Erica Picked Up A Rock By Chance 10 Years Ago. It Might Hold The Oldest Form Of Complex Life On Earth

July 3, 2024

Erica Barlow

Erica picked up a rock by chance 10 years ago. It might hold the oldest form of complex life on Earth

A fossil found inside a souvenir ‘pet rock’ could push the start of complex life on Earth back by about 750 million years.

It’s not every day that you find out you’ve been sharing your desk with an ancient life form.

It’s also not every day that this ancient life form turns out to be more than half the age of the Earth itself.

But for Dr Erica Barlow, a geobiologist who completed her PhD at UNSW Sydney, that’s exactly what happened 10 years ago, when she found the microscopic fossil of a 2.4-billion-year-old creature hidden inside a small rock on her desk.

After a decade of studying the ancient fossil, research has shown her former desk mate is more than just really, really old: it’s also the only one of its kind known to the geological record.

What’s more, it shows a ‘jump’ in the complexity of life around 2.4 billion years ago – and could rewrite our timeline of complex life on Earth altogether.

A chance discovery

Dr Barlow’s uncanny discovery started a decade ago, when she was on an undergraduate field trip with her research team in the Western Australian outback.

She was there studying large, layered limestone rocks called ‘stromatolites’ as part of her Honours research project, and spent her days in the Pilbara heat mapping the area and analysing the rock structures.

The field site was hot, dry and dusty, with barely a tree in sight.

Dr Barlow was walking back to the campsite one afternoon when a small, shiny black rock reflecting the sunlight caught her eye. It stood out to her in the otherwise red landscape, so she picked it up as a memento of her trip.

“I kept it on my desk as a kind of pet rock while I wrote my thesis,” she says.

The pet rock sat on Dr Barlow’s desk at the Australian Centre for Astrobiology for several months while she worked on her stromatolites project.

It wasn’t until her supervisor, UNSW Adjunct Professor Martin Van Kranendonk, walked past her desk one day that she learnt just how rare these rocks – called black chert – were.

“‘Did you know black chert are known to hold microfossils?’” she recalls him saying at the time. “‘You should really investigate that.’”

Given their tiny size, microfossils are notoriously easy to miss. Unlike larger macrofossils (think plant, mammal and dinosaur fossils), microfossils aren’t visible to the naked eye.

In fact, the only way to find out if a rock holds a microfossil is to take it back to a lab and slice out a piece of the sample. This slice is then attached to a glass slide and ground down until it’s thin enough to be see-through, before being put under the microscope.

Dr Barlow told Prof. Van Kranendonk she’d give it a shot – but as microfossil analysis was time-consuming and she was preoccupied with her thesis, she decided to put it off a little bit longer.

But after another round of encouragement, Dr Barlow realised it was worth pausing her thesis for. She took her pet rock to the lab and prepared a sample for the microscope.

What she saw shook her.

“I was just amazed,” says Dr Barlow. “I’d never seen anything like it before.”

Not only did Dr Barlow find a microfossil staring back at her, but a microfossil so unique that she didn’t recognise it.

Instead of long filaments, it was round. In fact, it reminded her of a soccer ball.

She called over Prof. Van Kranendonk for a second opinion. But he didn’t recognise the fossil, either.

Dr Barlow began cross-checking geological records trying to identify the type of fossil she’d found, but the results were fruitless. No matter where she looked, she couldn’t find a match.

This could only mean one thing.

“There was nothing else like the microfossil I found in the geological record,” says Dr Barlow.

“It was an entirely a new type of life.”

A new type of life

Dr Barlow finished her Honours project and dove into the PhD project of a lifetime: investigating the new type of ancient life she found.

She travelled back to the Turee Creek field site, which is on Puutu Kunti Kurrama and Pinikura land in the Hamersley Ranges, to retrace her steps in search of more rock samples.

The search didn’t take long: about 30 metres uphill from the sample she’d found was an entire rock wall embedded with black chert.

“The black chert layers extended for kilometres in each direction,” she says.

After documenting the site, Dr Barlow used a geological pick to pry out some additional samples.

Once she was back at university, she took them under the microscope – and sure enough, found more of the strange soccer ball-like microfossils hiding within them.

Independent dating of the rock layers surrounding the embedded black chert suggest the microfossils are about 2.4 billion years old.

This age estimate coincides with an important point in time known as the ‘Great Oxidation Event’: a volatile tipping-point in Earth’s history when oxygen levels on the planet’s surface dramatically and irreversibly increased.

The sharp, one-time rise of oxygen has been theoretically linked to the development of all complex life on Earth, but we haven’t had the fossil record to demonstrate this theory – until now.

“We’ve shown the first direct fossil evidence linking the changing environment during the Great Oxidation Event with an increase in the complexity of life,” says Dr Barlow.

“It shows a step up in the organisation of life at this time.”

A stepping stone to complex life

The only known life that existed before the Great Oxidation Event were ‘prokaryotes’: simple, single-celled organisms without nucleuses, like bacteria.

But the comparatively intricate shape and large size of the fossil Dr Barlow found suggests its life form may have been an early step towards a ‘eukaryote’: that is, a complex, generally multi-celled form of life that has a nucleus. Most of the visible life we see on Earth today, like animals, plants, and fungi, is eukaryotic.

