Prehistoric World

1.9 million

The first truly human-like hominid species was Homo ergaster ('workman'). This species evolved from either Homo rudolfensis or Homo habilis during an accelerated period of global cooling and drying that cleared more and more tropical rainforest from Africa and regularly created a desert in the northern half of the continent.

Until recently, the general consensus was that Homo habilis was the more likely ancestor, but finds from Kenya in 2007 revealed an overlap of about 500,000 years during which Homo habilis and Homo ergaster must have co-existed in the Turkana basin area, the region of East Africa where the fossils were unearthed. Their co-existence makes it unlikely that Homo ergaster evolved from Homo habilis.

If Homo ergaster had evolved from habilis and stayed within the same location then both must have been in direct competition for the same resources. Eventually, one would have out-competed the other. The fact that they stayed separate as individual species for a long time suggests that they had their own distinct ecological niches, therefore avoiding direct competition.

Homo ergaster typically possessed a thick, bony ridge across the eyes, large teeth sticking out from a vaguely apelike projecting mouth below a long, wide nose, and long limbs. They seem to have grown to a height of around 185cm (6'1") with a body shape that was perfectly adapted to an active life in the sun (just as with modern human populations living on equatorial grasslands today, such as the Masai in Kenya). This body shape creates a large surface area over which the body can cool itself more easily, preventing Homo ergaster from overheating under the blazing sun.

This hominid was probably the first to regulate its temperature through sweating. For creatures that must remain active at midday in a sunny, dry habitat, sweating is the most effective mechanism for maintaining safe body and brain temperatures. Homo ergaster's body was probably smooth and largely hairless, since heat loss through sweating occurs most efficiently through naked skin. Its skin was almost certainly dark, to protect it from the sun's harmful rays.

Homo ergaster travelled long distances on foot, as it worked hard to scavenge enough meat to feed its growing body and brain. In order to increase the energy efficiency of muscles involved in upright walking, ergaster developed a narrower pelvis. But its snake hips came at a price.

Firstly, the narrowing of the pelvis caused the lower part of the ribcage to narrow. In order to prevent constriction of the lungs, the upper part of ergaster's rib cage expanded, giving its chest a human barrel shape. Secondly, and more importantly, the narrowing of the pelvis constricted the female birth canal. This single anatomical change seems to have had profound consequences for human relationships.

A tight pelvis could have caused problems during birth. As brains increased in size, mothers had to push increasingly big-brained infants through an already tight pelvis. The solution was a trade off. While chimpanzees are born with their brains almost fully mature, humans are born with a comparatively immature brain. This makes human babies helpless and vulnerable during their first year of life as their brains make vital neural connections.

As a result, human mothers need to be well nourished to keep up with the demands of their babies, making them increasingly reliant on the support of their male partner and other members of their social group. Many experts regard this shift as the beginning of the nuclear family.

Climate variations in East Africa may also have influenced hominid development. Scientists have identified lake systems which formed and disappeared in East Africa between one and three million years ago, providing possible evidence that global climate changes were occurring.

There were three distinct periods during which extensive lakes covered the region and grew to depths of hundreds of metres. The growth of these lakes probably resulted from a moist local climate. The regional wet periods, which may have persisted for up to 100,000 years, occurred as much of Africa became increasingly dry.

The periods of wet weather in East Africa might reflect fluctuations of the Earth's climate as a whole. At the time when the lakes grew - and this period was one of them - glaciers and the atmosphere were also going through major transformations.

This provides strong support for theories in which early human species evolved and spread out in response to a rapidly changing environment.

At the same time, boisei's southern African relative, Paranthropus robustus ('robust') appeared, surviving until 1.4 million years ago. It is uncertain if robustus evolved from the same ancestor as boisei or if, as is argued by some researchers, robustus is a case of parallel evolution in that it may have descended directly from Australopithecus africanus.

Evidence of what robustus, was eating is less clear than for their northern 'relative'. They seem to have been consuming grass-eating insects, including termites. Archaeological finds show that robustus dug termites out of their mounds using sharpened animal bones. It could also have been eating the roots of plants like papyrus.

The species name was chosen to describe the skull, jaw, and teeth, which were much more dense and thicker than what had been seen in previous species. There were also many more ridges and crests located on the skull. The front teeth of robustus were smaller, but the molars in the back were larger than previous species. These dental characteristics support theories of the species' diet.

A new advancement in robustus was the presence of a sagittal crest, a ridge that ran from front to back on top of the skull in which muscles were attached. These muscles aided in moving the jaw so that chewing was possible. As more muscle was formed more powerful chewing was possible.

1.8 million

Shortly after Homo ergaster appeared, humans began to leave Africa for the first time and migrate to other continents, forced to hunt for new foodstocks by progressively cooler global temperatures at the start of the Pleistocene era. After Homo ergaster left Africa it became known as Homo erectus ('upright man'). It survived mainly in South Asia, while Homo ergaster remained in Africa as a direct ancestor of modern man.

(There is, typically, some debate about the validity of Homo ergaster as a separate sub-species, with many seeing it as no different to an African-bound Homo erectus, a generalised classification inherited from the days before more than two Homo species were known.)

Homo erectus' migration took place during a rather brief period called the Olduvai subchron (1.98 to 1.79 million years ago). The East African Rift and extreme Southeast Asia were endpoints on a grand east-west geotectonic pathway called the Tethys corridor, a feature which was extremely unstable. Homo erectus and companion mammals took advantage of open linear landscapes to migrate north from the Rift to the Caucasus, and then both ways across the Tethys corridor - west toward Gibraltar, and east to the Himalayan fore slope.

