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Rise of Humans 2 Million Years Ago Doomed Large Carnivores

The views expressed are those of the author and are not necessarily those of Scientific American.


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Lions are one of just six carnivores that remain in East Africa today, compared with more than 15 species that shared the landscape before the dawn of Homo. Image: Kate Wong

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.

 

About the Author: Kate Wong is an editor and writer at Scientific American covering paleontology, archaeology and life sciences. Follow on Twitter @katewong.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. erbarker 11:31 am 04/25/2012

    Good article. That’s the way natural selection and evolution works. One day it may get us to.

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  2. 2. hardboiled 11:45 am 04/25/2012

    We are little more than out of control rats!

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  3. 3. geojellyroll 3:33 pm 04/25/2012

    Well, perhaps in some of the world. humans have had no real impact on carvivores in the western world until the last few centuries. Even today, here in Canada, black bears, polar bears and wolves have high healthy numbers. Polar bear numbers are at an all time high according to environment Canada. Wolves by rhe thousands are culled in northern Canada and in Alaska. Cougar numbers are steady. Grizzlies a bit uncertain but fairly good numbers in most of their Canadian range.

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  4. 4. em_allways_right 4:30 pm 04/25/2012

    Because humans weren’t in Canada 2 million years ago!

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  5. 5. geojellyroll 5:19 pm 04/25/2012

    em…that’s my point. Carnivores were not ‘doomed’ because humans rose 2 milljion years ago..humans were restricted geographically. Even where humans were found most carnivores have done ok…large carnivores not so well recently (in the last blink of a geologic eye).

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  6. 6. Extremophile 5:49 pm 04/25/2012

    This theory builds on three assumptions:

    - Humans become “omnivorous” 2 Million years ago.

    - Humans were enough by numbers to kill all the carnivores.

    - Correlation (if it exists at all) is the same as causality.

    The first statement may be one of the biggest buses to Abilene in science. For both, there is no evidence around, only strong opinions.

    Wouldn’t it be a more scientific approach to discuss various options?

    Maybe, the extinction of large carnivores by whatever cause allowed the human populations to grow?

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  7. 7. jgrosay 6:27 pm 04/25/2012

    A nice reminder about how the world still works. Big carnivores did not only eat the cattle, pork, chicken, lamb and other things we wanted to eat, but when given an opportunity, they even eated some of us. The mankind had no scape: Us or them. We just need remembering that there’s enough of almost everything for all, and that eating your neighbour may finally be counterproductive to you.

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  8. 8. Gatnos 6:37 pm 04/25/2012

    Watch out readers! Don’t overlook the fact that this is a piece of fantasy! There is no science here, no facts presented. No evidence unearthed. It is pure fiction, pure speculation by the editors of this rag posing as scientists. It is as untrue as is the unproved theory upon which it is based.

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  9. 9. American Muse 9:58 am 05/1/2012

    “Gatnos” (6:37 pm 04/25/2012) may actually have been living two million years ago. You see, they were all hard-boiled creationists in those good old days.

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  10. 10. Chris Miller 1:19 pm 05/3/2012

    It’s striking that as soon as humans expanded into new territories, the megafauna quickly vanished. Northern Europe/Asia lost its mammoths and woolly rhinos; the Americas their giant sloths; New Zealand lost its moas only in the last 1,000 years when the Maori arrived.

    No doubt some of this may have been due, at least in part, to the same climatic changes that allowed humans to expand into these areas, but the circumstantial evidence does appear strong.

    The only area with a still thriving megafauna (Africa) was where humans evolved, giving animals there time to co-evolve a strategy of keeping away from the chimps with spears.

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  11. 11. upload70 8:56 am 10/5/2012

    I wonder why man didn’t use other predators such as the big cats to hunt for us rather than dogs if raised from kittens it should have been possible to domesticate / cave them. http://buysteroidsuk.co/

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  12. 12. DeadManTalking 1:09 am 12/24/2012

    While it’s perfectly possible that human activity helped wipe out large carnivores, it’s probably not quite as Ms Wong or Mr. Werdelin imagine. The idea that we starved large carnivores to death by chasing them away from their kills is a tad improbable. Chimpanzees show disdain for dead meat they haven’t killed; and we were probably the same. We didn’t start out as scavengers, and we didn’t end out as scavengers, so it’s unlikely that we had a scavenger interregnum. In any event, any scavenging on our part would have been opportunistic and not systematic enough to dent other predator populations.

    Secondly, we didn’t start using stone tools 2.5 million years ago; we only started to flake stone tools at that point. We’d been using stones for millions of years before that, most likely. Furthermore, we always had spears since prior to our separation from the chimps.

    Lastly, we didn’t abandon the trees because the forests were disappearing under us; that’s like asking the polar bears to adapt to the forest because their ice fields are disappearing. Can’t happen, doesn’t happen that way. When one’s eco-niche restricts, one’s population shrinks to accommodate that shrinkage. Environments change quicker than species can adapt; humans included. Sad, but true. (We didn’t adapt to changing environments, anyway, we learned to recreate our environment anywhere.)

    We could only have abandoned the trees because a new food source opened up that required us to abandon the trees; a food source so rich that it would make it worthwhile to, not only abandon the protection of the trees, but to give up the speed of escape of running on all fours. Even worse, we didn’t grown big fangs or claws to protect us, either. It would have been literally impossible for us to give up the food sources we depended on in the forest if we hadn’t switched to a new one, and the new one had to be better than the old. What could that food source have been, and why did we have to stand up to get it?

    I’ve never been able to think up but one answer to that question: meat was the food source, and carrying weapons on the hunt and food back made standing up necessary, possible, and a good strategy. We wouldn’t have stood up to scavenge. We wouldn’t have stood up to grub for roots and bulbs. We wouldn’t have abandoned climbing for fruits and nuts.

    There is a rule in evolution: one can only evolve towards something, not away from anything. Remembering it solves a lot of problems.

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  13. 13. American Muse 10:59 am 12/25/2012

    DeadManTalking, your talking now!

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