Africa’s Great Rift Valley is one of the geological wonders of the world. Formed by the violent activities of volcanoes and earthquakes, the tectonic history of this part of East Africa has been traced back more than 900 million years. But the area has another significance. It is the cradle of human evolution. Could these two strands of its past be interwoven? The findings from a new geological study suggest that perhaps they could.
Some 150 kilometres east of the Rift Valley lie Kenya’s Chyulu Hills, a 100-kilometre chain of several hundred volcanoes towering more than 1000 metres above the savanna. Last year, a team of geologists from Karlsruhe University in Germany completed a detailed analysis of two sites in this region, and their findings make interesting reading in the light of local anthropological records. What they discovered was evidence for a period of extreme volcanic activity that started no earlier than 1.6 million years ago. Perhaps environmental changes resulting from this volcanic period could have been a driving force behind the evolution of modern humans.
As Darwin envisaged it, evolution occurs over millions of years, with natural selection within populations giving rise to new species. But what we know about human evolution does not fit in with this idea. Almost all the fossil remains of the ancestors of modern humans come from sites in and around the eastern arm of the East African Rift Valley and from southern Africa. The evidence is limited, but what there is suggests there were periods when many species suddenly appeared, followed by times of very little change. Similar patterns are found in the fossil remains from many other groups of animals.
Advertisement
For the past two decades, support has been growing for the idea that all species undergo spurts in evolutionary change – a theory known as ‘punctuated equilibrium’. Proponents suggest that these leaps may have been prompted by the need for animals to adapt to changes in their environment. If environmental changes can be shown to correlate with periods of speciation, when rapid evolution leads to the generation of new species, then the theory is strengthened. Global climate fluctuation is the main force controlling environmental change, and its links with speciation are strong (‘Taking the pulse of evolution’, New Scientist, 24 July 1993). But climate changes cannot explain all the major leaps in human evolution; a second harbinger of environmental change is required. This is where the finds of tectonic activity in Kenya may come into the picture.
Four key steps
Piecing together the story of human evolution is no easy task. The anthropologist Richard Leakey has identified four key steps in our evolution from the earliest hominid to modern humans. First, the occurrence of bipedalism between 10 and 4 million years ago. Then the evolution of Homo, with its large brain and capacity to make stone tools – the earliest examples of which are 2.5 million years old. Next, the evolution of Homo erectus almost 2 million years ago, followed by its migration out of Africa into Eurasia. And finally the appearance of modern humans less than 150 000 years ago.
Through the 10 million years of human evolution, the Earth’s climate has changed considerably. During the period that Michael Sarnthein of Kiel University has called the ‘Golden era’ – up to 3 million years ago – the world was much warmer than it is now. Then conditions started to deteriorate, and there was a gradual build-up of ice at the poles. Around 2.6 million years ago the climate became cyclical (see ‘A change in the weather’): ice ages characterised by huge ice sheets covering much of North America and northern Europe were followed by interglacials, when conditions were comparable to those we see today. Elizabeth Vrba of Yale University, one of the most vigorous proponents of the idea of punctuated equilibrium, has shown that this change in the world’s climate 2.6 million years ago had sudden and dramatic effects in Africa. A predominantly warm and moist climate was transformed into one which was colder and more arid.
Leakey is only one among several eminent anthropologists to speculate that global cooling around this time was the driving force behind the evolution of Homo, and the subsequent development of tool-making skills. These anthropologists propose that as Africa’s climate became drier, the vegetation became sparser and hardier, and our ancestors were forced to increase their mobility and adopt a more varied diet.
