Born in Africa_ The Quest for the Origins of Human Life - Martin Meredith [52]
But further research by Sarich and Wilson, involving DNA molecules as well as proteins, confirmed the validity of their original approach. ‘One no longer has the option of considering a fossil specimen older than about eight million years a hominid, no matter what it looks like’, proclaimed Sarich in 1971.
For more than a decade, many palaeoanthropologists fought a rearguard action against the molecular school but steadily lost ground. By the early 1980s, a new family tree using molecular techniques had gained widespread acceptance. It showed the ancestor of gibbons splitting off first from the line leading to humans more than 20 million years ago; followed by the ancestors of orang-utans splitting off at 16 million years ago; by gorillas at about 10 million years ago; by chimpanzees at between 6 and 7 million years ago; and finally, by australopithecines splitting off from the line leading to early Homo at between 3 and 4 million years ago.
While the molecular revolution was gathering momentum, new schools of thought were emerging about the process of evolutionary change. Since the 1950s, the ruling doctrine to which most scientists subscribed was the evolutionary, or modern, synthesis, the theory that viewed evolution as the steady accumulation of small genetic changes over long periods of time under the guiding hand of natural selection. Its defining feature was gradualism. New species were the result of gradual change involving a whole population.
But in 1972, two American invertebrate palaeontologists, Niles Eldredge of the American Museum of Natural History and Harvard University’s Stephen Jay Gould, presented a counter-theory. From his studies of trilobites—fossil marine arthropods dwelling on the sea bottom—Eldredge had noticed a distinct lack of evolutionary change; for millions of years, one type of trilobite had remained unaltered until environmental change had enabled a new species to invade and replace it. Gould had observed the same kind of pattern from his studies of an ancient species of land snail. It appeared to both of them that evolutionary change occurred not so much as part of a gradual process but in relatively short sporadic episodes, with most change being concentrated in branching events in a geographically restricted subset of a population. Once evolved, new species, with their own peculiar adaptations, behaviours and genetic systems remained unchanged for long periods of time, often for several million years. In a paper published in 1972, they called this process ‘punctuated equilibrium’ and posed it as an alternative to the doctrine of ‘phyletic gradualism’.
They claimed that their theory provided a more convincing explanation of the gaps that seemed to occur in the fossil record than the one given hitherto by the ‘gradualist’ school. They pointed out that if evolution had occurred as a result of slow, gradual and continuous change over the generations, as the gradualists maintained, then the fossil record should show it. But the record appeared to contain gaps. In the past, these gaps had been attributed to the difficulty that researchers faced in finding enough fossils to fill in the record. But even though a plethora of fossils had since been discovered, the ‘gaps’ were still said to be there. The explanation, said Eldredge and Gould, was that evolution occurred in bursts of change followed by long periods of stasis, or non-change. What the fossil record showed, they argued, was ‘breaks’ in the pattern of evolution, not missing links; in some cases, changes may have occurred so rapidly that intermediate forms had not been preserved in the fossil record.
They also contested the gradualist assertion that human evolution had occurred as a straight line of continuous transformation of one species into the next. They argued instead that human evolution resembled a multibranched bush of diversity, with many species coming