Australopithecus afarensis

Lucy was the first fossil hominid to really capture public notice, becoming almost a household name at the time.

Current opinion is that the Lucy skeleton should be classified in the species Australopithecus afarensis.

Lucy is preserved at the national Museum in Addis Ababa, Ethiopia. A plaster replica is displayed instead of the original skeleton.

Physical characteristics

Chimp-sized brain

This was because during the period 1950-1970 it was believed that the development of a brain larger than an ape brain was the trigger that caused apes to evolve into humans. Before Lucy, a fossil called '1470', with a brain capacity of about 800 cubic centimetres had been discovered, an ape with a bigger brain, and if the 'big brain' theory was correct, then all humans should have evolved from 1470. However, it turned out Lucy was older than 1470, yet Lucy had bipedalism (she walked upright), i.e. she had developed a human trait without a big brain, which destroyed the 'big brain' theory. Lucy's brain was only around 400 cc.

Bipedalism

The so-called 'savannah theory' on how A. afarensis evolved bipedalism hangs on the evidence that around 6 - 8 million years ago there seems to have been a mass extinction of forest dwelling creatures.
This triggered a burst of 'adaptive radiation', an evolutionary characteristic that generates new species quickly.
Lucy's 'grandparents' were tree dwelling apes, but in Lucy's world the trees would have gone, and Lucy would have been forced to find a living on the flat treeless savanna.
Being bipedal would have had evolutionary advantages - for example, with the eyes higher up, she could see further than quadrupeds.
The disadvantages of bipedalism were great - Lucy was the slowest moving primate of her time, for example, but the advantages of bipedalism must have outweighed the disadvantages.

The minority Aquatic Ape Hypothesis makes the observation that when apes wade in water, they always walk bipedally.
It suggests that bipedal hominids evolved from a population of apes occupying a semi-aquatic habitat, so that they waded or swam all the time they were active, and rested in trees at night.
Over generations they became more and more upright, and lost the ability to walk quadrupedally.
By Lucy's time, the hominids had reoccupied land areas, but were now committed to upright bipedal walking.

Related work

Recently, an entirely new species has been discovered, called Kenyanthropus platyops. This has the same characteristics as Lucy, but is possibly an entirely different genus.

Another species, called Ardipithecus ramidus, has been found, which was fully bipedal, yet appears to have been contemporaneous with a woodland environment, and, more importantly, contemporaneous with Australopithecus afarensis. We do not yet have an estimate of the cranial capacity of A. ramidus, however.

See also