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Prehistoric World

Hominid Chronology

by Peter Kessler, 26 July 2005. Updated 10 August 2011

 

 

Rainforest Beginnings

Arboreal Beginnings
 

20-10 million

During the Early and Middle Miocene eras, dating from 20 to 10 million years ago, Africa had a much higher annual rainfall level than today. A single super-rainforest covered most of the continent from shore to shore. A hominoid (primate) ancestor common to all living apes and humans lived in the trees of this super-rainforest.

Africa first became connected to Eurasia around 18 million years ago, causing major geological upheavals, and transforming formerly flat landscapes by pushing up some of the world's youngest mountain ranges: the Pyrenees, the Alps, and the Zagros mountains, as well as the mountains and rift valleys of East Africa. Geographical transformations were accompanied by gradual climatic and environmental changes which may even have triggered the emergence of the first specialised hominoids.

The very earliest known representatives of the hominoids have been found only in Africa and were very different from living apes and humans. The oldest finds belong to a group of species in the genus Proconsul. Fossils of Proconsul have been found in Early Miocene deposits 22 million years old in Kenya, and along the rift valley, although it probably originated rather earlier, between 25 and 28 million years ago.

 

Middle Miocene coastline

Middle Miocene coastline

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Proconsul's skeletons are described as generalised in that it shows none of the particular features such as thickened tooth enamel or adaptation for knuckle-walking which characterised the later apes. It was adapted to living in trees and was about the size of a gibbon.

Proconsul suffered somewhat during Africa's collision with Eurasia. Volcanic activity reached a peak 18 million years ago, especially in East Africa. The Kisingiri volcano erupted, apparently with a pyroclastic flow, killing whole populations of Proconsul which lived in an environment that was semi-arid, covered mostly in dry, deciduous, single-canopy woodland, with some evergreen forest in restricted, low-lying areas.

Despite repeated volcanic activity around this period, the species persisted into the Middle Miocene (15 to 10 million years) when it overlapped with Kenyapithecus.

 

Another early hominoid also existed in Africa around 20 million years ago. Ugandapithecus major is known to have lived around the site of a now-extinct volcano in Uganda's remote north-east Karamoja region.

Scientists say preliminary analysis of a single specimen discovered there showed that the tree-climbing herbivore was roughly ten years old when it died. The skull was about the same size as that of a chimp, but its brain was smaller.

 

At about 17.1 million years, probably as soon as a land bridge was formed between Africa and Eurasia, some hominids migrated out of Africa. Once in Eurasia, they eventually found a fertile valley in what is now south-western Germany, north of the Alps. The region apparently had its own microclimate with a mean temperature of 20ºC, about 11ºC above present day conditions there. There was a swamp to the south of a lake which was full of reed beds, with a coastline of deciduous trees, palm trees (amongst them the climbing rattan palms), lianas, ferns and grasses. To the north was a slope covered by an evergreen forest.

These Engelswies hominids eventually died out as conditions dried and cooled, but they were the first in a series of migrations out of Africa.

 

The appearance of Kenyapithecus between 15 to 11 million years ago marked the point at which some specialisations began to appear. This hominoid also lived exclusively in the rainforests of the rift valley, and showed modifications in the teeth and limb bones, making them more like those of the living great apes.

Kenyapithecus africanus can be regarded as being part of the combined great ape and human group but cannot be linked directly to any one of the living hominoids.

However, evidence has emerged which strongly suggests that Kenyapithecus is not one genus but two. Kenyapithecus wickeri appears to have migrated out of Africa at the same time as many other species and lain the roots of the later European apes. Kenyapithecus africanus has been rechristened by some scholars as Equatorius africanus, because the Equator is where all its remains have been discovered.

 

By around 14 million years ago there were other hominoid variations in existence, and these had migrated out of Africa, with some representatives spreading east across into the Asian continent. By 14 to 11 million years ago Ramapithecus and Sivapithecus were in Anatolia and Pakistan (and had reached South Asia by 8 million years ago where they lived alongside a sister-species, the enormous Gigantopithecus).

They are closely related to the orang-utan, and differed from each other only in size, suggesting that they were also closely related to each other and may perhaps be males and females of the same species.

