A mountain of evidence has accumulated showing that our ancestors emerged in Africa. What is less clear-cut is what spurred their evolution
A mountain of evidence has accumulated showing that our ancestors emerged in Africa. What is less clear-cut is what spurred their evolution. The answer lies in the environments in which our predecessors lived, and the influence of technology, which hugely expanded their ecological niche.
Technological primate
Hominids are frustratingly rare in the fossil record, but at some time around 2.6 million years ago they began to leave calling cards, in the form of stone artefacts.
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At the adjacent archaeological sites of the Gona and Middle Awash in Ethiopia, there is now abundant and unambiguous evidence of the earliest stone tools made by hominids. The fossilised bones of large mammals bear definite traces of marks made by sharp instruments.
The production of sharp-edged stone flakes enabled hominids to eat large amounts of meat and marrow previously unavailable to primates. At the same time, the selective pressures associated with such activities 鈥� particularly for a bipedal primate operating cooperatively under the noses of abundant predators, from hyenas to sabre-toothed cats 鈥� would lead to dramatic anatomical change as the braincase enlarged in Homo.
Stone technology greatly widened our ancestors鈥� ecological niche, as well as their geographic range, enabling H. erectus to reach Europe and Indonesia more than 1.5 million years ago.
Cactus or bush?
The late American palaeontologist Steven Jay Gould wrote a classic essay in 1977 in which he predicted that the hominid family tree would prove to be 鈥渂ushy鈥�. Today, it is common to see lists of more than 25 different hominid species, and Gould鈥檚 prediction is often declared to be fulfilled.
Not so fast. Many of these species are 鈥渃hronospecies鈥�, which evolve from one to the other, such as the earliest two Australopithecus species, A. afarensis and A. anamensis. These names are merely arbitrary divisions of a single evolving lineage.
A modern biologist addressing the question of species diversity counts the number of related species existing at any one time. When we do the same thing across the hominid fossil record, what we get is not a bush but something like a saguaro cactus, with only a few branches or species lineages. Indeed, the greatest diversity among hominid species appears to be at around 2 million years ago, when as many as four different lineages 鈥渂riefly鈥� co-existed in Africa.
The key question turns out to be not how many species there were per se, but rather why species diversity has been so limited on our branch of the evolutionary tree compared with other mammals like fruit bats or South American monkeys? The reason is probably that our ancestors鈥� niche kept broadening, as a woodland omnivore 6 million years ago expanded ecologically into more open environments, and then again as technology further extended its capability and horizons.
Robust Australopithecus
Look at the skull on the right. Would you say it looked robust? That鈥檚 what palaeontologist Robert Broom thought when it was found in South Africa in the late 1930s, naming this hominid Paranthropus robustus. It has oversized molars, tiny canines and incisors, a massive lower jaw, dished face, small brain and, usually, a bony crest atop its skull. It came to be known as 鈥渞obust鈥� Australopithecus, and it appears in the fossil record more than 2.5 million years ago, in eastern Africa, with its last members some 1.2 million years ago. By that late date, our genus, Homo, had been on the scene for more than a million years. There are many mysteries about robust Australopithecus still to be solved, but one thing is clear: this side of 2.5 million years ago, our lineage was not alone.
鈥淭his side of 2.5 million years ago, our lineage was not alone鈥�
The savannah hypothesis
Many modern palaeoanthropologists invoke climate change as the motor for our evolution. But they are hardly the first to recognise the impact of the environment. Long before relevant fossils were found, an early proponent of evolution, Jean-Baptiste Lamarck, saw grasslands as pivotal in the evolution of our ancestors from tree dwellers to bipeds. He was followed by Raymond Dart in the 1920s, who argued correctly that the fossil child he named Australopithecus was adapted to open environments.
But the popularity of the 鈥渟avannah hypothesis鈥� began to wane in the 1990s, when Ardipithecus fossils were found in contexts suggesting a woodland habitat. Today independent lines of evidence suggest that the earliest hominids were indeed woodland creatures: climbing adaptations; diet as deduced from the shape, wear and isotopic composition of teeth; and the thousands of plants, insects, snails, birds and mammals that also prefer such habitats and are abundant in the same localities. Australopithecus came next, though, and does appear to have been associated with more open landscapes.
It has been known since the 1940s that the hip, knee and foot of Australopithecus were adapted to bipedality. However, it was the discovery of the 鈥淟ucy鈥� fossils (see right) in Ethiopia and fossilised footprints in Tanzania during the 1970s that established this genus as representative of the evolutionary phase from which later hominids evolved. By 3 million years ago, Australopithecus species had spread from north to south across much of Africa.
To 20th-century anthropologists, Australopithecus seemed like an unstable transition between ape and human. Now, however, in the light of the Ardipithecus discoveries, this genus is seen as a long-lasting phase of our evolution. As well as gaining the means for habitual two-footed walking, robust forms became adapted to heavy chewing and developed relatively large back teeth with thicker enamel (see 鈥淩obust 础耻蝉迟谤补濒辞辫颈迟丑别肠耻蝉鈥�). Some contemporary but less robust species is likely to have given rise to the Homo genus.
