Prehistoric World

3.9 million

In the area of Afar in Hadar, Ethiopia, Australopithecus afarensis, or 'southern ape of Afar', now possessed mixed human and chimpanzee characteristics. They had small brains, long, dangly arms, short legs and a cone-shaped thorax with a large belly. But the structure of their knees and pelvis show that they routinely walked upright on two legs.

Probably descended from Australopithecus anamensis, and just over a metre tall, afarensis foraged for fruit, nuts and seeds in a mixture of savannah and woodland. They may also have obtained animal protein from termites or bird's eggs.

The most famous afarensis remains discovered were those of 'Lucy' found by anthropologist Professor Donald Johanson and his student Tom Gray in a maze of ravines at Hadar in northern Ethiopia in 1974, and dated to 3.2 million years ago. The following year Michael Bush, one of Johanson's students, found the remains of more than 13 afarensis individuals buried together following a natural disaster – possibly a flash flood. The find yielded vital information about afarensis' social organisation, likening it to the social grouping found in chimpanzees.

Professor Robin Crompton of Liverpool University has used computer modelling to reconstruct how Lucy walked based on the proportions of her skeleton. He assumed that Lucy could either have walked upright with a bent hip and knees like a chimp, or with straight legs like a human. Orangutans live 20-40m above ground in the forests of Indonesia. They spend most of their time in an upright position, but suspend themselves from branches with their long arms. However, orangutans sometimes walk on branches without aid, raising their arms for balance. Orangutans are not as closely related to humans as chimps. But this behaviour was recently observed in wild chimpanzees living in dense forest, suggesting it could be an ancestral trait common to all great apes.

It is thought that the human lineage developed routine bipedalism as a strategy for living on the ground when climate change decimated the forest, leaving wide belts of open terrain with no trees. Crompton believes the forest canopy bipedalism shown by orangutans provided the kick-start for routine bipedalism when our ancestors came down from the trees and began living on the ground. Once our ancestors were forced to adapt to living on the ground, some drew on this behaviour from their existing repertoire as a method of terrestrial locomotion.

The challenges of spending more time on the ground would have favoured those hominids whose anatomy and behaviour gave them a reproductive edge over their peers, however slight. Hominids that were good bipedal walkers were clearly at an advantage in this terrestrial environment, because millions of years later, humans walk on two legs instead of four.

3.5 million

Australopithecus bahrelghazali appeared, and flourished until 3.0 million years ago.

The species designation of Australopithecus bahrelghazali is a new one, which has little support at present among most researchers. This is due to the small sample size, its recent designation, and the affinity of the specimen to afarensis material. However, it differs in several important aspects from afarensis (the geography being a major component in and of itself), and thus may be a new species, or at least a regional variation of afarensis.

The material designated as bahrelghazali was discovered by Michel Brunet's team in the ancient riverbed of Bahr el Ghazal in Chad, 2,500 kilometres west of the Rift Valley. This was the furthest west australopithecine material had been found in Africa.

No further specimens have been found so an impression of bahrelghazali is not yet possible.

A much shorter-surviving species, Kenyanthropus platyops appeared at the same time, but seems to have died out by 3.3 million years ago.

Discovered by Meave Leakey and her team in 1998 in the west of Lake Turkana in Kenya (near the Ethiopian border), Kenyanthropus platyops was described as a new genus dating back to the middle Pliocene period, positioning itself as a possible direct ancestor of modern humans. However, the skull found was very distorted, and some researchers believe this may not be a separate species at all.

3.2 million

Australopithecus afarensis continued to roam a mixed habitat of savannah and woodland beside lakes and floodplains, foraging for fruit, seeds and nuts and maybe even some meat. They probably climbed into trees to avoid sabre-toothed cats like dinofelis and to sleep in safety at night.

This hominid seems to have lived in social groups of between 20 and 30. These groups were probably like those of chimpanzees, with dominance hierarchies in which each individual knows their place. Male afarensis probably cooperated to drive away predators. Once mature, females may have joined other troops of afarensis.

3.0 million

As Australopithecus afarensis and bahrelghazali disappeared, the first hominid to replace them was Australopithecus africanus, a direct descendant of afarensis, which lasted until 2.4 million years ago.

The world of Lucy and Australopithecus afarensis had vanished. Hidden forces were transforming the Earth's climate, with devastating consequences for the African landscape. Temperatures in Africa plummeted and the air became stripped of moisture. By 2.8 million years, humid woodland had shrivelled away, leaving wide belts of open terrain in its place.

The cause of this environmental upheaval was to be found in space. The Earth orbits the Sun at a slight tilt, known as the axis of rotation. This means that as our planet spins, it points towards the Sun at some times and away from it at others. This is the origin of the seasons on Earth. Three million years ago, this axis was changing so that the Earth pointed away from the Sun for longer periods. This caused an overall cooling of the Earth, locking away moisture in ice at the North and South Poles. It also made the climate more seasonal, as Earth dried out.

