Back before there were land animals, we know that most life on Earth lived in the seas. Fish may have evolved from corals, which, like most things, probably came from microorganisms. In the very beginning, life probably started with single-celled organisms like bacteria.
Unfortunately, most early life forms are too small to be preserved in the fossil record. And too old — there are only really a few places on the planet with rocks ancient enough to carry that kind of fossil evidence.
As a result, some of the earliest traces of life we have detected on our planet are so faint that the fossil evidence that remains only reveals the movements of these microscopic organisms.
“It is a real detective story,” says William Schopf, a paleobiologist with the University of California, Los Angeles.
Rare, Ancient Rocks
Life is believed to date back several billion years at least — and in that time, the geological cycle of our planet has changed a lot, even for rocks. Rocks and the fossils they carry are often buried in sediment. Over time, tectonic movement pushes these rocks back up to the surface, where tides, winds and other processes erode them away. As a result, most of the fossils that were once preserved of the earliest life would have disappeared due to erosion over time. The older the rock is, the greater the chance the fossil no longer exists, Schopf says.
Furthermore, geological cycles often pressure cook rocks, wiping out the fossils preserved inside in the process. There are only a few places on Earth where rocks older than 3.5 billion years can be found that still carry fossil evidence. Parts of western Australia, Greenland and South Africa have ancient rocks like these, exposed.
Schopf and his colleagues found worm-like patterns preserved in the Apex Chert — a rock formation in Australia dating back to about 3.465 billion years ago. They first discovered these supposed organisms in 1993, but the idea that these patterns represented ancient life was controversial. In 2018, Schopf published a follow-up study in Proceedings of the National Academy of Sciences that used secondary ion mass spectroscopy technology to reveal the ratio of carbon-12 and carbon-13 isotopes. This ratio revealed that the shapes preserved in the chert were characteristic of biological matter.
During this time, the planet did not yet have oxygen, Schopf says. Ancient iron doesn’t start to show traces of rust — a telltale sign of oxygen in the atmosphere — until about 3 billion years ago. It becomes more abundant about 2.8 billion years ago and common roughly 2.3 billion to 2.5 billion years ago.
“I don’t think oxygen-producing organisms developed until about 3 billion years ago,” Schopf says.
As a result, microorganisms older than 3 billion years may have used simplified photosynthesis that produced methane rather than oxygen.
Still, the microorganisms Schopf discovered in the chert in Western Australia may not be the oldest fossil evidence of life on the planet. That honor goes to stromatolites, which are sometimes mushroom-looking formations. Some of these may date back hundreds of millions of years earlier than the Apex Chert fossils Schopf discovered.
Stromatolites aren’t exactly fossilized remains of ancient life forms themselves. Researchers believe that cyanobacteria often erroneously called blue-green algae (it’s not actually algae), would have acted like the organisms do today, spreading across the surface of water to absorb sunlight. Researchers believe the strange shapes of stromatolites were formed by cyanobacteria moving above the surface of sediment.
“It looks just like a rug, but it’s only 1-2 millimeters thick,” Schopf says.
Sediment would sometimes fall on top of these cyanobacteria mats, which would then push up above the sediment. As these processes repeated over and over again, it formed these strange hummocks or mushroom-like shapes that remain on the landscape today in places like Shark Bay in Australia.
Some researchers believe stromatolites dating back to 3.7 billion years ago discovered in Greenland represent traces of life, though this research is disputed by others who believe that geological processes may have caused these strange shapes to emerge in this case.
Due to its very nature, most of the fossils that remain of ancient life today are likely indirect, or traces of movement that act similar to ancient footprints. The trouble is we don’t really know what the foot looked like in the first place. Japanese researchers recently published a study in Nature stating that rocks in Labrador, Canada contain microfossils dating back 3.95 billion years ago, though these claims are also disputed by researchers, stating that there is no way to be certain.
Even Schopf is skeptical about some of these supposed ancient traces of life, which he believes may be the results of ripple marks made by tides or the wind. But in Greenland, climate change is causing the glacial ice cover over many ancient rocks to recede. He says that the exposure of more ancient rocks may yet reveal more evidence of ancient life.
“There’s a lot more to learn,” he says. “It’s not just the organisms. It’s the history of the whole planet."