Are Body-Snatching Aliens a Real Threat to Humanity?

It’s hard to imagine a more frightening existential threat than aliens landing on Earth, but one early sci-fi movie scared viewers on a visceral level by zooming into one eerily plausible way that extraterrestrials could invade our very bodies.

In 1978’s Invasion of the Body Snatchers — a remake of the 1956 movie of the same name — health inspector Matthew Bennell (Donald Sutherland) faces off with an alien after his scientist friend, Elizabeth, discovers pod-bearing pink flowers taking over Earth. By hitching a ride on these flower seed pods, alien microbes can invade and create duplicates of any sleeping human, effectively turning them into “pod people.”

According to Stuart Sumida, a biology professor at California State University, San Bernardino who consulted on movies like How to Train Your Dragon, Reign of Fire, and The Lion King, there are real-life comparisons to be made with the sci-fi classic that turns 45 this December.

“The invaders in Body Snatchers are sort of co-opting the bodies,” Sumida tells Inverse. “So they are somewhat parasitic, but they also take over the body machinery — sort of like the way a virus does.”

Alien vs. Humans

So what do evolutionary biologists make of the movie’s premise of alien microbes infecting humans and using them to propagate? It could happen says R. Alexander Pyron, the Robert F. Griggs Associate Professor of Biology at George Washington University. “There's no intrinsic reason that life elsewhere in the universe couldn't be similar enough to our own to infect us,” Griggs tells Inverse, “especially if it is a very simple organism like a virus.”

But other experts are quick to point out how unlikely it is. Beth Reinke, assistant professor of biology at Northeastern Illinois University, tells Inverse it depends on whether the parasite is a specialist — meaning they rely on one or few species to survive — or a generalist capable of adapting to a variety of conditions and infecting a wide range of hosts. The parasite would have to be either a generalist species or have a host “remarkably similar” to humans, Reinke says, which isn’t likely on any nearby planets.

Cross-contamination from alien microbes is a clear and present concern for space agencies, especially as more rovers have landed on Mars in recent decades. It’s also why astronauts are kept in isolation upon returning from space travel and space agencies have infection control protocols. Some microbes — and organisms like tardigrades — are known to survive the freezing temperatures and vacuum of deep space.

“From the beginning of space exploration, there has been a concern about importing alien microbes from space here to Earth,” William Schaffner, a professor of infectious diseases at Vanderbilt University, tells Inverse.

Adapting Alien Life to Our Planet

In Invasion of the Body Snatchers, the aliens can adapt and survive on Earth. But it’s up for debate as to how well a similar alien species might actually adapt to our planet’s environmental conditions and colonize our bodies.

One big factor determining whether an alien micro-organism could survive on Earth is plasticity. According to Reinke, plasticity refers to the variation in the trait of an individual. So, let’s say a microbe typically lives in an environment of 40°C (104°F) but could survive at temperatures ranging from 20-60°C (68°F- 140°F). Alien microbes with a lot of plasticity could adapt to Earth’s climate.

The second factor is generation time, or the amount of time it takes for microbes like bacteria to double in number. So it’s plausible a microbe — like the pods in Invasion of the Body Snatchers — could adapt to Earth in a matter of hours or weeks if it has a short generation time.

Reinke adds that “if not much adaptation is necessary because environmental conditions are similar, yes, I'd imagine they would be likely to survive if they had a food source.” (In the case of Invasion of the Body Snatchers, the fictional food source would probably be the sleeping human bodies)

“From the beginning of space exploration, there has been a concern about importing alien microbes from space here to Earth.”

Jason Wright, professor of astronomy and director of the Penn State Extraterrestrial Intelligence Center, isn’t quite convinced alien microbes will be coming for humans anytime soon. Viruses on Earth evolved in tandem with humans to hijack our bodies’ cells and evade immune responses — with SARS-CoV-2 being the most infamous recent example. Alien microbes, on the other hand, would not have co-evolved with and adapted to humans.

“We should not expect life that evolved on another planet to have any such adaptation,” Wright says. “It seems unlikely that Earth would just happen to have the conditions alien life needs to thrive.”

Sumida agrees, saying there is no “guarantee” alien microbes would adapt and survive on Earth — but they might be able to if they came from “a similar water-based, carbon-based habitat.”

Schaffner points out that microbes often multiply and mutate rapidly to adapt to environmental conditions. So, it’s “scientifically reasonable” to assume alien microbes may “grow more rapidly in our more lush circumstances” on Earth, especially if they originally adapted to reproduce on a planet with a harsher environment than ours.

Now that’s a truly chilling idea to contemplate — and another reason why Invasion of the Body Snatchers endures as such a terrifying sci-fi nightmare after all these decades.