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.
The Appearance of Mammals
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.
Primate Species Divergence
From the evolution of the first true life forms—prokaryotic unicellular organisms which appeared around 3,900 million years ago—humanity has spent an extremely short amount of time (relatively speaking) evolving from mammals into apes, from apes into humans.
It was just fifteen million years ago that our ape ancestors split into two species groups—the gibbons (lesser apes) and Hominidae, the great apes. This latter group would eventually split further into the orangutans, chimpanzees, gorillas, and humans.
However, this didn’t occur for several million years. In fact, it took a further two million years for the speciation of orangutans to occur. At this time, a species called Pierolapithecus catalaunicus was in residence. Believed to be a common ancestor of the humans and the great apes, this species was adapted for tree-climbing in the same way that humans are, with a wide, flat ribcage, flat shoulder blades lying parallel with the back, a stiff lower spine, and flexible wrists.
Over the next several million years, further speciation occurred, with Hominini (the tribe of which humans and chimpanzees are the only living members) splitting from the gorillas around ten million years ago. The last common ancestor of humans, chimpanzees, and gorillas is thought to be Nakalipithecus nakayamai, a great ape which lived in what is today known as Kenya.
Nakalipithecus lived on a diet composed chiefly of hard objects such as nuts and seeds, but little else is known about this ancient species of great ape—so far, the only fossils which have been excavated are a jawbone and eleven teeth. The existence of Nakalipithecus is important because it provides more evidence of the divergence of Hominina having occurred around this time. In addition, it supports the idea that the closest relatives of the Homininae were resident in Africa, contrary to a competing theory that Homininae was actually an Asian lineage which recolonized Africa after great apes had become extinct there.
Seven million years ago, Hominina diverged from the chimpanzees. (The subtribe called Hominina once contained a much larger number of species—including australopithecines, the Neanderthals, and other species—but for the last twelve thousand years, Homo sapiens sapiens has been the sole surviving member.) Chimpanzees are therefore our closest evolutionary ancestor. This is confirmed by DNA evidence, with human DNA and chimpanzee DNA being 98.4% identical.
The latest known common ancestor of both Hominina and chimpanzees is Sahelanthropus tchadensis, a species which is thought by some paleontologists to have been at least partially bipedal.
The Appearance of Early Human Ancestors
Somewhere between seven and four million years ago, Orrorin tugenensis—known as the “Millienium Man"—was in existence. This species is the earliest known ancestor to have lived after the speciation of chimpanzees, therefore making it the oldest known hominin ancestor from which modern humans are descended.
More significantly, it is the earliest species of hominid which is known to have been bipedal. Orrorin lived in a dry forest environment, interestingly enough—this discovery was intriguing because it had previously been thought that bipedalism evolved in open country, rather than in forested land.
Species such as Orrorin tugenensis are our closest non-human ancestors—close to being human, but not quite. Within the next few million years, several more species of these ape-humans would appear, eventually to be followed by the first of the Homo species.
They’re all Human
When discussing human evolution, the term ‘human’ doesn’t only refer to Homo sapiens. It also refers to other hominins, such as the australopithecines, as well as several now-extinct species of Homo. While we don’t necessarily think of those early species as human, it’s important to realize that in evolutionary terms, they definitely are.
Following the appearance of Orrorin tugenensis six million years ago, Ardipithecus evolved over the next million years. This early hominin shared a number of traits with the great apes, but its similarity to Australopithecus firmly places it in the ‘human’ category. Ardipithecus was around the size of a chimpanzee, and may have been at least partially bipedal.
Australopithecus was the next genus to appear. Three million years ago these early humans were widespread in northern and eastern Africa. By this time living in the African savannahs, they had already begun to evolve to the point where it would soon become fully bipedal, although they were unlikely to have been any more intelligent than modern apes (the fossil record turned on its head the previous theory that bipedalism had occurred after larger brains had evolved). During the same period in which they became fully bipedal, australopithecines also began to lose their body hair.
The “Human Humans"
During the period in which australopithecines was evolving in these two crucial respects, the Homo genus appeared. Homo diverged from the australopithecines around two million years ago, and interestingly enough, tools began to appear in the fossil record around this time. The appearance of these stone tools signals the beginning of the Paleolithic era. This Stone Age covers almost all of human history, but is predated by nearly 3,900 million years of evolution.
Several species of Homo, all but one of which are now extinct, evolved in various locations.
Homo habilis, the first species of Homo, lived between 2.4 to 1.4 million years ago, evolving in south and east Africa. Homo habilis is the first human species to be associated with the use of tools.
Homo rudolfensis and Homo georgicus are two species which lived between 1.9 and 1.6 million years ago, about which is known very little. It has been suggested that these may be species of habilis.
Homo erectus appeared between 1.5 and 1 million years ago, and from its origins in Africa migrated to Europe and Asia. During this time, Homo habilis living in some locations began to evolve larger brains, and to make and use more sophisticated stone tools. On the basis of these and other differences between the two, erectus was determined to be a separate species. Its locking knees and the placement of its foramen magnum (the location where the spine meets the skill) allowed Homo erectus to walk fully upright. Homo erectus may have used fire to cook meat.
Homo heidelbergensis lived between 800,000 to 300,000 years ago. Also known as Heidelbuerg Man, this species is believed to be an ancestor of Homo neanderthalensis.
Homo neanderthalensis appeared around 250,000 and became extinct fairly recently, somewhere between 25,000 and 30,000 years ago. While there has been considerable debate over the evolutionary relationship between neanderthalensis and sapiens, new mitochondrial DNA evidence indicates the two were a separate species with a common ancestor which lived around 600,000 years ago.
Homo floresiensis lived between 100,000 and 12,000 years ago, and has been nicknamed “hobbit" due to its small size. Floresiensis is particularly interesting because it represents an example of a Homo species taking a distinct and separate evolutionary path. The species has several traits in common with modern humans, but the most complete floresiensis fossil is just one meter high. However, it has also been theorized that floresiensis was not a separate Homo species, but in fact a sapiens which had suffered from dwarfism.
The first Homo sapiens appeared on the scene somewhere between 400,000 and 250,000 years ago, evolving from Homo erectus. Eventually, this new species would replace all other existing human species, and Homo sapiens is the only species of human still in existence. There are two theories to account for how this happened—the first says that Homo sapiens first evolved in Africa, then migrated to other locations and replaced populations of other human species, while the other argues that modern humans evolved from existing Homo species in multiple locations.
So far, there is not a sufficient amount of fossil evidence to prove or disprove either theory; however the “Out of Africa" model, in which sapiens evolved in Africa and migrated to replace human populations in other locations, is favored.