|If smart aliens exist, they may have visited Earth millions of years ago – and left signs of their technological prowess that should be cheap and easy for us to detect.
So says astrophysicist Paul Davies of Arizona State University in Tempe Traditionally, the search for extraterrestrial intelligence (SETI) has focused on listening for deliberate dispatches in radio signals But in 50 years of searching, this costly approach has drawn a blank.
So Davies suggests scouring our own planet, even our cells, on the off-chance that aliens visited and did what amounts to scrawling "We were here" on the walls "We can answer the question 'Are we alone?' without picking up messages," he says "We may see indirect evidence of alien technology based on the footprint it leaves."
If aliens did come to our cosmic neighbourhood, it was probably hundreds of millions of years ago Any "footprints" would have to last for eons to be detected by us That means a manufactured slab of rock in the style of 2001: A Space Odyssey (pictured) would be a poor candidate: it would erode, or at least get buried before we came to notice it.
But detritus from industry, such as mining, might last Former mines on the Earth, the moon or asteroids could show up in geological surveys, even if they have filled in or eroded since the aliens left, says Davies.
Nuclear waste is another possibility Davies suggests we search for plutonium-244 The element occurs naturally on Earth in trace amounts only and has a half-life of 80 million years So substantial deposits would be a sign of alien nuclear technology.
This echoes a suggestion by Frank Drake of the SETI Institute in California that aliens might have used radioactive materials to mark the location of a message.
Aliens may even have left deliberate messages in our DNA, Davies says Most DNA includes long sequences that seem to serve no biological function Aliens may have embedded code in this "junk DNA" – a sequence of prime numbers as in the movie Contact, say – to signal their existence to us.
Microbiologist Steve Benner of the Foundation for Applied Molecular Evolution in Gainesville, Florida, reckons such a message would get deleted pretty quickly "Paul is being a little too extravagant here," he says "He overestimated the robustness of non-functional DNA."
Gary Ruvkun at Harvard Medical School, who heads the Search for Extraterrestrial Genomes project, agrees: "You'd need a funny code," he says, one that would still be decipherable even after it mutated.
In 1978, Japanese researchers searched the DNA of the bacteriophage Phi X174 for such signs and failed to find anything.
Davies's paper has garnered a spectrum of further responses Drake thinks "being alert for artefacts of extraterrestrial origin, in space but particularly on Earth, is a good thing to do".
Seth Shostak, also at the SETI Institute, agrees, but emphasises that hunts on Earth can't be a substitute for searching beyond our planet "After all," he says, "the Earth tends to obliterate evidence on its surface."
David Deutsch, a theoretical physicist at the University of Oxford, says searching on Earth is unlikely to be fruitful but is certainly worth doing: "This is something where we really should leave no stone unturned." He doubts that aliens used nuclear power to reach Earth – something more powerful like antimatter is more likely, he says – "but maybe they used plutonium for their mobile phones".
Biologist Norman Pace at the University of Colorado, Boulder, on the other hand, says the article is "mainly bullshit" "I do have faith that there is a lot of biology out in the universe, but I also believe in travel limitations implicit in the speed of light and the vastness of the universe."
Davies admits that his idea is "wild and fanciful", but adds that it has the benefit of being cheap "It costs nothing to crawl through a database of genomes This stuff is free on the internet," he says "The chances are exceedingly small that anything will come out of it Nevertheless, the pay-off is exceedingly big."
He wants more people to have a shot at doing SETI "Instead of leaving it to a heroic band of radio astronomers, I want to say 'look, everyone can lend a hand'," he says "We can make this whole field much larger, and involve everybody."