The fossil record seems to support this idea. It shows major speciation of hominids – and other animal groups – concentrated around 2.5 million years ago. In 1985, for example, Leakey together with Alan Walker, who comes from Johns Hopkins University in Baltimore, were excavating west of Lake Turkana in northern Kenya when they found evidence of the first appearance of two hominid species, Australopithecus afarensis and Paranthropus boisei, both about 2.5 million years old. More recently, an international team from the Hominid Corridor Research Project found a jawbone in East Africa that they assigned to another new species, Homo rudolfensis, dating from around 2.4 or 2.5 million years. After a decade of field work, the team also believes that Australopithecus africanus dispersed to eastern Africa from southern Africa and evolved into Homo habilis during this period of rapid hominid speciation.
But global climatic change cannot explain all the major shifts in human evolution. The development of bipedalism, the origin and migrations of Homo erectus, and the appearance of modern humans, for example, do not coincide with significant changes in global climate. Local changes, caused by tectonic shifts do, however, correlate with some of these developments, and the recent results from Kenya seem to strengthen this theory for the period from around 2 million years ago.
Turbulent history
One of the pioneers investigating the geology of East Africa almost half a century ago was Robert Shackleton from Imperial College, London. He showed that 900 million years ago the region experienced a collision of tectonic plates similar to the much more recent collision that threw up the Himalayas. The event, which is known as the Pan African orogeny, produced a massive mountain range, which has been eroded to the flat basement plain we see in Kenya today. It also led to the Mozambique Belt, an extensive series of faults running the length of East Africa.
Just 20 million years ago, the Great Rift Valley started to open up under the influence of east-west stresses in the tectonic plates beneath eastern Africa. This reactivated the Mozambique Belt fault system, and led to an upwelling of semifluid rock from the mantle below. The resulting bulge created the Kenyan and Ethiopian domes, two mounds rising almost 2000 metres above the East African plateau. The presence of this high ground altered rainfall patterns in the area, producing a mosaic of environments, from rainforest to woodland and shrub to grassland. Subsequent faulting along the tectonic plate boundaries caused the rift to subside several thousand feet, creating a natural barrier between the diverse environments of eastern Africa and the rest of the continent.
Around 10 million years ago, when this landscape was still evolving, there were at least 20 species of apes. But sometime during the subsequent 5 million years, the number of species started to decrease and one of them underwent a unique evolutionary transformation to become the first bipedal ape. What prompted this change?
Peter Rodman and Henry McHenry from the University of California showed recently that modern chimpanzees use as much energy walking on all fours as they use when walking on two legs. It appears there was no energy barrier to the evolution of bipedalism, and by forfeiting the apes’ compromise between walking on the ground and climbing trees our earliest ancestors could have increased their efficiency for long-distance travel. Rodman and McHenry suggest that the fragmentation of the forest cover that started 10 million years ago meant that food became more dispersed. Apes that evolved bipedalism were able to forage more successfully in this altered environment and so had an advantage over their unadapted cousins. In addition, their hands were freed to become specialised for manipulation and, ultimately, tool-making.
This may explain early developments in the evolution of humans, but what of later evolutionary leaps such as the appearance of H. erectus, H. habilis and modern humans? Until recently, it was thought that there had been no major tectonic activity in eastern Africa during the past 5 million years or so. Results from the German geologists’ survey in Kenya seem to put these assumptions in doubt.
In the summer of 1991, Gerald Haug and a small team of geologists from Karlsruhe University made the first of many expeditions to the Chyulu Hills. Most of the team members were students of Manfred Strecker, whose field work in the Rift Valley over the three preceding years had convinced him that there had been dramatic and recent changes in the direction of stresses shaping the local geology. Strecker, now professor of neotectonics at Stanford University in California, saw signs that the direction of stress had shifted from east-west to northwest-southeast. Two sites in the Chyulu Hills were chosen to test the theory, because they showed the young volcanic layers very clearly. The two-year study included detailed surveys of the fault system, together with chemical analysis of volcanic rock samples.
