E.T. was the perfect extraterrestrial: Cute, smart and — best of all — a perfect pacifist.

Unfortunately, scientists aren't so sure that an actual intelligent alien would be so benign. In a recent interview with El País, famed physicist Stephen Hawking posited that an alien visitation would put Earthlings in the same position as Native Americans when Columbus landed on their shores.

"Such advanced aliens would perhaps become nomads, looking to conquer and colonize whatever planets they can reach," Hawking speculated. [7 Huge Misconceptions About Aliens]

The likelihood that intelligent life is out there is up for debate; less discussed are the conditions necessary to evolve a life-form that's both smart and nice. But the lessons from Earth suggest that intelligence and aggression might evolve hand-in-hand.

Evolving smarts

No one really knows how humans got to be so clever. What's clear is that hominin brains began expanding wildly about 2 million years ago. (Hominins include those species after the human lineage — the genus Homo — split from the chimpanzee lineage.) By around 100,000 years ago, humans made the never-before-seen leap to inventing language. And by at least 40,000 years ago, our ancestors were making art.

"We have brains that are three times bigger than those of our closest relatives," said Mark Flinn, an anthropologist at the University of Missouri who has researched the emergence of human intelligence. Humans have unprecedented abilities to think about each other's thoughts and motivations, he said, to play out social scenarios in their brains and to think about the past and future.

"The general presumption is that this is just sort of a natural outcome of the evolutionary process, but that's really giving short shrift to the very special circumstances of human evolution," Flinn said.

Huge brains are expensive. They take an enormous number of calories to grow and function (up to 50 percent of intake in infancy and childhood, Flinn said) and make humans basically helpless for years after birth.

"Our babies are born as larvae, basically," said David Carrier, an evolutionary biologist at the University of Utah.

Many anthropologists and evolutionary biologists have tried to pinpoint the special circumstances that make these huge brains worth the expense. Charles Darwin suggested that perhaps males developed cleverness to attract females, much as a male peacock developed showy tail feathers to prove to potential mates that he could strut his stuff. But if brains were just for sexual display, scientists would expect to see big differences between male and female intelligence — females, not having to attract mates, shouldn't waste so much effort on their brains, much as peahens don't waste effort on growing shiny feathers (theirs are dull and brown). And female humans are just as smart as males.

Social pressure

Would smart aliens have energy-intensive brains? Hard to say — perhaps E.T. could evolve a more efficient, yet just as clever, organ. But if aliens were sending signals into space or building rockets, they'd have to have achieved an intelligence that far exceeds what is needed to survive. [13 Ways to Hunt for Intelligent Aliens]

Humans have done the same, and researchers can't quite figure out why. The brain could have evolved to allow humans to use tools, but chimpanzees use tools without developing complex languages, art and culture. One provocative theory holds that pathogens play a role: The brain is vulnerable to infection, wrote Hungarian researcher Lajos Rózsa in a 2008 article in the journal Medical Hypotheses. Showing off one's cleverness may be a way of showing off how resistant one is to infection. After all, if you're smart enough to invent language and art, you must be pretty good at battling brain parasites.

So perhaps intelligent aliens might be subject to alien parasites. Flinn and his colleagues favor another theory, though. They argue that humanity underwent a runaway cycle of brain evolution because of hominins' social nature.

The ecological dominance-social competition hypothesis works like this: Human ancestors reached a point in which their interactions with one another were the most important factor in whether they'd survive and pass on their genes. Finding food and shelter was still important, Flinn said, but it wasn't the main factor determining evolutionary success. The difference between clever humans and, say, caribou, is that intraspecies relationships drove evolution the fastest in humans, Flinn said. A herd of caribou has social interactions, to be sure: Males have to fight for mates, for example. But a more pressing concern would be avoiding predators and finding food. For hominins, these external issues became relatively less important, the theory goes, while their ability to form coalitions, to have empathy and to behave in such a way as to win friendships from others became key to their survival. [10 Things That Make Humans Special]

In this heavily social context, it became very important to be smarter than the competition. Each generation got a little smarter and a little better at building complex social relationships, which created a feedback loop in which even smarter brains were beneficial.

