Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [113]
If alien life were to prove Earth-like, scientists would be faced with two possibilities: Perhaps the same biology emerged independently on different worlds. Or perhaps it went from one world to another.
Anaxagoras, a Greek philosopher who lived in the fifth century B.C., declared that all life on Earth originated from seeds that pervaded the cosmos. He called the process panspermia. In the twentieth century, Francis Crick and several other prominent scientists revived panspermia in various forms. They suggested that spores had fallen to Earth billions of years ago and given rise to all life. The panspermians met with skepticism because they had no clear evidence that life existed on other planets or that it could survive an interplanetary journey. Panspermia was unsatisfying as a theory, because it did not explain the origin of life. It just pushed the question back.
Panspermia still meets with skepticism, but scientists now regularly talk about it at conferences without being laughed off the dais. Early in the history of the solar system, large meteorites were crashing into planets quite frequently, launching material out into space. In some cases, that material could have reached other planets. The path from Mars to Earth is particularly easy because the planets are so close to each other and because Mars has a much weaker gravitational field. Even today an estimated fifteen meteorites from Mars land on Earth each year. Planets may trade bits of themselves over far greater distances. A few Earth rocks could travel all the way to the moons of Saturn and Jupiter. In fact, according to one estimate, a rock from Earth might strike Jupiter’s moon Europa once every 50,000 years. To us 50,000 years may be an unimaginably long time, but in the history of the solar system it’s like the patters of a hailstorm.
If these studies are correct, it’s possible that some E. coli rode a meteorite into space thousands of years ago. For most microbes this sort of journey would be fatal. Many would be destroyed by the harsh interplanetary radiation from which our atmosphere shields us. Still others would die in their blazing descent to another world. But a few microbes might survive. And as Lederberg and his colleagues recognized, it would take only a few microbes to populate a fertile planet. Some scientists have even suggested that these journeys might have kept life from disappearing from the solar system altogether. A big enough impact could boil off the oceans of Earth, leaving it sterilized. It would take millions of years for the water vapor to rain back down and allow a stable habitat to form. Life could hold out during that time on Mars or in some other refuge.
The most extreme form of panspermia was proposed in 2004 by William Napier, an Irish astronomer. He argued that some rocks lofted from our solar system might fly out of the solar system altogether. Once safely distant from the sun, the microbes they carried would no longer be harassed by ultraviolet radiation. Some of the rocks might wind up on planets orbiting other stars, and a few of the microbes might find a new place to grow. Of course, those planets would be hit by heavenly bodies as well, and their organisms would be passed on to other solar systems. Napier estimates that this interstellar infection could contaminate the entire galaxy in a few billion years.
Which brings me back to the dish of E. coli I hold up to the sky. On some nights at some places on Earth you can spot the International Space Station through a telescope. E. coli is up there. It floats inside the bodies of the astronauts, swims in their drinking water, and drifts inside droplets that cling to the space station walls. Has E. coli gotten any