By Maina Kiarie
Human evolution, or anthropogenesis, is the origin and evolution of Homo sapiens as a distinct species from other hominids, great apes and placental mammals. The term human in the context of human evolution refers to the genus Homo, but studies of human evolution usually include other hominids, such as the Australopithecines, from which the genus Homo diverged by about 2.3 to 2.4 million years ago in Africa. Scientists have estimated that humans branched off from their common ancestor with chimpanzees about 5-7 million years ago. Several species and subspecies of Homo evolved and are now extinct.
Divergence of the human lineage
Species close to the last common ancestor of gorillas, chimpanzees and humans may be represented by Nakalipithecus fossils found in Kenya and Ouranopithecus found in Greece. Molecular evidence suggests that between 8 and 4 million years ago, first the gorillas, and then the chimpanzees split off from the line leading to the humans; human DNA is approximately 98.4% identical to that of chimpanzees.
Ma (Megaannum)-is a unit of time equal to one million. It is commonly used in scientific disciplines such as geology, palaeontology, and celestial mechanics to signify very long time periods into the past or future e.g. 65 Ma=65 million years ago.
Ka (Kiloannum)-is a unit of time equal to one thousand
Homo habilis lived from about 2.4 to 1.4 Ma. Homo habilis, the first species of the genus Homo, evolved in South and East Africa in the late Pliocene or early Pleistocene, 2.5-2 Ma, when it diverged from the Australopithecines. The fossils of Homo habilis were discovered by Louis Leakey and nicknamed it ‘handy man’ due to its association with stone tools.
a) Smaller molars and larger brains than the Australopithecines
b) Made tools from stone and perhaps animal bones
Homo rudolfensis and Homo georgicus
Homo rudolfensis refers to a single incomplete skull from Kenya.
Homo georgicus, from Georgia, may be an intermediate between Homo habilis and Homo erectus, or a sub-species of Homo erectus.
Homo ergaster and Homo erectus
Homo ergaster and Homo erectus were discovered by Dutch physician Eugene Dubois in 1891 on the Indonesian island of Java. Homo erectus lived from 1.8Ma to about 70,000 years ago.
In the early Pleistocene, 1.5-1 Ma, in Africa, Asia, and Europe, some populations of the Homo habilis are thought to have evolved larger brains and made more elaborate stone tools. They were classified as a new species, Homo erectus.
Homo erectus was the first human ancestor to walk truly upright. This was made possible by the evolution of locking knees and a different location of the foramen magnum (the hole in the skull where the spine enters). They may have used fire to cook their meat.
Homo cepranensis and Homo antecessor
These two species lived in the intermediate period between Homo erectus and Homo heidelbergensis.
a) Homo antecessor is known from fossils from Spain and England that are dated 1.2 Ma-500ka.
b) Homo cepranensis refers to a single skull cap from Italy estimated to be about 800,000 years old.
Homo heidelbergensis lived from about 800,000 to about 300,000 years ago
Homo rhodesiensis, and the Gawis cranium
Homo rhodesiensis, estimated to be 300,000-125,000 years old.
In February 2006 a fossil, the Gawis cranium, was found which might possibly be a species intermediate between Homo erectus and Homo sapiens or one of many evolutionary dead ends. The skull from Gawis, Ethiopia, is believed to be 500,000 years old.
Homo neanderthalensis lived from 400,000 years ago
Homo sapiens (the adjective sapiens is Latin for “wise” or “intelligent”) have lived from about 250,000 years ago and the second interglacial period in the Middle Pleistocene. Around 250,000 years ago, the trend in skull expansion and the elaboration of stone tool technologies developed, providing evidence of a transition from Homo erectus to Homo sapiens.
The direct evidence suggests there was a migration of Homo erectus out of Africa, then a further speciation (evolutionary process by which new biological species arise) of Homo sapiens from Homo erectus in Africa. A subsequent migration within and out of Africa eventually replaced the earlier dispersed Homo erectus.
Homo sapiens idaltu, from Ethiopia, is a possible extinct sub-species who lived from about 160,000 years ago.
Homo floresiensis, lived from approximately 100,000 to 12,000 years ago. It is nicknamed ‘hobbit’ owing to its small size, possibly as a result of insular dwarfism. Homo floresiensis share a common ancestor with modern humans, but split from the modern human lineage and followed a distinct evolutionary path.
In 2008, archaeologists working at the site of Denisova Cave in the Altai Mountains of Siberia uncovered a small bone fragment from the fifth finger of a juvenile hominin dubbed the “X-woman” or the Denisova hominin. Artefacts, including a bracelet excavated in the cave at the same level were carbon dated to around 40,000 years ago.
Analysis of the DNA extracted from this fossil indicated that modern humans, Neanderthals, and the Denisova hominin last shared a common ancestor around 1 million years ago. Modern humans are known to have overlapped with Neanderthals in Europe for more than 10,000 years, and the discovery raises the possibility that Neanderthals, modern humans and the Denisovan hominin may have co-existed together.