Forget waiting for ET to call -- the most likely place to find an alien message is in our DNA, according to an expert in Australia.
Professor Paul Davies, from the Australian Center for Astro-biology at Macquarie University in Sydney, believes a cosmic greeting card could have been left in every human cell.
The coded message would only be discovered once the human race had the technology to read and understand it.
Writing in New Scientist magazine, Davies said the idea should be considered seriously.
For more than 40 years astronomers have been sweeping the skies with radio telescopes hoping to catch a signal from an alien civilization.
So far the search has been in vain. But Davies believes it is wrong to assume that extraterrestrials who may be hundreds of millions of years ahead of us technologically will have chosen to communicate by radio.
Leaving artifacts for humans to find once they are sufficiently evolved -- like the obelisk in the movie 2001: A Space Odyssey -- might be a more attractive strategy, he said.
But ensuring the survival of such an artefact over possibly millions of years would be difficult.
A better solution would be to incorporate information into the human genome, allowing it to be copied and maintained over immense periods of time.
One way to do this might be to deliver alien viruses which could infect cells with message-laden DNA, said Davies.
Scientists have recently discovered large sequences of "junk" DNA that contain no genes and appears to be very stable.
"If ET has put a message into terrestrial organisms, this is surely where to look," said Davies.
A computer could be used to find obvious attention-grabbing patterns within these stretches of DNA, he said. If a sequence of junk units of DNA were displayed as an array of pixels on a screen and produced a simple image "the presumption of tampering would be inescapable".
The DNA code was easily big enough to contain a decent-sized novel or a potted history of the rise and fall of an alien civilization.
Davies added: "Trying to second-guess alien communication strategies is fraught with uncertainty, so we should try everything we can afford. The truth may be out there somewhere. Or it could be a lot closer to home."
