Exactly when Homo sapiens first left Africa is hotly debated. Also debated is how cognition evolved in earlier modern humans in Africa itself: gradually or in spurts. Most early human remains discovered so far have been found in the Rift Valley, South Africa, and in North Africa. Now a cave has been found in Kenya that has been lived in for the last 78,000 years, a vast international team of researchers reported Wednesday in Nature.

Panga ya Saidi is actually a network of caves about a kilometer long in limestone hills: the main chamber is about 100 square meters (1,076 square feet) co-author Prof. Michael Petraglia of the Max Planck Institute tells Haaretz. It was used from the Middle Stone Age to this day, though people don’t live in it any more: now they use it for burials and rituals, he says. In any case, it was big enough to have supported hundreds of people.

During the first 10,000 years, people did live in the cave, but not many. Starting 67,000 years ago, the archaeologists could see a gradual change in stone work and symbolism. Stone arrowheads, blades and backed tools would appear.

“Backed tools are small stone tools with flaking, or retouching along an edge, possibly to dull the edge so the tool can be inserted, or hafted, into an arrow shaft, for instance,” Petraglia explains.

Open gallery view Savannah, with giraffe Credit: Khaled Kazziha, AP

Retro reversion in the Holocene

Panga ya Saidi lies in the zone where savannah morphs into lowland tropical forest. Today, the cave is 15 kilometers (9 miles) from the shore, but its distance from the sea would have varied with changes in sea level as the planet’s climate shifted. In any case, its dwellers don’t seem to have fished.

“Despite being relatively close to the coast, we do not have evidence that the hunter-gatherer populations occupying the cave were in any way dependent on coastal resources,” Petraglia says. “Instead, they were reliant on inland, terrestrial resources in their tropical forest and grassland ecosystem.”

In fact, most of the prehistoric human sites found in Africa so far have been inland, with a few on the coastal margin, he explained. Of course, most of Africa has yet to be archaeologically explored, so the model could change.

Meanwhile, the intense human occupation found at Panga ya Saidi undermines hypotheses that the Indian Ocean coasts served as a sort of "superhighway" channeling migrating humans out of Africa, Petraglia says.

Open gallery view Prof. Michael Petraglia. Credit: Sven Doering

Climate fluctuations seem to have largely spared the environment around the cave. Possibly because of that relatively stable environment, Panga ya Saidi was occupied more or less consecutively.

In climatic upturns, parts of East Africa receive increased rainfall, and grasslands form. In climatic downturns, these areas become arid and are not hospitable to humans. But the site continued to enjoy a high level of rainfall while other regions of Africa dried up, says Petraglia; the conditions in the cave would have been steadier than in its surroundings, enabling this extraordinarily long-term settlement.

As an aside, 74,000 years ago, a super-volcano called Toba erupted in today’s Sumatra, Indonesia. Many suspect that an ensuing volcanic winter all but wiped out the human race. Well, if it did, again Panga ya Saidi was spared: the archaeologists see no break in human occupancy at that time. From about 60,000 years ago, the cave seems to have housed bigger populations, they add.

Open gallery view Approach to the Panga ya Saidi cave. Limestone upland in background. The cave site is 15 km from the modern coast. Credit: Michael Petraglia

A better stone knife

Stone tools have been a thing before people even existed. Worked stones have been found going back over 3.3 million years. Humankind is only believed to go back around 300,000 years.

At Panga ya Saidi, tools go back to the earliest occupation 78,000 years ago. But the occupants’ technology changed markedly 67,000 years ago, with smaller, finer implements appearing, reflecting changes in hunting practices and skills.

After that turning point, the archaeologists observed a mix of technologies rather than sudden changes. That argues against a series of cognitive or cultural "revolutions" theorized by some archaeologists, they write.

Open gallery view Ostrich eggshell beads, bone tool, and close-up of the bone tool showing traces of scraping. Credit: Francesco D'Errico and Africa Pitarch

Their tastes in adornment also underwent evolution. Panga ya Saidi delivered the oldest bead in Kenya, about 65,000 years old. By 33,000 years ago, most beads were made of shells lugged from the beach. By 25,000 years ago, the people had shifted to making beads from ostrich eggshell.

Come the Holocene 10,000 years ago, the people had gone retro: they went back to using seashells.

At some point, art too began to develop, it seems: Layers dating from 48,000 to 25,000 years ago produced carved bones and tusk, a decorated bone tube and worked pieces of ochre, which indicate the development of symbolism.

Open gallery view Worked red ochre and bead made of a sea shell. Credit: Francesco D'Errico and Africa Pitarch

However, the intermittent appearance of such decorated pieces in the cave sequence argues against a behavioral or cognitive revolution at any specific point in time, write the archaeologists.

If there’s another lesson to be learned from the Kenyan cave, it’s just how adaptable people were. “The occupations at Panga ya Saidi occur in a tropical forest ecotone,” says Petraglia, by which he means the “seam” between the forest and the savannah. “This has not been shown before, so our findings are unique,” he adds.

"This information adds to our knowledge that Homo sapiens was living in a variety of environments and that it could adapt to a variety of settings," Petraglia says. "It is clear that humans entered into an array of environmental settings as they expanded within Africa and migrated outside Africa.”