As far as researchers can tell, evolution is not a constant march across time. Instead, it’s more like a series of dashes from mountain top to mountain top. The fossil record shows that there have been a few eras characterized by geologically speaking brief spurts of diverging animal species. What exactly is behind those spurts, however, is open to debate.
There are a lot of pressures that can cause an organism to evolve. A new pathogen or a new predator may arrive that forces an animal to better protect itself. Having a food source disappear can lead to animals having to develop other ways to feed themselves. And sometimes the environment around them can change, forcing adaptations to better equip the animals in new surroundings.
For years, the leading theory on human evolution has cited evidence for a long, slow process of environmental change. A new study from Penn State, however, is challenging that notion. Instead, researchers postulate that it was a series of fast environmental changes see-sawing between forests and grasslands that set the stage for the rise of Homo sapiens.
The evidence comes from digging up samples of lake sediments from Olduvai Gorge in northern Tanzania in East Africa. As the seasons passed, organic material was funneled to the lake, where it decomposed and became layers in the geologic record. The team of researchers analyzed tiny layers made up of that material, and searched for fossil molecules from ancient organisms and the waxy coating on plant leaves. They used gas chromatography and mass spectrometry to determine the relative abundances of different leaf waxes and the abundance of carbon isotopes for different leaves.
After analyzing many different layers, the team realized that rather than undergoing a long, drying process, the local environment had switched back and forth between grasslands and forest several times, each within a matter of 10 to 100 generations. And that much change in that short of time is exactly the sort of pressure that might cause a spike in evolution. Throw it all together with the fact that this was all occurring at a time and place, where humans are thought to have first arrived on the scene, and you have a potential driver for our species to emerge.
These sorts of fast changes are important for evolution. Imagine, if you will, that every species is undergoing mutations that are making it more and more of a perfect fit for its environment. Eventually, it will reach the top of its evolutionary mountain; no further mutations will help it survive better.
At least, no further small mutations.
There might be another “fitness” peak where the organism could do even better. However, small mutations cause small changes that cause animals to die. It would take too drastic of a mutation to leap over the canyon and start climbing the next peak over. Mutations like that happen only in comic books.
But when the environment is changed, suddenly the playing field is leveled. The specimins at the top of the mountain no longer have a large advantage over those with minor mutations, meaning that the freaks begin surviving in larger numbers. When you start throwing mutations upon mutations, it becomes possible to find new “best fits” for the environment. And there’s no reason why several new “best fits” could be found through different kinds of mutations. Then, as the environmental changes settle down, the new species start climbing up their evolutionary mountains again, with those best suited for the new environment reproducing and pushing the entire population up the slope once more.
Welcome to the world of evolution.
“Early humans went from having trees available to having only grasses available in just 10 to 100 generations, and their diets would have had to change in response,” said Clayton Magill, graduate student in geosciences at Penn State. “Changes in food availability, food type or the way you get food can trigger evolutionary mechanisms to deal with those changes. The result can be increased brain size and cognition, changes in locomotion and even social changes — how you interact with others in a group. Our data are consistent with these hypotheses. We show that the environment changed dramatically over a short time, and this variability coincides with an important period in our human evolution when the genus Homo was first established and when there was first evidence of tool use.”
The paper, “Ecosystem variability and early human habitats in eastern Africa,” was published in the Proceedings of the National Academy of Sciences by Clayton Magill, Katherine Freeman, professor of geosciences at Penn State, and Gail Ashley, professor of earth and planetary sciences at Rutgers University.