Once photosynthesising cells present in the oceans had produced enough oxygen to form the ozone layer conditions on land were much more favourable to life than before. Without exposure to ultraviolet radiation cells were much more likely to survive being washed ashore. After a major extinction event 488 million years ago plants and fungi started to grow in out and around the water. Genetic mutation and evolution would see life adapt to the new environment and go on to thrive. Arthropods are believed to be the first animals to appear on land, around 450 million years ago. These early creatures would have been able to feed from the new plants that were growing on land, albeit around the water's edge.
440 million years ago there was another mass extinction that would kick start evolution. Extinctions are perhaps naively thought of as bad for life, being that most of it is killed off. While it is bad for the organisms alive at the time for obvious reasons, a mass extinction actually has a major benefit for evolution in the path to intelligent life. Every mass extinction to date has been followed by a surge in new species and evolution. The reason for this is that evolution can get into a rut relatively easily, without a mass extinction it is likely that life would stay the same with little adaptation. Extinctions force adaptations and so accelerate evolution.
The result of the extinction event at the end of the Ordovician period, around 440 million years ago, was fish evolving into land-dwelling tetrapods. As their fins evolved into limbs they adapted to breathing air and eventually would live their entire life on land, only returning to water to lay eggs. These animals were the first amphibians and are essentially the genesis of much of life as we know it. During this period plants would evolve seeds and would spread across land, sparking the full colonisation of land by life. In another 20 million years animals would have evolved the capability to lay amniotic eggs as well as having diverged into many distinct evolutionary paths. Insects, reptiles, fish and bacteria were all present at this point.
The next major event has become known as the Great Dying. The Permian-Triassic extinction event is the most severe in Earth's history. It is estimated that around 96% of marine life and 70% of land dwelling vertebrates would die out. It is also the only known mass extinction of insects. The causes for this are thought to be numerous, including environmental change and a catastrophic event such as increased volcanism or meteorite impacts.
The result of this extinction event was the Triassic period and the start of the age of the dinosaurs. Extending from 251 million years ago until around 199 million years ago the Triassic age is bookmarked by two mass extinctions. During this time the first dinosaurs evolved and it is believed the first mammals, flying vertebrates and flowing plants evolved as well.
The most common land vertebrate during the early Triassic was the Lystrosaurus. It was a small reptilian herbivore about the size of a pig. It had a horny beak and tusks and accounted for 95% of the population of organisms at the time. The Lystrosaurus is notable for being the only species to dominate the planet in such a great number. It is thought that by surviving the Permian-Triassic extinction that they became free of predators and could reproduce with little resistance.
Seeding plants, conifers and cycads would go on to dominate and flourish in the plant world. This would lead to greater insect numbers and would allow greater biodiversity among vertebrates. Towards the end of the Triassic early dinosaurs had evolved along with pterosaurs in the skies and icthyosaurs in the oceans.
As dinosaurs grew in numbers they would force mammalian ancestors to live nocturnally on a diet of insects. It is thought that living in this way would lead mammals to evolve fur and their higher metabolic rates required to function in the cooler night time temperatures.
The Triassic period came to a close 199 million years ago during the Triassic-Jurassic extinction event. During this time around half the species on Earth died out, leaving vast ecological niches. These niches were quickly filled and gave rise to the domination of dinosaurs. The next two periods of time, the Jurassic and Cretaceous, would be inhabited by some of the largest animals that have ever lived and would set in motion the events that would lead to the evolution of modern mammals.
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