Homo erectus (1.9 million to after 200,000 years ago)
All paleoanthroplogists agree that the new human form, known generically as Homo erectus, evolved in tropical Africa. They point to the continuous record of human evolution in the continent, from Australopithecus afarensis to the later australopithecines, then Homo habilis and Homo erectus. However, bushlike the evolutionary tree, and it was much more complex than we know today, the best and longest fossil record comes from Africa. As we shall see the same continent also may have been the cradle of modern humans, Homo sapiens sapiens.
Current estimates place the emergence of Homo erectus (the earliest African form is sometimes called Homo ergaster,) probably, in Africa, at about 1.9 million years ago. That the human evolved from Homo habilis–like hominid seems unquestionable, but we still await the discovery of larger–brained transitional forms.
The Earliest African Forms
The early East African Homo erectus specimens come from northern Kenya. Skull KNM-ER 3733 from East Turkana dates between 1.6 and 1.5 million years ago. This fossil¸ with its massive brow ridges¸ enlarged brain size¸ and high forehead¸ is morphologically very close to examples of later Homo erectus specimens dating to a million years ago.
Another fossil find, this time from a 12–year old boy from the western shore of Lake Turkana, dates to about the same time period. From the neck down¸ the boy's bones are remarkably modern looking. The skull and jawbone are more primitive, with brow ridges and a brain capacity perhaps as high as 700 to 800 cubic centimeters, about half the modern size. The boy stood about 5 feet 6 inches (1.8 meters) tall¸ taller than most modern 12–year olds. The Turkana boy seems to confirm the theory that different parts of the human body evolved at different rates¸ the body achieving fully modern form long before the head.
Unlike its predecessors, Homo erectus spread out of Africa and adapted to a wide range of temperate, tropical, and cold environments throughout Europe and Asia. However, these archaic humans never crossed into the Americans
We know¸ that the earliest hominids could adapt to a variety of climates and habitats. Both Australopithecus and Homo habilis adjusted to plunging global temperatures during a glacial episode between 2.7 and 2.5 million years ago. The colder conditions turned much of Africa's moist woodlands into much drier¸ open savanna. The hominids thrived in these conditioned¸ as tree–dwelling primates yielded to bipedal forms better able to survive in the open. This adaptability let hominids move into new environments¸ where their mixed diet of meat and plant foods caused them to move over large home territories. Homo erectus was just as adaptable and mobile¸ but was the first human to use fire¸ fashion more elaborate tools¸ and leave Africa.
Homo erectus Radiates out of Africa
If the potassium argon dates for Homo erectus fossils in Indonesia are to be believed¸ then these new humans radiated out of Africa with remarkable speed¸ appearing on the savanna by 1.9 million years ago¸ and in the southeast Asian rainforest by 1.8 million years ago. However would a radiation Homo erectus population from Africa into Asia occur within such a short time frame as 100¸000 years? Such a rapid spread is theoretically possible. With¸ covering large open territories where food resources were scattered unevenly over the landscape¸ even an expansion of about 20 to 30 miles (30 to 50 kilometers)a year soon translates to hundreds¸ then thousands of kilometers within a few generations. Unfortunately¸ the archaeological and fossil evidence for this extraordinary radiation is still tantalizingly incomplete.
Why did this sudden movement occur ? Around 2 million years ago¸ hominids were adjusting to cyclical alterations among savanna¸ forest¸ desert¸ as the Ice Age began. They did so by migrating with changing vegetation zones¸ as many mammals did¸ or by adapting to new environments¸ changing their dietary emphasis from meat to plant foods. Finally¸ they could move out of tropical latitudes altogether¸ into habitats that human beings never occupied before.
Very likely Homo erectus adapted to changed circumstances in all these ways¸ radiating out of Africa by way of the Sahara¸ when the desert was capable of supporting human life. Geologist Neil Roberts has linked the Sahara to a pump¸ sucking in population during wetter savanna phase and forcing forages out northward to the margins of the desert during drier cycles. In radiating out of Africa¸ Homo erectus behaved just like other mammals in its ecological community.
Homo erectus was a carnivore and a plant eater¸ and thus was linked ecologically with other predators. There was widespread interchange of mammals between Africa and more temperate latitudes during Pleistocene. For example¸ a major change in the mammalian populations of Europe took place about 700¸000 years ago. Hippopotamuses¸ forest elephants¸ and other herbivores and carnivores like the lion¸ leopard¸ and spotted hyena–the animals with which hominids shared many ecological characteristics–were in the same direction as that taken earlier by Homo erectus. That the first successful human settlement of tropical Asia and temperate Europe coincided with radiations of mammalian communities out of Africa seems plausible. It may also have coincided with taming of fire.
Homo erectus may have domesticated fire as early as 1.6 million years ago. Early humans would have been familiar with the great grass and brush fires that swept across the savanna during the dry months. Fire offered protection against predators and an easy way of hunting game¸ with even insects and rodents fleeing from a line of flames. Perhaps Homo erectus developed the habit of conserving fire¸ talking advantage of long–smoldering tree stumps ignited by lighting strikes and other natural causes in order to kindle flames to light dry brush or simply to scare off predators.
It may be no coincidence that the radiation of Homo erectus out of tropical Africa into temperate environments in Asia and Europe occurred after the taming of fire and during a period of accelerated climate change. While most experts agree that Asia was settled soon after 2 million years ago¸ the evidence for the first settlement in Southwest Asia and Europe is more uncertain. To judge from the Southeast Asia dates¸ Homo erectus moved rapidly eastward out of Africa but settling the more northerly latitudes of Europe and Eurasia may have presented a more formidable challenge¸ especially during glacial cycles. Male and female skulls of Homo erectus have recently been discovered at Dmansi in southwest Tbilisi in Georgia (A republic of the former Soviet Union)in associated with crudely shaped choppers and flakes. The crania came from river deposits dated to about 1.7 million years ago¸ making them the earliest humans to be unearthed in Eurasia and predating the earliest Western Europeans by a ½ a million years. These new finds are evidence that archaic humans foraged widely in search of food their movements driven¸ perhaps as mush by hunger as by superior technology.
Homo erectus in Asia
The classic Homo erectus fossils come from Asia¸ from the Trinil area of Java¸ where they date to between 1.8 million and 600¸000 years ago and from northern and southern China¸ dating to between 600¸000 and 350¸000 years ago perhaps considerably earlier. These well–preserved specimens show that these archaic hunters had a brain capacity between 775 and 1330 cubic centimeters¸ showing much variation. It is probable that their vision was excellent and that they were capable of extensive reasoning. The Homo erectus skull is more rounded than that of earlier hominids. It also has conspicuous brow ridges and a sloping forehead. Homo erectus had limbs and hips fully adapted to an upright posture. Makes stood over 5 feet 6 inches (1.8 meters)tall and had hands fully capable of precision gripping and many kinds of toolmaking.
During Homo erectus long history¸ humanity adapted to a far wider range of environments¸ ranging from tropical savannas in East Africa to forested Javanese valleys¸ temperate climates in North Africa and Europe¸ and the harsh winters of northern China and central Europe. Homo erectus was certainly capable of a far more complex and varied life way than previous hominids. With such a wide distribution it is hardly surprising that some variations in population occur. For example¸ Chinese scholars claim that the Homo erectus fossils from the famous Zhoukoudian cave near Beijing display a gradual increase in brain capacity from about 900 cubic centimeters 600¸000 years ago to about 1100 cubic centimeters in 200¸000 year old individuals. In any case¸ Homo erectus was far more human than Homo habilis¸ a habitual biped¸ who had probably lost the thick hair covering that is characteristic of nonhuman primates.
