Human evolution—also known as anthropogenesis—spans a much shorter period of time than evolution in general. The first life appeared on earth around 3,900 million years ago, but the genus Homo, of which modern humans are the only surviving member, has been in existence for only two and a half million.
It wasn’t until around 315 million years ago that the first fish developed recognizable limbs and was capable of actually coming onto land. Acanthostega most likely evolved in shallow-water or swampy habitats, however, and was poorly adapted for life on land.
Fifteen million years later, the amniotic egg had evolved—enabling the first lizards to reproduce on land, rather than retreating to the water to reproduce and lay eggs. Several million years later, around 265 million years ago, the existing reptiles began to diverge into two groups—Diapsida, which became the ancestor of modern reptiles, and Synapsida, which evolved into reptiles with mammalian characteristics. These mammalian reptiles, known as Pelycosaurs, eventually evolved into Therapsida.
One hundred and twenty-five million years ago, Eomaia scansoria appeared—the descendant of all modern placental mammals.
In the intervening 140 million years, several developments had taken place. The Therapsids had evolved more obviously mammalian characteristics, with a group of animals called the cynodonts eventually evolving into a species which maintained a constant body temperature and had milk glands capable of feeding offspring. These early mammals also evolved the neocortex, the newest part of the cerebral cortex, and which is present only in mammals.
Following the appearance of Eomaia scansoria came a period of around sixty million years in which mammals continued to evolve more mammalian features, until around 65 million years ago.
At this time, a group of insect-eating, nocturnal mammals began to diverge into three distinct groups. These mammals, called the Euarchonta, began a process of speciation which eventually led to the formation of the flying lemurs, the treeshrews, and the primates.
The primates included an order called the Plesiadapiformes. While these animals retained characteristics such as clawed feet and side-facing eyes, they were also able to move more quickly on land than in the treetops. These primates began to spend more time on the ground and in the lower branches of trees, and it is thought that within the Plesiadapiforme order is the species which is the ancestor of all modern primates.
Over the next forty million years, several instances of speciation occurred within the primates. The primate group itself split into two—Strepsirrhini and Haplorrhini, with the latter group including tarsiers, monkeys, and apes.
Thirty million years ago, Haplorrhini split again, into Platyrrhini and Catarrhini, with the latter remaining in Africa, while the Platyrrhini migrated to South America.
Five million years later, the Catarrhini split again, into the Old World monkeys, and the apes. Ten million years after that (fifteen million years ago) the apes split into the gibbons (lesser apes) and Hominidae. This latter group would eventually become the great apes—orangutans, chimpanzees, gorillas, and humans.