Are the world's earliest fossils really fossils?
Hot on the heels of the discovery of the TRAPPIST-1 solar system comes another find that's rocking the scientific community: fossils, which scientists from University College London (UCL) believe to be the world's oldest. Thought to be between 3.8 and 4.3 billion years old, this makes them around 300 million years older than the 3.43 billion-year-old fossils of single-cell organisms that were found on a beach in Western Australia in 2011. Other scientists have since uncovered what they claim to be even older specimens but until now, the Australian specimens have remained the oldest reliable fossils to date.
If the new claims are true, they could have important implications for science, not least in the way we look for extraterrestrial life. But do these microfossils have a biological origin?
What we know so far
No one knows how life on earth began but there are multiple theories, one of which revolves around the idea that ancient deep-sea hydrothermal vents might have been one of the earliest places to support ancient life. To test this hypothesis, the researchers turned to Quebec's Nuvvuagittuq supracrustal belt, home to some of the oldest sedimentary rocks known on Earth.
"These rocks were of a period in time when we don't know whether there was life," said first author Matthew Dodd in an interview with The Guardian. "If we believe the long-standing hypothesis that life evolved from hydrothermal vents billions of years ago then these were the perfect rocks to look at for answering these questions."
Scientists believe these rocks likely formed part of an iron-rich deep-sea hydrothermal vent system that provided a habitat for Earth's first life forms between 3.7 and 4.3 billion years ago.
The evidence for early life
Encased in quartz layers, scientists found tiny filaments and tubes measuring half the width of a human hair, which were supposedly formed by bacteria that lived on iron. Through their examination of the microfossils and razor-thin sections of the rock in which they were found, the researchers concluded that the tubes and filaments, which are made of a form of iron oxide or 'rust' called hematite, were unlikely to have been made through non-biological methods such as temperature and pressure changes in the rock during burial of the sediments. Here's why:
First, these ancient haematite tubes and structures share striking similarities with fossils of microbes in younger rocks and modern microbes, says Dodd. Like modern bacteria living in mat colonies around deep sea vents, it is likely these ancient iron-oxidizing specimens left behind tube-shaped pits that eventually became part of the fossil record.
"We found the filaments and tubes inside centimeter-sized structures called concretions or nodules, as well as other tiny spheroidal structures, called rosettes and granules, all of which we think are the products of putrefaction," explains study lead, Dominic Papineau, a lecturer at UCL's Precambrian Group. "They are mineralogically identical to those in younger rocks from Norway, the Great Lakes area of North America and Western Australia."
Secondly, they were found alongside graphite and minerals including carbonate and apatite, which are often formed from chemical compounds that organisms release when they die.
"The structures are composed of the minerals expected to form from putrefaction, and have been well documented throughout the geological record, from the beginning until today," says Papineau.
What the skeptics say
As is the case whenever someone announces a discovery of this magnitude, this study has divided the scientific community. In the case of its critics, the skepticism stems not from the belief that life could not have blossomed so early in the Earth's history, but from the researchers' approach to looking for early life in these ancient sea vents.
Evolutionary biologist William Martin of Heinrich Heine University in Dusseldorf, Germany, believes that it is "likely" there was life in the hydrothermal vents during this period; he is of the opinion that LUCA (the Last Universal Common Ancestor), a single-cell organism from which all living things evolved, can be traced to deep-sea vents. However he doesn't think what the researchers found is evidence of early life.
"I doubt that the microstructures they found in those ancient vents are biological," he tells New Atlas. "I do think that hydrothermal vents would likely have been inhabited that long ago, but the new paper does not provide clear evidence that the structures are fossilized cells. I am skeptical that they have evidence for life in the vents, the carbon isotope signature that one would like to see as evidence for life is not there. The microscopic structures in the vent could be anything. Abiotic processes can readily produce such microscopic structures."
For Frances Westall, who specializes in the study of ancient fossil bacteria at France's Centre de biophysique Moléculaire, two things about the discovery stand out as red flags: the orientation of the structures as well as their size. "The thing that bothers me most about these structures is the fact that they all seem to be extremely oriented," she said to The Guardian. "They are parallel to each other and microbes don't grow parallel to each other."
In addition, the filaments are far too large for organisms that supposedly lived in an environment that had no oxygen. Those found in 3.3 billion year-old rocks were much smaller – not surprising given that they would have grown very slowly in such an environment.
She believes, as does Martin Van Kranendonk, co-author of the Greenland fossils study that was published last year, that high temperatures and pressure exerted on the rocks are more plausible reasons for the filaments' formation. That's not to say she doesn't think life could not have existed more than 3.8 billion years ago. But given that these rocks were formed when the planet was basically one giant fireball, such distinct structures are unlikely to have survived in the rocks.
Others strike a more neutral tone. Geomicrobiologist David Emerson, an expert in modern microbial mats at the Bigelow Laboratory for Ocean Sciences thinks the unique structures in the rocks could be identified as biological. "The problem is that when you go back in time it becomes harder and harder to interpret the evidence," he tells New Scientist.
That said, the lukewarm reception to these new findings should not be seen as a blow. "The ancient vents alone are a good story," says Martin, adding that these findings should prompt further investigations to find more ancient fossilized hydrothermal vents in search of evidence for life.
The researchers discuss their findings in the video below.
The study was published in Nature.