Evolution is a process by which organisms change over time through the work of natural selection. Through evolution, lineages of organisms branch out from common ancestors (such as a family tree) and become distinct from each other as they acquire different biological and behavioural traits. From an evolutionary point of view, extant great apes such as chimpanzees, gorillas and orangutans, are our distant cousins, with whom we share an apelike ancestor a long time ago. Our closest evolutionary cousin is the chimpanzee, with whom we share more than 98% of our DNA. Scientific evidence suggests that the most recent common ancestor of humans and chimpanzees existed between 6 and 7 million years ago.
Since our split from the chimpanzee lineage, the human evolutionary branch underwent many great changes, including bipedalism (habitually walking upright on two feet), an increase in brain size, and the ability to make and use tools. Much of our evolution took place in the African continent; the dispersal of humans across the globe was comparatively recent. Based on skeletal morphology, scientists (palaeoanthropologists) have classified early humans into many different genera (plural of genus) and species. We collectively refer to these different human groups as “hominins”. A major challenge in the study of human evolution is that early human fossils are exceedingly rare, especially when we go back further in time. Below, we will introduce three hominin genera that are important for understanding the evolution of our species: Australopithecus, Paranthropus, and Homo.
Australopithecus is a hominin genus that comprises a number of different species (such as A. anamemsis, A. afarensis, A. africanus, A. garhi, and A. sediba) that existed in Africa between about 4 and 2 million years ago. These species share a mixture of ape and human features: they typically have flat faces, protruding jaws, large back teeth, small braincases, curved finger bones adapted for tree climbing, and lower limb morphologies indicative of regular upright walking. More direct evidence of bipedalism among Australopithecines includes footprints found at the site of Laetoli, Tanzania (dated to about 3.7 million years ago) that have been attributed to A. afarensis. Recent studies have reported stone artefacts and cut-marked bones in East Africa dated to 3.4–3.3 million years ago, implying the later Australopithecus species (A. garhi) may have been capable of making and using stone tools (although the timing and validity of this evidence is still debated among scientists). Australopithecines are also likely the common ancestor of the two genera described below: Paranthropus and Homo.
The Paranthropus genus existed between about 2.7 and 1.0 million years ago, and include species of P. boisei, P. aethiopicus and P. robustus. In contrast to Australopithecines which were comparatively more gracile, members of the Paranthropus group exhibit “robust” features: very large back teeth with thick enamel, large and wide cheek bones (zygomatic arches) that allowed greater opening for the passage of large jaw muscles, and a sagittal crest (bony ridge on the top of the skull as seen in gorillas) that anchors the jaw muscles. These skull adaptations allowed Paranthropus hominins to chew and grind down tough, fibrous food resources. The flaring cheekbone also gave them a characteristic wide, dish-shaped face.
The Homo genus evolved from Australopithecus (independently of Paranthropus) about 2.8 million years ago, and includes Homo sapiens and other extinct species, such as H. habilis, H. erectus and H. neanderthalensis. Major biological and behavioural changes occurred during the evolution of the Homo genus. The earliest, unequivocal evidence of systematic stone artefact production (dated to 2.6 million years ago in Ethiopia) has been attributed to H. habilis. Between 2 million and 800, 000 years ago, H. erectus emerged in Africa and dispersed to Europe and Asia (including modern-day China and Indonesia), leaving behind characteristic stone artefacts, such as handaxes and cleavers, as well as archaeological traces of fire use. With the appearance of H. erectus, hominin brain size began to increase rapidly. H. neanderthalensis, which appeared about 400, 000 years ago in Europe and then southwest and central Asia, exhibited advanced stone working skills and the ability to control fire. Some archaeological evidence also indicates that Neanderthals possibly buried their dead and made symbolic or ornamental items. The oldest fossils of our species, H. sapiens, appeared in East Africa about 200, 000 years ago. Modern humans (Homo sapiens) have very large brains (relative to body mass) housed in globular skulls with flat, near-vertical foreheads and receded jaws. The earliest convincing evidence of modern human behaviour, including symbolic objects and complex tools, emerged about 100, 000 years ago in southern Africa.
Homo sapiens dispersed out of Africa and arrived in Australia by about 50, 000 to 60, 000 years ago, and in Europe by about 45, 000 years ago. Coinciding with the arrival of our species in these regions, other existing Homo species, such as H. neanderthalensis, quickly declined and disappeared across Eurasia. Recent genetic studies demonstrate that non-African populations of people living today have between 1 and 4% Neanderthal DNA in their genome, suggesting genetic exchange between Neanderthals and the ancestors of all non-African populations soon after the first group of H. sapiens exited Africa.
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