An ancient lake whose shores vacillated between lush forests and dry savannahs shows how the changing climate may have shaped humanity's dawn in eastern Africa, according to new research.
Scientists studying organic remains dating back 2 million years in Olduvai Gorge in Tanzania tracked how plant life adapted to the regional climate as it shifted from regular monsoons to scorching dry spells. The researchers published their findings last week in the Proceedings of the National Academy of Sciences.
The gorge was home to some of humanity's earliest hominid ancestors, and the surrounding landscape provides some of the best glimpses of the conditions they lived in from fossil remains, tools, artifacts and plant residues.
"It's an unusual and almost extreme situation," said Gail Ashley a co-author and a professor of earth and planetary sciences at Rutgers University. "[The Olduvai Gorge is] like a perfect environment because it was a closed basin and it filled up with sediment, and those sediments recorded everything around it, just like a book."
It was in these sediments that Ashley and her collaborators found waxes from prehistoric plants and algae, collected in samples over a decade from Olduvai. The team examined residues from 2 million years ago spanning a 200,000-year time frame, around the dawn of Homo erectus.
Clayton Magill, a geochemistry graduate student at Penn State University and a co-author, explained that by measuring isotopes in these waxes, the team painted a picture of what kinds of plants grew in the gorge and what environments they lived in.
"With carbon, we can delineate between grasses and trees," Magill said, noting that different plants have different carbon signatures. Hydrogen isotopes, on the other hand, measure aridity. "Heavier [hydrogen] isotopes are associated with drier conditions," he said. Water with lighter hydrogen isotopes tends to evaporate faster, so plants end up accumulating heavier hydrogen when the ground dries up.
How brain development connects with climate
From these measurements, the researchers traced what kinds of plants grew in the gorge over time and compared them with how the climate changed, constructing a continuous record of plant and water fluctuations. "What we find is that the period between 2 million and 1.8 million years ago is associated with extreme environmental variability," Magill said.
Grasslands gave way to woody forests and back again while water levels in the gorge rose and fell, often very quickly by geological time scales. "There was evidence that there was contraction and expansion over time," said Katherine Freeman, a co-author and a geosciences professor at Penn State. "What we show is a repeated transition from the driest to the wettest on the scale of a few thousand years."
Though not as dramatic as a towering black monolith, these changes may have spurred human evolution by forcing early hominids to adapt to a rapidly changing climate, driving them to develop new strategies to hunt, gather and survive with changes in food and fresh water. "That's where the connection has been; the development of the brain, food gathering, might have been triggered by the continually changing climate," Ashley said. These changes also created selection pressures in other species like birds and reptiles.
"Our findings are consistent with variability as a driver of human evolution," Freeman said. "Rather than a slow and steady change, what we see is a pretty intense variability." This challenges views that hold that a slowly drying continent forced early hominids to evolve and disperse.
Does this have any implications for modern climate changes? It may to an extent. "The speed at which modern climate is changing is fairly unique," Magill said. "We're not really quite sure what's going to happen."