The search for the origins of life on Earth is a pretty interesting topic, at least if you’re a biologist. Part of the problem in looking for the earliest signs of life is that, for the most part, the earliest life forms were like the vast majority of life forms that inhabit this planet today – microscopic and squishy, which doesn’t lead to good fossils. But the study goes on, and has pushed the timeline of the earliest life forms back to as much as 3.8 billion years ago; that’s 83% of the Earth’s 4.5 billion year lifespan. Excerpt:
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.
It’s interesting to think about what the Earth would have been like when these earliest organisms were around. The planet then was uninhabitable by our standards: Almost no oxygen in a toxic atmosphere, no plants or animals, the ocean nearly sterile, the land completely so. Earth was a pretty hostile place for quite a while – at least to us.
But not to these first organisms.
What’s really cool, though, is that some of the candidates for the earliest life forms have adapted to changing conditions and are still around today: We see evidence of them as stromatolites, rock-like formations produced by cyanobacteria.
Biologists and paleontologists keep pushing back this boundary for the first signs of life on our world, and the knowledge gained thus may help us find signs of life in other places; Mars, for example, that was once a warmer, moister place that may have been friendlier to life.
I was educated as a biologist and, while I haven’t worked in the field in decades, I do try to stay current. My own assessment, based on the diversity of life here on our planet and the variety of environments where life flourishes, is that life probably arises anywhere where conditions allow it – including places other than Earth. There’s no evidence of such life yet, of course.
But it’s the “yet” that makes this interesting.