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 probably 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."
advertisement
