Prehistoric World

6.7 million

Sahelanthropus tchadensis

Sahelanthropus tchadensis flourished between 6.7 to 6.3 million years ago.

This very recent discovery (in 2001) in the central African state of Chad, in the southern Sahara desert (Brunet et al 2002, Wood 2002), was poised to upset the human family tree.

The fossil skull which was unearthed was nicknamed Toumaï. It is as old as any hominid fossil found to date (by 2016), yet its features appear much more human-like than those of other contenders for the title of human ancestor.

It was discovered by Ahounta Djimdoumalbaye. Based on faunal studies, it is estimated to be between 6 and 7 million years old, and more likely in the older part of that range. It consists of a mostly-complete cranium with a small brain (between 320 and 380cc), comparable in size to that of a chimpanzee.

No bones below the skull have yet been discovered, so it is not known whether Toumaï was bipedal or not. Brunet agrees that it was likely to be a habitual biped because it shares characteristics with other hominids which are certainly known to have been bipedal.

Other scientists have pointed out the foramen magnum (the hole through which the spinal cord exits the skull) of Toumaï is positioned towards the back of the skull as in apes, indicating that the skull was held forwards and not balanced on top of an erect body.

Brunet's camp considers Toumaï to be a hominid - that is, on the human side of the chimpanzee-human divide and therefore more closely related to modern humans than to chimpanzees. This is not at all certain.

Some scientists think it probable while others have suggested that it may come from before the point at which hominids separated from chimpanzees (around 7 million years ago). To further muddy the water, Brigitte Senut (one of the discoverers of Orrorin tugenensis) has suggested that it may be an early gorilla.

It could have adapted to bipedalism due to the thinning of the vast African forest, caused as the climate dried out, by standing up to reach the most inaccessible fruits at the tops of trees. That would have given it a noticeable advantage over its competitors.

It seems to be impossible to know how Toumaï is related to hominids (on the human side of the family tree) until other fossils can be found from the same time period. Whatever it is, all scientists agree with its finders that Toumaï is of major significance, and subsequent discoveries tend to support the view that Toumaï is a hominid.

6.2 million

Orrorin tugenensis

Orrorin tugenensis (otherwise known as 'Millennium Man' thanks to the date of its discovery) appeared between 6.2 and 5.8 million years ago.

This species was named in July 2001 from fossils discovered in western Kenya (Senut et al 2001). The fossils include a left femur, pieces of jaw with teeth, isolated upper and lower teeth, arm bones, and a finger bone. Preliminary analyses suggests the hominid was an agile climber and that it walked on two legs when on the ground.

The tentative date of six million years, taken from the age of the deposits in which the fragments were discovered, indicate a date very close to the common ancestor of humans and chimpanzees, although this date may now need to be pushed back (or at least moved back to the earliest point of its window, around 7 million years ago).

The limb bones are about 1.5 times larger than those of Lucy, suggesting that it was about the size of a female chimpanzee. It probably climbed trees but may also have walked upright on the ground (although for how long is unclear).

As no complete skull can yet be formed from the finds, an artist's impression of Orrorin tugenensis is not realistically possible.

5.8 million

Ardipithecus ramidus

Ardipithecus ramidus proved to be the second longest-surviving hominid species (known to date), flourishing between 5.8 and 4.4 million years ago as the Middle Miocene became the Late Miocene.

While Ardipithecus ramidus is not the sought-after 'Missing Link' - the so-far undiscovered creature which lived at the cusp of the evolutionary division between man and chimpanzee - Yohannes Haile-Selassie, a doctoral candidate (in 2005) at the University of California at Berkeley, said the hominid certainly is very close to the branching point.

Seventeen Ardipithecus ramidus fossils had been located by the end of 1993 from a cluster of localities to the west of the River Awash, within the Afar Depression in Aramis, Ethiopia. The physical attributes of this hominid show a range of primitive traits, which are most likely character retentions from the last hominid/chimpanzee ancestor. At the same time, some hominid innovations are apparent.

The currently known traits of ramidus can in general be placed within two categories: ape-like traits and Australopithecine-like traits. The creature's teeth share more characteristics with all known hominid finds from dates later than this period than they do with the teeth of all fossil and modern apes. The relatively large back teeth and narrow front teeth indicate that ramidus ate less fruit and more soft leaves and fibrous food than his chimpanzee contemporaries, who were specialised frugivores.

Vertebral differences also suggest that if ramidus was not bipedal in the modern sense, it at least had key adaptations toward a similar end, almost certainly walking on two legs when on the ground.

Haile-Selassie believes that ramidus was about the size of a modern-day chimpanzee and about twenty percent larger than the 'Lucy' specimen. Because neither the skull nor intact limb bones of ramidus have been found an artist's impression of the creature was impossible at this time (2005).

