Advanced ground and space-based telescopes are discovering new planets around other stars almost daily, but an environmental scientist from England believes that even if some of those planets turn out to be Earth-like, the odds are very low they'll have intelligent inhabitants. In a recent paper published in the journal Astrobiology, Professor Andrew Watson of the University of East Anglia describes an improved mathematical model for the evolution of intelligent life as the result of a small number of discrete steps.

Evolutionary step models have been used before, but Watson (a Fellow of England's Royal Society who studied under James Lovelock, inventor of the "Gaia hypothesis") sees a limiting factor: The habitability of the Earth (and presumably, other living worlds) will end as the sun brightens.

Like most stars, as it progresses along the main sequence, the sun's output increases (it is believed to be about 25 percent brighter now than when the Earth formed). Within at most 1 billion years, this will raise the average temperature of the Earth to 50 degrees C, rendering the planet uninhabitable.

Four major steps
Applying the limited lifespan to a stepwise model, Watson finds that approximately four major evolutionary steps were required before an intelligent civilization could develop on Earth. These steps included the emergence of single celled life about half a billion years after the Earth was formed, multicellular life about a billion and a half years later, specialized cells allowing complex life forms with functional organs a billion years after that, and human language a billion years later still.

Several of these steps agree with major transitions that have been observed in the archeological record.

Watson estimates the overall probability that intelligent life will evolve as the product of the probabilities of each of the necessary steps. In his model, the probability of each evolutionary step occurring in any given epoch is 10 percent or less, so the total probability that intelligent life will emerge is quite low (less than 0.01 percent over 4 billion years).

Even if intelligent life eventually emerges, the model suggests its persistence will be relatively short by comparison to the lifespan of the planet on which it developed.

The mathematical methods Watson used assume that each evolutionary step is independent of the others, though they must occur in sequence. Watson considers this "a reasonable first approximation for what is, after all, a very idealized sort of model, deliberately simplified enough that the math can be solved analytically."

Critical changes
Watson also suggests that some of the critical steps may have changed the biosphere irreversibly.

The development of photosynthetic plants, for example, led to an oxygen atmosphere, which was a necessary precursor to the development of complex land animals. Once this transition occurred, any further evolutionary step would have to take place in an oxygen atmosphere, which may have limited opportunities for non oxygen-breathing life to evolve.

Watson says in the conclusion to his paper: "... only on those rare planets on which complex creatures happen to evolve can there exist observers who ask questions about evolution and care about the answers." Asked if an advanced, space-faring civilization might be able to survive the brightening of its star by migrating off the planet where it evolved, Watson agrees that's possible: "the model predicts only when 'intelligence' can arise based on the time available. Once the observers exist, they might do all manner of things to find new places to live."

Seth Shostak, Senior Astronomer at the SETI Institute, had this comment on Watson's work: "We have, of course, only one example of intelligent life (indeed, of life of any type). That means we cannot possibly estimate from this single instance what is the probability of life on other worlds unless we are completely confident we understand all the relevant evolutionary processes.

Watson argues that intelligent life will be dismayingly rare: there is no way to prove that is true. On the other hand, if the converse is the case - if the galaxy is home to many intelligences - that is amenable to proof. We should do the experiment."