What would aliens really look like? What biology says, and where the grey alien came from

Everything we know about life comes from one case: life on Earth. NASA's working definition is "a self-sustaining chemical system capable of Darwinian evolution," and it is deliberately broad. It says nothing about shape, color or size.

The first discovery, if it comes, will probably be modest: traces of microbes in Martian rock, in the oceans under the ice of Enceladus or Europa, or a gas in a distant planet's atmosphere. In September 2025 NASA announced a "potential biosignature" in a Mars rock that has not been confirmed. And if an intelligent civilization is found, it will most likely be through a signal from a distant star, the kind SETI has hunted for decades, in which case we might never see the senders at all.

In 2017 Oxford biologists led by Samuel Levin published a paper called "Darwin's aliens." Their argument: any complex life would probably have gone through natural selection, because it is the only process we know that produces complex design without a designer. That allows some general predictions. A complex alien would be built from parts that cooperated until they became one body, the way cells joined into organisms, and it would be well adapted to its surroundings.

Earth offers strong hints about what selection produces again and again. Biologists estimate that eyes evolved independently at least 40 times, and perhaps as many as 65. Flight evolved four times, in insects, pterosaurs, birds and bats, and the lens-and-retina "camera" eye evolved separately in octopuses and in us. Intelligence arose at least twice in very distant lineages, in mammals and in squid and octopuses. This is convergent evolution, and it suggests aliens might look less alien than we imagine: senses clustered near the brain and mouth, limbs of some kind and a way to grab food.

How close? That is an old argument. Paleontologist Stephen Jay Gould held that if we "replayed the tape of life" we would get something entirely different; evolutionary biologist Simon Conway Morris replies that good solutions keep coming back. Both agree on one thing: the details would differ.

Stronger gravity favors short, sturdy bodies, more legs, or life in water. A thick atmosphere makes flight easy; a thin one makes it hard. The star's light sets the colors: in 2007 Nancy Kiang of NASA's Goddard Institute for Space Studies found in her models that plants around dim red dwarfs might look black to us, because they would soak up every bit of light, and that blue is the least likely color for their leaves.

The chemistry is probably familiar. Carbon forms more stable compounds than any other element, so most researchers expect other life to be carbon-based too, with liquid water as the natural solvent. Silicon, science fiction's favorite substitute, forms weaker and less varied bonds. These are expectations, not laws.

Our bodies are the product of an accidental history: four limbs because our fish ancestors had four fleshy fins some 375 million years ago, five fingers, and upright walking that evolved in Africa. The odds that a different chain of events on a different planet would end in a head, two eyes, a nose and two hands are slim.

So why do movie aliens look like people with extras? Mostly because an actor in a suit is cheaper than a machine, and because we relate to faces. What might recur is function, not humanity: eyes, but not necessarily two of them; grasping organs, but not necessarily hands.

Philosopher Susan Schneider, who spoke on "alien minds" at a 2015 conference held by NASA and the Library of Congress, argues that the most advanced civilizations are probably post-biological: artificial intelligence built by creatures like us that then surpassed them. The logic is simple. We built radio about a century ago and AI is already reshaping our lives; a civilization a million years older than ours would almost certainly be something else. The SETI Institute's Seth Shostak holds a similar view.

There is a practical side to this. A probe arriving from another star would almost by definition be a machine, not a creature. The idea has been around since 1960, which is why astronomers scrutinize every object that comes from another star, such as comet 3I/ATLAS. How we might talk to such a thing is the subject of a separate guide.

The familiar figure, short and hairless with a big head, no nose or ears and huge black eyes, did not begin with a sighting. As early as 1893, H. G. Wells described "The Man of the Year Million" with an enormous head and no hair, nose or mouth. The modern version was born with Betty and Barney Hill, a New Hampshire couple who said they were abducted in September 1961. Barney described the beings' eyes and sketched them under hypnosis on February 22, 1964. In 1990 researcher Martin Kottmeyer pointed out that twelve days earlier, TV's "The Outer Limits" had aired an episode featuring an alien with similar wraparound eyes.

From there it spread: into Steven Spielberg's "Close Encounters of the Third Kind" (1977), into stories of bodies at Roswell that surfaced in the 1980s, and onto the cover of Whitley Strieber's "Communion" (1987). In 1995 producer Ray Santilli circulated an "alien autopsy" film supposedly from Roswell; in 2006 he admitted it was a staged "reconstruction." Today the grey is the most common figure in abduction accounts, especially in the US.

On July 26, 2023, former intelligence officer David Grusch testified to Congress that the US had recovered non-human craft along with "non-human biologics." He presented no public evidence, and the Pentagon's UFO office said in 2024 that it had found none. Also in 2023, Mexican journalist Jaime Maussan showed Mexico's Congress two "non-human" mummies from Peru. Of similar bodies he had promoted earlier, Peruvian prosecutors concluded in 2017 that they were "recently manufactured dolls" covered in a paste of paper and synthetic glue.

If real remains are ever presented, there is a way to check. Independent labs would analyze the biochemistry. Is there DNA, and if so, is it like ours? Do the amino acids have the same "handedness" as in all life on Earth? A creature that evolved on another planet should differ at the molecular level. Until then, every "real alien photo" online is a doll, a drawing, an effect or AI. Here is how to check a video.