On February 11, Abderrazak El Albani, a sedimentologist at the University of Poitiers in France, published a study in Proceedings of the National Academy of Sciences containing a spectacular claim: Life on Earth began making relatively large-scale active movements some 2.1 billion years ago.
That time is well within the era in which Earth’s biosphere was almost exclusively composed of single-celled organisms thought to be capable of only the most minuscule motion, and predates the generally accepted advent of large-scale biological motility by roughly 1.5 billion years. The evidence for this controversial conclusion is a collection of tiny burrowlike fissures in rocks found in the west African nation of Gabon.
Determining whether microscopic details in ancient rock formations are evidence of early life or the result of abiotic—or nonliving—processes is extremely difficult. Trace fossils are as elusive as they are open to interpretation, and disagreements among those who study them—ichnologists—are common. What looks like evidence of life can have myriad abiotic origins. As a result, pinning down specific time frames for major events such as the emergence of eukaryotes (organisms whose cells contain a membrane-bound nucleus) or the rise of multicellular life is an unenviable task. The Proterozoic—the geologic eon that preceded the Cambrian period and spanned the time from the appearance of oxygen in Earth’s atmosphere to the emergence of complex life—is itself nearly two billion years long. “We’re in the business of trying to figure out what was going on in the depths of time, and the data are extremely sparse,” says Nick Butterfield, a paleontologist at the University of Cambridge. “The history of Precambrian paleontology is strewn with mistakes and misidentifications.”
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“When you are trying to claim the oldest anything, the bar is very high,” says Williams College paleontologist Phoebe Cohen, who peer-reviewed the paper from El Albani and colleagues. She says the thoroughness of the team’s methods makes their results “compelling, even though I might not agree with their specific conclusions.” Perhaps the most ambitious of these was the assertion the fissures in ancient Gabonese rocks could have been made by an organism analogous to slime molds. These amoebozoans—a phylum of single-celled eukaryotes—spend most of their lives as individual organisms, but will gather into motile multicellular formations to search for food and reproduce. The study proposes the fissures could be the result of a similar organism moving through sediment and leaving behind mucus trails. “It’s not impossible,” she says. But molecular clocks—which infer the age of evolutionary lineages by estimating the accumulation of genetic mutations over time—place the last common ancestor of all eukaryotes at around 1.8 billion years ago, she notes. That’s about 300 million years after the putative slime molds would have made the channels.
A virtual fly-through of a 2.1-billion-year-old rock specimen revealing fissure-like microfossils that may have been produced by living organisms. Credit: © A. El Albani & A. Mazurier, IC2MP, CNRS and Université de Poitiers
El Albani made headlines in 2010 with the discovery of the oldest fossils of multicellular organisms in a 2.1-billion-year-old shale bed called the Francevillian Formation in Gabon. (That claim was controversial, however.) In the latest study he led a team that examined features in the same formation with a variety of imaging techniques. They also measured the rocks’ sulfur isotope content, which could offer additional clues that the cracks were biotic, or made by living organisms. Using mass spectrometry, a technique that measures the chemical content of specimens, they determined the Francevillian rocks contain an abundance of light sulfur isotopes—just what would be expected if it had been home to colonies of anaerobic microorganisms that “breathed” sulfate rather than molecular oxygen, which at the time was largely absent from Earth’s biosphere. Pedro Marenco, a geochemist at Bryn Mawr College who was not involved in the study, says the sulfur isotope values reported by El Albani’s team are indeed consistent with that interpretation. But is this sufficient evidence to indicate the Gabonese microfossils have biotic origins? “That’s a whole different question,” Marenco says.
Several researchers who study ancient life and were not involved in the study expressed varying degrees of skepticism about the team’s conclusions while simultaneously praising their exhaustive methods. “They’re making an extraordinary claim, and they don’t have extraordinary data,” Butterfield says. “There’s a litany of pseudo-fossils in the literature, and this is one of them.”
“The evidence they’ve lined up could indeed be consistent with having an affinity to amoebozoan organisms or the mucus trails they create,” says Lidya Tarhan, a sedimentologist at Yale University. “I think the challenge is being confident that we can definitively link these structures to not only slime molds, but to more complex life more broadly.” Murray Gingras, an ichnologist at the University of Alberta who commented on the review of the study prior to publication, says he would have interpreted the specimens differently. Where El Albani’s team saw fossilized burrows, Gingras sees abiotic cracks in the sediment that were stabilized and compressed by algal mats. “In particular, in Neoproterozoic and older rocks we see such features,” he says.
Fabien Kenig, a geologist at the University of Illinois at Chicago, says he is not convinced by the paper. “However, I am also sure that the discussion will be very productive,” he says. “Other manuscripts will be written, reinforcing or contradicting the conclusion of this one. This is science at work.”
El Albani says the team was aware of the difficulties surrounding their claim. “That’s why we spent a lot of time and used the skills of a lot of people to be clear about the interpretation,” he says. University of Saskatchewan geologist and study co-author Maria Mángano notes the team assessed some 80 specimens from several sections of sediment. “It is clear that we are looking at recurrent patterns, not just a weird-looking curiosity,” she adds. Co-author Luis Buatois, an ichnologist also at Saskatchewan, says the team was extremely careful in drawing conclusions about the data. “We started with multiple hypotheses and we tested them,” he says. “We also were quite careful on theoretical grounds, aiming to be consistent with what we know from the fossil record and molecular clocks.”
A discovery of this magnitude can often represent a major breakthrough, but occasionally it may be an overreach. “The good thing about science is that it’s inherently critical,” says Tanja Bosak, a geobiologist at Massachusetts Institute of Technology who was not involved in El Albani’s study. “Claims have to be constantly reevaluated and tested by more data and new experiments.” Because of this, Gingras urges caution when reporting major claims. “I think the biggest mistake that the media and the public make is the inference that if it’s published in a high-end journal, that the paper represents some fundamental truth,” he says. “More and more, we understand that’s simply not the case.”
Given the boldness of El Albani’s claim and the fact it is supported by various lines of evidence, Tarhan says it is still beneficial to communicate it to the public. “It is a very exciting find, and hopefully future fossil and geologic evidence will place it in a better context,” she says. Gingras agrees: “Science is ultimately furthered by putting interpretations out there for broader discussion, whether the concept is ultimately accepted or not,” he says.