"... there must be extraterrestrial civilizations ... because the laws of nature that led to the development of life and intelligence on Earth must be the same as those prevailing elsewhere in the universe."
— Robert Zubrin
Astronomer Frank Drake proposed what became known as the Drake Equation in 1961 to provoke discussion on how best to go about a search for extraterrestrial intelligence. In order to gauge the likelihood of finding extraterrestrial civilizations by listening for galactic radio transmissions, Drake started with the estimated number of stars in our galaxy (maybe 400 billion), multiplying by a subset of these, then subsets of subsets: the odds of a star being stable and long-lived; of such a star having planets favorable to life; of likelihood of life actually arising on such a planet; and then of multicellular followed by intelligent life. Plus the critical parameter "L": the lifetime of an intelligent civilization, starting when a species can first communicate across space and ending when it becomes extinct. In the early 1960s the possibility of all-out nuclear war seemed quite likely, so Drake and others estimated L pessimistically, assuming that once a civilization was smart enough to build radio telescopes, it would soon be smart enough to destroy itself. (More recently, skeptic Michael Shermer came up with an L of 420 years, based on historic civilizations on Earth.)
Curiously, the equation is particularly sensitive to the value of L. All the other factors look to be fairly well defined within an order or two of magnitude, assuming primitive life will sometimes lead to increasingly complex life forms. So it's worth taking a good look at L and the most obvious flaw in the equation: It assumes intelligent life will arise on a planet just once and when it's gone, it's gone for good. This can't be right; if intelligent species can happen once, surely other such species will subsequently arise again and again, because intelligence seems to be a useful evolutionary trait. For instance (the only instance we have), once we started on the path to big brains and technology a mere 2 million years ago, there was no stopping us.
Suppose a catastrophe occurred now, either natural (asteroid) or man-made (nuclear winter). If just some of us survived, it's a safe bet that civilization would recover in a few hundred years. And if humankind were entirely wiped out? My money's on a re-run of the post KT recovery. Sixty-six million years ago, the "KT event" caused the extinction of the remaining dinosaurs along with three-quarters of all animals and plants. It was probably initiated by a Manhattan-Island-size asteroid impact on a really bad day for our planet. Yet it took Earth's resilient biosphere just 5 million years to recover, and soon novel menageries of mammals and reptiles emerged from the calamity.
The point is, whereas the Drake Equation incorporates L, the average lifetime of a planet's one and only intelligent technological civilization, we shouldn't discount subsequent civilizations arising phoenix-like if the first self-destructs or is wiped out by natural causes. The lifetime of planets like ours is of the order of 10 billion years, plenty of time for numerous intelligent species and civilizations to come and go. I take comfort in the thought that if we manage to eliminate ourselves, taking all our technology along with us, something will reemerge from the ashes. I've always thought that tarsiers, after a few million more years of evolution, could do a better job than us anyway.
Given the abundance of planets in the Milky Way, the resilience of life and the vast time scales available for evolution to play out, I'm confident that ETs are out there. I'm not so confident we can ever detect them: the galaxy isn't just old, it's huge. And stars are far, far apart.
Barry Evans (firstname.lastname@example.org) wrote this on an even day of the month. On odd days, he's sure we're the only intelligence in the galaxy.