experiment, none of these things are related to anything around today’.

“We know that some of these organisms gave rise to the great diversity that we have today. But this was a major time of body plan experimentation… A lot of these body plans came and went, but some of them – just a few of them – survived to give rise to the animals we have today.”

The molecular clock tells us that the first animal evolved near or before the beginning of the Ediacaran. It’s this ancestral line that palaeontologists like Droser, along with long-time collaborator Jim Gehling from the South Australian Museum, are working to uncover.

Most specimens are in the shapes of discs, ribbons or fronds. Some, like Dickinsonia, at least have clear bi-radial (two-sided) body symmetry; others like Tribrachidium have tri-radial symmetry, like the Mercedes-Benz logo. Some – such as a group called rangeomorphs, best known from incredibly preserved specimens at Mistaken Point in Newfoundland, Canada – grow fractally. Their bodies consist of repeating branching patterns that make many palaeontologists suspect they were not animals.

The tricky thing is, with no shells, bones or exoskeletons to speak of, Ediacaran fossils are only moulds of the outside of their bodies. For palaeontologists, it’s like looking at a shadow and figuring out what cast them. “We don’t know what most of these things are, to be honest,” Droser confesses.

To get an idea, the team at Nilpena uses gear that runs the gamut of high and low tech. On the hillside, Droser tosses me a plastic egg full of silly putty and tells me to find myself a fossil; I press it into a Dickinsonia imprint the size of a 50-cent piece and come away with a 3D animal in my hand, complete with delicate ridges streaming away from its midline. The team also uses 3D laser scanning to build models of the impressions, and photogrammetry to record the spatial distributions of fossil beds as a whole.

There is, Droser says, an incredible amount of imagination necessary to work within this period – scientifically rigorous imagination, but imagination nonetheless.

Droser often dreams of snorkelling this ancient sea; over the years she and her team have had hundreds of hours of conversations about what they would have seen. She imagines fields of upright tubular animals called Funisia, anchored to the sea floor; pancake-flat Dickinsonia crawling over the organic mat and chowing down on it; tiny worm-like Ikaria burrowing down into the sediment; the fronds of Aspidella growing up towards the surface; the grape-sized Attenborites swimming up into the water column; frond-like Arborea waving in the current; mollusc-like Kimberella scraping at the microbial mat for food.

She imagines, too, all the things they have missed.

While the soft body parts of the Ediacarans have long since decayed, in very rare cases, fossils can preserve within them “biomarkers” – traces of molecules that were created by a living creature.

About a decade ago, several of these exceptionally rare fossils – including an exquisitely preserved Dickinsonia – were found halfway up a 100-metre cliff in the White Sea, in north-western Russia.

The specimens were excavated by Ilya Bobrovskiy, now a geobiologist at the California Institute of Technology in Pasadena, but back then he approached ANU’s Jochen Brocks to propose a radical PhD project.

“He explained to me, ‘I’ve found Ediacaran biota fossils that are organically preserved – it’s not only rock, there’s actually an organic film of the original creatures’,” Brocks says. “I simply did not believe him.”

Brocks had previously studied the chemistry of fossils, but never from this long ago. He thought the chance that Ediacaran fossils would contain bio-markers was practically zero.

“The Ediacaran has been quite thoroughly, deeply buried,” he explains. “In particular, the ones in the Flinders Ranges went through pretty high mountain building… These fossils were buried under kilometres of rock, pushed so deep that they were thoroughly heated and cooked. No molecule would survive that at all.”

But the fossils found by Bobrovskiy in the White Sea had never been buried to the same extent; the sandstone is still “fresh”, Brocks says, despite the fact that it’s the same age as the rocks at Nilpena. As the sandstone formed, the fossilised creatures were compressed, sandwiching within the rock a thin layer of organic matter just one micrometre thick.

The fossils – including one of a Dickinsonia specimen 1.4 metres in diameter – had to be destroyed in order to extract the organic molecules from them; Bobrovskiy used a scalpel and tweezers to harvest the organic film and analyse the molecules within.

The results were astonishing.

In 2018, the team announced they had identified the signature of cholesterol – a fat type found only in animals’ cell membranes – confirming that Dickinsonia was unequivocally an animal. The finding, Brocks says, was beyond his wildest dreams. “It was just a beautiful, clear signal… This thing’s loaded with cholesterol – that’s what animals do.”

The specimen could be specifically dated to 558 million years ago thanks to an abundance of volcanic ash within the rock, adding Dickinsonia to the small but growing list of earliest confirmed animals.

