When Did Life Colonize the Land?
The colonization of land Terrestrial environments were probably first colonized by crusts, or mats, of microbes at the waters edge; the microbes were most likely Cyanobacteria, though it is not known for sure because the fossil evidence is chemical – localized areas enriched in organic content. Terrestrial life persisted in this form until the evolution of the group that includes the vascular plants, probably in the middle Ordovician. Vascular plants originated in the Ordovician and by the end of the Devonian the land was covered in plants in a way that architecturally (though not taxonomically) resembles the modern world. In the Devonian there is also the earliest evidence, from body fossils, of terrestrial animals. It seems that animals took some time to evolve adaptations for herbivory because it was well into the Carboniferous before definite herbivores appeared; these were arthropods. In the Carboniferous there were also gigantic predatory insects, including dragonflies with wing-
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 po
