omelette, and so on. In the same way, similar genes, combined according to different molecular recipes, can create animals as radically different as humans and chimpanzees.
But regulatory genes show only that it is possible for small differences in DNA to create two species as different as human beings and chimpanzees. They do not tell us
how it happened
. For that there is no substitute for the fossil record.
Two legs good …
Charles Darwin came up with a possible story of human evolution. Our ancestors, he claimed, first stood up on two legs. This freed their hands to make tools. Making tools required more mental power, which in turn caused their brains to grow. It is an eminently plausible story except for one thing: it is contradicted by the fossil record.
Millions of years before they left any tools
and
millions of years before they had big brains, our hominin 2 ancestors in Africa were walking on two legs.
Australopithecus anamensis
, for instance, was a hominin barely more than a metre high with anape-sized brain. There is evidence from a fossil shinbone, or tibia, that,
before
4 million years ago, it was walking on two legs, or bipedal, much of the time. The rest of the time, presumably, it was still hanging around in the trees. Then there is a close relative,
Australopithecus afarensis
. According to the evidence of a fossil leg bone, it was walking upright by about 3.5 million years ago.
But surely one of the most evocative discoveries in all of palaeoanthropology was made by Mary Leakey at Laetoli in Tanzania in 1976. Some time, around 3.6 million years ago, three
australopithecines
padded on two legs across a bed of freshly fallen volcanic ash. They left their fossilised footprints there for all of posterity. ‘Who does not wonder what these individuals were to each other, whether they held hands or even talked, and what forgotten errand they shared in a Pliocene dawn?’ says Richard Dawkins. 3
Walking on two legs releases the hands to carry food or offspring, to fashion tools and to brandish weapons. It also allows an ape to range further afield for food and to spot predators at a greater distance. Darwin was right in believing that bipedalism has distinct advantages. The difficulty is in explaining how it came about. The only changes perpetuated by evolution by natural selection are ones that are
immediately advantageous
. 4 However, being on two legs requires a major change in the structure of the leg bones – longer thigh bones, and a shorter and wider pelvis – and the development of a powerful bottom muscle, or
gluteus maximus
, to keep those bones upright and power a running gait. Until both of these changes have been made – which is likely to have taken many generations – there appears to be no survival advantage to being on two legs.
One intriguing possibility is that the first steps towards bipedalism were taken not on the ground but up in the trees. Gibbons and orang-utans often saunter on two legs along branches so they can reach the juicier leaves and fruit at the very tips. Our ancestors might have learned this same trick. Then, when they descended to the ground, they continued the walking habit, bounding on two legs between trees.
The naked ape
Eventually, something drove our ancestors to stay on the ground permanently, where they perfected their unusual bipedal mode of locomotion. The most likely thing is that the climate gradually became drier. The dwindling rains caused the forest habitat of our hominin ancestors to shrink and be replaced by open grassland across vast tracts of their African homeland. When other creatures adapted to this new habitat, the lure of the vast grazing herds simply became too great. First cautiously, then with gathering boldness, our ancestors ventured from the leafy shadows out into the unforgiving sun.
It was
Homo erectus
, between about 1.8 and 1.9 million years ago, that made the transition to a recognisably human body shape. Walking upright was an
But regulatory genes show only that it is possible for small differences in DNA to create two species as different as human beings and chimpanzees. They do not tell us
how it happened
. For that there is no substitute for the fossil record.
Two legs good …
Charles Darwin came up with a possible story of human evolution. Our ancestors, he claimed, first stood up on two legs. This freed their hands to make tools. Making tools required more mental power, which in turn caused their brains to grow. It is an eminently plausible story except for one thing: it is contradicted by the fossil record.
Millions of years before they left any tools
and
millions of years before they had big brains, our hominin 2 ancestors in Africa were walking on two legs.
Australopithecus anamensis
, for instance, was a hominin barely more than a metre high with anape-sized brain. There is evidence from a fossil shinbone, or tibia, that,
before
4 million years ago, it was walking on two legs, or bipedal, much of the time. The rest of the time, presumably, it was still hanging around in the trees. Then there is a close relative,
Australopithecus afarensis
. According to the evidence of a fossil leg bone, it was walking upright by about 3.5 million years ago.
But surely one of the most evocative discoveries in all of palaeoanthropology was made by Mary Leakey at Laetoli in Tanzania in 1976. Some time, around 3.6 million years ago, three
australopithecines
padded on two legs across a bed of freshly fallen volcanic ash. They left their fossilised footprints there for all of posterity. ‘Who does not wonder what these individuals were to each other, whether they held hands or even talked, and what forgotten errand they shared in a Pliocene dawn?’ says Richard Dawkins. 3
Walking on two legs releases the hands to carry food or offspring, to fashion tools and to brandish weapons. It also allows an ape to range further afield for food and to spot predators at a greater distance. Darwin was right in believing that bipedalism has distinct advantages. The difficulty is in explaining how it came about. The only changes perpetuated by evolution by natural selection are ones that are
immediately advantageous
. 4 However, being on two legs requires a major change in the structure of the leg bones – longer thigh bones, and a shorter and wider pelvis – and the development of a powerful bottom muscle, or
gluteus maximus
, to keep those bones upright and power a running gait. Until both of these changes have been made – which is likely to have taken many generations – there appears to be no survival advantage to being on two legs.
One intriguing possibility is that the first steps towards bipedalism were taken not on the ground but up in the trees. Gibbons and orang-utans often saunter on two legs along branches so they can reach the juicier leaves and fruit at the very tips. Our ancestors might have learned this same trick. Then, when they descended to the ground, they continued the walking habit, bounding on two legs between trees.
The naked ape
Eventually, something drove our ancestors to stay on the ground permanently, where they perfected their unusual bipedal mode of locomotion. The most likely thing is that the climate gradually became drier. The dwindling rains caused the forest habitat of our hominin ancestors to shrink and be replaced by open grassland across vast tracts of their African homeland. When other creatures adapted to this new habitat, the lure of the vast grazing herds simply became too great. First cautiously, then with gathering boldness, our ancestors ventured from the leafy shadows out into the unforgiving sun.
It was
Homo erectus
, between about 1.8 and 1.9 million years ago, that made the transition to a recognisably human body shape. Walking upright was an
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