If you've ever seen a lion or a polar bear on the hunt, you know how powerful predators can be. Life may well have been troubled by these killer species since its very beginning, over 3.5 billion years ago, and they have wrought untold death and destruction. As a result they get a bad press: even the word "predator" stems from the Latin term to rob or plunder. Small wonder that, when people imagine paradise, it normally doesn't have any predators in it.
But this reputation misses something important: out of predatory destruction, constructive things can emerge. Predators may have played a big role in many of the biggest leaps forward in the history of life, from the origin of animals to the evolution of skeletons. Predation may even have helped shape the evolution of humanity itself, driving us to become ever more intelligent.
These ideas are inherently difficult to prove, because we are talking about changes that took place in ancient ecosystems that no longer exist. Researchers can and do legitimately quibble with most of these notions. But whenever biologists try to explain big leaps forward in evolution, predators emerge as a possible factor. That might well be more than just coincidence. It may be that what does kill you makes you stronger - and bigger, better defended, and more intelligent.
The origins of life on Earth remain a mystery. The life story might have begun around hydrothermal vents at the bottom of the primordial ocean, or in hot, dry valleys on the earliest continents. It may even have come to Earth from Mars. What we do know is that, by 3.43 billion years ago, biological cells had appeared, marking the beginning of Earth's fossil record.
All living things are made of cells – apart from viruses – so the appearance of cells was one of the most important events in the history of life. And that event might have been triggered by the threat of predators.
Before cells, life probably consisted of naked molecules floating around in water. But those chemical inhabitants of the primordial soup were not necessarily peaceable. Some researchers believe that some of these molecules behaved like predators or parasites. They attacked and broke down - "killed" - other molecules. The best way to guard against these attacks was with a solid defence to keep the predators away. The first membranes, which packed organic molecules into the first cells, might have been ideal for the job.
"The defence against predator/parasites is one of the major justifications behind the models of early compartmentalisation," says Armen Mulkidjanian, an origin-of-life researcher at the University of Osnabrück in Germany. We can't rule out that cellular life evolved by some other mechanism - but we may have predators to thank for the existence of the cells in our bodies.
Those first cells were probably prokaryotes - one of the two large divisions of life, which includes all bacteria, plus a group of less well-known microbes called archaea. Prokaryotes are microscopic but by no means insignificant: they outnumber larger organisms, and probably - collectively - outweigh them too. But the plants, animals and fungi we see and interact with are not prokaryotes. All these large organisms belong to the second, more complicated division of life: the eukaryotes.
The consensus view is that the first eukaryotes appeared about 2 billion years ago. It was not an easy step for life to take. Their cells are more complex, so they need much more energy to keep going, say William Martin at the University of Düsseldorf, Germany, and Nick Lane at University College London in the UK. That means the very first eukaryotes must have been getting a boost in the form of tiny sausage-shaped structures called mitochondria that generate energy. But how did the first eukaryotes gain their mitochondria?
Genetics offers a clue. Many eukaryote genes are similar to archaeal genes, while mitochondrial genes look more like bacterial genes. So one idea is that eukaryotes began as predators. In this scenario, an archaeal predator swallowed a bacterial prey. By some quirk of fate, the bacterium avoided being digested and settled down to a new life as a mitochondrion within the larger cell.
There are problems with this predator-prey model, though. Not least of them is the fact that prokaryotes don't seem to swallow cells today. It's more likely, says Martin, that it was the potential for mutual metabolic benefits, and not predation, that brought the bacterium and archaeon together.
Not everyone is willing to give up on the predator model, though. It seems unlikely that an archaeal predator swallowed a bacterial prey, but there's an alternative: maybe a bacterial predator invaded an archaeal prey.
Edouard Jurkevitch at the Hebrew University of Jerusalem in Rehovot, Israel, has spent years studying prokaryotic behaviour. His work has revealed that prokaryotic predators are more common than most biologists appreciate, and they tend to be smaller than their prey, which they attack by tunnelling their way inside. Both observations make a predatory origin for the eukaryotes a plausible option again, he says.
Eukaryotes are more sophisticated than prokaryotes, but many - like the amoebae - are still pretty simple by our standards. They are single-celled and usually microscopic. To grow larger, animals and plants had to become multicellular.
We know that predators can encourage single-celled organisms to turn multicellular, because biologists have seen it happen in the lab. Fifteen years ago, Martin Boraas and his colleagues at the University of Wisconsin-Milwaukee showed that a single-celled eukaryote called Chlorella vulgaris evolved into a multicellular form within 10-20 generations of being introduced to a predatory microbe.
This doesn't necessarily mean that predators drove the original evolution of multicellularity, says Stefan Bengtson at the Swedish Museum of Natural History in Stockholm. But it provides a neat demonstration of how predators could, in principle, encourage such a dramatic change.
And here's another. In 2007, Nicole King at the University of California in Berkeley and her colleagues studied the genome of Monosiga brevicollis, a predatory single-celled organism. It belongs to a group, the choanoflagellates, that are closely related to multicellular animals.
King found that M. brevicollis has genes coding for 23 proteins called cadherins. Animals use cadherins to help bind their cells to one another – but a single-celled organism like M. brevicollis shouldn't have to worry about that.
