How Many Origins of Life? f_l

In scientific circles there's much less concern now than in the past about the value of f_l, the fraction of habitable planets on which life evolves. The molecular building blocks of life — complex organic compounds and even amino acids — are abundant in the universe. They have been discovered in meteorites, comets, and interstellar gas and dust. There are vastly more amounts of amino acids, for instance, in interstellar space than in the Earth's biosphere. Although hydrocarbons and amino acids are not living organisms, there's little doubt that a lot of prebiotic evolution is going on in the dark clouds between the stars.

Most significant are the recent discoveries that microorganisms appeared on Earth only moments (geologically speaking) after the last devastating, ocean-vaporizing impacts of the planet's youth some 3.9 billion years ago. There is good evidence that mats of photosynthetic organisms were already around by 3.4 billion years ago, and there's more disputed evidence of bacteria from 3.7 and 3.85 billion years ago. Apparently, given the right conditions, the origin of life is a rather straightforward process that happens easily — at least when given a planet-sized laboratory and millions of years for the experiment to run.

If the process were rare or difficult, one would not expect it to have happened at the first possible opportunity on our home planet, but (given our existence at all) somewhat later in Earth's history instead. Biologists now discuss whether life may have arisen several times separately on the early Earth. There's every reason to think that all living things today have a common ancestry, but other, independent lines could have formed and been wiped out (or eaten) early. If life does form wherever it can, then f_l = 1.

Intelligence, f_i

That leaves us with three remaining unknowns. How likely is the evolution of intelligence (f_i)? How confident can we be that at least some intelligent extraterrestrials will broadcast radio or other signals we can detect (f_c)? And what is the average lifetime of radio-capable civilizations (L)? These biological and sociological factors in the Drake equation are subject to greater scientific debate and uncertainty than the astronomical ones.

According to many life scientists, it is naive to suppose that evolution on another planet should necessarily result in intelligence as we know it. In his bestseller Wonderful Life, the late paleontologist Stephen Jay Gould (Harvard University) asserts, "We probably owe our own existence to . . . good fortune. Homo sapiens is an entity, not a tendency." Evolution is unpredictable, undirected, and chaotic. Gould pointed out again and again that if we could rewind the tape of biological evolution on Earth and start over, it is impossible that humans themselves would again appear on the scene. We are the result of too long a chain of chance flukes and happenstance.

Others counter, of course, that humans are not what we are looking for. No one expects to find men among the stars (little green ones or otherwise). Rather, the issue is whether any species evolve enough symbol-based intelligence to use tools, store and manipulate information, and develop societies that grow large and complex enough to discover the principles of science and electronics. To optimists this seems like a difference only in degree, not in kind, from the levels of intelligence, tool use, and purposeful behavior that have evolved independently in widely divergent species of animals on Earth, from apes to octopi.

But Gould notes that there is no overall pattern in evolution, no preferred direction. If some recently evolved animals are bigger and smarter than earlier ones, that could just be a fluke. Human levels of planning and technology may be even more so.

To some biologists and SETI proponents, the phrase "survival of the fittest" implies that greater intelligence inevitably boosts a species' chance to survive and spread by natural selection. But the late biologist Ernst Mayr argued that many astronomers and physicists are too optimistic concerning the emergence of intelligence. "Physicists still tend to think more deterministically than biologists," wrote Mayr in the May 1996 issue of The Planetary Report. "They tend to say that if life has originated somewhere, it will also develop intelligence in due time. The biologist, on the other hand, is impressed by the improbability of such a development."

This divergence stems in part from different specialists' intellectual backgrounds. To a biologist, something that happened once in 4 billion years is terribly rare. Astronomers take a wider view: something that happened once in less than a single planet's lifetime seems reasonable for planets generally.

Optimists have pointed out that by some estimates, Earth has 1.1 billion good years ahead before it will get broiled to death by the expanding Sun. This is several times longer than the time since the first simple creatures crawled out of the sea onto land. If the emergence of intelligence were difficult and rare, the optimists argue, it would not have happened relatively early in the time available for it to do so on Earth. Given humanity's early arrival in the long era expected for land life, it seems likely that entirely different intelligent creatures will emerge a few more times in the coming geological ages (and they will find our fossils!). This argument parallels the point drawn from the rapid emergence of microorganisms on the young Earth.

Pessimists reply that we don't really know how long the Earth will remain clement. For all we know, the Earth's habitable climate may be the result of a long run of lucky flukes that could give out at any time, geologically speaking. If so, humans have arisen late in the total span of time available. Given the fact that we are here at all to ponder the question, a late emergence in the timespan available would indicate that the birth of intelligence is an improbable event.

Contrary to popular belief, the fact that intelligence has arisen once tells us nothing whatsoever about how often it happens — for the simple reason that we ourselves are the one case! We are a self-selected sample of one. Even if intelligent life is so improbable that it appears just a single time in one remote corner of the universe, we will necessarily find ourselves right there in that corner observing it, because we are it.

Strangely enough, both camps accept the so-called Copernican principle, which claims that humankind enjoys no preferred position in time or space. Skeptics like Mayr say it is anthropocentric to believe that humanlike intelligence has appeared over and over again in the universe. Believers like Drake are unwilling to accept our uniqueness, because this would put us on a very un-Copernican pedestal.

Christopher Chyba, chair of the SETI Institute's Center for the Study of Life in the Universe, sums it up: "It's an argument that turns on the comparative importance of contingency versus convergence in evolution." In other words, how many evolutionary tendencies are random flukes, and how many of them drive repeatedly in a particular direction? "Are there data sets that we can analyze to actually help focus and quantify this argument?" Chyba continues. "The answer, it appears, is a resounding 'yes.' We don't have to guess about these questions, but can begin to quantitatively assess some of them using well-understood, quantifiable tools." The SETI Institute is funding researchers to tackle this problem.

For now, however, f_i is one of the most controversial factors in the Drake equation. Some scientists believe it is almost certainly next to zero; others are convinced it's close to one. There seems to be no middle ground.

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