“To Think or Eat?” Extreme Environment Stress Gave Rise to Human Evolution

Habilis When extreme climate change made the going tough in prehistoric East Africa, one branch of humanity's closest relatives opted for bigger jaws to eat tough tubers and nuts, rather than bigger brains to figure out better ways to adapt, now feted as Homo habilis -the direct ancestor of the humans who would eventually migrate out of Africa.

It seems that when our ancestors evolved, the climate of Africa rapidly switched from wet and humid periods when the landscape was dominated by huge ephermeral lakes over 300 meters deep to arid barren stretches when rainfall was highly seasonal.  Over 80% of the new hominid species in Africa emerge during these periods of  extreme environmental stress. As a result of the formation of the Rift Valley, shifts in global climate caused brief periods during which these huge lakes appeared and disappeared in East Africa. These rapid shifts from very wet to very dry may have been the environmental stress necessary to push our ancestors down a human path.

In 1985, Elizabeth Vrba of Yale University, showed a significant shift from woodland/forest species to those found in a more open environment — and made a compelling argument that increased aridity led to widespread grasslands in southern Africa around 2.7 million years ago and a shift from hominins such as australopithecines who were still at home in the trees to the more fully erect species of Homo better adapted to life on the plains.

This “savanna hypothesis,” the idea that the incredible change in the landscape of East Africa sometime after 6 million years ago caused the earliest hominins to evolve bipedalism. Before this time, food was plentiful in the rainforest. After this time, though, the hypothesis goes that the spread of grasslands broke up the forest and forced adaptation. For an ape, walking upright is an extremely efficient way of covering vast distances between food sources, which may have been scarce after the vegetation shift.

However, the story is probably more complex, as early bipedal hominins such as Ardipithecus ramidus or Australopithecus afarensis have been found as far afield as Ethiopia. The savanna hypothesis implies that hominin evolution occurred first in East Africa with subsequent migration. But the fossil record is so sparse and difficult to date that it is next to impossible to confirm whether fragmenting of the East African landscape actually led to our ancestors getting up and starting to walk.

Because grasses did not fully dominate these landscapes until much later, Loic Ségalen of the University of Pierre and Marie Curie in Paris concluded that the early presence of these grasses allowed for experimentation and adaptations that may have been employed later, when grasslands began to dominate the ecosystem. So it seems that as grasslands appeared, hominins were taking advantage of new opportunities, but the appearance of grasslands alone was probably not enough to cause bipedalism (nor a major turnover of new hominin species).

The fossil record supports Mark Maslin's theory revealing two starkly differentiated methods adopted by different hominin species: brain expansion to think and manage environmental stresses, and massive jaws to eat anything, whatever the conditions.

Yet another example is the periodic extreme climate change that can come from orbital forcing, the wobble of Earth on its axis of rotation and orbit around the sun. Geologists have known for decades that orbital forcing controls the waxing and waning of ice ages, but we now realize that it may have profoundly affected tropical climates as well. The vivid geological record at the East African sites allowed scientists to document the link between orbital forcing and ancient lake levels.

The formation of the East African Great Rift Valley, a depression a kilometre above sea level,

saddled between two massive mountain ranges running north to south, each about 2 kilometres high that rose up out of the east African plateaubetween 10 million and 5 million years ago forever changed Africa’s landscape and climate and, it seems, the course of human evolution. Most early hominin fossils discovered in Africa are along the Rift, which runs 6,000 kilometers north-south and 600 kilometers east-west In less than 5 million years, East Africa went from a relatively flat area with abundant rainforests to a region of extreme topography. As the rift opened, a region of mountains, plateaus and deep rift valleys formed, creating the so-called cradle of humanity.

Over the last 3 million years, the East African Rift Valley has become more arid, but scientists now know that this long-term trend was punctuated by short episodes of alternating periods of extreme wetness and aridity. Such periods of extreme climate variability occurred three times in the last 3 million years. 

Each of these periods coincided with a major global climate change during which East Africa became more locally sensitive to orbital forcing at precessional scales. This resulted in rapid shifts from wet to dry conditions, as moisture from the tropics alternated depending on orbital configurations. These periods of “pulsed climate variability” are characterized by the precession-forced appearance and disappearance of large, deep lakes in the East African Rift Valley. 

These paleolakes lakes were huge and deep — well over 1,000 square kilometers and 300 meters deep. Lakes in the Suguta Valley in northern Kenya appeared and disappeared. Theb geological record shows that the scale of paleo-Lake Suguta was astonishing — only 13,000 to 10,000 years ago, there was a lake 300 meters deep covering 1,600 square kilometers.

The extreme swings in environment was a source of tremendous evolutionary pressure. "You needed an ability to migrate and move from food source to food source," Mark Maslin of University College London told New Scientist. "One way or another," he suggested "that led to a seminal moment in primate development around 6 million years ago: one species learned to stand and walk on two feet."

"Either you had to think your way out, or eat your way out," added Maslin. About 2.5 million years ago, evolution took two turns: one towards bigger brains to figure out better ways to adapt, the other towards bigger jaws. The first strategy was the winner.

The “Pulsed Climate Variability” hypothesis integrates climate variability with major climate events, and provides a potential explanation for the rapid evolutionary innovation during this time period. Whether this directly led to Homo Habilis, and later migration out of Africa, is still an unanswered question. 

Casey Kazan via http://www.geotimes.org and newscientist.com


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