"The thing about social competition is it's a dynamic challenge and it's also creative," Flinn said. "You need to have the better mousetrap every time. The competition adjusts to the current winning model, so you need to be one better than the current winning strategy."

The model seems to work with other clever animals, too, he added. Dolphins, orcas and chimps all form social coalitions with each other and depend on their social groups to survive. It's possible that this social factor would hold for species on other planets, too.

The evolution of aggression

A key part of this theory is competition. Chimps form coalitions that battle against other chimps. And humans are far from peaceful. So if an alien species were to evolve intelligence, would aggression be an inevitable part of the package?

Perhaps. The evolution of aggression is a question unto itself. Fights to the death occur only in species where the options are mate or die, Carrier said. [Fight, Fight, Fight: The History of Human Aggression]

"If you can walk away from a fight and reproduce another day, you do that," he said. "But if circumstances are such that your ability to reproduce is threatened by a competitor, in that situation it makes sense to fight."

Environmental factors may determine whether a mate-or-die system emerges. For example, chimpanzees are a particularly homicidal (chimpacidal?) species, Carrier said. Work by primatologist Richard Wrangham at Harvard University and colleagues finds that chimp "wars" arise from a chimpanzee's territorialism. Small groups of foraging chimps may come into contact with other chimpanzees; killing these competitors (particularly when the foragers have numbers on their side) can be beneficial by opening up access to more resources.

Deadly male-male competition is less of a way of life for bonobos, humankind's other closest primate ancestor. Male bonobos stick by their mothers and the species is less territorial than chimpanzees. Bonobo foraging groups are also larger, perhaps because their food sources are more abundant, studies have found. Would aliens act more like bonobos or chimps? Hard to say. Researchers are even split on whether humans are more inherently aggressive or inherently peaceful.

A controversial theory holds that aggression was a driving force in human evolution. The "Killer Ape" hypothesis argues that the human ancestors who thrived were those better adapted for fighting. For example, Carrier said, modern humans can form fists, which our closest primate relatives cannot. This particular hand configuration may have evolved primarily for better manual dexterity — but it also could have come in handy as a club. Likewise, when human ancestors started walking on two legs, their face bones also evolved to be stronger and less delicate. This could be due to diet, Carrier said, but male face bones are more robust than female face bones, a sign that male-male competition could be at play. In other words, thick facial bones could be a defense against the fist, a weapon that would have become available once human ancestors became bipedal.

Kind aliens

If intelligence evolves in the context of social competition, and aggression is the natural outcome of competition, it's hard to imagine that clever aliens could also be kind. Is this the end for hopes of sweet little E.T.?

Maybe not. The social competition model doesn't work without cooperation, after all. Humans fight, wage war and sometimes murder each other. But humans also form coalitions, care for each other and even build coalitions of coalitions, such as nation-states.

"There are two sides to our nature," Carrier said. "It's not that one is any more real than the other. It's just who we are."

Humans are unique among Earth life in forming long-lasting alliances between groups, not just individuals, Flinn said. Chimpanzees can't pull that off, he said, so it's not clear that aliens could, either.

"On Planet X, it may not be inevitable that social competition results in a morality and a creativity of the sort that allows these intelligent life-forms to negotiate with us for a mutually beneficial outcome," Flinn said.

On the other hand, chimpanzees don't explore space. Perhaps a civilization that can band together to reach for the stars has to be cooperative by definition. If that's the case, humanity might be a greater threat to aliens than aliens are to humanity. Luckily, evolution has given humans the tools for peace.

"We can, in effect, rise above the design, potentially," Flinn said. "If we understand what our brains are designed to do, we are going to be way more capable of rising above those tendencies that we have."

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