The search for signs of Earth’s earliest forms of life isn’t quite like looking for dinosaur bones protruding out from desert outcrops. The oldest species on our planet were microscopic, nothing more than itty-bitty specks. Detecting them, and verifying their identity, is a complicated task that often relies not only hunting town tiny remains but also on chemistry and perceiving how those microorganisms modified their environments.
“Every time I have ever told anyone I look for fossils, I then have to follow up that I’m lot looking for fun things like bones or shells or claws or teeth,” says University of Kansas geologist Alison Olcott. But charismatic creatures like dinosaurs are only a small part of our planet’s story. “Even now, the majority of life on Earth is squishy and microscopic,” she says. The trick is following that fossil trail to its source.
Signals in the Sands of Time
Picking out the evidence of Earth’s earliest life is more than a needle-in-a-haystack problem. The entire planet is the metaphorical haystack, while the needles are no more than microscopic cells or faint chemical traces. Even when scientists can pin down possible candidates, it’s can be hard to know for sure when something is a signature of ancient life rather than a plain-old geologic phenomenon.
Consider the case of extremely old stromatolites, layered underwater mounds created by cyanobacteria. Bacteria have been building these mounds for billions of years and in some places, such as Utah’s Great Salt Lake, they continue to do so.
In 2016, a team of researchers from Australia proposed that they had found evidence of stromatolites being formed about 3.7 billion years ago, which would make them a remnant of some of the earliest known organisms. The Earth itself, after all, is only about 4.5 billion years old.
But other experts weren’t convinced. The fossils preserved only the stromatolite structure, not the organisms that created them, and some researchers argued that the rocks were formed by other geological processes. However, a study published just last year put forth stronger evidence, including geochemical analysis, that suggests some 3.5-billion-year-old stromatolites found at a different site in Australia do record and contain evidence of some of Earth’s oldest microbes.
The search for the signs of early life, what experts call biosignatures, often focuses on four different lines of evidence, says University of New South Wales astrobiologist Tara Djokic. The first kind of evidence is relatively large and can be seen with the naked eye, such as the stromatolites. Other traces are much harder to find, and that group includes chemical traces of fossils, degraded biological compounds, and fossils of microorganisms themselves.
Parsing these clues is no small task. “It is very difficult to fossilize something with no mineralized parts,” Olcott says. In addition, some non-biological processes and phenomena can mimic the shape and chemical signatures of microbial life.
Finding any one of the four markers is a good hint. But, Djokic says, “finding more than one of these together would of course strengthen the argument.” That’s because it’s not always easy to tell what’s a real sign of early life and what’s a trick from deep time. Sometimes folds in rock or other non-living features can look like stromatolites. What might look like a group of tiny cells could be some kind of sedimentary structure. Billions of years have passed since the time Earth’s first life originated. The rocks containing signs of early life, Djokic says, “have been deformed, and metamorphosed by heat, stress, and hydrothermal alteration and then subject to billions of years of weathering processes” such as erosion. And there aren’t many of them left exposed.
“Thanks to billions of years of the rock cycle, there are not so many rocks which are even suitable to search in for signs of early life,” Olcott says. Searching for early life requires understanding—and correcting for—the factors that may obscure the truth. “It’s like coming to a crime scene and having to piece together what has happened and who was involved,” Djokic says.
The Race for the Oldest
At present, the starting date for life on Earth is still rough. “There is no specific site that is considered [to be] 100 percent proof of the earliest signs of life on Earth,” Djokic says. One of the oldest, least controversial sites is the 3.4-billion-year-old Strelley Pool Formation in Australia, Olcott notes. Researchers have found stromatolites, microbial mats, chemical signatures indicative of life and more. Older sites hold potential but are controversial. A roughly 3.8-billion-year-old site in Greenland may contain even older traces of life, but this spot is more contentious, Djokic says, because the rocks there have been through the geological ringer and are more difficult to interpret.
The search, and ensuing debate, continues. Different research groups have their favored candidates for the oldest signs of life on Earth, with an informal race to find the oldest. But, Olcott notes, “I think a large part of the lack of consensus is the lack of an unambiguous sign of life.”
What researchers propose as the earliest signs of life hinge upon what we know of living and non-living phenomena on Earth, and how these can lead to similar results that can be challenging to tease apart. “A field site could contain carbonaceous microstructures that look like fossils but also chemical signals that are not consistent with life, or vice versa,” Olcott says, with additional threads of evidence needed to determine whether signs of life are present.
Researchers from a diversity of backgrounds keep going back to the slivers of Earth’s ancient rocks to keep searching. The hunt isn’t just for the fossils themselves, Djokic points out, but humanity’s origins. Determining the timing and nature of early life tells us about where we came from, and the details of life for most of Earth’s history. Such clues tell us what to look for as we search other planets for life.
“These studies have given us an idea of how to search other planets,” Olcott says, helping to refine what lines of evidence to look for and collect. Those plans are already underway, to be launched with the Mars 2020 Perseverance rover this year. “Decades of fighting over evidence and interpretations on Earth, have hopefully prepared us to mount a search for potential signs of life elsewhere,” Olcott says.