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Becoming Human

Humankind's destructive streak may be older than the species itself

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Some scientists have proposed designating a new geological epoch, the Anthropocene, that would cover the period since humans became the predominant environmental force on the planet. But when would you have it begin? Some geologists argue that the Anthropocene began with the Industrial Revolution, when fossil fuel consumption started influencing climate. Others point back several thousand years earlier to the onset of agriculture, when humans cleared swaths of forest to make way for neat little rows of cultivated crops.

But the roots of the Anthropocene may stretch even farther, deep into the Stone Age:  Humans have been driving other species to extinction since before we were even human, back at least 2 million years to the early days of the genus Homo.

The human family grew up in East Africa during an age of carnivores. Species like Australopithecus afarensis foraged for fruits, nuts and seeds amid large (and probably scary) meat-eating mammals such as saber-toothed cats, giant hyenas and otters as big as bears.

Those creatures had their peak about 3.5 million years ago. About a million years after that, hominids invented stone tools. Early species of Homo started to scavenge and eventually hunt other animals, muscling in on the carnivores’ culinary turf. Our ancestors became so good at killing or stealing prey that some meat eaters couldn’t compete, and they died out. Since hitting their peak, the large carnivores of East Africa have lost 99 percent of their diversity.

Lars Werdelin of the Swedish Museum of Natural History and Margaret Lewis of the Richard Stockton College of New Jersey uncovered this decline while studying 29 species of large carnivores that lived in East Africa over the last 3.5 million years. Their sample includes members of the dog, cat, bear, weasel, hyena and civet families.

Werdelin and Lewis didn’t simply add up all the carnivores that went extinct. They also looked for changes in what they call “functional richness” — broadly speaking, a measure of the number of ecological niches that carnivores inhabited. The more niches, the more varied the species must have been. In this case, the researchers assessed how varied carnivore diets were during different periods of time. Analyzing differences in the size and shape of the teeth, the researchers looked for changes in how carnivores chewed, which reveals what kind of foods the animals could eat.

A clear pattern emerged. Starting around 2 million years ago, East African carnivores became less omnivorous. That left a population of “hypercarnivores,” those truly devoted to red meat. But after 1.5 million years ago, even the hypercarnivores started to thin out, Werdelin and Lewis report March 6 in PLOS ONE.

The researchers argue that these events track important milestones in human evolution. They suggest that East Africa’s big, omnivorous mammals were squeezed out when largely plant-eating hominids started to incorporate animal protein into their diet, initially by scavenging. Once our ancestors, probably Homo erectus, became dedicated hunters and fashioned more sophisticated tools, hypercarnivores such as the saber-toothed cat felt the competition and their numbers also dwindled.

Hominids may not have been the sole driver of carnivore extinction. East Africa became much cooler and drier some 3 million to 1 million years ago, and grasslands replaced forests. But time lags between pulses of intense climate change and carnivore declines may make hominid competition the more likely culprit, Werdelin and Lewis say. The idea could be further tested, they suggest, by investigating whether similar carnivore changes coincided with the emergence of hominid hunting in nearby southern Africa.

We already know what happened many times over when prehistoric Homo sapiens entered a region that had never before been home to a hominid: Animals died in large numbers. After our species ventured into North America for the first time around 15,000 years ago, about two-thirds of the continent’s large mammal genera disappeared. A similar thing happened in South America. With current evidence, however, climate change can’t yet be ruled out as a player in these extinctions.

More recently, as humans colonized the islands of the Pacific Ocean roughly 700 to 3,500 years ago we annihilated nearly 1,000 species of land birds, researchers recently calculated. In Madagascar, humans arrived about 2,500 years ago, and soon after all of the island’s endemic large animals vanished.

That brings us to today. Biologists claim we’re witnessing Earth’s sixth mass extinction, with species dying off at rates more than 100 times normal background levels. We can’t blame an asteroid strike for this carnage, only our own behavior. The level of destruction is unprecedented, but not unexpected given our genus’s history. The difference now is that we’ve come to a point where we’re not only capable of destroying life, but of choosing to save it, too. 

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