The first attempts of life colonizing the land were microbial mats, large flat colonies of photosynthetic microbes, fossilized remnants of which have been dated to 2.6 billion and 2.7 billion years ago. For billions of years, microbes were the only forms of life colonizing the land (and the only life in general). These microbes lived mostly on the coasts of the oceans and streams, and would appear as nothing more than a green slime. It would take many more millions of years for life to colonize the land in earnest.
The first possible tracks on land are dated 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 these tracks are thought to have been made by early arthropods, and some of them are quite large for the time — as much as 10 cm (4 in) wide. These tracks may have been left by sea scorpions traveling from one tide pool to another.
The first widespread terrestrial fossils didn’t emerge until about 425 million years ago, during the Silurian period. The first life to colonize the land 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 land fungi, which left fragmentary fossils. These plants started off very short, just a couple of inches tall, but left large “forests” of fossils. They did not yet have differentiated stems or leaves.
When the land started to build up a nice layer of soil, more plants could grow, causing a positive feedback cycle of land colonization. By the end of the Silurian period, a simple terrestrial ecosystem had emerged, including millipede herbivores, centipede and arachnid carnivores, worm detritivores, and fungal decomposers. Nematodes were also likely present but have not left fossils. It took several tens of millions of years for life to colonize the land further.