Bones of contention
This is a shortened and updated version of an article written by Matthew Reynolds in 2000
In the original article, Matthew Reynolds remarked: 'The story of human evolution is based on a consensus of palaeontologists, but don't be surprised if a small discovery next year changes the whole sequence of events.' Little did Matthew know that that 'small discovery' was actually taking place as he wrote: the finding of a dozen bone fragments in Kenya really did change everything...
• A short history of human evolution
• Find out more
For
the first archaeologists concerned about our origins in the wake of Darwin,
the story of human evolution was based on a handful of finds and was therefore
very simple. But as more and more fossilised human bones were discovered
during the next 100 years, the picture became increasingly complicated.
The only way now to interpret the evidence is to see evolution as a complex
event, with several different species of semi-humans and pre-humans alive
at the same time, perhaps competing for natural resources. Out of this
complex picture, the modern human evolved.
A short history of human evolution
Orrorin tugenensis
In October 2000, experienced fossil hunter Ezra Kiptalam Cheboi made a discovery in the Tugen Hills of north-west Kenya – part of the Great Rift Valley – that changed the timeline of human evolution. Dr Martin Pickford and Professor Brigette Senut of the National Museum of Natural History in Paris, working with the Community Museums of Kenya, identified Cheboi's find – part of a jawbone with teeth. Along with a femur (thighbone), a humerus (upper armbone) and other bone fragments found later, the jawbone belonged to one of at least five individuals who had lived 5.8-6.1 million years ago – the oldest pre-human remains found so far.
Because of the timing of the discovery, the press dubbed this creature 'Millennium Man', but the scientists eventually called it Orrorin tugenensis – 'Orrorin' means 'original man' in the local Kenyan dialect.
The important thing was to discover whether this species was bipedal – that is, did it walk on two feet and so was a hominid? Unfortunately the femur was broken off above the knee (which would have been a major clue), but a groove below the hip joint suggests that this is where a ligament would have been that would have angled the femur towards the knee – a prominent sign of bipedalism. In addition, a CAT scan showed that the neck of the hip joint was thicker at the bottom than at the top, which it would have been if it had been stressed from the weight of an upright body.
Orrorin had back teeth that were human-like, but its front teeth resembled those of chimps. It would have been an omnivore, eating nuts, berries, fruit, insects and, possibly, meat.
Powerful muscles would have been attached to the flare at the lower end of Orrorin's humerus, which suggests that the creature would have been a good climber. This is even more likely because the finger bones found were permanently curved, due to grasping and pulling up on branches.
The scientists now think that Orrorin lived among the trees, before this part of Africa changed from thick forest into savannah. This goes counter to the previous theory that walking on two legs occurred after this environmental change. The scientists point out that living in trees may have helped to develop the acute sense of balance needed for bipedalism. Present-day orang-utans give hints as to how walking on two feet could have developed among the branches.
Ardipithecus ramidus
Found in 1993 at Aramis in Ethiopia by palaeontologist Tim White, Ardipithecus ramidus was also a bipedal hominid and, like Orrorin, appears to have been a forest dweller when it lived 4 to 5 million years ago. The evidence is a few fossilised skull fragments, but a great deal has been learned from these. Standing at an estimated 122 centimetres (4 feet), Ardipithecus ramidus's teeth are intermediate between those of apes and other archaic humans that developed later.
Australopithecus anamensis
A little later, 4.2 million years ago, Australopithecus anamensis arrived. Its remains were found in 1965 in Kenya by palaeontologist Bryan Patterson. This species may have coexisted with Ardipithecus ramidus for some time, but then outlived its cousin. A. anamensis has a mix of primitive skull features and advanced body features. Although the skull looks relatively ape-like, the legs give strong evidence for walking bipedally and the arms are extremely human-like: Patterson found a well-preserved humerus. A. anamensis probably managed to survive for about 350,000 years, but it was eventually outlived by another hominid that shared some of its time on earth.
Australopethecus afarensis
This creature walked the earth for nearly 1 million years, with the earliest examples dating from about 3.9 million years ago. Before A. anamensis and A. ramidus and, now, Orrorin tugenensis were discovered, A. afarensis was seen as the first archaic human. This species came from Ethiopia and includes 'Lucy', a fossil of a 25-year-old female. Found in 1974, Lucy provided one of the first pieces of evidence for bipedal walking and gave scientists a major breakthrough.
With a low forehead, bony ridge above the eyes, flat nose and protruding jaw, A. afarensis would have looked very robust. Its pelvis and leg bones closely resemble those of modern humans. These hominids were very strong and some were 152cm (5ft) tall. They also exhibited sexual dimorphism – that is, the females were smaller than the males. Finally, they had a cranial capacity of between 375 and 550 cubic centimetres – and the larger brain suggests greater survival abilities.
Australopithicus africanus
Roughly 3 million years ago, another Australopithecus entered the record and was to survive a further million years. Discovered by Raymond Dart in 1924 in South Africa, A. africanus – known familiarly as the 'Taung Child' – comprised an exceptionally well-preserved skull. In 1947, Robert Broom found further bones, including vertebrae, ribs and pelvis, which completed the picture.
