``There is now unambiguous evidence for the presence of clumps of living cells in air samples from as high 41 kilometers (25 miles), well above the local tropopause (16 kilometers up), above which no air from lower down would normally be transported,'' Professor Chandra Wickramasinghe, an astronomer at Cardiff University in Wales, said in a statement.
He presented the findings to a meeting of the International Society of Optical Engineering in San Diego, California.
``A prima facie case for a space incidence of bacteria on to the Earth may have been established,'' the statement said.
Wickramasinghe and scientists from India collected the space bugs from samples of stratospheric air using the Indian Space Research Organization's cryogenic sampler payload flown on balloons from a launch pad in Hyderabad, southern India.
Using a fluorescent dye the scientists detected living cells in the sample and estimated by the way their distribution varied with height that they are falling from space.
As much as a third of a ton of the biological material is raining down over the entire planet daily, by their estimation.
Space Bacteria
Professor David Lloyd, a microbiologist at Cardiff University who examined the space bugs and co-authored the report, said they look like common terrestrial bacteria but there is no explanation of how they could have risen so high.
``There would have to be some unusual event which would take particles from the Earth to a height of 40 kilometers,'' Lloyd said in a telephone interview.
The bacteria could have hitched a ride on a rocket or satellite into space or they really could be from another planet.
``We have no evidence for one or the other as yet,'' said Lloyd. ``The most likely possibility is that the bacteria have arrived from another planet. I'd like to think that, at any rate.''
Lloyd has tried, so far unsuccessfully, to grow the bacteria in culture but said he hasn't found the right conditions yet.
``It's the first pointer that it is possible to get evidence that there is life on other planets,'' he added.
Wickramasinghe is convinced the space bugs provide strong support for the panspermia theory -- which suggests that life may have come from outer space in the form of germs or spores.
``We have argued for more than two decades that terrestrial life was brought down to Earth by comets and that cometary material containing micro-organisms must still reach us in large quantities,'' he said.
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