Stromatolites, layered structures created by photosynthetic bacteria, are the oldest known fossils at 2.74 billion years old. They exist in isolated ecosystems and are considered “living fossils.” Cyanobacteria played a significant role in changing the Earth’s atmosphere by converting carbon dioxide into oxygen. The oldest animal fossils date back to 600 million years ago, appearing after the largest ice age in history.
The oldest known fossil is a stromatolite (meaning “mattress rock” in Greek) dating back 2.74 billion years. A stromatolite is a layered accretionary structure created when bacteria, especially photosynthetic cyanobacteria, trap layers of sediment between the biofilms they create.
Stromalites come in a variety of different shapes and sizes, including cones, columns, and branching types. Stromatolites were long thought to not exist in the present (too many grazing organisms), until they were discovered in isolated, highly saline ecosystems such as Shark Bay and Lake Thetis in Australia and Cuatro Cienegas in Mexico. Like horseshoe crabs, due to their age, stromatolites are considered “living fossils.”
The oldest known fossil of stromatolites is nearly five times older than the oldest fossil of a multicellular organism, which dates back to about 600 million years ago. There are a few even older stromalite-like formations dating back more than 2.74 billion years ago that are also candidates for the oldest fossil, but none are confirmed by the scientific community.
Stromatolites are signs of the organisms that may have played a more significant role in changing the Earth than any other organism: cyanobacteria. These photosynthetic bacteria converted carbon dioxide into oxygen on a massive scale, changing the composition of the atmosphere and causing a so-called “oxygen catastrophe” that would have killed many of the other organisms alive at the time, for which the oxygen was poisonous.
There are non-biological stromatolite-like structures that can be confused with the oldest known fossil, but mean nothing more than the precipitation of tiny crystals. These have not been observed today, but it is thought that conditions in past oceans may have been ripe for their creation.
The oldest known animal fossils do not appear until the Middle Ediacaran Period, about 600 million years ago. These include a variety of simple sponges, cnidarians (such as modern hydras), and even simple bilaterians (organisms with bilateral symmetry) that look like tiny balloons. These animals emerged shortly after the largest ice age in the history of the planet, which caused the formation of sea-level glaciers on the equator.
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