A VORACIOUS dinosaur chases its prey and tears it limb from limb. Big cats stalk the herds of the Serengeti before pouncing on a terrified gazelle. If you’ve ever wondered why we find gruesome scenes like this so compelling, it may well be because we have something in common with those unfortunate victims.
Put aside the idea that our hominid ancestors emerged from the jungles with weapons in hand and a range of hunting tactics at their disposal. A growing number of researchers think it’s time to revise the old notion of primitive humans as killer apes turned hunters.
Richard Coss, a psychologist at the University of California, Davis, argues that far from moving smoothly to the hunter stage, our evolution was dramatically shaped by a long struggle to outwit predators of our own. Until our ancestors gained the upper hand, humans were a hunted species, and that long period of history spent as a potential meal has shaped the way we are. If this new thinking is correct, the prey phase in our evolution could help explain not just our fears and phobias, but some of the key characteristics that make us human, including our ability to intuit each other’s thoughts, our social values of loyalty and friendship, perhaps even our language and technology.
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Popular versions of hominid history highlight the importance of our prehistoric “Fall” about five million years ago when long-term climate change shrank the African rainforests, driving our ancestors earthwards from their treetop Eden. What many accounts fail to emphasise is the hellish nature of this new environment. In the forest lurked sabretooths, leopards and giant carnivorous bears. More dangerous still was the ever-expanding savannah, patrolled by pack-hunting predators – giant flesh-shearing hyenas, blade-toothed dogs the size of wolves, lions and yet more sabretooths. There seems little doubt that these predators – already practised at hunting animals faster, larger and better-armed than hominids – would have found the conspicuous, puny and slow early humans a tempting meal.
Primates face the same dangers today. Researchers looking at modern patterns of predation regularly find primate remains in the droppings of large carnivores. What’s more, Kim Hill and Anna Magdalena Hurtado from the University of New Mexico in Albuquerque have found that among the Amazonian Ache foragers, 6 per cent of deaths in young adults are due to jaguars. And a recent review by Adrian Treves, a biologist at Conservation International in Washington DC, revealed that lions, leopards and spotted hyenas killed or injured 393 people in Uganda over an 80-year period ending recently. “These numbers undermine the long-standing assumption that large carnivores do not attack humans unless they are maneaters as a result of a disability or old age,” says Treves.
Coss says this would have applied directly to our prehistoric ancestors. “Once leopards and lions learn the ease of rasping flesh off human bones, they can become voracious killers, focusing almost exclusively on humans,” he says. “Such effects could have decimated ancestral human populations at night.” Coss points to the fossil record to support this view. Carnivore lairs in South African caves, dating from between one and three million years ago, have turned up the fossils of 324 baboons and 140 australopithecines – ancestors of humans – many bearing marks typical of tooth and claw damage by both big cats and predatory hyenas. The very high proportion of primate fossils at some of these sites suggests at least one predator species was specialising in baboons and early humans.
Being on another animal’s menu invariably has evolutionary implications. An arms race develops between hunter and hunted which can lead to changes in the prey animal, such as increased body size, higher sprinting speeds and improved sensory faculties. It can also produce various behavioural adaptations, including herding, heightened vigilance and predator mobbing. So, if predation was a substantial evolutionary pressure facing early humans, how did it shape us?
Coss is convinced that we still bear the marks of our time as a hunted creature. Inherited instincts and intuitions can remain hardwired into the brain for a very long time, he says, even once they have become obsolete. This might explain relic behaviours such as the Moro response – a reflex grasping action displayed by newborn babies when startled. It resembles the way in which an alarmed infant monkey clings to the furry belly of its mother. “This ancestral infant response, which disappears at about six months of age, is therefore at least six million years old,” says Coss.
Looking for further evidence, Coss generated a virtual rocky outcrop in a prehistoric African savannah. It included three possible refuge places: a boulder, a crevice and a thorn tree. He then gave a tour of the site to a group of pre-school children, before introducing a virtual lion to the scene and asking them to choose the safest place. Most opted for the crevice or the thorn tree. Only about one in six kids went for the boulder – the worst place to escape to, since lions like to climb such rocks to sun themselves. So how did most urban four-year-olds know to avoid this? For Coss, the answer is the intense predation pressure on our ancestors. Individuals who chose unsafe refuge sites would have perished, together with any gene combinations that made them behave that way.
Some scientists doubt that obsolete traits would persist for so long, but Coss believes he has direct evidence that they can. He studied Californian ground squirrels, which react to predatory snakes with a characteristic elongated frozen posture and by throwing sand and leaf litter. Although this behaviour is not found in the related Arctic ground squirrels, which have lived without snakes for 3 million years, it is still present in Californian ground squirrels from isolated high-altitude populations that have not encountered snakes for 300,000 years. By estimating the corresponding number of generations for humans, Coss calculates that our anti-predator adaptations could have persisted in the absence of significant predation for well over a million years – long enough for them still to be with us today.
Other researchers argue that some of our irrational fears are evolutionary hangovers from the dangers of our ancestral environment. “Spider phobias, for example, are much more common than car phobias,” says Clark Barrett, an anthropologist at University of California, Los Angeles. “But the probability of being killed by a car is much higher in modern urban environments than the probability of being killed by a poisonous spider.”
Barrett believes that if predators have been an important influence in our evolution then we should be well attuned to understanding them. And that’s exactly what he’s finding. For instance, children from different cultures – urban ones included – learn facts about how dangerous animals are faster than information about, say, their habitats. And they seem to have an insatiable appetite for information about predators – even extinct ones. Barrett calls this “Jurassic Park syndrome”. What’s more, very young children seem to be well aware of the true motivations of predators even if they have been bombarded with false images such as the friendly lion in The Lion King.
