Microbial mats were the first life forms to colonize Earth, followed by arthropod trace fossils during the Cambrian period. Mosses and lichens were the first widespread terrestrial fossils during the Silurian period, followed by simple vascular plants and fungi. A simple terrestrial ecosystem emerged by the end of the Silurian period.
The first attempts at life to colonize the earth were microbial mats, large flat colonies of photosynthetic microbes, the fossil remains of which have been dated to 2.6 billion and 2.7 billion years ago. For billions of years, microbes have been the only life forms to colonize the earth (and the only life in general). These microbes lived mostly on the coasts of oceans and streams and would appear to be nothing more than green slime. It would take many more millions of years for life to really colonize the earth.
The first possible traces on land date back to 530 million years ago, during the Cambrian period. These trace fossils, known as Protichnites and Climactichnites, appear as a series of faint bumps and long grooves resembling motorcycle tracks, respectively. Both of these tracks are thought to have been made by early arthropods and some of them are quite large for the time, up to 10 cm (4 in) wide. These tracks may have been left by sea scorpions traveling from one tidal pool to another.
The first widespread terrestrial fossils did not emerge until about 425 million years ago, during the Silurian period. The first life forms to colonize the earth were mosses and lichens. These were followed by simple vascular plants, such as Cooksonia (mostly from the Northern Hemisphere) and Baragwanathia (from Australia), which were quickly followed by terrestrial fungi, which left fragmentary fossils. These plants were born very short, only a couple of inches tall, but they left large “forests” of fossils. They did not yet have differentiated stems or leaves.
When the land has started to form a nice layer of soil, more plants may grow, causing a positive feedback loop of land colonization. By the end of the Silurian period, a simple terrestrial ecosystem had emerged, comprising herbivorous centipedes, carnivorous centipedes and arachnids, worm detrituses, and fungal decomposers. Nematodes were probably also present, but left no fossils. It took several tens of millions of years for life to further colonize the earth.
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