Jaime Maussan, who has led investigations into alien phenomena for decades, stood with scientists to unveil two corpses in what he called a 'watershed' event in front of Mexican Congress on Tuesday.
The researchers made the extraordinary claim that the corpses, presented in windowed boxes and supposedly recovered from Cusco in Peru, were not part of 'our terrestrial evolution', with 30 per cent of their genetic composition still 'unknown', according to Mexican media.
Carbon dating by the National Autonomous University of Mexico (UNAM) found the bodies, pictured with three-fingered hands, no teeth and stereoscopic vision, were more than 1,000 years old, Maussan claimed.
The bizarre presentation has triggered a frenzy of excitement among conspiracy theorists online - but it has also drawn scepticism. Unusually for fossils that have been subjected to analysis, the specimens were coated in what appeared to be sand.
But Maussan - who has been associated with debunked alien theories in the past - insisted under oath on Tuesday: 'These specimen are not part of our terrestrial evolution [...] These aren't beings that were found after a UFO wreckage. They were found in diatom [algae] mines, and were later fossilized.'
He later added: 'Whether they are aliens or not, we don't know, but they were intelligent and they lived with us. They should rewrite history.'
'We are not alone in this vast universe, we should embrace this reality,' he said at the event.