Did The Planet's Tectonics Trigger Human Evolution?

Posted by Casey Kazan.

“Tectonics was ultimately responsible for the evolution of humankind.”

Royhan and Nahid Gani of the University of Utah, Energy and Geoscience Institute.

The scientists argue that the "Wall of Africa" - an accelerated uplift of mountains and highlands stretching from Ethiopia to South Africa- blocked much ocean moisture, converting lush tropical forests into an arid patchwork of woodlands and savannah grasslands that gradually favored human ancestors who came down from the trees and started walking on two feet – an energy-efficient way to search larger areas for food in an arid environment.

The East African Rift runs about 3,700 miles from the Ethiopian Plateau south-southwest to South Africa’s Karoo Plateau. It is up to 370 miles wide and includes mountains reaching a maximum elevation of about 19,340 feet at Mount Kilimanjaro.

The rift “is characterized by volcanic peaks, plateaus, valleys and large basins and freshwater lakes,” including sites where many fossils of early humans and their ancestors have been found, says Nahid Gani (pronounced nah-heed go-knee), a research scientist. There was some uplift in East Africa as early as 40 million years ago, but “most of these topographic features developed between 7 million and 2 million years ago.”

“Because of the crustal movement or tectonism in East Africa, the landscape drastically changed over the last 7 million years,” says Royhan Gani (pronounced rye-hawn Go-knee). “That landscape controlled climate on a local to regional scale. That climate change spurred human ancestors to evolve from apes.”

Hominins – the new scientific word for humans (Homo) and their ancestors (including Ardipithecus, Paranthropus and Australopithecus) – split from apes on the evolutionary tree roughly 7 million to 4 million years ago. Royhan Gani says the earliest undisputed hominin was Ardipithecus ramidus 4.4 million years ago. The earliest Homo arose 2.5 million years ago, and our species, Homo sapiens, almost 200,000 years ago.

Plate Tectonics – movements of Earth’s crust, including its ever-shifting tectonic plates and the creation of mountains, valleys and ocean basins – has been discussed since at least 1983 as an influence on human evolution.

But much of the previous debate of how climate affected human evolution involves global climate changes, such as those caused by cyclic changes in Earth’s orbit around the sun, and not local and regional climate changes caused by East Africa’s rising landscape.

“Although the Wall of Africa started to form around 30 million years ago, recent studies show most of the uplift occurred between 7 million and 2 million years ago, just about when hominins split off from African apes, developed bipedalism and evolved bigger brains,” the Ganis write.

“Nature built this wall, and then humans could evolve, walk tall and think big,” says Royhan Gani. “Is there any characteristic feature of the wall that drove human evolution?”

“Clearly, the Wall of Africa grew to be a prominent elevated feature over the last 7 million years, thereby playing a prominent role in East African aridification by wringing moisture out of monsoonal air moving across the region,” the Ganis write. That period coincides with evolution of human ancestors in the area.

Royhan Gani says the earliest undisputed evidence of true bipedalism (as opposed to knuckle-dragging by apes) is 4.1 million years ago in Australopithecus anamensis, but some believe the trait existed as early as 6 million to 7 million years ago.

The Ganis speculate that the shaping of varied landscapes by tectonic forces – lake basins, valleys, mountains, grasslands, woodlands – “could also be responsible, at a later stage, for hominins developing a bigger brain as a way to cope with these extremely variable and changing landscapes” in which they had to find food and survive predators.

For now, Royhan Gani acknowledges the lack of more precise timeframes makes it difficult to link specific tectonic events to the development of upright walking, bigger brains and other key steps in human evolution.

“But it all happened within the right time period,” he says. “Now we need to nail it down.”