Evolutionary Challenge: The origin of every hominid genus, including our own, appears to fall within one or another window of climatic variability. Now, with planetary warming occurring at a breakneck pace, human adaptability is likely to face its biggest test. Image: Sebastian Niedlich/Flickr
In the 5 million years since early hominids first emerged from east Africa's Rift Valley, the Earth's climate has grown increasingly erratic. Over cycles lasting hundreds of thousands of years, arid regions of central Africa were overrun by forests, forests gave way to grasslands and contiguous landscapes were fractured by deep lakes.
It was within the context of this swiftly changing landscape that humans evolved their sizable brains and capacity for adaptive behavior, said Rick Potts, director of the Human Origins Program at the Smithsonian Institution National Museum of Natural History. In such a world, the ability to think creatively, to imagine novel solutions to survival threats, proved to be a major asset, he said.
"The evolution of the brain is the most obvious example of how we evolve to adapt," he explained. "But in the modern era, we know that in the human genome there are all kinds of interactions that allow human organisms to have plasticity -- the capacity to adjust is itself an evolved characteristic."
Man had two key advantages, he said: our brains and our capacity for culture.
"Our brains are essentially social brains," he added. "We share information, we create and pass on knowledge. That's the means by which humans are able to adjust to new situations, and it's what differentiates humans from our earlier ancestors, and our earlier ancestors from primates."
This adaptive ability not only allowed our progenitors to ride the massive seesaws of climate shifts but subsequently helped them to colonize new habitats. The earlier hominid species Homo erectus ranged across much of Africa and Asia. Meanwhile, Homo neanderthalensis -- Neanderthals -- occupied large parts of Europe. Our own species, Homo sapiens, dispersed to even more far-flung corners of the globe, employing boats to reach Australia more than 50,000 years ago.
The species that went into the cold
"You had Homo sapiens going into colder environments than even the Neanderthals could tolerate, at the same time that they were migrating into deserts, tropical forests, steppes and glacial environments," Potts said. "How this thin, long-limbed hominid could make it in all these different environments, to me that is a story about how you become adaptable."
The theory of "variability selection," which Potts first described in 1996, doesn't just relate to humans and their brains but can be applied to any species passing through periods of environmental instability. Generalist traits such as a wide-ranging diet would be an advantage at such times, Potts said, whether for grazing animals or for their predators.
"All organisms have to be able to maintain homeostasis within some range of conditions that are not completely stable," Potts explained. "The genome itself is an evolved structure, and that means that all forms of life have some degree of adaptability to them."
The idea that adaptability might itself be an evolved characteristic is a relatively new concept. When Potts first described his theory nearly two decades ago, it was met with a healthy dose of skepticism from evolutionary geneticists who understood evolution to be a process of matching animals to their environments.
It had been largely understood among paleoanthropologists -- including, formerly, Potts -- that humans had evolved during a period of gradual change from colder, wetter climates to a more arid environment.
The idea that major developments in human evolution happened not gradually, but in fits and spurts during periods of heightened climatic variability, seemed to fly in the face of the scientific consensus. But the theory of variability selection did have one big advantage: It could be tested.