Homo erectus reached Dmanisi, which is 80 kilometres (50 miles) south-west of the Georgian capital, Tbilisi, at around 1.8 million years ago. Here, they encountered cool, seasonal grasslands where African animals such as ostriches, rhinoceros and giraffes mingled with Eurasian species such as wolves and the sabre-toothed cat megantereon.

Homo erectus quickly spread further east to the emergent Sunda continental shelf off East Asia's present south coast, before rising sea levels cut the shelf into a series of islands of which the modern Indonesian island of Java is the southernmost. Populations existed throughout subtropical Asia, but extended no further north than the Himalayas or southern China. Instead, erectus learned to survive in the bamboo forests that covered this region of Asia. The paucity of stone tools from Southeast Asian hominid sites suggests that erectus may have created a technology based on bamboo, a strong and versatile material.

"They may have used bamboo to make spears for hunting and poles to knock animals down from the tall trees," says Professor Russell Ciochon of the University of Iowa. Homo erectus shared these bamboo forests with pigs, a type of elephant called Stegodon and the biggest primate that has ever lived: the giant vegetarian ape Gigantopithecus - a cousin of the earlier Ramapithecus. It's possible that Gigantopithecus may even have been hunted by early humans in Asia. "They probably wouldn't have taken on the big adults, but they may have targeted juveniles. If we look at people who live in forests today, they also eat apes," says Ciochon.

Dates for the arrival of Homo erectus in subtropical Asia are controversial. While erectus was clearly established throughout the region by 1.8 million years ago, some sites suggest an even earlier date for its arrival. A hominid jaw and stone tools unearthed at Longuppo Cave, China may date to as early as 1.9 million years ago. Similar dates have been established for hominid sites at Mojokerto and Sangiran in Java. This newfound wanderlust may have been dictated by an increasing reliance on meat for food. Carnivores generally need much larger home ranges than similar-sized herbivores because carnivores have fewer total calories available to them per unit area of their territory.

Shortly after settling in its new Asian homelands, from about 1.6 million years ago Homo erectus began to diverge from Homo ergaster populations. This divergence may be related to the first full scale Ice Age, which occurred around 1.5 million years ago, following half a million years of continued cooling of the world climate.

1.77 million

Once in the Caucasus, Homo erectus may have discovered that it was not the only hominid living in the region. There is also evidence to suggest that a group of hominids of a much smaller stature were established there.

A collection of finds from Dmanisi in Georgia in the same layer of sediment as Homo erectus finds has brought to light these little "people" - who stood at around four feet tall - but this has caused a lively debate amongst palaeoanthropologists. So far it has been tricky to work out exactly what species they are. Common thought is that Homo erectus was the first to venture out of Africa and spread around Asia. But the Dmanisi hominids were not typical of the tall-standing, big brained erectus - instead they were short, long-armed, small-brained, and thin browed, with a far smaller brain cavity - half the size of a modern human - and the huge canine teeth and thin brow of an ape. However, the tools found alongside fossils were basic choppers and cutters, just like those found at the sites of early, primitive men in Africa.

This has led some to believe they may have been Homo habilis. But the relatively ape-like habilis was not thought to have lived outside Africa. Other researchers have coined the term Homo georgicus to describe the finds, and this seems to be sticking, for now.

1.6 million

Less differences between the sexes in Homo ergaster than in previous hominids may reflect a distinctively human pattern of sharing and cooperation between males and females. Homo ergaster probably communicated using gestures combined with a limited range of sounds. Their vertebral canals do not seem developed enough to have given them the control over breathing needed for complex speech.

Ergaster also seems to have relied more than previous hominids on stone tools for processing food. To begin with, ergaster used primitive 'Oldowan stone tools,' which are little more than chipped rocks with sharp edges. But by around 1.6 million years ago, ergaster developed symmetrical, heart-shaped hand axes known as 'Acheulean bifaces', which gave the hominid greater control over the butchering of meat for food.

Again, this seems to be related to changing conditions caused by the occurrence of the first Ice Age. Hereafter, Ice Ages occurred at fairly regular intervals of 80,000 to 100,000 years: dramatic falls in temperature and the formation of extensive ice sheets, especially in the northern hemisphere, alternated with warmer periods when temperatures were similar to those of the present day.

1.2 million

Paranthropus boisei eventually paid the price for being a specialist in a changing world.

Despite its successful way of exploiting the savannah - from its taste for termites to the wide range of vegetation it had specialised in eating - boisei became a footnote in human prehistory. They were driven to extinction, probably by an intense period of cooling and drying caused by the Ice Age.

1.0 million

Erectus was still dominant in South Asia, living in small, semi-isolated groups. One such erectus group seems (according to available but still-controversial evidence) to have managed to make its way by sea - despite sea travel previously being thought beyond erectus' capabilities - to the island of Flores in Indonesia (one of a chain of islands stretching east from Java), where by 800,000 years ago it was making stone tools. By around 100,000 years ago the limited resources on the island seem to have forced this group to evolve into Homo floresiensis.

Other erectus groups were beginning to move into Europe, although their presence there would never be very large.

Fossil finds in Kenya dated to 930,000 years ago suggest that Homo ergaster was still dominant in Africa. It seems to have become the first hominid to use fire, which enabled it to eat food more easily and for the size of its jaws and teeth to reduce. This resulted in some variation in skull sizes occurring, marking out demonstrable differences at this time between ergaster and Homo erectus.

It is possible that these changes were partly the result of the new Ice Age, which was already bringing climate changes with it. Whatever the cause, the appearance of Homo heidelbergensis was imminent.

All images copyright © BBC or affiliates unless otherwise stated. No breach of copyright is intended or inferred. Text copyright © P L Kessler, adapted from numerous sources and notes, most notably the BBC tv series, Walking with Cavemen. An original feature for the History Files.