Monkeys’ Attraction to Burned Grasslands May Offer Clues to Human Ancestors’ Mastery of Fire

When human ancestors first made fire, they turned a force of nature into a handy tool. The pivotal breakthrough allowed our predecessors to cook, clear land and fend off fierce predators.

Scientists will never know with certainty the story behind that initial spark lit by hominins—living and fossil species on the human branch of the primate family tree. But some researchers assume clever hominins learned to control flames by watching wildfires that tore through their African homelands. Perhaps human ancestors tasted a naturally seared carcass and became determined to light their own paleo-cookout 1 million years ago or so.

Since scientists can’t ask ancient hominins what drew them to fire, several years ago anthropologist Nicole Herzog turned to the next best thing. The professor at the University of Denver began studying primates who live near frequently ablaze habitats to see if our evolutionary cousins reap any benefits from scorched lands. “Fire's a frightening and dangerous and scary thing,” she says. “What would motivate a critter to actually want to go near it or associate with it in the way that humans do?”

One key benefit may be safety from predators, according to Herzog’s latest study, published this month in the Journal of Evolution. For the study, the anthropologist tracked the behavior of South Africa’s vervet monkeys before, during and after fires. It turns out the monkeys sensed fewer predators in fire-swept savannah compared to unburned grasslands, where leopards and other stealth killers hide. The openness of burned ground may deter carnivores and offer a safe haven for these primates.

The finding that living primates benefit from blazes in this way means it’s possible ancient hominins did too, according to Herzog and her coauthors. The scientists think human ancestors initially ventured into fire prone grasslands, at least in part, to avoid predators. That could explain why hominins came to frequently encounter and eventually master fire.

Archaeologist Sally Hoare of the University of Liverpool praised the study for documenting primates that face multiple predator species in a savannah environment, which might be similar to the habitats of some hominin species. She says studying non-human primates is “the only way we can actually look at how the early uptake of fire may have occurred, what may have attracted hominins to these environments.”

Others show more skepticism. “It’s an important question how primates in general approach fire and fire areas,” says biological anthropologist Rick Potts of the Smithsonian’s National Museum of Natural History. “It may have relevance to human evolution.”

Herzog conceived the study with colleagues from the University of Utah while she was a doctoral student there. They had read reports, mostly anecdotal, describing primates’ reactions to fire, but they had not seen any studies attempt to measure the benefits of burned lands.

To do so, Herzog focused on a troop of 25 vervet monkeys that roamed freely in South Africa’s Loskop Dam Nature Reserve. The plucky monkeys, with salt-and-pepper pelts and black faces, are famous among anthropologists for their language-like communications. Vervets sound distinct alarm calls for different predators and troop members respond with appropriate defensive moves. They climb high trees after the leopard call, scan the brush after the snake call, and dive into vegetation upon hearing an eagle call. “They have this vocal repertoire,” Herzog explains. “When you hear those calls you have a good idea of what's going on.”

In 2012, park staff lit controlled fires to improve habitat conditions and reduce the risk of natural wildfires. The researchers observed the animals during the two days the fires raged, plus 90 days before and after. Over that 6-month span, during 107 hours of logged observations, the scientists documented the troop members’ minute-by-minute locations and activities like feeding, resting and bickering. But the scientists were most interested in behaviors signaling the monkeys had either spotted or were looking out for predators. In addition to the alarm calls, when a vervet first senses danger, it may scale a tree or stand on its hind limbs and visually scan the surroundings.

During the study, Herzog’s team didn’t witness any lethal attacks. But they did glimpse vervet killers, including black mamba, python, viper snakes, leopards and baboons—bigger monkeys that sometimes eat vervets. The reserve is also home to African wildcats, jackals, eagles and crocodiles. Based on earlier research, vervets’ most common killers are leopards, which hunt by sneak attack in dense vegetation. Herzog predicted fire would transform treacherous grasslands into open ground, where monkeys could spot predators well in advance. What she didn’t know is if the monkeys themselves would realize that charred earth offered protection in this way.

The results suggest the monkeys did in fact feel safer from carnivores in the burned zones. Of the 72 anti-predator behaviors observed, only 10 happened on burned ground. They logged eight scans of the surroundings, two instances of fleeing and zero alarm calls. This could mean there were actually fewer carnivores in the areas post-fire. Or, the monkeys relaxed their vigilance measures, knowing they could spot a predator before it posed serious danger. The vervets also entered grassy parts of the park, now charred, where they’d never been seen before. Generally the troop congregates near rivers lined with canopy trees. But, “burning that grass was like turning this key and it opened up an entirely new area to them,” says Herzog.

