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.
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