When did human ancestors begin leaving Africa and why did it take them so long to start?
The first hominids arose in Africa in response to environmental changes about 5 million years ago. These new species were adapted to walking upright on two legs. And, since they no longer needing their forearms for moving about, their dexterous hands were useful for a variety of new tasks.
Then, about 2.5 million years ago the Earth entered a long period of climatic instability. Conditions became much more variable. Africa's climate became drier at an accelerated rate. Grasslands continued encroaching upon forests. These changes were the impetus for another series of momentous adaptations.
In just 300,000 years some 29 species of forest antelope went extinct. At the same time, new antelope species emerged and proliferated. They were specialized to life on the open grasslands.
The hominid family tree would spin off its own new genus. It included several species adapted to exploit the new opportunities that were developing. Each species retained characteristics of its australopithecine ancestors while developing their own unique attributes.
Homo habilis (handy man), or one of its close kin, was one of the first species in this genus. These were the first hominids to use tools.
But, H. habilis did more than simply select stones with useful shapes. They learned to shape tools for specific needs. These primitive tools are representative of what has come to be known as Olduwan technology. It better enabled hominids to hunt and butcher animals, increasing the proportion of energy-rich meats and fats in their diet.
This was important for development of larger brains. Brains require disproportionately large amounts of energy. The brains of H. habilis were a little larger than those of australopithecines. The trend toward larger brains accelerated in subsequent species.
One descendant of H. habilis was Homo erectus. Until recently, H. erectus was thought to have been the first hominid to leave Africa about a million years ago. Java Man and Peking Man are typical of the many H. erectus specimens that had been found in Southeast Asia and China.
More recent discoveries in Java show H. erectus survived until about 40,000 years ago. It is impressive to think this species survived nearly 2 million years and occupied a host of environments across both Africa and Asia.
Other fossil discoveries in the 1990s from Java and the former Soviet republic of Georgia suggest hominids left Africa at least 1.7 to 1.8 million years ago Also, Olduwan-like tools have been found in Pakistan dated from 1.6 million years ago.
Some argue that some of these specimens are from another species, Homo ergaster. More probably, H. ergaster preceded H. erectus in leaving Africa and is closer to the direct lineage of modern humans.
There is little doubt H. erectus was a dead end. Though it was among the most successful hominids, it was a side branch to the direct lineage of modern humans.
The most complete hominid fossil ever found was unearthed in 1984. Turkana boy, as he has come to be known, was an H. ergaster male youth who lived 1.8 million years ago As an adult, he would have stood 6 feet tall.
His lanky body has proportions more typical of modern humans. Evidence suggests he was much less hairy than his H. habilis predecessors.
The shape and arrangement of bones in this individual's neck and base of skull show he had not acquired the capacity for speech. As an adult, his brain volume would have been 900 cc cubic centimeters. Modern humans have an average brain volume of 1350 cc.
About 1.5 million years ago hominids in Africa began making more sophisticated tools. This Acheulean technology includes the use of hand axes. No tools of this type ever reached the eastern parts of Asia. This is evidence the first hominids populating East Asia left Africa before development of Acheulean technology.
H. ergaster and H. erectus may well have left Africa at different times or in several waves. Importantly, expansion did not occur until hominid species arose with nearly modern human bodies.
Such developments came with more diverse, energy rich diets. Hominids were no longer tied to the forest, but could easily walk upright and were able to scavenge and hunt animals on the open plains.
Homo antecessor and Homo heidelbergensis are two species that arose subsequent to H. ergaster. Each contributed further technological advances. Though there is some evidence H. ergaster/H. erectus used fire, its controlled use became more prevalent with these later species.
Genetic analysis of lice that inhabit clothes indicates human ancestors began wearing clothes about 170,000 years ago. Clothing and control of fire made habitation of harsher environs possible.
Changes in the base of the skull associated with the ability to speak are found in fossils of H. heidelbergensis. These fossils date from about 500,000 years ago.
Each new adaptation promoted greater social interaction. The essentials for creating modern societies and culture had taken root.
Environmental changes provided the impetus for hominid adaptation throughout their 5 million year history. Each species, over the course of many generations, left heirs better adapt to nature's gradual changes.
None was capable of altering their environments to the extent Homo sapiens have. Will the rapid pace of changes we are bringing about overtake this last remaining hominid?
Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at firstname.lastname@example.org.