Under the Microscope Prof William RevilleModern paleoanthropology began in the 19th century with the discovery of Neanderthal Man in 1856.
It was obvious that humans are similar to some great apes and the idea of general biological evolution was formalised when Charles Darwin published On the Origin of Species in 1859. The notion of human descent from an ape-like ancestor became an established interpretation of the theory of evolution. Fossil remains of such ancestors were found in Africa in the early 20th century.
In 1924 Raymond Dart found a famous fossil in South Africa known as the Taung child. The fossil was classified as Australopithecus africanus. The Taung child had a well-preserved skull and, although its brain was small, its shape resembled a modern human brain more than that of chimp or gorilla. The skeleton also had short canine teeth, as have humans, and the position of the foramen magnum, the hole at the base of the skull through which the spinal cord passes, indicated bipedal locomotion. Dart was convinced he had found a bipedal human ancestor. But it took another 20 years before Dart was taken seriously, after more fossils resembling his were found.
In December 2000, paleoanthropologists made a remarkable find at Dikika in Ethiopia. They found the fossilised remains of a three-year-old female Astralopithecus afarensis (A afarensis) who lived 3.3 million years ago. A afarensis is believed to be the immediate predecessor to the genus Homo to which modern humans belong. This latest find comes from the same locality where another celebrated find was made in 1974, the 3.2 million-year-old skeleton of the young female A afarenesis nicknamed Lucy. The December 2000 find has been nicknamed Selam, meaning peace in a number of Ethiopian languages. Study of Selam is yielding valuable information on the evolution of upright waking. The story is summarised by Kate Wong in Scientific American, December 2006.
Some decades ago there was a debate amongst paleoanthropologists as to whether the evolution of a large brain predated the evolution of upright walking in human evolution or vice-versa. The debate is now settled - bipedalism came first and the discovery of small-brained, upright Lucy helped to settle this matter.
Selam's skeleton confirms this conclusion, but is also helping to answer another unresolved question. Did A afarensis spend all its time on the ground or did it, like the modern ape, live a partially arboreal life?
Selam is remarkably well-preserved. The skull and torso are pretty much complete and parts of the arms and legs are well preserved. She exhibits the characteristic features of A afarensis, eg narrow nasal bones, and is easily differentiated from the closely related A africanus species described by Dart.
The A afarensis remains discovered so far, including Selam's, all show adaptations to bipedalism in the lower body, but the upper body seems to be adapted for tree-living, eg long curved fingers for gripping tree branches. Two possibilities suggest themselves - either A afarensis simply retained some evolutionary baggage inherited unmodified from a previous arboreal existence, or else these tree-climbing adaptations indicate that climbing trees was still important to A afarensis.
Selam's remarkably preserved skeleton shows features not seen before. She has two shoulder sockets that her discoverer Zeresenay Alemseged thinks resemble the shoulder sockets of gorillas. These sockets facilitate raising the hands over the head as primates do when they climb.
Selam's inner ear canal was examined by computed tomography imaging and resembles the inner ear of the African apes more than that of the human. The inner ear is important for maintaining balance. Wong reports that this indicates that A afarensis was not as quick and nimble on its feet as humans and had limited ability to independently move its head and its torso. It is thought that flexibility in this latter regard gave early humans a capacity for endurance running which was an important advantage.
It would not be at all surprising if A afarensis spent some time off the ground in the trees. The environment it lived in at Dikika was a wet delta rimmed by forests and with grasslands nearby.
The commonest interpretation of the significance of the suitability of the lower body for bipedalism and the upper body for tree-dwelling in A afarensis is that evolution moved in a piecemeal fashion with natural selection choosing for bipedalism in the lower body first and leaving changes to arms and shoulders to come later.
Studies of Selam's skull also indicate piecemeal change. She has an ape-like voice box but her brain seems to be developing more slowly in size than the rate characteristic of a chimp. This slower rate of development is more human like.
So much for the past, but what is the future of human evolution? One possibility is the concept of "transhumanism", a term coined by the biologist Julian Hurley in 1957 but prefigured as early as 1769 by the French philosopher Denis Diderot. This means deliberately transcending the limitations of our human bodies and brains through the use of technology.
We are now within striking distance of embarking on this road if we so choose using pharmacology, nanotechnology, biotechnology and artificial intelligence. We should soon be able to extend lifespan, enhance our senses, master tiredness, gain greater control over our emotions, and so on. We could merge our minds and bodies with computer power. The big question is - if we do this will we remain human and therefore, should we go down this road at all?
William Reville is associate professor of biochemistry and public awareness of science officer at UCC - http://understandingscience.ucc.ie