Life from the Heavens?
The conventional wisdom is that Earthly life began… on Earth. A few decades ago, many scientists believed (as did Charles Darwin) that terrestrial life first appeared in "some warm little pond." Today's astrobiologists are less fond of ponds, and more likely to suggest that biology began in the hot, sulfurous thicket of a deep sea vent.
Credit: SETI Institute
But there is a controversial alternative to this life from hell scenario. It's life from heaven. Or if not from heaven, at least from the stars.
About 25 years ago, two British astronomers, Fred Hoyle and Chandra Wickramasinghe, proposed that comets might be the Johnny Appleseeds of life, carrying vital spores from star system to star system, an idea that is known today as panspermia. If the tail of such a life-loaded comet were to brush the Earth, it might pass some of its frozen microorganisms into the atmosphere where they could descend to our planet's surface. The two astronomers ventured that this might account for the start of life on Earth.
They also made the disturbing suggestion that panspermia could spread disease.
Now you might wonder whether life from space, as intriguing as the idea might be, solves the mystery of how biology got started in the first place. Or does this theory merely push the problem of life's origin into someone else's lap?
Well, of course, to some extent it only accomplishes the latter. But there is an appealing aspect to panspermia: it allows life to be widespread, even if the genesis of life is a difficult and rare event. After all, humans cover the planet, even though Homo sapiens got his start in only one locale (Africa, presumably.) Life might blanket the Galaxy even if it only sprung up on a small number of worlds.
|Time series of meteor's fireball trail. Is this the interplanetary carrier of life's seeds?
Credit: Aflin photo
Great. But is there any evidence for panspermia, or is it just a seductive idea with a sexy moniker?
Jayant Narlikar, of the Inner-University Centre for Astronomy and Astrophysics in Pune, India, claims to have data in support of panspermia. Narlikar recently flew an experiment in a high-altitude balloon. On board was a cryogenic sampler consisting of 16 cylinders that were pumped out and decontaminated before launch. As the balloon climbed into the Indian sky, puffs of air were sucked in. One by one, the cylinders were automatically filled with samples from various altitudes, ranging from 25 to 41 km.
Once the payload returned to Earth, it was examined in biology labs in Cardiff and Sheffield, England. To their amazement, the researchers found evidence for live cells in the samples from 41 km. Even more interesting, these "bacteria" recovered at high altitude were non-culturable. This doesn't mean that they didn't appreciate opera, but rather that they couldn't be grown in laboratory Petri dishes. According to Narlikar, this was important in ruling out laboratory contamination of the samples - the cells found were clearly not a common lab bacterium.
Narlikar emphasizes that this is the first attempt to demonstrate that biological systems exist at such heights. But he admits that he still doesn't know if the biology he's found comes from above or below. After all, perhaps all he's found are high-flying, single-celled earthlings. But the Indian researcher seems confident that they've come from above, because atmospheric scientists have told him that even during the strongest volcanic eruptions, ash is ejected to heights no more than 32 km. So the fact that he's found something at 41 km, and during a period when there were no volcanic eruptions, speaks to the real possibility of an extraterrestrial origin.
|Spores, like the one shown above, are sensitive to UV rays that damage their DNA.
Credit: S. Pankratz
Needless to say, panspermia remains a "radical" theory, and Narlikar says that colleagues greet this idea in one of two ways. Either they protest violently, evidencing a sort of geocentric mind set as he calls it, or they are cautiously encouraging, speaking of an interesting result that should be explored further. But he does admit that the reaction to panspermia is less hostile today than it was 25 years ago, because in the interim bacteria have been found living in extreme conditions in the Earth, and in the lab they've been experimentally subjected to harsh conditions and survived. So the idea of panspermia is no longer thought to be as bizarre as it once was.
What about the possibility that a disease like SARS may have come from space? "We sent a letter to Lancet, the medical journal," Narlikar says, "suggesting that these things are percolating from atmospheric heights down to Earth. Clearly, they would reach the tallest peaks first (the Himalayas - and thanks to the wind - into China.) So we suggested that this might be the origin of the SARS virus." It's a disturbing thought.
How would panspermia affect our view of the origin of life? Narlikar laughs: "Well, we could all be ETs!"
Related Web Pages
Panspermia Revisited [pdf]
Analogies of Nature: Simon Newcomb on Panspermia