麻豆传媒

When early humans came down from the trees, they clambered up into tectonically active areas

THE course of human evolution was directed by the shifting and shaking of the Earth鈥檚 crust.

鈥淎s a species we evolved the way we did as a result of living in very dynamic landscapes that selected for adaptability,鈥� says archaeologist of the University of York in the UK, who along with of the Paris Institute of Earth Physics in France has spent over 20 years amassing evidence for the theory.

鈥淗umans evolved as a result of living in very dynamic landscapes that selected for adaptability鈥�

Our ancestors evolved into modern humans while inhabiting tectonically active regions, the researchers say. Intelligent species would have thrived in these deformed landscapes, exploiting the topography to hunt, avoid predators and competitors, and build defensible homes. Eventually they developed large brains, a prolonged childhood and the use of advanced tools and weapons. Less smart species would not have had the ability to use the uneven ground to their advantage.

The idea is similar to that put forward by of the University of California, Los Angeles, in his best-selling book Guns, Germs and Steel. Diamond argues that factors such as the shapes of the continents and the location of mountain ranges were crucial in shaping modern history.

Bailey and King鈥檚 work extends this to the millions of years of human evolution, but a lack of conclusive evidence has hampered its acceptance. With two new papers out this month, this could be about to change.

The pair began collaborating in the 1980s, after King pointed out that the ancient Greek settlements Bailey was excavating were all in tectonically active regions. 鈥淲e realised there was something going on,鈥� says Bailey.

The Earth鈥檚 surface is divided into plates, which move around over the millennia. Where they grind against each other, pressure builds up, and this can trigger earthquakes and volcanic eruptions. Most dramatically, the massive Toba eruption 74,000 years ago in what is now Indonesia chilled the climate around the world and came close to killing off the human race.

But Bailey and King are concerned with subtler effects. 鈥淭ectonic movements create and sustain landscapes in ways that are beneficial to human occupation,鈥� Bailey says.

In active regions, the folding and faulting of the crust, combined with regular earthquakes and volcanic activity create a disrupted landscape with many hills, valleys and cliffs, criss-crossed with solidified volcanic lava.

Bailey argues that these complex landscapes were perfect for early humans, who were not fast runners or particularly strong, but were intelligent and adaptable. For instance, even though weapons such as spears had not been invented, early hunters could kill large animals by exploiting the irregularity of the landscape.

What鈥檚 more, since humans evolved from tree-living primates, they would have found it easy to switch to clambering around hills and valleys. 鈥淗umans are adapted for complex topography,鈥� says King. By contrast, they would have been at a disadvantage on flat, open plains like the African savannah, which is dominated by fast-running predators like lions and hyenas.

Tectonically active landscapes are also more likely to have reliable water sources, because earthquakes can trap water behind barriers of rock, forming lakes, and underground water can rise through faults to form springs. These water supplies would support plants and attract animals. Barriers like cliffs and ridges would have made life safer by allowing early humans to hide from predators and defend themselves against invaders (, vol 80, p 265).

So much for the theory. If it is right, we should find that early humans were clustered in tectonically active regions. When Bailey and King superimposed the locations of human fossil sites throughout Africa with satellite images that show the roughness of the land, they found that they lined up neatly (see map).FIG-mg27864001.jpg

In fact, 93 per cent of the fossil sites are in regions of high or medium surface roughness. For example, most of the classic human fossil sites, like Olduvai Gorge and Laetoli, are found along the East African Rift, where two continental plates are slowly coming apart. Bailey and King have found similar patterns of fossil sites in Arabia, which humans colonised later (, in press).

However, it is by no means case closed for the tectonic theory. Buried remains are more likely to be thrown up onto the surface if they are in an earthquake-prone region, so the results could be misleading, says of the University of Oxford.

So Bailey and King, with of the University of the Witwatersrand in Johannesburg, South Africa, have extended their studies to South Africa, where human remains have been found in sites like Taung and Makapansgat (, in press). Rather than having been revealed by tectonic activity, the remains were found in caves. 鈥淭here are hundreds of these caves, but only some have remains 鈥� and those are in regions that were tectonically active,鈥� Bailey says.

鈥淚t is a fascinating piece of work that will make people sit up and reconsider,鈥� says , a physical geographer from the University of Exeter, UK. Petraglia points out that shifts in the climate, like the desertification of the Sahara, would also have affected where our ancestors lived. 鈥淭here are more factors to consider,鈥� he says, 鈥渂ut their ideas are very interesting.鈥