The turbulent environment created the conditions for leaps in human evolution...

At one location in Kenya, my colleagues and I have been working on this puzzle for decades. It is a place where great changes can be seen in the archaeological and fossil records hundreds of thousands of years ago. But what external factors have led to the emergence of behaviors that are typical of our species homo sapiensinteracts with its environment?

We wanted to know if we could relate what was happening in the environment at that time to these technological changes and the human species living there. Based on our analysis, published in Science Advances, we conclude that the roots of homo sapiens‘Evolutionary adaptations result from our ability to adapt to environmental changes.

Missing time in the archaeological record

The famous Olorgesailie prehistoric site is located in southern Kenya. It is located in the Rift Valley, a seismically active area where lakes and streams produced sediments that accumulated over time, burying and conserving fossilized bones and ancient stone tools.

At Olorgesailie, our scientific team has found evidence that may be related to the origin of homo sapiens in the form of a critical transition from one technology to another.

The older technology is characterized by large, oval cutters called hand axes. Typical of the so-called Acheulean stone technology, almost two dozen layers of this handaxe and other Acheulean tools were discovered in Olorgesailie. They stretch over an immense period of about 700,000 years and cover a time when fossil remains show that the hominin species are Standing man and Homo heidelbergensis inhabited East Africa.

The last archaeological sites of the Acheuleans in Olorgesailie are 500,000 years old. At this point there is a frustrating 180,000 year gap in these sediments caused by erosion. Archaeological records begin again about 320,000 years ago when sediments began to fill the landscape.

But the Acheulean was gone. In its place was Mesolithic technology, which typically consisted of smaller, easier-to-carry devices than the clunky Acheul hand ax. Elsewhere in Africa, Mesolithic technology is associated with the earliest African homo sapiens.

These toolmakers often used sharp-edged black obsidian as their raw material. Archaeologists Alison Brooks, John Yellen, and others have chemically traced the obsidian back to distant outcrops in various directions, up to 59 miles from Olorgesailie. They concluded that the distant sources of obsidian provide clues to the exchange of resources between groups, a phenomenon unknown in Acheul times.

Our Mesolithic excavations also contained black and red dyes. Archaeologists see such pigments as a sign of increasingly complex symbolic communication. Think of all the ways people use color – in flags, clothing, and in many other ways people visually claim their identity as part of a group.

So here we had the extinction of the Acheulian way of life and its replacement with dramatic new behaviors, including technological innovations, exchanging obsidian between groups, and using pigments. However, we had no way of examining what was happening in the 180,000 year void when this transition was taking place.

We had to restore that time. We began strategizing how we could dig up sediments from somewhere nearby that would have mapped the environments and survival problems associated with this shift in early human adaptation.

Let’s turn to geology for clues about early humans

Various types of sediment are deposited in lakes, streams, and soils, and the sediment layers tell the story of changing environments over time. Geologists Kay Behrensmeyer and Alan Deino accompanied me in the field in southern Kenya to find out where we could drill for sediments that could fill the time gap in Olorgesailie.

We hypothesized that the key to understanding the great transition would be under a flat grass plain about 15 miles south of our excavations at Olorgesailie. Together with colleagues like René Dommain and employees of the National Lacustrine Core Facility, we drilled in September 2012 until we reached the volcanic rock floor of the Rift Valley.

The result was a 139-meter-deep core with a succession of ancient habitats and soils at the lake and lake edges, all dotted with volcanic layers that we could date to achieve the most accurately dated East African environmental record of the past 1 million years.

With the advice of geologist Andy Cohen and other colleagues, I assembled an international team of earth scientists and paleoecologists to study and analyze the core. We have found ways to convert many different measures of the past environment – microscopic pieces of plants, unicellular diatoms from ancient sea resources, and various chemical signals – into ecological measures of freshwater availability and vegetation cover. The newly published study provides our results.

Environments during the time gap

The sediment record showed that during the period from 1 million to 500,000 years ago when Acheul toolmakers were employed in the Olorgesailie Basin, ecological resources were relatively stable. Fresh water was reliably available. Grazing zebras, rhinos, baboons, elephants and pigs changed the regional vegetation of forested meadows to create short, nutritious plains of grass.

And then what happened in the time gap?

The core is very well preserved in the previously mysterious time interval. We have found that a critical environmental transition occurred around 400,000 years ago. In a relatively stable environment, we have seen repeated fluctuations in vegetation, available water, and other ecological resources on which our ancestors and other mammals depend.

According to the anthropological literature, today and in recent history, hunters and gatherers respond to times of uncertain resources by investing time and energy to refine their technology. They connect with remote groups to maintain networks of resource and information exchange. And they develop symbolic markers that strengthen those social connections and group identity.

Sound familiar? These behaviors are similar to the difference between the ancient Mesolithic Olorgesailie lifestyle and the Acheul lifestyle.

It is also noteworthy that the large grazing species typical of the Acheulean era died out after 500,000 years. 360,000 to 300,000 years ago, ecologically flexible herbivore species, which were smaller, less water-dependent, and dependent on both short and tall grass and tree leaves, had replaced specialized grass-eaters such as the now-extinct zebras and the giant baboon.

These changes in the animal community reflect the benefit of an adaptable diet, in parallel with how our Mesolithic ancestors adapted to the insecurity of the environment.

For the past two decades, many researchers of human origin have viewed climate as the primary, if not only, driver of the adaptive evolution of hominins. However, our new study draws attention to several factors in the transition from the Acheulean-Middle Stone Age in southern Kenya.

Yes, the rainfall was very different after the environmental change 400,000 years ago. However, the terrain in the region has also been broken by tectonic activity and covered with volcanic ash. And large herbivores exerted different influences on the vegetation before and after this transition.

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The result was an ecological cascade of changes that included the early humans who practiced the Mesolithic way of life. We propose that all of these factors taken together triggered this critical evolutionary shift.

The Mesolithic could be a lesson for today. Given that humanity is now facing an era of environmental insecurity around the world, is our species nimble enough to use social networks, new technologies, and reliable sources of information to adapt to the environmental disruptions ahead?