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Traces of ancient life found in 1.6-billion-year-old rocks in Australia
A previously unappreciated group of lifeforms are changing what we know about the evolution of life on Earth.
While one lineage of these organisms would go on to become the ancestor of all modern animals and plants, the rest would become extinct and fade into obscurity.
Ancient molecules found in Australian rocks have provided some of the earliest evidence of complex life.
The 1.6-billion-year-old rocks of the Barney Creek Formation located in northern Australia were found to contain chemical traces of early life. The compounds are thought to have been used by early eukaryotes, the group of organisms that would later give rise to plants, animals and fungi.
While it’s not known what these lifeforms, collectively named the Protosterol Biota, looked like, the new research will help to explain how life as we know it evolved.
Dr Benjamin Nettersheim, a co-author of a new paper revealing these organisms, says, ‘Modern forms of eukaryotes are so powerful and dominant today, but physical remains of their early relatives are extremely scarce.’
‘Our study shows that the Protosterol Biota were hiding in plain sight and were in fact abundant in the world’s ancient oceans and lakes all along. Scientists just didn’t know how to look for them – until now.’
Dr Keyron Hickman-Lewis, a Research Fellow at the Museum who specialises in the signs of early life, says ‘This study provides a new record of biomarkers in the Proterozoic, helping to answer questions about the lack of a fossil record of aerobic eukaryotes as well as interpreting and reinterpreting the controversial cellular fossil record of this period of Earth’s history.’
The findings of the study were published in the journal Nature.
The protosterols were detected in rocks from marine ecosystems all over the world, and dating back more than a billion years. Image © The Australian National University.
What are eukaryotes?
Eukaryotes are the group of life which includes all animals, plants and fungi, as well as other related organisms. Their cells are characterised by multiple membranes, which allow them to have organelles such as the nucleus and mitochondria.
While there are some single-celled eukaryotes, the membranes also allow eukaryotes to become multicellular. How exactly this happened is something of a mystery, amid wider questions over eukaryote evolution.
Timings have been a particular issue. While fossils of eukaryotic cells have been found from around 1.6 billion years ago, these remains are scarce until around 900 million years ago. This appears to go against genetic studies which suggest that the last ancestor of all eukaryotes, known as LECA, evolved in the last 1.2-1.8 billion years.
If this was the case, then there should be evidence of an evolutionary build up to LECA, rather than it suddenly appearing out of nowhere. Given the current lack of these body fossils from early eukaryotes, researchers are instead looking for chemical traces of these organisms.
These traces include sterols, a group of compounds which help maintain the structure of cell membranes and which are almost exclusively found in eukaryotes.
But confusingly, common sterols found in modern organisms, like cholesterol, don’t turn up in the fossil record until less than a billion years ago. While a variety of explanations have been offered for this, the researchers behind the current study came up with a simpler one – these molecules simply weren’t being made at the start.
Instead, they’ve suggested that rather than making the complex sterols used by modern eukaryotes, these ancient ancestors might have been using simpler forms known as protosterols. Therefore, ff researchers could detect protosterols, it would go some way towards showing that early eukaryotes were present after all.
The researchers believe that the Protosterol Biota may have preyed on bacteria, similar to modern eukaryotes like Paramecium. Image © Lebendkulturen.de/Shutterstock.
What happened to the Protosterol Biota?
By grinding up samples of rock from the Barney Creek Formation into powder, the researchers were able to identify the different chemicals it contained. While the scientists are not the first to attempt to detect chemical signs of life in rocks, Keyron says that the study seems to have addressed the issues previous studies faced.
‘Previous studies concerning the biomarker record have been subject to considerable controversy.’ Keyron says. ‘For example, much older biomolecules from earlier periods of Earth history have since been shown to be the result of modern contamination.’
‘The authors of this study have conducted a comprehensive analysis of certain residues from these rocks, showing that apparently primary biomolecules are preserved. It is particularly encouraging that these biomolecules have forms consistent with our understanding of the timing and biochemistry of early eukaryotic cells.’
Their analysis revealed the traces of almost 100 different protosterol compounds, not only in the Australian rocks but also from samples taken all over the world.
Professor Jochen Brocks, the first author of the study, says, ‘Scientists had overlooked these molecules for four decades because these kinds of molecules weren’t being searched for.’
‘Once we knew what we were looking for, we discovered that dozens of other rocks, taken from billion-year-old waterways across the world, were also oozing with similar fossil molecules.’
The team have interpreted this as a sign that the Protosterol Biota were widespread around the world during this period. Jochen suggests that these early eukaryotes may have been among the first predators on Earth, consuming bacteria and competing with each other until LECA evolved.
However, another scenario is possible. While the chemical signature of the protosterols suggests a eukaryotic origin, the researchers can’t rule out that they were actually made by early bacteria. If this was the case, then early eukaryotes may have been very rare, hanging on in bacteria-dominated ecosystems.
In any case, the eukaryotes began to take on more recognisable shapes during the Tonian Period between a billion and 720 million years ago. This is when the first fungi and algae appear in the fossil record, setting the stage for the beginnings of terrestrial life.
While it’s not yet known when the Protosterol Biota became extinct, it seems likely that they eventually died out as LECA’s descendants became increasingly widespread.
‘Just as the dinosaurs had to go extinct so that our mammal ancestors could become large and abundant, perhaps the Protosterol Biota had to disappear to make space for modern eukaryotes,’ Jochen says.
Future research building on this study could help to shed more light on how the conditions of the Proterozoic lent themselves to the origins of the eukaryotes.
‘We know that the mid–late Proterozoic was a time of major environmental change, and such globally and regionally important stresses might have influenced the evolutionary trajectory of biology at that time,’ Keyron says. ‘In this regard, the study is another important datapoint that can inform us about the early stages of the emergence of eukaryotes.’
