Evolution of Humans
Timeline of Human Evolution: From Primate Origins to Modern Man
Sahelanthropus and the First Upright Walkers (7–5 Million Years Ago)
Fossils found in Chad, dating to around 7 million years ago, belong to a species called Sahelanthropus tchadensis. This early hominin had a mix of ape-like and human-like traits. Its brain was small, but the position of its foramen magnum—the hole where the spinal cord connects to the skull—suggests it held its head upright, indicating the possibility of bipedalism.
Around 6 million years ago, in what is now Kenya, a creature called Orrorin tugenensis also walked upright. Then came Ardipithecus ramidus, around 4.4 million years ago. Discovered in Ethiopia, “Ardi” had feet adapted for walking as well as grasping trees. This suggests a transitional lifestyle: partly arboreal, partly terrestrial
When humans first ventured out of Africa some 60,000 years ago, they left genetic footprints still visible today. By mapping the appearance and frequency of genetic markers in modern peoples, we create a picture of when and where ancient humans moved around the world. These great migrations eventually led the descendants of a small group of Africans to occupy even the farthest reaches of the Earth.
Our species is an African one: Africa is where we first evolved, and where we have spent the majority of our time on Earth. The earliest fossils of recognizably modern Homo sapiens appear in the fossil record at Omo Kibish in Ethiopia, around 200,000 years ago. Although earlier fossils may be found over the coming years, this is our best understanding of when and approximately where we originated.
By 4 million years ago, a new genus had emerged: Australopithecus. Among the most famous of its species is Australopithecus afarensis, best represented by “Lucy,” a fossilized skeleton discovered in Ethiopia and dated to 3.2 million years ago. Lucy walked upright and had relatively long arms, suggesting she still spent time in trees.
Around 2.4 million years ago, the first members of the genus Homo appeared. Homo habilis, or “handy man,” was named for the stone tools found near its fossils. These early humans had larger brains than Australopithecus and were likely more dependent on tools for survival. They lived in complex social groups and may have scavenged or hunted small animals. Their teeth were smaller, suggesting a shift in diet. The use of tools allowed them to crack open bones for marrow—a rich source of calories that may have fueled further brain growth.
Around 1.9 million years ago, Homo erectus emerged in Africa. This species was larger, stronger, and more advanced than its predecessors. Its brain was about two-thirds the size of a modern human’s, and its body proportions were similar to ours. Homo erectus used more sophisticated stone tools, possibly controlled fire, and built shelters.
From Homo erectus came a variety of species adapted to different environments. In Europe and western Asia, Homo neanderthalensis—the Neanderthals—evolved around 400,000 years ago. In eastern Asia, another group emerged: the Denisovans, known primarily through genetic evidence and a few fossil fragments.
Neanderthals were not brutish cavemen, as once believed. They had larger brains than modern humans, buried their dead, cared for the injured, and created art and tools. Their lives were hard, marked by cold climates and dangerous hunts, but they were intelligent survivors.
Homo sapiens seems to have appeared in East Africa around 200,000-300,000 years ago. The oldest individuals found left their marks in the Omo remains (195,000 years ago) and the Homo sapiens idaltu (160,000 years ago), that was found at the Middle Awash site in Ethiopia.
When modern humans reached the Near East 125,000 years ago, evidence suggests they retreated back to Africa, as their settlements were replaced by Neanderthals. It is now believed that the first modern humans to spread east across Asia left Africa about 75,000 years ago across the Bab el Mandib connecting Ethiopia and Yemen. From the Near East, some of these people went east to South Asia by 50,000 years ago, and on to Australia by 46,000 years ago at the latest, when for the first time H. sapiens reached territory never reached by H. erectus. H. sapiens reached Europe around 43,000 years ago, eventually replacing the Neanderthal population by 40,000 years ago.
East Asia was reached by 30,000 years ago. Archaeological and genetic data suggest that the source populations of Paleolithic humans survived in sparsely wooded areas and dispersed through areas of high primary productivity while avoiding dense forest cover. The date of migration to North America is disputed; it may have taken place around 30 thousand years ago, or considerably later, around 14 thousand years ago. The oldest DNA evidence of human habitation in North America, radiocarbon dated to 14,300 years ago, has been found in fossilized human coprolites uncovered in the Paisley Five Mile Point Caves in south-central Oregon. Colonization of the Pacific islands of Polynesia began around 1300 BCE, and was completed by 1280 CE (New Zealand). The ancestors of Polynesians left Taiwan around 5,200 years ago.
More recent migrations of language and culture groups within the modern species are also studied and hypothetised. The African Epipaleolithic Kebaran culture is believed to have reached Eurasia about 18,000 years ago, introducing the bow and arrow to the Middle East, and may have been responsible for the spread of the Nostratic languages. The people of the Afro-Asiatic language family seem to have reached Africa in 6,200 BCE, introducing the Semitic languages to the Middle East.
From there they spread around the world. An initial venture out of Africa 125,000 years ago was followed by a flood out of Africa via the Arabian Peninsula into Eurasia around 60,000 years ago, with one group rapidly settling coastal areas around the Indian Ocean and one group migrating north to steppes of Central Asia.
There is evidence from mitochondrial DNA that modern humans have passed through at least one genetic bottleneck, in which genome diversity was drastically reduced. Henry Harpending has proposed that humans spread from a geographically restricted area about 100,000 years ago, the passage through the geographic bottleneck and then with a dramatic growth amongst geographically dispersed populations about 50,000 years ago, beginning first in Africa and thence spreading elsewhere. Climatological and geological evidence suggests evidence for the bottleneck. The explosion of Lake Toba created a 1,000 year cold period, as a result of the largest volcanic eruption of the Quaternary, potentially reducing human populations to a few tropical refugia. It has been estimated that as few as 15,000 humans survived. In such circumstances genetic drift and founder effects would have been maximised leading to a rapid racial differentiation after that date. The greater diversity amongst African genomes may be in part due to the greater prevalence of African refugia during the Toba incident.
