Late Miocene fossil-bearing strata of Lothagam in West Turkana. Credit: Christian Rowan.

In Eastern Africa, the cradle of humankind is tearing apart given Earth’s underlying crust there has significantly thinned.

Eastern Africa’s Turkana Rift is both a hotbed for fossil discoveries of our earliest ancestors and a literal hotbed of volcanic activity caused by shifting tectonic plates.

Given the ongoing and future plate tectonics, which will see the eventual break-up of the African continent, researchers are offering a new perspective on how Turkana’s world-famous fossil record of human evolution came to be.

East African Rift System

Scientists have long been fascinated by the Turkana Rift, a 500-kilometre-wide, low-lying region that spans Kenya and Ethiopia.

This rift is part of the larger East African Rift System, which runs from the Afar Depression in northeastern Ethiopia to Mozambique in the south, with the African tectonic plate on one side and the Arabian and Somali plates on the other.

At the Turkana Rift, the African and Somali plates are drifting apart at a rate of about 4.7mm per year.

In the process, known as rifting, Earth’s crust is stretched horizontally, causing it to buckle and fracture, thus releasing magma from deep below.

Not every rifting episode ends in continental breakup. The Turkana Rift, however, appears destined for that fate.

Study lead author Christian Rowan, a PhD student at Columbia University’s Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School, said: “We found that rifting in this zone is more advanced, and the crust is thinner, than anyone had recognised.

“Eastern Africa has progressed further in the rifting process than previously thought.”

Homo erectus crania from the Turkana Rift. Left: WT 15000, ‘Turkana Boy’ from West Turkana. Right: ER 3733 from East Turkana. Credit: Photo: John Rowan.

The study of human evolution

Rowan and colleagues used a unique dataset of high-quality seismic measurements collected by industry partners and acquired in collaboration with the Turkana Basin Institute, a research organisation founded by the late paleoanthropologist Richard Leakey to further the study of human evolution and its geological context in Kenya.

By studying how acoustic waves used in the measurements were reflected by subsurface layers, then combining their interpretations with other deep subsurface imaging, the researchers visualised the structure of sediments and how deep the top of the crust is within the Turkana Rift.

Along the rift’s axis, the crust – the rocky outermost layer that rests atop Earth’s shifting mantle – is about 13km thick.

That is significantly thinned compared to the more than 35km-thick crust further from the rift’s centre and is a telltale signature of a process called ‘necking’.

Rowan said: “The thinner the crust gets, the weaker it becomes, which helps promote continued rifting.” Eventually the crust breaks apart.

Anne Bécel, a geophysicist at Lamont and co-author of the study, said: “We’ve reached that critical threshold. We think this is why it is more prone to separate.”

But this will play out in geological time, so ‘critical’ is relative.

The Turkana Rift started pulling apart about 45 million years ago, and the researchers estimate that necking began after an episode of widespread volcanic eruptions roughly four million years ago.

It will take a few million more years before necking gives way to oceanisation, the next stage in rifting, when magma will surge through the cracks and create a new seafloor for water that pours in from the Indian Ocean to the north.

The researchers also found evidence of an earlier period of rifting that did not culminate in continental breakup but left the crust weakened and thinned, thus contributing to the present rifting phase.

Rowan said: “It challenges some of the more traditional ideas of how continents break apart.”

Critical phase of continental separation

The Turkana Rift is the first identified active continental rift undergoing necking, making it important for the study of tectonic processes associated with this critical phase of continental separation.

Co-author Folarin Kolawole, who is also with Lamont, said: “In essence, we now have a front row seat to observe a critical rifting phase that had fundamentally shaped all rifted margins across the world.”

Those rifting processes in turn connect to other Earth systems; understanding them helps scientists reconstruct past landscapes, vegetation and climate.

Bécel said: “Then we can use that knowledge to understand what’s going to happen in our future, even on shorter time scales.”

Human evolution

The findings also have implications in a very different domain: the study of human evolution.

The Turkana Rift has yielded more than 1,200 hominin fossils spanning the last four million years.

That’s one-third of all such fossils found in Africa, and many paleoanthropologists have argued that this scientific ‘Garden of Eden’ was a hotspot of evolution for humanity’s ancestors.

Rowan and colleagues think their findings may suggest a different narrative.

Following widespread volcanism around four million years ago, necking initiated subsidence of the Turkana Rift, where fine-grained sediments favourable for fossil preservation rapidly accumulated.

Rowan said: “The conditions were right to preserve a continuous fossil record.”

It’s possible, then, that the Turkana Rift was not uniquely important in the evolution and diversification of our hominin ancestors, but rather a place where conditions lent themselves to documenting them.

Although still only a hypothesis, Rowan said: “But other researchers can now use our results to explore those ideas.

“In addition, our results can be fed into tectonic models that are coupled with climate to really explore how shifting tectonics and climates influenced our evolution.”

The findings were published in Nature Communications.