Mind readers
To investigate the way young children think about predators and prey, Barrett invented new versions of a standard test used by psychologists to assess children’s ability to understand what another person is thinking. The Sally-Anne test is normally acted out with two dolls. Sally hides a sweet in the presence of Anne, then moves it to a new location when Anne’s back is turned. A child with the mental sophistication to see the world from another’s perspective will realise that Anne will search for the sweet in the original hiding place.
Barrett wanted to see what would happen if he did the experiment with toy predator and prey, so he replaced the dolls with a toy zebra and lion. He gave both the Sally-Anne test and the zebra-lion test to two groups of children – one from rural California, the other from an Amazonian tribe called the Shiwiar – and found that their insight into the lion’s mind was at least as good as their grasp of Anne’s viewpoint. By age four they could all answer both versions of the test correctly. What’s more, younger children who passed only one of the tests always got zebra-lion right and Sally-Anne wrong.
Celia Heyes, a psychologist from University College London, points out that since modern children rarely encounter predators, their sophisticated reasoning about them is unlikely to be the result of learning. “The expertise could only come from one place: natural selection,” she says. Barrett, who has carried out lots of experiments similar to the zebra-lion test, believes they show we do have a special way of thinking about predators. “Predators are not just represented by the mind as a kind of human with nasty intentions,” he says. More radically, the fact that some young kids performed better on the zebra-lion test than on Sally-Anne has led Barrett to question one of the orthodox views about how “mind-reading” evolved.
Humans and a few other highly intelligent animals possess what psychologists call a “theory of mind” – the ability to put themselves in another’s position and attribute feelings and motivations to them. This helps individuals live in social groups, and it’s also known as Machiavellian intelligence because it is frequently used to outwit others. Because of these advantages, mind-reading is thought to have evolved as an adaptation to a social lifestyle. But Barrett points out that this isn’t necessarily so. “It is at least plausible that predator mind-modelling evolved earlier in our lineage,” he says. Perhaps our slow and vulnerable ancestors were forced to evolve mental abilities to outfox their deadly predators, as they couldn’t run away or overpower them. Once in place, these abilities could easily have been adapted to social situations. “I’m very much in favour of this idea,” says Klaus Zuberbühler, a primatologist at St Andrews University.
But it’s going to take more than supposition to overthrow the old theories. “The idea that sophisticated social-cognitive abilities are required to understand predators seems far-fetched,” says Michael Tomasello from the Max Planck Institute for Evolutionary Anthropology in Leipzig. “It seems most plausible that the key evolutionary pressures for our unique brand of theory of mind come from cooperative interactions with members of our own species.”
There is no doubt, however, that the threat of being eaten has directly affected our social lives. “It is the driving force for primates to live in groups,” notes Louise Barrett, a primatologist from Liverpool University. By foraging in groups, baboons and chimpanzees improve their chances of detecting predators and escaping in the ensuing confusion. Groups of primates even mount counter-attacks – mobs of baboons can seriously injure leopards and cheetahs, while chimpanzees may pursue, corner and kill prowling leopards.
It’s not just a question of safety in numbers. Being dish of the day also affects behaviour within the group. “Calmer, less conflict-ridden social organisations are safer,” says Treves. That’s because infighting groups are noisy and conspicuous, and less able to look out for danger. Such vigilance would have been essential, and Treves has found that primates are unique in that a group’s ability to look out for danger is not linked to its size but instead to the physical proximity of the members. He concludes that ancestral human groups would probably have been physically close-knit. “Safety might depend on establishing familiarity, trust and reciprocity with only a few individuals.” In other words, our vulnerability probably cultivated behaviours that are key to human civilisation.
There’s even a possibility that two of the hallmarks of humans – language and tool-making – may have evolved as defence mechanisms against predators. Several species of monkey including vervets, Diana monkeys and Campbell monkeys produce alarm calls that identify different sorts of predator such as eagles, leopards and snakes. “Judging by their behaviour,” says Zuberbühler, “these monkeys treat predator alarm calls as truly semantic signals.” Although the vocalisations are largely innate, some people regard this as a proto-language. “I think that primate alarm calls are important for the discussion on the origins of human language,” says Zuberbühler.
There are also records of chimps using sticks and stones as weapons in counter-attacks against leopards. “I suspect that alarm-calling and mobbing behaviour – in particular shouting and throwing stones – pre-dated walking upright in ancestral humans,” says Coss, who is sympathetic to the idea that language and tools may have arisen in this way. Louise Barrett agrees. “It is surely more reasonable to imagine that early humans discovered weapons as a means of driving away predators, then adopted them for scavenging and then hunting, than that we spontaneously ‘invented’ hunting tools,” she says.
Some of these claims might seem radical, but is it so far-fetched to argue that avoiding being eaten has helped shape the way we think? Some people find the mere suggestion provocative, and many anthropologists don’t accept that looking at the environment in which ancient hominids evolved can say anything about our minds today. “Yes, our ancestors were predated upon, but whether this has any substantial relevance to the things that fascinate us about ourselves today, I am dubious,” says Ian Tattersall, curator of the American Natural History Museum in New York. “Though such ingredients make for a dramatic story, they are inherently untestable. How could we place a probability on them, or show them to be wrong?” Louise Barrett is also wary about putting too much emphasis on a single factor to explain evolutionary change. “But I do think that the influence of predation on human evolution has been greatly underestimated,” she adds.
Coss, Clark Barrett and Treves know they’ll need to do more to shore up their idea, but they also believe that it has great potential to increase our understanding of human evolution. Perhaps the time is right to recognise that we may owe our strength to our ancestors’ vulnerability. “We tend to think of prehistoric humans as hunters,” says Louise Barrett, “whereas they probably spent a lot of time hiding in holes.”