Digging deeper, King's team discovered that the cadherins in M. brevicollis are mostly located around the base of the cell, which anchors it to a surface, and also around its tentacles. These latch onto passing bacteria and pull them in, providing M. brevicollis with its food. This suggests that cadherins, which today hold the cells in our bodies together, originally helped single-celled predators hold onto their prey.
"It certainly is likely that proteins involved in cell adhesion in animals had different roles in the ancestors of animals," says King. "I like the idea that cadherins might have been involved in recognition or capture of prey." But it is just an idea for now, she says.
Animals probably evolved about 635 million years ago. Within just 100 million years they had exploded into a vast array of forms, many with tough skeletons. Once more, we might have predators to thank.
The evidence that predators encouraged animals to defend themselves with skeletons is pretty strong, says Susannah Porter at the University of California in Santa Barbara. "A really good case can be made, partly because so many different types of animals evolved mineralised structures in a brief interval of time, and many of them seem to serve a defensive purpose."
For instance, some early skeletons have the appearance of chainmail, says Porter. Her colleague Michael Vendrasco has shown that the microstructure of some of those early skeletons would have made them particularly resistant to crushing by predators.
The conquest of the land is one of the most famous events in evolutionary history. Fish began moving onto land, and evolving into amphibians and reptiles, around 370 million years ago. A century ago it wasn't at all obvious how fish fins became arms and legs, but a wealth of fossil discoveries since then have made the transition one of the most complete in the fossil record.
This evolutionary event is often described as the "invasion" of land. But some researchers see it differently. John Maisey at the American Museum of Natural History in New York says it could just as easily have been a way to escape from predators in the water.
It's an idea worth considering, says Bengtson. But again, proving it actually occurred is tricky. "Also the alternatives always need to be considered," he says. Other animals such as millipedes, insects and snails had already moved onto the land well before fish started crawling out. So maybe the fish were lured out of the water by the promise of easy prey.
Within about 150 million years of those first four-legged animals, the group had diversified and flourished, giving rise both to dinosaurs and mammals. Some dinosaurs would later grow into the largest land animals that ever lived. What drove them to do so continues to baffle palaeontologists, who have come up with several ideas to explain the giants' size. Somewhere near the top of the list is the suggestion that predators played a role.
Dinosaurs were probably not warm-blooded in the same way that mammals are, so they could evolve to large sizes without overheating. Plant-eating dinosaurs would have gained by doing so, because growing large protected them against predators. At least, it offered protection until the predators evolved to be larger themselves, which would in turn have encouraged herbivores to evolve even larger bodies, and so on, until both predator and prey were gigantic.
It should be clear by now that whether biologists are looking at the origins of the first tiny cells or the evolution of the largest animals that ever lived, the one constant in their studies is the idea that predators were somehow involved. Little wonder, then, that predation may have had a part to play in our own species' rapid and spectacular success - and in more ways than one.
Our hominin ancestors first walked on two legs perhaps 7 million years ago, but it was only about 2 million years ago that these upright apes gained one of our most important features: a really large brain. Predation might be the ingredient that separates those first true humans from their small-brained predecessors.
The idea is simple. Around 2 million years ago, our ancestors suddenly switched to a much more nutritious, easy-to-digest diet. That meant they no longer needed to work so hard to process their food, so, over countless generations, the human gut began to shrink. The energy our ancestors saved by no longer having to maintain a long gut was channelled into brain development instead.
If the idea is correct, researchers should find evidence that ancestral diets really did change about 2 million years ago. Last year, they found that evidence.
At the 2-million-year-old site of Kanjera in Kenya, which was once occupied by early humans, Joseph Ferraro at Baylor University in Waco, Texas, and his colleagues found the bones of many antelope species, both large and small. The larger animals tended to be represented by partial skeletons that were probably scavenged. But the smaller antelope skeletons were often complete, suggesting these animals had been brought to the site intact. That implies early humans had caught and killed them.
"We showed that hominins may have been able to meet the increased energetic requirements associated with increasingly large brains via persistent carnivory," says Ferraro. To put it another way, early humans might have grown bigger brains by turning predator.
Even the way we use those big brains might be down to the action of predators. Famously, we divide some of our neural activity between the left and right hemispheres of our brains. You may have heard it said that creative people are "left-brained", which isn't actually true, but the divide between your left and right brain does matter. Individuals who can't divide their brain activity can end up developing stutters or dyslexia, among other things.
Over the last few decades, it's become clear that many animals divvy up brain activity in a similar way. Lesley Rogers at the University of New England in Armidale, New South Wales, Australia, thinks she knows why. Early in their evolution, she says, animals found it useful to deal with routine activities with the left hemisphere, freeing up the right to specialise in detecting and responding rapidly to unexpected objects - things like predators.
"One side of the brain was specialised for pursuing food, including prey, and the other for rapid response to predators," she says. "This enabled dual processing and a more efficient brain."
Her research backs up the idea. For instance, a decade ago she showed that chicks lose the ability to multitask – to forage for food while keeping track of the threat from a predator - if they don't divide neural activity between their hemispheres.