TRULY UNCORRELATED ?

A well-known argument due to Brandon Carter suggests that intelligent life in the Galaxy is much rarer than a conventional probabilistic reasoning would suggest. A crucial assumption in that application of the anthropic reasoning is that the biological timescales for the development of life and intelligence are entirely independent (”uncorrelated”) of astrophysical timescales for habitability of planetary ecospheres around Main Sequence stars. This assumption may be too naive extrapolation from our state of relative ignorance. We discuss the impact of several plausible mechanisms inducing a correlation between the two timescales, some of them of fairly recent origin, such as the impact of local (”galactic”) gamma-ray bursts. Although the results are still far from conclusive, due mainly to our poor understanding of biogenesis and noogenesis, we hope to set up a long-term research programme aimed at addressing these uncertainties in a quantitative manner. Explosive development of astrobiology (e.g. Darling 2001) coincided with the recent resurgent interest in catastrophism in terrestrial and life sciences (e.g. Clube & Napier 1990; Clube 1995; Raup 1999). Hereby, we discuss an important link between the two: correlation introduced by catastrophes of cosmic origin between the astrophysical and biological timescales. Why is this important for an astrobiologist? There are several relevant reasons, but the one we wish to concentrate here on pertains to the role of such correlations in probabilistic arguments attempting to answer the question about the frequency of (higher forms of) life and intelligence in our Galaxy. The well-known argument due to Carter (1983), and developed by various authors (e.g. Barrow and Tipler 1986), goes as follows. If astrophysical (τ∗) and biological (τl) timescales are truly uncorrelated, life in general and intelligent life in particular forms at random epoch with respect to the characteristic timescale of its astrophysical environment (notably, the main-sequence lifetime of the considered star). In the Solar system, τ∗ ≈ τl, within the factor of two. However, in general, it should be either τl >> τ∗ or τ∗ >> τl. In the latter case, however, it is difficult to understand why the very first inhabited planetary system (that is, the Solar System) exhibits τ∗ ≈ τl behaviour, since we would then expect that life (and intelligence) arose on Earth, and probably at other places in the Solar System, much earlier than they in fact did. This gives us probabilistic reason to believe that τl >> τ∗ (in which case the anthropic