The troop appeared nonplussed near active, raging fires, says Herzog. She fondly recalls one “decrepit old male” that perched on a branch and watched a fire, seemingly for entertainment, until it nearly reached the tree’s base. This fits with observations of chimpanzees around brush fires in Senegal, previously reported by Herzog and Texas State University anthropologist Jill Pruetz. Both the chimps in Senegal and vervets in South Africa appeared to grasp fire safety. Near a blaze, they continued feeding, grooming and resting without signs of stress or fear—for example, having their hairs stand upright. When the flames came close enough to pose danger, the groups calmly relocated. They “key into things about each particular fire that tell them whether they need to get out of there, or whether they can sit and watch it pass,” says Herzog.

Herzog cautions that the new study tracked one troop of vervets for several months. She says more studies are needed on diverse primates in different environments before scientists can make any big claims about fire in human evolution.

Still, the vervet data adds one brick of support to a big idea posed by the study authors a few years back. According to their pyrophilic primate hypothesis, hominins faced frequent wildfires 2 to 3 million years ago. Eventually human ancestors adapted to and benefited from these conditions for several possible reasons: Burning made grasslands easier to traverse, exposed hidden food resources like seeds and tubers, or reduced predator risks—as seems to be the case for vervets today.

The last point may have been especially important a few million years ago, when the African carnivore guild was far more terrifying that today. Archaeologist Sally Hoare points out that predators were bigger, like Pachycrocuta, the largest hyena to ever walk the earth, and Agriotherium, a carnivorous bear nearly twice the size of a lion. Hoare says predators were also more diverse. At least ten genera of large mammal predators stalked Africa then, compared to five today. As for hominins, roughly three-to-four feet tall Lucy-like Australopiths, Hoare remarks, “You kind of wonder how any of them actually survived.”

Potts takes issue with the timing of this scenario. Carnivore numbers begin to drop around 2.8 million years ago, whereas fire prone grasslands don’t appear widespread until after 2 million years ago, he says.

The date when human ancestors mastered fire is hotly contested. Archaeologists identify hominin-made fires by using biochemical analyses to identify sediment, bones and artifacts that show signs of high-heat alteration. Using these methods, scientists have shown human ancestors built campfires by nearly 800,000 years ago, based on circular concentrations of burned wood, seeds and stone tools at the site of Gesher Benot Ya’aqov, Israel. Some scientists think hominins mastered fire between 1 and 1.5 million years ago because of scattered burnt bone, plant ash or reddened sediments detected at African sites, including South Africa’s Wonderwerk and Swartkrans and Kenya’s Koobi Fora and Chesowanja. But for these sites more than 1 million years old, researchers debate whether hominins ignited their own flames or wildfires burned the materials.

Some scientists push our lineage’s mastery of fire earlier than that in-the-dirt evidence. Harvard University anthropologist Richard Wrangham and others contend hominins became pyro-masters around 2 million years ago based on changes in the appearance of fossils. Around this time hominin body and brain size nearly doubled, while teeth and jaws got smaller. Wrangham and colleagues attribute these changes to the invention of cooking, which makes food easier to chew, explaining the smaller dentition, and more energy-rich, to fuel bigger bodies and brains. They say these hominins must have mastered fire for cooking.

“I vigorously disagree about that because ‘the must have’ argument is not evidence,” says Potts. He sees direct physical evidence for hearths and burned artifacts as the only way to set the date for hominin control of fire. And other scientists propose the skeletal changes, which form the basis of Wrangham’s cooking hypothesis, were because hominins started hunting and regularly eating meat 2 million years back

Even if scientists do eventually settle when our ancestors mastered fire, that won’t necessarily explain the how and why of it. Nor will watching vervets and other primates provide a definitive answer. But the hope is such studies will spark hypotheses about what drew our ancestors to the flames, and that’s important because there’s at least one point anthropologists agree upon: mastery of fire was a major milestone that set human ancestors apart from other species. “The way that we take advantage of fire and dependence that we have on it” says Herzog. “Probably has been incredibly influential in shaping human evolution.”

Bridget Alex | | READ MORE

Bridget Alex is an anthropologist and science writer based in Pasadena, California. Her stories can be found in outlets including Discover, Science, and Atlas Obscura. She tweets @bannelia.