Blackwell Publishing

Fossils and the history of life - When did life colonize the land?


The colonization of land

Terrestrial environments were probably first colonized by crusts, or mats, of microbes at the water’s 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-spans of over 60 cm. Vertebrates show a similar pattern to arthropods; the first terrestrial tetrapod fossils date to the late Devonian, and they were predators (they ate fish and insects); the first herbivorous tetrapods did not evolve until the end of the Carboniferous and the early Permian.

One of the important evolutionary steps enabling vertebrate life on land was the origin of the amniotic egg; reptiles, birds, and mammals are amniotes, and members of these groups, unlike most amphibians, do not return to water for the early stages of the life cycle. The origin of egg types cannot be traced directly in the fossil record; however, at the origin of the reptiles there were changes in skeletal morphology as well as egg type and there is good evidence for the former.

The reptiles probably evolved in the Carboniferous; an early definite reptile, for example, is the small lizard-like creature called Hylonomus that comes from deposits in Nova Scotia. After the origin of the reptiles the two main events in vertebrate evolution were the origin of flight in birds and the origin of the mammals. The late Jurassic fossil Archeopteryx (pictured opposite) is a good intermediate stage in the evolution of flight.

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