A. africanus may have had a slightly larger brain than A. afarensis, one that was a little bigger than modern-day chimps', but it was still not advanced enough in areas necessary for speech. Its teeth were smaller than those of A. afarensis.
The Australopithecines
Our evolution appears to have received a boost some 2-2.5 million years ago, when several new Australopith types came on the scene. It is almost as if it became a competition to see which species would survive. These hominids developed robust features with the advent of Australopithecus robustus (found in South Africa by a Mr Fourie in 1950), A. aethiopicus (found in Kenya by Alan Walker in 1985) and A. boisei (discovered by Mary Leakey in 1959 at the Olduvai Gorge in Tanzania). All these types had a cranial capacity of about 530cc. A more graceful offshoot called A. garhi, a 1990s discovery, also existed for a short time. And our own genus, Homo, appeared for the first time.
Homo habilis
The first H. habilis – or 'handy man' – existed between 2.4 and 1.5 million years ago. Although similar to the Australopithecines in many ways, its cranial capacity could reach 800cc and its brain shape was more human-like. There are also signs that the Broca area of the brain, essential for speech, was developing and that H. habilis may have been capable of rudimentary speech.
H. habilis was originally a controversial species. Many palaeontologists did not accept its validity because the few remains that were found seemed too similar to Australopithecines. However, following further discoveries (such as those made by Bernard Ngeneo in Kenya in 1972), it is now accepted as a species.
The H. habilis skull is rounder than that of its Australopithecine relations, its teeth are reduced in size, its arms are shorter and its legs are longer. We also have evidence for the use of tools: H. habilis knocked chips from rocks to create what are now called 'Oldowan tools'.
Homo erectus
After H. habilis had been around for about 600,000 years, another Homo species arrived. H. erectus existed between 300,000 and 1.8 million years ago. Like H. habilis, its face had protruding jaws, thick brow ridges and a long, low skull, but its cranial capacity could reach 1,225cc. Its skeleton was more robust than those of modern humans, implying greater strength, and there is evidence that H. erectus would have been more efficient at walking than we are today because our bodies have since adapted to allow the birth of larger-brained infants.
Apart from being fairly certain that H. erectus could speak, we also have evidence that they used fire and were making more advanced tools than H. habilis. 'Acheulian' tool sets (large cutting tools, especially hand axes and cleavers) arrived at this time and teardrop-shaped axes were produced. H. erectus was also quite adventurous. All of the species described so far have been found in Africa, but H. erectus has been discovered in Asia and Europe, too.
Then followed a phase, covering roughly 500,000-200,000 BC, the evidence for which consists mainly of skulls. Different features on them relate to both H. erectus and modern humans – for instance, they had an average 1,200cc cranial capacity (ours today is about 1,350cc). These individuals have been grouped into a banding called Homo sapiens archaic. The skulls are more rounded than that of H. erectus and the other skeletal remains lead us to believe that they were more slightly built, although still more bulky than modern humans.
The Neanderthals
Our closest cousins – Homo sapiens neanderthalensis – appeared some 230,000 years ago, about 70,000 years after H. erectus died out, but coexisting with Homo sapiens archaic. Although their heavier bones undoubtedly made the Neanderthals fairly stocky, they were starting to look like humans today and stood at about 168cm (5ft 6in). Their brain size was slightly larger than ours, but this has been attributed to their greater bulk. It has even been suggested that their intelligence was as advanced as ours.
Found mainly in colder climates, the Neanderthals lived in Europe and the Middle East. A large number of tools and weapons are associated with them, and we also find the first evidence for burial of the dead. They obviously cared for each other and must have related on an emotional level.
Modern humans
After Homo sapiens neanderthalensis had been around for some 110,000 years, existing – and probably competing – with Homo sapiens archaic, another competitor joined the race. About 120,000 years ago came the first evidence for Homo sapiens sapiens, or modern humans.
About 40,000 years ago, the Cro-Magnon culture came into being, leaving behind advanced tool kits using a variety of materials. There is also evidence for engraving, sculpting and making clothing. After about 10,000 years of coexistence, the Neanderthals and the archaic Homo species began to die out, and both faded from the record about 30,000 years ago.
At this point, we were on our own. Over the next 20,000 years, H.
s. sapiens developed fine artwork in the form of beads, decorated
tools, carvings of animals and humans, cave paintings and even musical
instruments. And human development has not stopped. Mesolithic humans
had a face, jaw and teeth 10% more robust than ours – which means
that humans have adapted to eating more refined foods over the past 10
millennia.
Find out more
Modern Human Origins
www.modernhumanorigins.com
Check out the 'Hominid index' on this site, which lists all our prehistoric
forebears. By clicking on any of the names – including Orrorin
tugenensis – you will find an article describing in detail all
the remains of that particular creature and what they signify.
