Gerberding said SARS is most explainable by natural evolution of coronaviruses, either from an animal or a poultry source, or possibly a coronavirus that's evolved in a human. "We have many hypotheses that are far more plausible than meteorites," she said.
About 100 tons (90 metric tons) of comet debris reach the Earth on a daily basis, and Wickramasinghe says there is evidence for life arriving with this debris.
In an experiment two years ago, he and scientists from the Bangalore-based Indian Space Research Organization sent sterile collecting devices into space and brought back large quantities of microorganisms in samples of stratospheric air captured at an altitude of 26 miles (41 kilometers). The researchers were able to culture, or grow, two types of microorganisms, which proved similar to known terrestrial species.
"Our findings lend support to the view that microbial material falling from space is, in a Darwinian sense, highly evolved, with an evolutionary history closely related to life that exists on Earth," said Wickramasinghe.
He estimates that a ton (0.9 metric ton) of bacterial material falls onto Earth every day, translating into more than 1,000 types of bacteria. Most of this material simply adds to the unculturable or uncultured microbial flora present on Earth.
The researchers argue that the random nature of epidemics adds credence to their argument that disease-causing bacteria and viruses may be evolving in outer space, parallel to those on Earth.
"New epidemics have a record of abrupt entrances from time to time," said Wickramasinghe. "The patterns of spread of these diseases are difficult to explain simply on the basic of endemic infective agents."
He says the unusual nature of major epidemics, such as the influenza epidemic of 1917-19, where infection rates were not easily explained by epidemiological modeling, bears the hallmarks of a space incident.
"Although person-to-person spread occurred in local areas, the disease appeared on the same day in widely separated parts of the world on the one hand, but, on the other, took days to weeks to spread relatively short distances," said Wickramasinghe.
The Origins of Life on Earth
The idea that SARS may have originated from space is based on the argument that life first appeared on Earth some four billion years ago when the Earth suffered a prolonged period of comet impacts.
This is a controversial theory. Most scientists believe that life on Earth began as a "primordial soup" in a lake or pond some 3.5 billion years ago: Chemicals from the atmosphere combined with some form of energy necessary to make amino acids (the building blocks of proteins) to create the first primitive organisms, kicking off the evolution of all species on Earth.
Wickramasinghe disagrees. He believes life did not start on Earth, but evolved on a much bigger scale, in a cluster of galaxies. Earth merely became an "assembly station" for all the cosmic genes.
"If Earth evolved in a closed-box situation, the argument we're making about SARS would not carry much weight," he said. "But if you accept the argument that life on Earth started from the outside, then all viruses originated from the outside."
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