By Andrew Thompson
In July 2001, a mysterious red rain started falling over a large area of southern India.
Dr Wickramasinghe thinks life could have originated in space
Locals believed that it foretold the end of the world, though the official explanation was that it was desert dust that had blown over from Arabia.
But one scientist in the area, Dr Godfrey Louis, was convinced there was something much more unusual going on.
Not only did Dr Louis discover that there were tiny biological cells present, but because they did not appear to contain DNA, the essential component of all life on Earth, he reasoned they must be alien lifeforms.
"This staggering claim is that this is possibly extraterrestrial. That is a big claim I know, but all the experiments are supporting this claim," said Dr Louis.
His remarkable work has set in motion a chain of events with scientists around the world debating the origin of these mysterious cells.
The main reason why Dr Louis's ideas have not been immediately laughed out of court is because they tie in with a theory promoted by two UK scientists ever since the 1960s.
The late Sir Fred Hoyle and Professor Chandra Wickramasinghe have been the champions of "Panspermia", the idea that life on Earth originated on another planet.
They speculate that life was first brought here on the back of a comet. Over the last decade, Panspermia is being taken ever more seriously.
The US space agency (Nasa) is now increasingly interested in searching for extra-terrestrial life.
A new robotic submarine is being developed to explore the oceans of one of Jupiter's moons. This submarine is on test at the moment in a lake in Texas.
Finding life elsewhere in the Solar System would be a vital bolster to the Panspermia theory.
Another section of Nasa is devoted to the study of bacteria found on Earth that can survive extreme conditions.
Finding these types of bacteria makes it more likely that micro-organism could survive the hardships of travelling through space on the back of a meteoroid.
Professor Wickramasinghe explained: "Bacteria have got to endure the extreme cold of space, the vacuum of space, ultraviolet radiation, cosmic rays, X-rays.
"That sounds like a tall order but bacteria do that. From what we know survival out in space is more or less ensured. Bacteria seem to me to be born space travellers."
From another place
Last summer, Horizon had exclusive access to a trip taken by Professor Wickramasinghe to India to investigate at first hand the red rain phenomenon.
He met Dr Louis and together they visited the people who had witnessed the red rain.
He was able to see the recent work of Dr Louis which shows that the red rain can replicate at 300C, an essential attribute of a space micro-organism that might have to endure extreme temperatures.
All this has convinced Professor Wickramasinghe that the red rain is a form of alien life.
Bacteria might survive the journey through space
"Before I came I had grave doubts as to whether the red rain was really an indication of life coming from space; new life coming from space," he said.
"But on reflection and after talking to Godfrey, I think I would now fairly firmly believe that it did represent an invasion of microbes from space."
Many scientists remain highly sceptical, however, but if Wickramasinghe and Louis are correct it will be the strongest evidence so far that the theory of Panspermia might be true.
It also raises the intriguing possibility that if life first originated on another planet then it must mean all Earth organisms, including humans, evolved from alien life.
Horizon - We Are The Aliens is broadcast on BBC Two on Tuesday 14 November at 2100 GMT