Geobiologist Discovers Oldest Complex Fossil – In Her Pet

While a student at the University of New South Wales, Dr. Erica Barlow found a rock that would change her life and, quite possibly, our view of the history of life on Earth. It took her a long time to figure out what she had, and even today, no one can be sure whether the rock contains what Barlow and others suspect.

Barlow was in Western Australia’s Pilbara region studying stromatolites, some of the oldest fossils we know of. The trip involved long walks between the campsite and the fossils; on a return trip, Barlow noticed a shiny black rock that caught the setting sun on the region’s famous red earth. She picked it up as a souvenir of the trip. “I kept it on my desk as a sort of pet rock while I wrote my (bachelor’s) thesis,” Barlow said in a statement.

Barlow was still working on stromatolites when her thesis advisor, Martin Van Kranendonk, noticed the rock and identified it as black chert. Kranendonk explained that black chert contained microfossils from the earliest stages of life on Earth (although this is debated) and suggested she check it out. Immersed in her thesis, Barlow needed some encouragement to take the time to prepare and examine a sample, but she was surprised when she did.

The fossils in the chert were unlike anything Barlow had seen before. Moreover, no one else had seen anything like it. Given the age of the chert, if there were microfossils inside, they were expected to be those of single-celled organisms. The microfossils Barlow had found looked more like a football: almost round, but with a complex outline and a honeycomb-like internal structure.

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

This is a significant claim in any case, but it is considerably more significant when one considers that chert dates back to long before complex life is thought to have arisen.

The stromatolites Barlow studied are made up of thousands of cells that collectively form layered structures from their own bodies and sand. However, they do not correspond to what we think of as complex life.

For all we know, the first complex life forms were hundreds of millions of years younger than this discovery. Barlow’s discovery could be a precursor to eukaryotes, the branch of the tree of life that includes all animals, plants, fungi, and algae. Or it could be an evolutionary dead end, an early flowering of complexity that was snuffed out. Or it could simply be an illusion, mimicking complexity in a way we can’t explain.

There was only one solution: to make the rock the subject of his thesis.

Barlow first wanted to know if the chert was unique. Back at the collection site, she answered that question almost immediately. Barlow discovered a rock face studded with thousands of black chert nodules 30 metres (100 feet) up a nearby slope. Like the Pilbara itself, the wall stretched out of sight in both directions. Barlow told IFLScience she has since measured the formation at 12 kilometres (7 miles) long, all laden with chert nodules averaging 20 centimetres (8 inches) wide and 7 centimetres (3 inches) high.

Many of the chert samples do not appear to contain any fossils. Others contain organisms that resemble those found around the world at that time – “either long thin filaments or single-celled – like bubbles,” Barlow told IFLScience. A scientist who collected a small sample could easily go home thinking there was nothing unusual there.

Realising the potential importance of her discovery and the need to replicate it, Barlow collected hundreds of samples. Back in Sydney, she found several specimens that resembled her original, some even with amber spheres in the honeycombs. She has now expanded the number of specimens to 19, including half a dozen from a single rock. Barlow’s hundreds of samples also contain specimens that might have looked similar originally, but are too degraded for her to be sure. Had she chosen one of them, she probably would not have recognised its value.

The chert samples are clearly of the same age, and independent tests have confirmed that they all formed around 2.4 billion years ago. Crucially, this coincides with the date that geologists have now – after much debate – agreed to be the Great Oxidation Event. This is when oxygen levels in the atmosphere and oceans rose so dramatically that it became possible to breathe, paving the way for complex life.

There was previously an unexplained gap of about 750 million years between the time oxygen became available and the first eukaryotic fossils, showing that something was taking advantage of it.

Unfortunately, none of the specimens discovered by Barlow can be considered predecessors of eukaryotes.

“When you’re working with material from this era, it’s really difficult to prove or disprove something like this because we just don’t have enough preserved information,” Barlow said in the statement.

Geneticists date major advances in life using “molecular clocks,” but Barlow told IFLScience that these clocks produce “a huge range of estimates” of when eukaryotes appeared. Some of them are close to the age of her chert, but others are hundreds of millions of years later. “One of the problems is that molecular clocks are informed by the fossil record, which makes it a bit fuzzy when you go back that far, where the fossil record is so patchy,” she said.

6 to 700 million years ago represented by a handful of sites on the planet.

Dr. Erica Barlow

In theory, chemical analysis could provide valuable evidence. “If we can identify the type of carbon, that could tell us what the organism has been eating,” she said, potentially proving its complexity. However, that’s nearly impossible because samples are so easily contaminated with carbon from the modern world.

“Working with such tiny fossils, with so little carbon, if we got a positive result, the (scientific) community wouldn’t believe it because of the risk of contamination,” Barlow told IFLScience.

Future technology could improve the process, but in the meantime, Barlow’s work has struggled to be recognized. The remoteness of the location may be part of the problem. When the oldest animals were discovered in the Ediacara Hills of South Australia, many paleobiologists refused to believe they were real until they saw them in person. The location made the process slow.

If similar fossils were found elsewhere in the world, it could help Barlow, especially if something later showed signs of further development. So far, nothing has been found. Barlow admitted to IFLScience that this may be the only evidence of an early experiment with complexity that was stifled and not repeated for a long time.

The lack of another site, however, is not particularly surprising, given how few sites preserve fossils older than 1.6 billion years. “There’s 6 to 700 million years ago represented by a handful of sites on the planet,” Barlow told IFLScience. It’s not easy to preserve a fossil site, but Barlow thinks such an extreme dearth could be a consequence of the state of plate tectonics at the time.

If these specimens are ancestral eukaryotes, they wouldn’t have looked very interesting by modern standards. “From what we can tell, the life would have been soft, spongy, and slimy — kind of like the slime you see on the edge of a pond,” Barlow said in the statement. Still, Van Kranendonk noted the similarity to modern eukaryotic algae,

While waiting for something to happen that might help us learn more about her discovery, Barlow completed a postdoctoral fellowship with NASA at the wonderfully named Agnostic Biosignatures Laboratory. There, she tried to devise ways to identify life on other worlds if it didn’t resemble life on Earth; she may have had the best training there is for such a task.

Barlow’s most recent study of the discovery is published open access in Geobiology.