NOW YOU see it, now you don't. The photo on the left, taken in 1976 by the Viking spacecraft, shows an area of Mars known as Cydonia. See the face, apparently carved out of the rocks?

Now take a look at the other picture, taken a few days ago by the Mars Global Surveyor (MGS) spacecraft. The US space agency actually planned the flyby specially, because ever since the Cydonia pictures were published in the Seventies, there have been people who have seized on it as evidence of intelligent life out there - a sign to tell us something. But what?

Yet the MGS picture shows nothing: no face. It doesn't even look like an eroded statue (or whatever) of a face, which you would expect if the area being pictured was an artefact rather than an entirely natural formation - which of course it is.

So, why do we see a face in the first picture? It's a conjunction of a trick of the light and some fantastic neural machinery that evolution has bestowed on us. We see a face in the Viking Cydonia image because there are areas (more correctly, volumes) of nerves in the right hemisphere of our brains which are specialised to pick up the distinct shape of human faces: oval shape lit from above, eyes at top of nose, mouth below nose, chin.

Why have those developed? Because it is a survival characteristic for babies to be able to recognise human faces over other species, since they rely absolutely on humans for their survival. Other animal species have similar abilities to recognise shapes of their own species and predators.

That this function begins with babies is demonstrated because we can recognise faces in any orientation, in many sorts of light. We can infer the presence of a face from just a few visual clues - again, a useful characteristic for babies, whose eyes can only focus a short distance at first. If a baby can recognise a blurry oval with some eye-like shapes and mouth in the right place as a potential parent, it will be able to attract attention - and perhaps get fed - more easily.

By contrast, it's helpful but not essential for adults to be able to pick out human faces from vague shapes. The Cydonia photographs show what can happen if you try to draw conclusions without remembering how fallible we humans can be as experimental observers. But it vindicates Nasa's decision to send the MGS for more data - a splendid example of science debunking myth.