If we take Homo habilis as the prototype, then Homo erectus was the line-defining human - the Model T Ford of the new genus. Even more successful than the Model T, they dominated the planet for a million and a half years. With a sad, wary face, a flat nose, and, initially, a rapidly growing brain, Homo erectus was just like us from the neck down. They had stone tools - simple retouched pebbles at first, but later more sophisticated hand-axes. Their African progenitor Homo ergaster was the first human to leave Africa, 1.95 million years ago, to become the Asian Homo erectus. The latter were slightly smaller than ourselves, and rapidly spread to the Middle East, Russia, India, the Far East, and Southeast Asia, carrying with them their so-called 'pebble-tool' technology.
There are controversial claims that the smaller predecessor on the tree Homo habilis also made this leap at the same time. There is better evidence, however, that all subsequent human species made it out of Africa at the first available interglacial warm-up between ice ages. Homo erectus types then dominated the world for nearly a million years until another terrible series of ice ages dried up much of Africa over a million years ago and brought about the emergence of a new, more specialized family. The first African representative of this new model was Homo rhodesiensis. The same size as us and with a brain volume of as much as 1,250 cm3, they used a more sophisticated stone tool kit known as Acheulian, named after a village in France near where the style was first found. Acheulian tools included largish flat stones shaped on both sides to form teardrop-shaped pointed hand-axes. This new arrival first made it out of Africa to Europe, and possibly to China, during a brief warm-up about half a million years ago, and carried the Acheulian technology with them.
Then, 350,000 years ago, another severe ice age struck, perhaps forcing yet another large-brained human onto the African stage around 300,000 years ago. They are known to some as archaic Homo sapiens, and to others as Homo helmei. To avoid confusion I shall use the latter name. Beetle-browed, the same size as us, and with an average brain volume slightly larger than ours at 1,400 cm3, they represented the plateau as far as dramatic brain growth was concerned. They were also associated with the start of one of the most important revolutions in human technology, known as the Middle Palaeolithic. Some have gone so far as to suggest that if brought up in a modern family, these heavy-browed creatures might fit into our society.
A larger and longer out-of-Africa movement, during a warm period, saw Homo helmei spreading throughout Eurasia 250,000 years ago. Homo helmei may have given rise to Homo neanderthalensis in Europe and Asia and had several possible relatives in India and China from the same period. The source human family containing our own ancestors remained in Africa, for the time being, physically separate from their Neanderthal cousins in Europe.
Our own species, Homo sapiens, was born over 170,000 years ago, out of what was nearly a human extinction in which the total population fell to an estimated 10,000 in a mother of all ice ages. Although Homo sapiens duly made it out of Africa to the Levant at the next interglacial, 120,000 years ago, the genetic evidence indicates that their descendants died out there without issue in the ice age after that. (The Levant - an old-fashioned label, but useful in this context - comprises modern Syria, Lebanon, Israel, Palestine, and Jordan: the Mediterranean Near East minus Egypt.) When modern humans finally spread out of Africa to the rest of the world around 70,000-80,000 years ago, Eurasia was still inhabited by several other human species. The European Neanderthals, and possibly the Southeast Asian Homo erectus, persisted until less than 30,000 years ago, but no genetic traces of them remain in living humans.
Significantly, both Neanderthals and those modern humans living before the last ice age 20,000-30,000 years ago had rather bigger brains than do people living today.16 It seems that the magic brain enlarging effect of ice ages had played itself out before the time of our birth as a subspecies of Homo helmei. Maybe the obstetric risks of large heads were limiting. Either that, or brain size was no longer the most important determinant of success, and something new that we were doing with our brains - some other behavioural or cultural innovation - had taken over. Once we had left Africa, although our brains had stopped growing, the climate continued to dominate human expansions and inventions right up to the modern age. It may be no exaggeration to say that the forces driving the waves of human technical innovation advancing across Eurasia from 80,000 years ago were more a result of stress and relief than of any biological improvement in the human computer. For example, the spreads of new technologies labelled by archaeologists as Early, Middle, and Late Upper Palaeolithic, Mesolithic, and Neolithic all coincided with dramatic ameliorations of Europe’s climate and population expansions into new territories. These events were mirrored in Southeast Asia with expansions and advances of boat-building and sailing in response to the flooding of continental shelf as the sea level rose and fell.
In summary, then, rapidly increasing brain size was a key feature that set humans apart from the walking apes that lived before 2.5 million years ago. Since then our brains have trebled in volume. This increase was not gradual and steady: most of it came as a doubling of volume in Homo erectus 2 million years ago. In other words, the greatest acceleration in relative brain size occurred before 1.5 million years ago, rather early in our genus, and then gradually slowed down. The paradox is that our apparent behavioural explosion is mostly recent and is accelerating.
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