Origins

Science writer and author of Microcosm: E. coli and the New Science of Life Carl Zimmer wrote the "On the Origin of Life on Earth"  last month. Today he discusses evolution on other worlds.

Imagine you spent your whole life on a tiny island, with only some tortoises and snails to give you a clue to what life was like. You'd be forgiven for failing to imagine a Venus flytrap or an armadillo. Evolutionary biologists are in much the same bind. They are, for the time being, stuck on a planetary island, only able to study life on Earth. While life on Earth takes many forms, every living thing is nevertheless a variation on the common theme of DNA, RNA, and protein. What kind of life, if any—exists on other planetary islands we don't know?

If we do discover life someday on another planet, evolutionary biology would leap to a new level. Biologists would be able to compare how evolution played out on two separate planets. If life began independently on another world and ended up a lot like life on Earth, that might mean that evolution must follow certain rules no matter where on the universe it plays out. Or perhaps evolution has the potential to be a lot weirder than we know, because we're stuck here on our little island of life. The closest place where it makes sense to look for life is Mars. Its surface may have been warm and wet in the past, and puffs of methane discovered in recent years just might be a sign that microbes are still thriving deep under the surface. The best way to see if that's the case is to drill into the Martian soil and find them.

But Chris McKay of NASA warns in this week's Science that in our search for a second biosphere, we may contaminate it with our own. As McKay points out, space scientists were already concerned about contaminating other planets in the 1960s. NASA completely sterilized the Viking Probe that landed on Mars in 1976, but the results of that mission suggested that the Red Planet was so harsh that no life could survive and so fewer protections were necessary. The Mars rovers that we've all watched wandering across the Martian landscape probably brought hundreds of thousands of bacteria with them.

Yet, over the years, scientists have grown more concerned again. The surface of Mars is clearly an awful place for even the hardiest microbe. But if we start drilling down into the ground, we might well be injecting microbes from Earth down into a Martian ecosystem. We unfortunately know all too well what happens when we accidentally introduce species to new places. At worst, the new species becomes invasive and drives native animals and plants extinct. At best, native ecosytems are dramatically altered. Do we have an ethical obligation to protect what McKay called "indigenous biospheres"?

Later this year, a meeting will be held to consider just this question. We do need to take responsibility for our actions, but we also should not forget another lesson of evolution here on Earth: Invasive species don't always need people to deliver them to a new home. Darwin himself first recognized that seeds and eggs can been carried to distant islands on the feet of birds. In space, meteorites may act as interplanetary birds, bringing microbes from Earth to Mars—or perhaps the other way as well. Even if we take every possible precaution, the life we find on Mars may turn out to be invasive after all. It just invaded Mars a billion years ago.

—Carl Zimmer