At the time of its existence, ramidus lived in a forested flood-plain environment - a far cry from Ethiopia's present day environment of harsh desert surroundings. The area in which the hominid dwelled was as much as 457 metres higher in elevation than today and it was much cooler and wetter.

However, the hominid lived at a time at which Africa was in the throes of continental change. The area was peppered with active volcanoes and intense earthquakes related to the formation of the rift valley (caused by the continent's collision with Europe and Asia, and its continued northerly movement).

The Awash region then was showered with pulses of thick, hot volcanic ash from nearby volcanoes. 'It's hard to imagine that life would go on under such hostile environmental conditions,' Giday WoldeGabriel, a geologist with the Los Alamos National Laboratory, said. 'Ardipithecus and the other animals inhabiting the region were real survivors.' The researchers found that numerous animals lived during the time of ramidus.

The research team found more than 1,900 fossil specimens comprising the remains of more than sixty identified mammal species. The fossils included primitive elephants, horses, rhinos, rats, and monkeys. Researchers found the remains of more than twenty primitive elephants together at one site.

5.7 million

Ardipithecus kadabba

This was the second Ardipithecus species to be found for this period. Ardipithecus ramidus appeared around 5.8 million years ago, while kadabba first cropped up at about 5.77 million years ago. It lasted until about 5.54 million years ago.

Ardipithecus kadabba was bipedal - one bone from the large toe has a broad, robust appearance, suggesting its use in bipedal 'push-off'. It was probably similar in body and brain size to a modern chimpanzee.

It possessed canines which resemble those of later hominins but which still projected outwards. Only a few post-cranial bones and sets of teeth have been found to date, so information on this species is generally lacking.

Discovered in 1997 in the Middle Awash region of Ethiopia, a total of eleven specimens were found for at least five individuals. The fossils included hand and foot bones, partial arm bones, and a clavicle (collarbone). While most are dated to 5.8 to 5.6 million years, one of the specimens, a toe bone, is dated to 5.2 million years.

Animal fossils from the site indicated that these early humans lived in a mixture of woodlands and grassland, just like A ramidus. They had plenty of access to water via lakes and springs.

In 2002, six teeth were discovered in the Middle Awash at the site known as Asa Koma. The dental wear patterns confirmed that these fossils were unique and not a subspecies of A ramidus.

Based on these teeth, palaeoanthropologists Yohannes Haile-Selassie, Gen Suwa, and Tim White allocated the fossils in 2004 to a new species A kadabba ('kadabba' means 'oldest ancestor' in the Afar language).

4.4 million

During the Pliocene era (between 5 million and 1.8 million years ago), hominids were walking upright on a permanent basis, allowing them to fully leave behind their former arboreal habitats and survive on the ground in an increasingly treeless environment in Africa. This also freed their hands for important new tasks, such as food-gathering.

4.2 million

Australopithecus anamensis

Australopithecus anamensis appeared in Pliocene Africa, surviving until 3.8 million years ago. It was probably descended from Ardipithecus ramidus.

First discovered in the Kanapoi region of eastern Lake Turkana, Kenya, in 1965, anamensis comes from 'anam', meaning 'lake' in the local Turkana language. This species was named in August 1995 (Leakey et al 1995).

The material discovered at that time consists of nine fossils, mostly found in 1994, from Kanapoi, and twelve fossils, mostly teeth found in 1988, from Allia Bay in Kenya (Leakey et al 1995).

A general similarity to other Australopithecus species seems to exist in anamensis. It had a mixture of primitive features in the skull, and advanced features in the body. The teeth and jaws are very similar to those of older fossil apes, as is its ape-like crania, although this clearly marks it out as a bipedal hominid.

A partial tibia (the larger of the two lower leg bones) is strong evidence of bipedal behaviour, and a lower humerus (the upper arm bone) is extremely humanlike. Note that although the skull and skeletal bones are thought to be from the same species, this is not confirmed.

This reconstruction of Australopithecus anamenis shows a fairly chimp-like facial type, but this creature walked upright, much more like a human than an ape

Curiously, the tibia and humerus of anamensis may be more similar to those from members of the genus Homo than they are to Australopithecus afarensis. This has not been decisively shown but, if true, would bring up a very interesting possibility.

It may be the case that Homo sapiens is more closely related to this four million year-old hominid than to the widely successful later hominid - afarensis. For the moment this is pure speculation, but it exists as a possibility.

Anamensis was superseded around 3.9 years ago by the aforementioned Australopithecus afarensis. Small populations continued to exist for some time after that, quite possibly in isolation in a landscape of steep hills, volcanoes, lava flows, and rifts.

An overlap of at least 100,000 years has been put forward, muddying the waters when it comes to establishing a clear line of descent for later Homo species.