The question of when life began on earth is a developing story. But the most common answer is that the earth is about 4.5 billion years old and the oldest uncontroversial evidence of life flourished some 3.4 billion years ago. These ancient forms of early life were bacteria – specifically, cyanobacteria. The evidence in question for establishing the earliest life at 3.4 billion years comes from fossil stromatolites in Australia.
If you aren’t a stromatolite enthusiast, then picture something like a big head of cabbage that gets buried in sediments and turned into rock. When you break open the rock and see it, 3.4 billion years later, you can recognize it as the rounded shape of a bioherm composed of thin layers that stack up one-on-another.
The original bioherm was composed of cyanobacteria (blue-green algae) that trapped sediment on the outer layers of the mound, thus giving it the layered, cabbage-like look. So, stromatolites are considered one of the world’s oldest life forms, but they still exist today. Shark Bay, Australia, is famous for its present-day stromatolites.
Fossil or rock
No scientific investigations are without controversy. So, this raises the question of how do you tell a 3.4 billion year old layered fossil from a layered rock? Stramatolites don’t actually preserve the algae cells for observation. Instead, they record the ability of the live cyanobacteria to trap sediment in layer after layer within the bioherm. But, there are other purely geological processes that can produce layering in a rock.
Investigations into early life must rely on a matrix of evidence, called biosignatures, since this increases the certainty that signatures in the rock are biological, not geological, in origin. Four major clues to life are:
Patterns in the rocks seen with the naked eye
Chemical traces of fossils
Chemical traces of biological compounds
Fossilized remains of microorganisms
The more boxes ticked on this list, then the more certainty that the rock sample is actually a fossil.
Old rocks
In the search for ancient life, complications arise as the age of a rock formation increases. So, time works against those researchers investigating the origins of life. The earth is a dynamic planet due to plate tectonics. The plates are constantly in motion. Igneous activity creates new rocks, and older rocks are buried deep within the earth where heat and pressure metamorphize them. Metamorphism can transform the rock, thus destroying any trace of ancient cyanobacteria.
As time passes, the chances decrease that well-preserved fossil evidence will remain. So after 3.4 to 4 billion years, there is very little fossil evidence left of early life. However, sites in Greenland, south Africa, and Australia will certainly yield more clues as
ArcheanWeb:
The Seeds of Life (Source: ArcheanWeb) – https://archeanweb.com/2019/12/01/the-seeds-of-life/ Also:
The Kaapvaal Craton and early life on planet earth (Source: ArcheanWeb) – https://archeanweb.com/2020/04/15/the-kaapvaal-craton-and-early-life-on-planet-earth/ Also:
Sources:
Why It’s So Difficult to Find Earth’s Earliest Life? (By: Riley Black; Smithsonian Magazine) – https://www.smithsonianmag.com/science-nature/earth-earliest-life-fossils-stromatolites-180974442/ Also:
Feature Image: Stromatolites in Sharkbay (By: Paul Harrison) (Modified) – https://commons.wikimedia.org/wiki/File:Stromatolites_in_Sharkbay.jpg – This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license. – https://creativecommons.org/licenses/by-sa/3.0/deed.en
