Evidence is now overwhelming that life on earth was brought here by comets, according to one of the originators of the theory.
Professor Chandra Wickramasinghe, Director of the Cardiff Centre for Astrobiology, first propounded the theory of panspermia with Sir Fred Hoyle back in the 1970s and 80s.
In a new paper, he argues that his case is backed up by recent discoveries.
"As we enter a new decade – the year 2010 – a clear pronouncement of our likely alien ancestry and of the existence of extraterrestrial life on a cosmic scale would seem to be overdue," he says.
Astronomy continues to reveal the presence of organic molecules and organic dust on a huge cosmic scale, says Wickramasinghe, with around a third of interstellar carbon tied up in this form.
Just as the overwhelming bulk of organics on Earth are derived from living matter, he argues, it seems likely that interstellar organics had a similar origin.
Professor Wickramasinghe concludes that the evidence of the past 30 years strengthens the case that life first came to Earth when comets carrying organic matter impacted the planet around 3.8 billion years ago.
However, he argues that 'cultural barriers' still exist to admitting the connection. After all, plenty of people have trouble with the idea that we evolved from apes, so it's hardly suprising that little alien bacteria are a step too far for some.
His paper appears in the International Journal of Astrobiology.