If future research confirms this theory, it would make this fossil the oldest-known evidence of complex life on Earth – but it may be a while before technology is at a stage where this can be assessed.

“When you’re working with material from this time, it’s really hard to prove or disprove something like this, because we just don’t have enough information preserved,” says Dr Barlow.

“With the current technology, we’re at an impasse.”

In the meantime, the microfossil shows a jump in complexity of life after the Great Oxidation Event – whether it’s considered a step towards a eukaryote or not.

“This finding pushes back evidence for the start of the complexification of life by 750 million years,” says Prof. Van Kranendonk.

“It’s a major development in the understanding of the evolution of life on our planet.”

A ‘soft, squishy and gooey’ life

To the human eye, the fossil is tiny, about the width of a human hair. But compared with other microfossils of its time, it’s considered quite large.

Under the microscope, the fossil’s honeycomb-like divisions become visible. But they’re not on its surface, as you’d expect on a soccer ball-like structure – instead, they’re internal.

“There’s no bone – we’re long before bone,” says Dr Barlow. “There’s not even a harder skeleton like a shell or anything. It’s just carbon.

“From what we can tell, the life would have been soft, squishy and gooey – kind of like slime that you see at the edge of a pond.”

Prof. Van Kranendonk, a geologist and astrobiologist who studies early forms of life, says the life form is a lot like modern algae.

“The discovered microfossils bear striking resemblance to existing eukaryotic algae, more so than to any other type of life,” he says.

“This suggests that this microfossil was a stepping stone to our own ancestry.”

The making of a fossil

Dr Barlow’s chance discovery is made even more exciting when considering the fact this fossil exists at all.

Firstly, way back when the creature was living, it had to be in a type of environment where it could be preserved, entombed and encapsulated after its death.

It then had to be buried by other sediments, survive the temperatures and pressures of being underground, before being lifted to the surface again – and somehow avoid being folded, altered and otherwise changed to the point of being unrecognisable.

Then, finally, a researcher had to stumble across it in the outback and identify it as a fossil.

“There are so many opportunities for this not to have happened, for this rock not to be available at the surface for us to study today,” says Dr Barlow, who finished a NASA postdoctoral program at Pennsylvania State University last year.

The Turee Creek field site is the only known well-preserved fossil site from this point in time, says Dr Barlow.

“It gives us this direct insight into what life was doing during the environmental change around the Great Oxidation Event,” she says.

Prof. Van Kranendonk says the quality of preservation of these microfossils is extraordinary for their age – and the only example of its type and age anywhere in the world.

“It is a geological wonderland,” he says.

The world 2.4 billion years ago

The face of the Earth looked so different 2.4 billion years ago that we probably wouldn’t recognise it.

For starters, the ocean probably wasn’t blue, says Dr Barlow.

“Because of the chemistry of the oceans, they may have actually been green, due to the type of iron in them,” she says.

“The sky probably also wasn’t blue, because the composition of the atmosphere was completely different,” says Dr Barlow.

“It’s hard to say exactly what the sky would have looked like, but it might have been orange, from all the methane, and was probably packed with carbon dioxide and sulphur-rich gases from all the volcanic activity.”

It’s not just the sky and the sea that would have looked different, either – even on the land, there wouldn’t have been any green in sight.

“Trees and plant life don’t develop for at least another 1.5 billion years,” says Dr Barlow. “The landscape was likely black from all the volcanic activity.

“Our young planet would have been entirely alien to us.”

The sudden increase in oxygen during the Great Oxidation Event would forever change this landscape.

It would also prove to be a very catastrophic event for many life forms of the time, who needed low-oxygen environments to survive (it’s no wonder, then, the event is sometimes referred to as the ‘Oxygen Catastrophe’).

“This would have created a lot of turmoil for life at the time,” says Dr Barlow.

But the growing oxygen levels would also mean that other, more complex life forms – potentially including this microfossil – could thrive, and in turn, help bump evolution up a notch.

Paving the way for future research

The time the team spent working on this project – a decade in total – is a testament to the complexity of studying something as ancient as this microfossil.

The project took Dr Barlow to interstate and international universities, where she worked with specialised equipment and external experts to help her better understand the fossil and the life it once held.

“It’s been a 10 plus year journey to get to get here,” says Dr Barlow. “But we wanted to be sure of our finding.

“Even though it’s a beautiful fossil and is very well preserved for its age, compared to modern microfossils, it’s quite degraded.”

Prof. Van Kranendonk says that over time, new technologies may enable a closer, more definitive analysis of the microfossil, which could help us better identify the type of life form it represents, and whether it’s an early step towards a eukaryote.

Until then, further investigations of the samples might be able to teach us more about how the organism lived and reproduced. For example, a recent study has fine-tuned the approximate age estimate of the black chert layer by an additional decimal point, to 2.37 billion years ago.

“Hopefully, our findings open it up for other researchers to think about complexity in this older age,” says Dr Barlow.

“There must be so many other fossils out there that we are yet to find. I hope this work is only going to encourage more research into this area.”