One Million Years Ago, Our Human Relatives May Have Challenged Giant Hyenas for Carcasses

In southern Europe, a million years ago, a horse or elephant kill was a big event. It meant a nutritious, fatty group feast for Homo erectus, made possible by the skill and tenacity of the hunter who brought down the beast, which all surely appreciated. But in this era before advanced weapons, the successful hunter may not have been human at all. Predators like saber-toothed cats dominated the food chain, and our ancestors and their relatives may have frequently relied on scavenging their felled prey as part of the prehistoric diet.

New research in Scientific Reports suggests that our ancient human relatives might have survived, at least in part, by adeptly feeding on the meaty carcasses left by big cat predators when they were plentiful. Models of predator, prey and scavenger relationships suggest hominins were most successful when banding together to exploit and defend such finds from adversaries like giant hyenas (Pachycrocuta brevirostris) that were seeking their own easy meals.

Researchers’ computer simulations theorized conditions under which routine scavenging would have been a successful strategy for early humans, and when it wouldn’t have been. The work explored a sometimes overlooked but likely important part of our ancestors’ wider prehistoric diet.

Hunting is often considered a more advanced human behavior than scavenging, says Jesús Rodríguez Méndez, a paleoecologist at the Spanish National Research Center for Human Evolution and a co-author of the study. “Actually, virtually all large predators are also scavengers,” he notes. “Instead of that negative impression, we view scavenging as a product of the behavioral flexibility and cooperative abilities of the early hominins.”

Saber-toothed cats were formidable early Pleistocene predators making regular meals of ungulate species from elephants and woolly rhinos to the ancient relatives of deer, horses and antelope. But research shows that because of their iconic teeth, the big cats didn’t pick the carcasses clean and routinely left lots of meat on the bone. Those carcasses represented nourishing meals for the taking. The question is whether hominins were able to regularly capitalize on them, or whether they were outcompeted by other scavenger species like the giant hyena.

Rodríguez and colleagues modeled conditions and competition during the late-early Pleistocene era, some 1.2 million to 800,000 years ago, in southern Europe’s Iberian Peninsula, now home to Spain and Portugal. The researchers first estimated just how much meat and fat would have been left on a typical carcass left behind by two species of big cats, and how often the felids would hunt. The study suggested that if Megantereon made one kill a week, the saber-toothed cat would only consume about one-third of the energy available in the carcass before moving on to a new kill and leaving the rest for opportunistic scavengers.

The study also simulated scavenging competition between hominins and giant hyenas by estimating both the energetic costs and the potential returns that each species would incur seeking such meals. The researchers ran simulations with various ecosystem parameters, featuring higher or lower populations of predators and prey, which determined the number of available carcasses. Six different experiments were each run 70 different times.

In experiments where the ecosystem had more predators, both giant hyena and hominin populations also grew to larger sizes. When fewer saber-toothed cats were present, giant hyenas survived in smaller numbers, and hominins often couldn’t source enough carcasses to survive on scavenging alone. When the European jaguar was introduced to such a system as a third predator, enough carcasses were left to support populations of both scavengers.

“Baring having a time machine, these sorts of models and simulations I think are really useful in getting a broad sense of parameters of what we think happened in the past,” says paleoanthropologist Briana Pobiner of the Smithsonian’s National Museum of Natural History, who studies the evolution of human diet and wasn’t involved in the new study. “I think it shows that scavenging could have been feasible.”

Rodríguez acknowledges that simulations can’t reveal the real-world behavior of past hominins with the kind of certainty that the archaeological record might. However, he notes, that record is scarce and fragmentary, and evidence that can clearly depict a behavior like scavenging after one million years is extremely hard to come by. “In this context, our aim is only to contribute new arguments to the debate, by showing whether it was feasible or not that saber-tooths produced abundant carcasses and that hominins were able to exploit them in the presence of giant hyenas,” he says. “We cannot demonstrate they did it; we only try to show that they might have done it under certain conditions.”

Among the most important of those conditions seems to have been social cooperation. The simulations suggest that humans got the upper hand only when they organized into scavenging groups big enough to fend off competitors but small enough so that each individual still got plenty of payoff from each kill.

The researchers arbitrarily set a group size of five as necessary for the ancient humans to chase off a solitary giant hyena scavenger. During all simulations in which the hominins banded together in groups of fewer than five individuals, hyena populations outcompeted them. Only when the number of predators was high—and thus more carcasses were available—were hominins in groups of less than five able to survive on scavenging alone. When hominins banded together in groups larger than five, their populations grew at the expense of hyenas, although the hyenas were able to survive in every case. This positive impact was seen up until hominin group sizes reached 13. After that point, the results suggested there wasn’t enough meat to go around and make it worthwhile for so many individuals to work together. “I really like the way they look at how much meat needs to be out there to make scavenging profitable, and when is it not,” Pobiner says. “I haven’t seen another study that looks at hominin group size in terms of scavenging.”

Rodríguez notes that while researchers don’t know what types of social groups early hominins used, cooperation was likely required. “And this is important, because people tend to think [of] scavenging as a ‘simple’ behavior, not requiring any complexity, in comparison to hunting. But we defy this interpretation,” he adds. “Unless you assume that a single hominin was able to chase away a giant hyena, cooperation is necessary. Did scavenging contribute to promote social organization? Probably yes, but just as one of many other factors.”

The study is based on a set of assumed parameters, but the model allows for adjustments that could allow researchers to explore other theories and conditions. For example, the scimitar-toothed cat Homotherium is modeled here as a solitary predator. If the cats were social, as some scientists suggest, then the packs of the predators might have left little meat on the bones for scavengers.

Giant hyenas were also modeled as solitary scavengers, but many scavenger species don’t behave that way. Modeling them as pack animals might change the apparent scavenging success rates of our own ancestors when in competition with the canids.

Importantly, while the models include only scavenging, the authors stress that wouldn’t have been the only way early humans sourced food. Homo erectus and relatives were opportunists and omnivores. “Our intention is not to say that early hominins were not able to hunt, or that they did not gather vegetables and small animals,” Rodríguez says. “The intention is only to show that scavenging could also be part of their food procurement strategies.”

While early humans were crafting spears and using them to hunt large animals at least 500,000 years ago, evidence also exists of them butchering such animals more than two million years earlier. Much of the meat early humans enjoyed before the development of more advanced weapons might have been scavenged; taking advantage of leftovers from the era’s very formidable hunters might have been a key part of human history.

Pobiner says scholars need to abandon the mindset that scavenging is a less advanced behavior than hunting, and the modeling study suggests more reasons why that might be the case. “Scavenging may have required some pretty sophisticated behavior, cognition and communication,” she says. “In some ways it’s really smarter, and safer, to let someone else do the killing for you.”

Brian Handwerk | READ MORE

Brian Handwerk is a science correspondent based in Amherst, New Hampshire.