Tracing Human Evolution: From Early Primates to Homo Sapiens
II.2.2. Human Evolution
Geologists divide Earth’s history into eras, periods, and epochs. Life began about three or four billion years ago. Around 600 million years ago, the first animals large and hard enough to leave abundant fossils appeared. The phylum of chordates, the subphylum of vertebrates, and the superclass of tetrapods were present in the Devonian Period (300-400 million years ago). Mammals appeared during the Mesozoic Era (200-150 million years ago). Towards the late Mesozoic or early Cenozoic (70-60 million years ago), during the Oligocene (40-25 million years ago), anthropoids began to multiply. In the following epoch, the Miocene, hominoids spread widely. Unmistakable early hominids appeared in the Pliocene. The genus Homo emerged in the transition from the Pliocene to the Pleistocene, and our own species, Homo sapiens sapiens, in the late Pleistocene. If the evolutionary clock were scaled to a year, humans would appear around 8 pm on New Year’s Eve.
Hominoids originated in Africa at least 23 million years ago and were varied and numerous across Africa, Asia, and Europe. A rapid decline began 10 million years ago, and by 7 million years ago, most species had disappeared. Currently, only Homo sapiens sapiens is abundant and widespread. Two reasons for this decline are:
- Loss of habitat due to ecological changes.
- Competition with other Old World monkeys: the cercopithecoids.
Climate change and a reduction of atmospheric carbon dioxide over 8 million years may have favored the expansion of the savannah.
Until recently, Ardipithecus ramidus (around 4 million years ago) was considered the first hominid. It was a forest-dwelling species with a diet similar to modern chimpanzees. Evidence from the base of the skull suggested bipedalism. However, the discovery of a nearly complete primate skull in Chad, estimated to be 7 million years old, has challenged existing classifications. This skull, named Sahelanthropus tchadensis (or “Toumai”), suggests the differentiation between humans and chimpanzees began earlier. Analysis of the remains indicates:
- Skull size larger than a chimpanzee.
- Cranial capacity between 320 and 380 cc.
- Oval-shaped foramen magnum (where the spine connects to the brain), suggesting upright walking.
- Thicker tooth enamel than chimpanzees, indicating a diet with less fruit.
Around 4 million years ago, Australopithecus anamensis existed. These hominids walked upright, and their dentition shows a shift in their ecological niche, incorporating plant products from drier environments. In the following million years, fossils are assigned to Australopithecus afarensis. Their intelligence was close to that of modern chimpanzees. They likely inhabited the dry forests of eastern Africa, evolving into bipedal forms with a diet including hard vegetables. They spread westward, and we have a more complete fossil record for this group, including the famous “Lucy” skeleton. Lucy was a female, 105 cm tall and approximately 27 kg (males could reach 135 cm and 45 kg). Cranial capacity and ability to transmit information or build complex tools are uncertain. Fossilized footprints of three individuals at Laetoli provide further evidence. The fossil record suggests our species originated in eastern Africa, particularly in the Rift Valley.
Between 3 and 2 million years ago, various hominid forms existed. Australopithecus africanus lived in South Africa, with fossils found in caves, likely left by predators. Around 2.5 million years ago, the group divided into two main types:
A) Paranthropus
Australopithecus afarensis appears to be their most direct ancestor. Australopithecus and Paranthropus exhibited sexual dimorphism and may have lived in communities with several male relatives and harems of females. A climate crisis around 2.8 million years ago may have contributed to the disappearance of Australopithecus africanus (forest-adapted) and led to the emergence of species adapted to open environments: the first representatives of the genera Homo and Paranthropus. These two genera originated around the same time and coexisted for about 1.5 million years.
B) Hominids
Hominids developed larger brains and made stone tools. The earliest stone tools, found in Ethiopia’s Afar region, are estimated to be 2.3 million years old. Initially, they were not very different from Australopithecus.