Because it’s psychological, not biological.

In the last few days, the Internet has found both a ghost woman and a crab on the surface of Mars. Or perhaps it’s a ghost woman made out of crabs, or a ghost of a crab that was once a woman, it’s not clear. What we do know for sure is that, despite what oddities can be found in the photos, if you give a human brain a complex image with a lot of random noise, it will reliably find a signal.

First up, the crab. The photo below was beamed across light-minutes of space-time by NASA’s Curiosity Rover back in July:

Let’s play spot the alien space crab ghost face-hugger beast.

Very quickly, media outlets and UFO enthusiasts noticed something odd. Tucked under an outcropping of Martian stone (to the right of the photo’s center), there was what looked like a crab, or at least an object out of place. Zoom in further, and the feature certainly does look like a face-hugger.

Then the “ghost woman”. Another recent photo from Curiosity apparently shows a ghost woman drifting languidly over dead, red dust:

But assuming for a second that Mars isn’t actually populated entirely by spirits, robots, and crustaceans, could there be an alternative explanation?

Why do we see animals in the clouds or Jesus’ visage on toast or a face on Mars? Psychologists have dubbed the experience pareidolia—the tendency to find distinct patterns in random or vague stimuli. Most of the time, pareidolia is a fun quirk of our brain’s visual processing systems. In fact, there are whole Twitter accounts devoted to finding faces in everything, and it’s delightful.

Finding faces in everything could even be evolutionary advantageous. The more quickly and accurately you can identify the face of another person, or so the evolutionary just-so story goes, the easier it will be to distinguish friend from foe. The human brain even has an area that lights up specifically in response to faces, the fusiform gyrus.

But when the same psychological quirk is at work on the surface of Mars, bringing cries of controversy and cover-ups, everyone seems to forget that it is the same process that makes these flower petals look like a skull or this washing machine look like it’s throwing up your clothes.

None of this is to say that people aren’t actually seeing a ghost crab woman alien beast on Mars—pareidolia or not, you certainly do see what you see. However, if you see a Martian crustacean that could just as equally be a geologically interesting outcropping of rock, perhaps it’s better to defer to psychology rather than conspiracy.