The geologists discovered evidence of extremely young tectonic activity overlying the much older Mozambique Belt fault structure. Field studies of fault zones in the different lava flows showed signs of five major changes in the direction of stress and intensity of volcanic activity. Chemical analysis revealed heavy contamination with silica, another sign that the orientation of the stress field had indeed been changing. While the stresses remained in their original east-west orientation, silica would not have accumulated in the rock because the fault lines that built up would have allowed any upwelling magma to move straight to the surface. But as the direction of the stress shifted, these vents would no longer be orientated in such a way as to allow upwelling magma to escape. It would, therefore, collect in chambers under the surface and become contaminated with silica from surrounding rocks. The team found increasing contamination in younger magmas.
A time and a place
From detailed mapping, Haug suggested that this volcanic activity began around 1.6 million years ago. More accurate radioisotope dating by Alan Deino at the University of California in Berkeley seems to confirm this. With further sampling, the German geologists hope to date the five shifts in volcanic activity that their study identified.
‘Our work was conducted to understand the relationship between young volcanism and changes in the regional tectonic stress field,’ says Strecker. ‘It never attempted to link the development of volcanism in the Kenya Rift with human evolution.’ Nevertheless, the team’s findings fit with the idea that environmental change around 1.6 million years ago may have prompted a surge in human evolution. It seems that the lowering of the Rift Valley 10 million years ago is still influencing the tectonics of eastern Africa and its vegetation.
The data are still very patchy. But the recent geological findings do seem to hint at a link between tectonic shift in East Africa and the evolution of H. erectus and H. habilis. Changes caused by volcanic activity may even help explain the migration of H. erectus out of Africa – although the recent dating of three skulls in Java suggests that this could have happened much earlier than the previously accepted date of 1.6 million years ago (see ‘Human origins: the challenge of Java’s skulls’, New Scientist, 7 May). The pieces seem to be coming together. But as Vrba and others caution, even a definite correlation between environmental and evolutionary change cannot prove a causal link.
Mark Maslin is a research scientist at the Geological Institute of Kiel University in Germany.
* * *
A change in the weather
A leap in the evolution of the ancestors of modern humans took place 2.6 million years ago. At the same time, there was massive cooling in the world’s climate, which led to the appearance of the northern hemisphere’s first ice sheets. What caused that climate change? Debate among geologists rages: tectonic activity is the major contender, but there are several rival theories.
Cooling may have occurred when the Panama isthmus emerged, separating the Pacific Ocean from the Caribbean. Recently, however, Gerta Keller from Princeton University in New Jersey dated this event to 2.5 million years ago – too late to have caused the dramatic global climate change.
So was cooling caused by the opening of the Bering Strait linking the Arctic Ocean and the North Pacific? It seems not, as this event occurred up to 1 million years before the global cooling. It may, however, have contributed to a longer-term cooling which started 3.2 million years ago.
There are problems even for the most famous theory – that gradual uplift of the Tibetan-Himalayan and Sierran-Coloradan regions altered global atmospheric circulation, reducing the amount of heat transported to high latitudes and encouraging the build-up of ice sheets (see ‘Did Tibet cool the world?’, New Scientist, 3 July 1993). Most recent estimates put the date of the Tibetan-Himalayan uplift at 4 million years ago or earlier, making it much too old to be implicated in the global cooling. Debate continues on the effects of mountain formation on atmospheric circulation.
At Kiel University in Germany, we have come up with another mechanism. We suggest that tectonic changes started the gradual global cooling 3.2 million years ago and brought the climate system to a threshold almost a million years later. The climate would then have been cold enough for ice sheets to form.
But a source of moisture was also required and, for this, we looked to the oceans. Here research by Nick Shackleton at the University of Cambridge has been of help. His studies suggest that a little more than 2.6 million years ago, circulation of global deep water was greatly reduced. With cold, deep water no longer circulating in the North Pacific, surface waters would have become warmer, increasing evaporation. This moisture would have travelled on the prevailing winds to North America, Greenland and Europe – exactly the areas where ice sheets started to expand. The theory is in its infancy, but it could yet resolve the mystery of climate cooling 2.6 million years ago. Mark Maslin and Gerald Haug