Recently, we’ve heard a great deal about the impacts of humans on warming the planet and on altering the future of the Earth’s climate system. However, emerging research, which I find interesting, suggests the reverse over past millennia: natural fluctuations in the climate of prehistoric Earth may have had an effect on the evolution that shaped the transformation of our earliest ancestors into modern-day humans. This possibility is discussed in a recent National Research Council report, Understanding Climate’s Influence on Human Evolution.
Some Species Carry on, Some Don’t
A highly simplified summary of human evolution over the past 8 million years.
The path to modern-day human has been a long one—scientists estimate that the split that lead to separate humans and chimpanzees lineages occurred between 6 and 8 million years ago. Since then, a succession of hominid species have come and gone, with changes such as the ability to walk upright, the development of stone tools, and the emergence of social behaviors happening over time.
It’s not hard to imagine that environment played a role in the evolution of the earliest humans, as it has for all species.
The earth system—the combination of land, atmosphere, and oceans that make up our environment—is constantly changing. Some changes are predictable, like seasonal shifts in temperature or the daily transition between hours of light and dark. But over the millions of years that it has taken humans to evolve, the Earth has endured some large-scale fluctuations in climate that in turn caused vast changes in habitat, such as shifts between ice ages and hot, arid conditions.
Adapting to Surroundings
Struggling to survive as conditions changed, some species became extinct, while others moved to new locations with more preferable habitats, and still other species gradually evolved to become better adapted to the new conditions.
For example, as cooler and drier climate caused the expansion of grasslands across Africa, one species of prehistoric antelope evolved teeth that could chew through the tough grass that grew on the plains— an advantage over pre-existing species that could not eat the coarse grass.
Similarly, scientists think that humans also evolved specific traits that could give an advantage in their habitat. This idea was the basis of one of the earliest theories of human evolution, the savannah hypothesis. The theory stated that many important human adaptations arose as a result of the expansion of grasslands across Africa – for example, as the landscape became more open, with fewer trees, our ancestors developed the ability to walk upright in order to move across the savannah more easily.
Evolution: a process in which genetic changes accumulate over time
Versatility is Key to Survival
Although the savannah hypothesis remains well respected, in recent years several other ideas about human evolution have emerged. One alternative hypothesis is based on the realization that ancient climate change didn’t occur in just one direction. By analyzing ancient sediments, layers of rock, and the fossilized remains of plants and animals, scientists have pieced together a record of prehistoric climate that suggests conditions fluctuated greatly over thousands of years, from monsoon to ice age and back again.
Under this hypothesis, simply becoming specialized to suit a specific environment wasn’t always useful in these conditions of changing climate. Many species, such as some types of musk oxen (see box) went extinct as shifts in climate changed landscapes. In contrast, the species that fared best were those that could cope with new conditions, not those adapted for just one specific habitat.
The Decline of the Musk Oxen
Studying the DNA of long-extinct species of musk oxen, scientists have found that populations of the animals began to decline in the midst of climate oscillations. This suggests these species were unable to survive as climate change made critical resources such as food and water scarce.
Scientists favoring this alternative hypothesis think that this prehistoric climate change may have shaped human evolution by driving our early ancestors to become specialists in versatility. Proponents of the idea point out that during the last six million years – the time when modern-day humans were evolving – there have been periods of particularly variable climate, although in the past 100,000 years climate has been relatively stable.
They believe that in the struggle for survival, the ability to cope with changing climate gave modern-day humans an edge over other species. For example, the Neanderthals are close evolutionary cousins of modern-day humans, and had relatively large brains, the ability to use tools, and even adapted to changing weather conditions such as ice ages. Yet the Neanderthals went extinct, while Homo sapiens – our species — went on to dominate the planet.
Scientific proponents of the new hypothesis think Neanderthals lacked the ability to innovate solutions to problems when their habitat changed. In the 200,000 years the species existed, their technologies didn’t develop beyond making and using of a few simple tools. Humans, on the other hand, devised new ways to overcome the challenges they faced. For example, early humans developed different tools to help solve different problems, built shelters to protect themselves from the cold, used fire to clear trees from the land to make hunting easier, and developed social behaviors. With these abilities, humans were no longer simply reacting to their environment; they could also exert some degree of control over their surroundings.
The skulls of several early human species. From left to right: Australopithecus africanus, 2.5 million years old; Homo rudolfensis, 1.9 million years old; Homo erectus, about 1 million years old; Homo heidelber-gensis, about 350,000 years old. The last skull is from Homo sapiens, the modern-day human species, and is estimated to be about 4,800 years old. SOURCE: Courtesy of the Human Origins Program; photo credits include Chip Clark, Jim DiLoreto, Don Hurlbert, all of the Smithsonian Institution.
Unanswered Questions Remain
The alternative hypothesis is gaining favor, but has not been fully tested and accepted across the scientific community—and so the story is by no means complete. Scientists still face major limitations in resolving questions about our origins and history because fossil records of our earliest ancestors are sparse and understanding of past climate is incomplete. Recommended research directions are outlined in Understanding Climate’s Influence on Human Evolution.
But over the next few years, just as we’re sure to learn more about the impact of humans on modern Earth’s climate system – and the steps we can take to minimize such impacts — hopefully, we’ll also get more information about the impacts climate had on shaping the human species.
Hi. I’m Barbara Schaal, plant evolutionary biologist, Vice President of the National Academy of Sciences, and Chair of the Division on Earth & Life Studies. In this column, I’ll be writing about topics the division has covered in its expert reports, which are written by committees convened to advise the nation on such topics as climate change, environmental health, agriculture, biosecurity, and many others. My goal is both to share my excitement about many of the topics and, when I can, to offer particular insights that may help people understand the relevance of the scientific findings to their daily lives.