In Africa, some animals that relied on the forests for their food died out. But others evolved to exploit different dietary sources. For example, many evolved physical adaptations to graze on the new species of plant life called grass that colonised the deforested terrain. Wildlife in Africa diversified as new animal species evolved to exploit different sources of food in a new mosaic of environments. The same seems to have happened to our ancestors, who had previously relied on forest foods such as soft fruit. After three million years, new ape-men were springing up all over the continent.

2.7 million

Paranthropus aethiopicus, a species not directly related to modern man, appeared in East Africa, and survived until 2.2 million years ago.

It was a short-lived side-shoot of Australopithecus africanus, and existed alongside it.  This and subsequent robust species of early human represented very different morphologies than were seen in the known Australopithecus specimens.

Adaptations of the cranium were associated with a "heavy-chewing complex." This complex is thought to have made it possible for these early humans to eat large amounts of tough, fibrous foods. Another feature of the robust skull is the presence, at least in males, of a prominent sagittal crest, a bony ridge that runs along the length of the top of the skull. This bony ridge provides an anchoring point for the large temporalis muscles.

These unique adaptations led Robert Broom to place the robust early humans from southern Africa into their own genus Paranthropus. Several species names have been proposed, including P. robustus or P. crassidens. In the 1960's palaeoanthropologists began to note similarities between all of the early human species before the appearance of Homo. As such, many researchers began to place all early human species into a single genus (Australopithecus) and described each species as either a "gracile" or "robust" Australopith. The robust specimens from southern Africa were then placed in the species Australopithecus robustus.

In recent years, many researchers have sought to emphasize the uniqueness of the heavy-chewing adaptations seen in at least three separate species of early human. Many favour the separation of these species into a robust genus of early human, for which the name Paranthropus was the first used, and therefore has seniority over all other names.

This particular species was eventually superseded by Paranthropus boisei.

2.5 million

The short-lived species Australopithecus garhi appeared, descended either from afarensis or africanus, it isn't clear yet which.

Garhi discoveries were made at Bouri, in the Middle Awash area of Ethiopia's Afar desert (about 70 kilometres south of the Lucy afarensis discovery at Hadar). The area was verdant at this time, with herds of antelope grazed the plains surrounding a lake. Volcanic ash and lava are interbedded with the fossil-bearing strata, providing accurately datable material. The ash just below the fossils erupted at this time.

The research team reported evidence of the earliest traces of animal butchery at the same site. Antelopes had been butchered with the world's earliest known stone tools. There is no direct evidence that garhi used stone tools, but the proximal cut-marked bones provide circumstantial evidence.

Garhi represents one of evolution's many question marks. It was contemporary of africanus but had a smaller brain than africanus and retained much of the primitive facial structure of afarensis.

Nevertheless, its mouth was more comparable to modern humans than africanus. Researchers have long assumed that evolution occurs in a linear fashion, with younger species possessing more "advanced" features than older ones. Garhi complicates this assumption as it seems to be a evolutionary step backward.

The specimens found include an associated set of cranial fragments, frontal, parietals, and maxilla with dentition. Garhi's teeth were larger than those of the earlier afarensis. Its braincase, face and palate were more primitive than those of later Homo. The known specimens demonstrate that the thigh bone had elongated a million years before the Homo forearm shortened. The derived human-like humeral/femoral ratio together with the ape-like upper arm-to-lower arm ratio evidence the mosaic-like evolution of the features that characterize modern humans.

Garhi was probably the direct ancestor of both Homo rudolfensis and Homo habilis, but vitally, it is not yet certain if Homo rudolfensis was the ancestor of later Homo species, or if Homo habilis was.

2.4 million

Homo rudolfensis ('Rudolf man') first appeared at this time, and flourished until 1.6 million years ago, living in East Africa.

Probably descended from Australopithecus garhi, rudolfensis may have lived over a wide area of East Africa, from Ethiopia in the north to Malawi in the south. It was a very tall species, towering above other hominids alive at the time – especially the stumpy Homo habilis. Their faces were large and flat in comparison with those of habilis, with flared cheekbones and big molar teeth, and their brains were 56% the size of those of modern man.

Like the slightly later Homo habilis, rudolfensis probably combined foraging with scavenging meat from animal carcasses. This probably placed the two species in competition with each other. The large body size of rudolfensis may have made it more effective at scaring away other animals from kills, turning it into a better scavenger.

Both habilis and rudolfensis survived for approximately 300,000 years after the appearance of the species that replaced them, and there is still some debate as to rudolfensis' exact classification -  and even whether it was really a species fully separate from Homo habilis - but neither species was directly related to the other hominid type which flourished at the time: Paranthropus boisei.

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.