Both also share some of the characteristics of Kenyapithecus, as well as particular features which show they must be related exclusively to the orang-utan and not to humans. This evidence indicates that the orang-utan was the first to split away from the general hominoid pool in Africa.

 

Dryopithecus emerged in Africa and Europe at around the same time (and persisted in Central and North Western Europe into the Late Miocene).

The connection between the two continents was more direct than it is today (see map, above). Much of the bulk of southern Europe was still a relatively thin corridor of land no more than twice the width of modern Italy in most places, while the Tethys Sea was twice as big as the Mediterranean Sea it would later become.

Dryopithecus remains have only been discovered in the past thirty years or so, and the creature, which was more like Proconsul, is highly unlikely to be related to any of the living hominoids, but there is a suspicion that it could have a place in early hominid ancestry.

 

10-7 million

At around 10 million years ago a large number of the first apes had blossomed into a huge radiation of species that reached across Europe and much of Asia. Over subsequent time the majority of them became extinct, mostly due to climate changes and extensive glaciation.

Further geological upheavals now occurred as the sub-continent of India moved north to collide with Asia. This created the Himalayas, causing weather patterns to change. Massively increased rainfall in India (the Monsoon) stripped the air of moisture so that the air currents that reached Africa were no longer wet, but dry.

In East Africa this caused drier conditions during the Late Miocene era (10 to 5 million years ago). Temperatures began to increase, resulting in a spread of grasslands as the super-rainforest began to die back over the next 5 million years. Animals had to adapt in response to these changes.

Between 7.0 to 5.8 million years ago, hominid transitional species appeared as the ancestors of man and chimpanzee divided. Hominid refers to the family of primates that includes all species on the "human" side of the evolutionary tree after that split. The chimpanzee's ancestors remained living in the remnants of the great forests, while early hominids placed an increasing reliance on surviving outside the shrinking forests.

Unfortunately, conditions in Africa between around 11 million to 5 million years ago were very detrimental to the preservation of fossils. There are very few finds made from this period, and very little evidence from which to build up an accurate picture of evolutionary events. However, some finds have been made more recently...

 

Proconsol skull

Primitive hominoids

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Earliest Hominids
 

     

6.7 million

Sahelanthropus tchadensis flourished between 6.7 to 6.3 million years ago.

This very recent discovery in the central African state of Chad, in the southern Sahara desert (Brunet et al. 2002, Wood 2002), is poised to upset the human family tree. The fossil skull that was found, nicknamed Toumaï, is as old as any hominid fossil found to date, 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 in 2001. 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. This is a mostly complete cranium with a small brain (between 320 and 380 cc) 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 concurs that it was likely to be a habitual biped because it shares characteristics with other hominids known to be 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 us than to chimpanzees. This is not at all certain. Some scientists think it probable; others have suggested that it may come from before the point at which hominids separated from chimpanzees, while 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 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 appeared between 6.2 to 6.0 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. The limb bones are about 1.5 times larger than those of Lucy, and suggest that it was about the size of a female chimpanzee.

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

 

5.8 million

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

While Ardipithecus ramidus is not the sought-after "Missing Link" - the so-far undiscovered creature that lived at the cusp of the evolutionary division between man and chimpanzee - Yohannes Haile-Selassie, a doctoral candidate 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 West of the Awash River, 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 Ardipithecus 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 later-discovered hominids than with the teeth of all fossil and modern apes. The relatively large back teeth and narrow front teeth indicate that Ardipithecus 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 20 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 is impossible at this time.

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 where the hominid dwelled was as much as 1,500 feet higher in elevation than today and it was much cooler and wetter. But the hominid lived at a time when 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 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 60 identified mammal species. The fossils included primitive elephants, horses, rhinos, rats and monkeys. Researchers found the remains of more than 20 primitive elephants together at one site.

 

4.4 million

During the Pliocene era (5 million to 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 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 East 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 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.

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 that Homo sapiens is more closely related to this 4 million year-old hominid than to the widely successful later hominid - Australopithecus afarensis. For the moment this is pure speculation, but it exists as a possibility.

Anamensis was superseded around 3.9 years ago by Australopithecus afarensis.

 

 

     

 

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, and subsequent archaeological discoveries. An original feature for the History Files.