There are always other interpretations, of course – perhaps, Brocks suggests, it’s another, now-extinct, non-animal form of evolutionary experimentation that (in parallel to animals) became bigger, multi-cellular and capable of producing cholesterol.

Though this is entirely possible, it is not the simplest explanation based on the other evidence gathered – which, Brocks notes, is why palaeontologists are absolutely crucial to this process.

“If the palaeontologists had not done all the work on these creatures before, [our] interpretation would not have been possible,” he says. “People like Jim Gehling and Mary Droser – they’re the heroes, they’re the people doing the hard work. They collected 99% of everything we know about these organisms.”

Brocks explains that chemists like himself – along with geophysicists, geneticists and other scientists – are contributing to the debate by solving problems that palaeontologists can’t with their own tools, adding independent lines of evidence.

The “molecular fossil” they found, he says, “is something totally new – it’s a new pillar, a different type of knowledge, which is…entirely independent from all the other knowledge that others have contributed before”.

Biomarkers aren’t limited to the Ediacarans – other researchers have attempted to find even earlier molecular fossils. A decade ago they thought they hit the jackpot when they found rocks from the bed of an ancient sea in the Middle East. These rocks held some lipid molecules, which were traces of a type of steroid that are today only produced by sponges. Further examples of these traces were subsequently found in more than 20 global locations, dating back 640 million years, but no body fossils were associated with them.

Then, last year, Brocks and his team published a study that independently delved into the origin of these traces and showed they could have been produced by algae. Over eons, as the algae transformed into rock, it underwent chemical processes that produced animal-like biomarkers, so these kinds of molecular fossils can no longer be used as a marker for early sponges or any early animals.

The Ediacarans, from Dickinsonia squelching across the sea floor to Ikaria burrowing through the sand, remain the oldest evidence for animal life.

Brocks suspects that even if sponges existed earlier than the Ediacarans, it will be very difficult to find fossil evidence due to the soft nature of their bodies. But just this year, sponge-like structures were discovered within an ancient reef, dating back to 890 million years ago. If these are verified as body fossils, it could mean that animals emerged before Earth’s oxygen increased to modern levels – and then shivered through global ice ages.

And it’s still feasible that more fossils will surface, pushing the physical evidence for animals back further in time. “The palaeontologists have been very good at finding more and more stuff,” Brocks says. “I’m very impressed they actually keep finding things.”

In the red-dirt remnants of an ancestral sea, Droser and her small team at Nilpena continue their discoveries. Recently, they pieced together a sandstone bed about nine metres long that is so rich with diverse fossils that they’ve dubbed it “Alice’s Restaurant” for that song’s chorus: “You can get anything you want, at Alice’s Restaurant,” Droser jokes.

The bed revealed two previously unknown fossils with celebrity names – Attenborites, potentially the first pelagic organism, and Obamus, a spiral-shaped bottom feeder – as well as hundreds of individuals from many more known species. Over several field seasons, the group carefully extracted and cleaned the fossils to release this alien menagerie from the rock – and to fathom an ecology of the early sea floor. The next generation of palaeontologists is now enthusiastically taking up this task, including Droser’s PhD student Phil Boan.

Because of Nilpena’s fantastic preservation of whole communities, Boan tells me that he can take spatial tools normally used in modern forestry or marine biology and apply them to these ancient communities. “It’s not a one-to-one comparison – a tree is not the same as an Aspidella,” he says.

But it doesn’t really matter what the critters are; the statistical methods he’s using don’t care “if it’s a plant or animal or galaxy or whatever. It’s all just numbers.”

These analyses can reveal patterns in the fossil assemblages – whether species or individuals lived close together, spread apart, segregated or thinned out – and from this, their behaviour can be inferred.

“I can start to figure out how they’re competing with each other, how they’re reproducing with each other,” Boan explains. “It’s a lot of sitting around looking at equations, but the outputs kind of bring order to these seemingly insane patterns.”

Like the afternoon sun giving depth to the wrinkles and ridges of the fossils on the Nilpena hillside, these new approaches could further illuminate and flesh out the Ediacaran biota, and therefore the origins of us all.

“People always say, ‘Oh, are our ancestors here?’” Droser tells me that afternoon, sitting on the golden sandstone slab that holds the impressions of the burrowing Ikaria. “Our relatives are here, but whether we are direct descendants of something that’s here, we don’t know.

“I don’t think we’ll get to a point where we say, ‘That’s Grandma!’, but I think we’re learning a lot.”