and Patterns of Hominid Dispersal
When and how new hominid species appeared, and how they affected or displaced already existing species, are questions that many lines of research are helping to answer. Patterns of human migration form a key part of the solution.
Recently, with more powerful geological dating methods based on fluctuations in the earth's magnetic field or electron spin resonance, the earliest Homo erectus fossils from China have been dated to 1.9 million years ago. This requires an even earlier date for the emergence of Homo ergaster in Africa, implying humans first evolved about 2.5 million years ago.
Climate studies show a cooling of the climate in Africa from around 3 million to 2.4 million years ago. This cooler, drier climate would have cut back a significant part of the lush northern African forests, opening up vast areas of grassy plains and restricting the range of australopithids adapted to forest habitats. These grasslands and the many resources they afforded for opportunistic scavenging were the ecological niche invaded by the new species Homo.
But scavenging is a migratory life, tied to the hunting patterns of predators and the herds of game they prey on, to fluctuations in water supplies, to seasonal shifts in the requirements for shelter and plants. The increased stature and brain size of Homo rudolfensis and Homo ergaster suggest humans adapted rather quickly to these new rigors. Successfully coping with fluctuating environments created the skills to adapt to a wider range of new environments, making it feasible for the migratory humans gradually to follow resources and herds of game into an ever widening range of habitats across the Middle East and southern Asia.
This was only the first of
many human migrations, which fall into three distinct geological periods:
- Hominid species up through early Homo habilis and the Australopithecines have so far been located only in the interconnected mountain valleys of eastern Africa. (A single australopithid fossil has also been retrieved from the Central Sahara.) This forms the baseline ecological and geographical condition of the earliest hominids.
- From about 2 million up to 1 million years ago, early Homo ergaster migrated outside this valley system into a larger area of Africa, and from there into central and eastern Asia -- evolving along the way into Homo erectus. The sheer geographic distances involved reduce substantially the gene flow in the total hominid population, making regional subspecies (such as eastern Homo erectus) more likely to appear. Besides climate change and fluctuating resources, hominid tools, diet and childrearing now emerged as factors that increasingly could affect human group organization and patterns of dispersal.
- Around 800,000 years ago a large brained, robust hominid, Homo heidelbergensis, appeared in Africa and migrated throughout that continent and into Asia -- but significantly also became the first hominid to colonize Europe. Although it utilized the same Acheulean tool industry as western Homo erectus, Homo heidelbergensis probably was a more systematic hunter and relied more extensively on the economic processing of animal resources.
- Finally, with the emergence of Homo sapiens around 120,000 years ago, hominid hunting pressures drove many large mammals to extinction, and human industry and cultures (including early language) evolved and diverged. Scarce resources and cultural distinctiveness further accelerated group segregation and interspecies competition for game, shelter and harvestable territories.
Around 120,000 years ago Homo
sapiens emerged as a new species, most likely in central East Africa,
and from there migrated into the Middle East, south Africa, Europe, central
Asia, and finally into the New World. To reach the Bering Strait from Africa
by 14,000 years ago, humans would have had to wander no more than one mile
every eight years. -- The timing of Ice Age coolings, and the amount they
lowered ocean levels, specifies the geologic periods in which it was possible
to migrate to land masses otherwise separated by water.
The map above shows the likely paths of human migration out from the location in east Central Africa where modern humans probably first appeared. The already-occupied range of Homo erectus is shown for comparison. (Not shown is the radiation of Homo heidelbergensis into Europe.)
The map below shows in greater detail the major fossil and archaeological sites in the Old World. Careful geological dating of these sites, and close comparison of the tools and bones found at different sites, are helping paleoanthropologists to map when and where different species appeared, and to tie these sites together into migratory patterns. Climate data and the migratory patterns of other fossil mammals provide corroborating evidence for the overall sequence of human movements.
Both maps present the ocean coastlines and inland tracts of vegetation as they likely appeared around 20,000 years ago, when the last Ice Age reached its maximum and sea level had dropped more than 20 feet below where it is today. By that time modern humans had radiated out of Africa into Europe and most of Asia, including Australia. Climate affected vegetation and rainfall to a considerable degree, and these wide variations stimulated human evolution through human migration. The backwater of human evolution, eastern Homo erectus, lived in parts of Asia where ecology varied much less with climate than it did in Africa and Europe.
It's interesting that several fossil sites from Homo erectus up to modern humans were located near seacoasts at the time of their habitation. There is good evidence that oceangoing boats were constructed by humans by 60,000 years ago. We tend to think of early humans as hillside "cave dwellers" and in inland areas they often were; but they were also early voyagers and harvesters on the waves.
Unfortunately it is very difficult to assess the importance of the oceans to human migration. Because sea level during the ice ages was much lower than it is today, most prehistoric coastlines are now covered by water. The few places where littoral habitations have been recovered are in areas where caves were located in seaside cliffs or hills.
