April 25, 2012 | 13
The impact of Homo sapiens on the environment over the past few hundred years has been so profound that some scientists term this chapter of Earth’s history the Anthropocene. But humans may have begun wreaking ecological havoc far, far earlier than that. A new theory suggests that a shift in the technology and diet of our ancestors around two million years ago led to the downfall of a number of large carnivore species in East Africa, which would have triggered cascades of ecosystem disruption.
East Africa today has six carnivores that weigh upwards of 21.5 kilograms and are thus considered large-bodied: the lion, leopard, cheetah, spotted hyena, striped hyena and wild dog. (The term carnivore is used here to specifically refer to members of the Carnivora order of mammals, whose extant members include animals ranging from the vegetarian panda bear to the almost exclusively meat-eating lion.) All six are hypercarnivorous, consuming a diet that is more than 70 percent meat. But once upon a time as many as 18 large carnivore species that occupied a broader range of dietary niches shared the East African landscape, including omnivorous bears and civets, saber-toothed cats that specialized in big prey, and bear-size otters that were more terrestrial than modern otters. Looking at the fossil record of 78 East African carnivore species over the past 3.5 million years, Lars Werdelin of the Swedish Museum of Natural History in Stockholm found that the diversity of the large-bodied species began dropping precipitously around two million years ago. On April 19, at a symposium on human evolution and climate change hosted by Columbia University’s Lamont-Doherty Earth Observatory, he outlined his explanation for this decline.
If climate change (or climate-related environmental change) was the culprit, Werdelin reasoned, then the smaller carnivores should have taken a hit too, since they are generally more sensitive to such shifts than their larger counterparts are. But examination of the fossil record of the smaller carnivores showed no such decline. Likewise, the pattern of loss in the fossil record bore no resemblance to the pattern of climate change-related losses that modern carnivores have been experiencing. Based on those two lines of evidence, he argues that the apparent downturn in large carnivores starting some two million years ago is “a dramatic difference that has nothing to do with climate.”
If the large carnivore decline was not the direct result of climate change or climate-related environmental change, then early hominins (members of the group that includes humans and their extinct relatives) are probably to blame, Werdelin posits. He admits that he’s blaming hominins by default, but the timing is striking: it coincides with a transition among early members of our own genus, Homo, to a greater reliance on stone tools and an omnivorous diet that included significantly more meat than their predecessors consumed. It’s unlikely that these hominins killed the large carnivores outright though. Rather, Werdelin says, they drove the carnivores away from kills during scavenging.
Details of the large carnivore decline evident in the fossil record support Werdelin’s theory: the species that went extinct were specifically those species that were in direct competition with the hominins or that were threats to them—namely, omnivores with diets similar to that of the hominins and hypercarnivores with a narrow range of prey. Hominins would have been formidable competitors against large carnivores not only because they were armed with stone tools, but because as dietary generalists they had access to a wider variety of lower-energy foods to fall back on during lean times than many large carnivores did.
That the loss of top predators can transform an ecosystem is well known. In the simplest arrangement, the disappearance of the predator allows populations of their prey species to expand, which alters the plants those prey species eat. Biologists call the domino-like chain of events a trophic cascade. The reintroduction of wolves into Yellowstone National Park in the mid-1990s nearly a century after they were exterminated provides a striking example of the far-reaching effects of top predators: not only did the resident elk population shrink back down to a more manageable size, but the aspen and cottonwood trees began to recover, as did the willows, which brought back the beavers whose dams create ponds. How East African ecosystems were affected by the loss of large carnivores two million years ago remains to be determined, however.
Werdelin’s presentation impressed other scientists at the symposium. The carnivore extinction pattern Werdelin describes “is very strong, and does not seem to be driven by sample size, but by real diversity changes,” comments René Bobe of George Washington University, an expert on the ecology of East African mammals from this time period. He contends that the cause or causes of the decline is less certain, though, noting “there is only a very broad correlation with the emergence of the genus Homo.” Pinning the blame to a particular event in the evolution of Homo is tricky because the timing doesn’t quite work. “If Homo and the earliest stone tools date to about 2.5 – 2.6 [million years ago], then the decline in carnivores comes about half a million years later,” Bobe explains. “If Homo erectus/ergaster [a species that originated around 1.9 million years ago] is responsible, then the carnivore decline begins too early.” Homo could well have played a role, he observes, but “the causes need to be well documented.”
Clarification may come with more fossil data. For this study Werdelin used 500,000-year time slices. “As we get better resolution we can hopefully capture more detail,” he says. In the meantime, his theory makes several testable predictions about the relative abundance and scarcity of specific size classes of prey mammals that paleontologists should expect to see in the fossil record if he is right about the decline in large carnivores.
Yet even if humans were responsible for the demise of those animals, we can still blame climate change. Shifting conditions between three million and two million years ago fueled the spread of grasslands in Africa, forcing our earliest ancestors out of the trees and onto the open savanna, which is how they ended up facing off against the large carnivores in the first place.