Prometheus 101: The Science Behind Ridley Scott’s Vision of Aliens Populating Earth

It's not such a crazy idea. Research has long supported the theory that life originated elsewhere

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20th Century Fox

The best thing about writing fiction is that you get to make stuff up. And when the fiction you’re writing is science fiction, you don’t even have to obey the laws of biology, physics or common sense. Want to have a Stay Puft Marshmallow Man go to war with a couple of crazed Transformers? Have at it.

Director Ridley Scott takes full advantage of this license in his new blockbuster Prometheus — and never more so than in the opening scene. Scientists have a lot of theories about exactly when in Earth’s history life first emerged and precisely where and how it occurred. But you can be pretty sure no one’s suggested that it happened when a naked alien landed on the planet, drank a fantastical potion and then threw up into the nearby waterways, seeding a virginal Earth with his DNA. A few million years — and a few jillion special effects — later, human beings venture into space, meet our progenitor race and discover that, yes, we’re the aliens after all. (If you’ve already seen the film, you’ll enjoy TIME’s attempt to answer all the Big Prometheus Questions)

O.K., so maybe this wasn’t peer-reviewed by the National Academy of Sciences. Still, it’s not a fully crazy idea. Indeed, it’s one scientists have explored — in a far more measured way than Scott — for a long time.

Panspermia is the wonderfully descriptive name for the idea that life — or at least the precursors of life — may be common in the universe and that for many planets, including Earth, biology did not just arise de novo. Rather, it was imported aboard incoming meteorites, comets or asteroids. There’s a wonderfully visual quality to this idea, as science illustrators discover when they paint an image of a life-carrying comet striking a sterile Earth and come up with something that looks like a sperm cell fertilizing an egg. Of course, simple imagery, no matter how serendipitously satisfying, has no bearing on whether the theory has anything behind it, but multiple studies over the years have backed it up.

(MORE: TIME’s Complete Prometheus Coverage)

Astronomers have long believed that while water appears to be ubiquitous in the universe, it was not terribly common on the boiling rock that was the primordial Earth. Incoming comets — which are mostly rock and water ice — might have solved that problem, colliding with the planet, hydrating its surface and eventually filling its oceans and seas. A 2011 study gave greater weight to this idea, showing that the ratio of heavy to light water aboard Comet Hartley 2 is a lot closer to the ratio in earthly water than we ever knew, making it likelier that our oceans were indeed filled up from afar. Water, of course, is a sine qua non for life as we know it, so even if there was nothing pre-organic aboard any incoming comets, they still helped get the planet biologically started. But organics are out there too.

In 1969, for example, a 220 lb. (100 kg)  meteorite landed in Australia, lighting up the night sky and crashing to the ground in the town of Murchison. Promptly dubbed the Murchison Meteorite, it was found to have been geologically altered in a way that suggested its home world — wherever it was — had plenty of water. Much more important, the rock turned out to be rich in amino acids, which serve as building blocks of proteins and peptides.

Not everyone was sold on the idea that the acids were extraterrestrial in origin, mostly because the meteorite lay on the ground long enough to have been contaminated by earthly chemistry before it was found. In 1979, however, investigators found a pair of meteorite fragments in Antarctica that had landed about 200,000 years earlier, and these too carried amino acids. The permafrost of the South Pole region is not a place likely to contain biological contaminants, and the researchers thus concluded that the amino acids were authentically extraterrestrial.

Over the years, the case for the cosmos as a biological incubator has only grown. Amino acids have been found in interstellar clouds — a fact that is true of water too. Of the 20 amino acids that exist on Earth, eight have been detected in meteorites. Tellingly, while amino acids can form in either a left-handed or right-handed configuration — essentially mirror images of each other — life on Earth uses just the left variety. And of the eight acids brought in by meteorites, only one — glycine — is a righty. Biologists have never been able to explain why earthly life prefers left-handed amino acids, since both types should work equally well. The answer may be that they are simply the kind that happened to rain down on us, and we used what we got.

The case for E.T.-as-biological-dad got even more compelling last year, when geologist and mineralogist Christopher Herd from the University of Alberta analyzed a meteorite that landed in British Columbia’s Lake Tagish in 2000 and found that it contained amino acids and other organic compounds and that the rock had been continuing to cook them up as it flew through space. The materials were present in the meteorite in five different stages of their development, progressing from simpler to more sophisticated states. Herd believes that traces of water in the matrix of the rock, coupled with heat from gravitational accretion and radioactive elements, allowed the meteorite to serve as a kind of flying incubator.

“It could have operated almost like a hydrothermal cell, with water circulating through the asteroid,” he said. “After a while, the heat would run out and that process would shut off, and you’d be left with different stages of organic materials.”

(MORE: How to Weigh an Asteroid — and Why You Should Care)

While we’ll never know the worlds on which the organics that may have given rise to us originated, it’s possible that one very familiar planet is responsible: Mars. It’s not that hard for the Red Planet to hurl the occasional rock at us. An asteroid strike would send Martian debris into space, which would slowly spiral in until it was snared by the gravity field of Earth. As recently as last July, 15 lb. (7 kg) of Mars rock landed in Africa, making a total of 240 lb. (110 kg) of Martian material that has been collected over the years. It’s the chemical makeup of both the rock itself and the traces of Martian atmosphere captured in it that pegs its origin, as well as the fact that the rocks are comparatively young — millions of years old, compared with the billions of years that asteroids have been around. This suggests that they come from a geologically active place, which also fits the Martian profile.

It was on Aug. 6, 1996, that the most famous Martian rock of all made headlines. NASA announced that a meteorite known as ALH84001 that had landed in the Antarctic in 1984 appeared to contain microscopic fossils of Martian bacteria in the form of tiny carbon shells. That was, by rights, stop-the-presses stuff, and the midday news conference at which NASA announced the findings made global headlines.

“Today rock 84001 speaks to us across all those billions of years and millions of miles,” said then President Bill Clinton. “It speaks of the possibility of life. If this discovery is confirmed, it will surely be one of the most stunning insights into our universe that science has ever uncovered.”

Alas, the discovery wasn’t confirmed. Too many other explanations and unresolved questions poked holes in the thrilling theory: the globules could have been formed volcanically; they were too small to have contained RNA; they could have been the result of earthly contamination. In recent years, the rock has picked up more believers, and some scientists have concluded that if the announcement that Martians had been found was too breathless, the backlash against it was too pitiless. Either way, findings from the Martian rovers Spirit and Opportunity have confirmed that Mars was indeed once a very wet and perhaps fecund place, and while neither of those Mars cars confirmed the existence biology, the new Curiosity rover that will land this summer — on the 16th anniversary of the 84001 announcement, as it happens — could well turn up something.

None of this means that human beings carry so much as a scrap of extraterrestrial organics, or if we do, that it’s any more than a meaningless soupçon of exobiology in an otherwise domestically grown breed. What it does mean is that in its own silly, sweeping, cinematic way, Prometheus is right: our remote little world, sealed in by the film of its atmosphere, will always be part of the much larger universe — its physics, beauty and biology included.

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