So if the Earths history is anything to go by, we should not even expect any higher life in a planet which is still youngit may take, even if life has appeared, and all goes well, many billion years for evolutionary processes to get that far. Moreover, all these "steps"the evolution of aerobes from anaerobes; of eukaryotes from prokaryotes; of multicellular organisms from unicellular ones; and of complex beings with intelligence and consciousness from the first multicellular formsmay have occurred against daunting orders of probability, as Stephen Jay Gould and Richard Dawkins, in their different ways, have brought out. Gould speaks here of life as "a glorious accident," and Dawkins of evolution as "climbing Mount Improbable." And life, once started, is subject to vicissitudes of all kinds, from meteors and volcanic eruptions to global overheating and cooling; from dead ends in evolution to mysterious mass extinctions; and finally (if things get that far) from the fateful proclivities of a species like ourselves.
Yet since there are microfossils almost four billion years old, life must have appeared within one or two hundred million years of the earths cooling off sufficiently to have liquid water. This makes one think that life may develop readily, perhaps inevitably, given the right physical and chemical conditions on a planet.
Such conditions could easily have occurred on Mars (before it dried and froze) or Venus (before is boiled). For Mars was once wet and warm, with seas and hydrothermal vents, and perhaps deposits of clay and iron ore, and it is especially in such places that we might hope to find evidence of past life. NASA has examined samples on Mars with its robot Explorers, but it has not been possible yet to make on-the-spot searches for microfossils, only to test for biogenic oxidation or metabolic products. No Martian sample has been returned to Earth. (There was, of course, great excitement a few years ago when it seemed that ALH 84001, the Martian meteorite, might contain minute fossilized structures.)
There must be thousands of Martian meteorites on Earth, and the notion of "seed-bearing meteoritic stones" was raised by Lord Kelvin in 1871, and the notion of free spores drifting through space and seeding life on other planets ("panspermia") was postulated by the Swedish chemist Arrhenius a few years later (an idea revived in the twentieth century by Francis Crick and Fred Hoyle). The idea was considered implausible for more than a century, but is once again a hot subject for discussion. For now, it is evident that the insides of sizeable meteors do not get heated to sterilizing temperatures, and that bacterial spores, or other resistant forms, could, in principle, survive within them, protected by the body of the meteor not only from heat but from radiations deadly to life. Meteors were being flung in all directions during the period of Heavy Bombardment four million years ago. Chunks of the Earth must have been ejected into space then, as well as chunks of Mars and Venusa Mars and Venus which might, at the time, have been more hospitable to life than Earth itself.
But we may not need to look too far afield for such meteors. We already know, from the samples returned by the Apollo missions, that there are early Earth and Martian meteors on the moon in considerable quantities. Now, perhaps, the time has come to plan a new mission to the moon, to allow mining and soil-concentration experiments that utilize technology unimaginable in the 1970s (such as portable polymerase machines to search the soil for ancient DNA). Here, perhaps, more easily than anywhere else, we may hope to find traces of the earliest life forms from Earth, or Venus, or Mars, and determine how life first started in our solar system.
And yet a romantic part of us cries out for more, for evidence of higher life, of human-order beings who can communicate their existence to us directly. So we need to keep SETI, our electromagnetic ears, open for more distant signs of life, as well as sampling our neighbors in this solar system. Who is to say what the next few years or decades will uncover?
For myself, since I cannot wait, I turn to science fiction on occasion; and, not least, back to my favorite Wells. Though it was written a hundred years ago, "A Lunar Morning" has the freshness of a new dawn, and it remains for me, as when I first read it, the most poetic evocation of how it may be when, finally, we encounter alien life.
(c) 2002 Oliver Sacks
Reprinted by permission of the NASA Arts Program
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