Aliens could have roamed the Earth at the same time as the dinosaurs.
New research poses the theory that humans could detect extraterrestrial life by looking for compounds that were on Earth during prehistoric times.
This is because 30 per cent of the planet's atmosphere consisted of oxygen approximately 300 million years ago, which has since balanced out to 21 per cent.
The scientists hypothesise that oxygen levels are a vital clue for spotting alien life on different planets and noted that the makeup of Earth in the present day is what has been used when comparing other planets for signs of aliens.
Astronomer Lisa Kaltenegger, of Cornell University in New York, said: "Modern Earth's light fingerprint has been our template for identifying potentially habitable planets, but there was a time when this fingerprint was even more pronounced - better at showing signs of life.
"This gives up hope that it might be just a little bit easier to find signs of life - even large, complex life - elsewhere in the cosmos."