"There are other types of artificial signatures of alien technology that we might look for," Professor Paul Davies told News24.
Davies is the chair of the Seti (Search for Extraterrestrial Intelligence): Post-Detection Science and Technology Taskgroup of the International Academy of Astronautics and a professor at Arizona State University.
He said that in the same way that an extraterrestrial intelligence could tell that there was life on Earth, we could identify structures in space that would give away the presence of "ET".
"For example, large scale astro-engineering projects. On Earth, human beings have impacted on their environment. From a thousand light years away, ET can tell that we're burning fossil fuels because global warming will be detectable."
Man-made global warming is a result of human impact on the planet for only the last 100 years, and Davies suggested that an ancient civilisation might have made a bigger impact on their planet and solar system.
"You might imagine that a civilisation that's been around tens of millions of years might not only have impacted its planetary environment, but its entire astronomical neighbourhood in some way," he said.
He cited the theory of the Dyson sphere first proposed by physicist Freeman Dyson in 1960.
Dyson's theory suggested that a civilisation would use planetary material or orbiting structures to surround its host star to capture all of its energy as the energy demands of the home planet outstripped supply.
Davies said that astronomers would be able to detect such a sphere if it existed, but that current attempts were limited by technology and funding.
"It's a big universe out there and you can only search into a certain limited level of precision."
Instead, Davies said that an easier way to search for extraterrestrial intelligence was to look for evidence closer to home.
"But there's another strategy entirely that we could adopt. That is, we could start searching a bit closer to home; it's easier to search in our astronomical backyard for evidence of ET.
"In this game, you have to think of very large amounts of space, but also very large amounts of time.
"The universe is 13.7 billion years old so there's been plenty of time for stars and planets to form and for life to emerge long before Earth even existed. So you have to think on an immense time scales," he said.
Davies suggested that if alien beings had visited the Earth in its history, it is likely to have been in prehistoric times, given the age of the Earth of 4.7 billion years.
"It's a favourite science fiction theme; it's a favourite of conspiracy theorists who think we are being visited now, but I come back to this point: We have to think of immense time scales.
"So what alien footprint might survive 100 million years? I came up with three things that we might actually still find after a 100 million years."
He said that if we could find evidence that would not degrade over a long period of time, we could identify clues of an alien visitation.
"One of these is nuclear waste; famously it would last for almost an eternity. If ET had used spacecraft or nuclear fuel or had been doing some nuclear technology and dumped that stuff, we could find it.
"It turns out that there is nuclear waste - two billion years old nuclear waste - in West Africa, in Gabon. It's a uranium mine and it was discovered some years ago.
"This uranium deposit went critical some two billion years ago and nobody's suggesting that this is some extraterrestrial technology... but you get the point - that we have discovered a nuclear [waste site] two billion years old," Davies said.
He also said that large-scale mining activity could suggest an ancient alien visitation.
"The second thing is any sort of large quarrying or mining activity especially on nearby bodies, but even on Earth. In principle we could find buried quarries or mines right here on Earth, but they would stand out much more - especially on the moon."
On Earth, weathering and erosion makes the identification of ancient rock formations difficult.
There is debate, for example, among experts on whether the submarine Yonaguni Monument which was discovered in the 1980s off Japan is artificial.
On the moon where there is no atmosphere, quarrying activity should be easily exposed.
"There's one place we can look really rather effortlessly - and that is on the moon. There's a satellite called the Lunar Reconnaissance Orbiter that's currently mapping the moon, which at a half metre resolution which is pretty good," said Davies.
He added that if indeed ancient aliens had visited Earth, they could have implanted a message in our genetic code that we would only be able to decipher when the technology was advanced enough to accomplish it.
Davies has had a long career in theoretical physics, cosmology and astrobiology, and has authored several books, from The Physics of Time Asymmetry in 1974 to, most recently, The Eerie Silence.
In his role for Seti, he is tasked with proposing policies when contact is made with an advanced extraterrestrial civilisation and has an asteroid 6870 Pauldavies, named after him.
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