Could your favorite Earthling also be your favorite Martian?
Allow me to respond: The answer is "yes." It's possible that billions of years ago, tiny bits of biology quit the Red Planet and infected ours. If so, your family tree -- and that of every other terrestrial life form -- has its deepest roots not in the ancient oceans of Earth, but in the vanished seas of Mars.
The mechanism by which biology can spread through space without the benefit of expensive space-agency hardware is known as panspermia. Life hitches a ride on sunlight or inside rocks -- not rockets.
This is more than a curiosity. It has important implications for the search for life in the solar system -- a search that's heating up.
Panspermia is hardly a new idea: the philosopher Anaxagoras was the first to publish on the subject more than two millennia ago. But its current vogue can be traced to thought experiments by the Swedish chemist Svante Arrhenius at the beginning of the twentieth century. He figured microorganisms, which can be tougher than old boots, might be pushed from one world to the next by the radiation pressure of stars.
That idea might work if the emigres are tiny and don't insist on going far. But a much better bet is to be a protoplasmic pilgrim inside a dirt clod kicked into space by a meteor impact. Sometimes called "lithopanspermia" for reasons that are obvious if you studied Greek, this mode of transport has the benefit of a protective environment. That's a necessity if your travel time is really long - hundreds of thousands or millions of years. After all, space is hardly benign: cosmic rays, extreme temperatures, and prolonged desiccation will relentlessly corrode any biology that takes too much time en route. Being inside a rock helps.
But is this Johnny Appleseed mechanism for spreading life between worlds likely to be for real?
To answer that, you first need to ask whether enough rocks are actually kicked off a planet to ensure that at least a few will accidentally land on another world. And second, would any of their microscopic passengers survive the trip?
Consider the panspermia prospects between Mars and Earth. Scientists estimate that in the early days of the solar system, billions of rocks between an inch and a yard in size were involuntarily shuttled from the Red Planet to ours. Their travel time might have been as short as a year, but most would have taken much longer, more like a million years. A rock can wander around the inner solar system for quite a while before actually hitting anything.
So could any biology within these space-borne lumps survive such an extended trip? After all, the conditions on board are worse than RyanAir. The rocks and their passengers would have been bombarded by radiation, cut off from water, and subjected to temperature extremes as bad as the moon. But it turns out that some hardy earthly microbes could survive these steerage-class conditions. Astrobiologists have identified terrestrial bacteria able to zone out in spore form for a million years. If you eventually put them in contact with water, they'll come back to life like sea monkeys.
It seems that panspermia was possible between Mars and Earth roughly four billion years ago, assuming there was any life on the Red Planet to make the trip. And perhaps there was. In its youth, Mars was wetter and warmer than now, and could have spawned living things at a time when Earth was as lifeless as an octogenarian slumber party. Because so many martian rocks were kicked into space, it's highly probable that at least some would have come from an inhabited patch of Mars -- assuming it had inhabitants. And some of those would have landed in a suitably welcoming patch of Earth.
In this way, our planet may have garnered its biota -- not as the result of any processes here on Earth, but thanks to a rain of rocks from Mars. If sometime in the next few decades we discover the remnants of ancient life on the Red Planet that are based on DNA, then we'll have good reason to believe that terrestrial biology is an import. We could say that not only men are from Mars; we all are.
The possibility that Earth's carpet of life might not be indigenous may sway our priorities in the search for life in the solar system. Should we continue to place our heavy bets on Mars, or would it be better to explore the moons of Jupiter and Saturn? Biology would be far more isolated on these latter worlds, and unlikely to be related to us. They would be true aliens -- perhaps the most interesting sort of life to find.
As intriguing as it is, panspermia doesn't offer any clues about life's origins. Indeed, it only seems to push the problem of biology's beginnings to another planet. But there's this: If life can spread, then countless worlds could be encrusted with biology even if generating it in the first place is difficult or highly improbable.
Turning hydrocarbons into protoplasm might be a semi-miracle, but life itself could be as common as fast food.