2.3. Coupling of Biological and Astronomical Timescales

We find more specific clues to factors influencing our time coordinate by a closer examination of local natural history, both in the fossil record (e.g. Knoll 1999) and the genomic one (e.g. Woese et al. 1990, Doolittle 1999). The oldest sedimentary rocks (from 3.9 Gya, where Gya = 10⁹ years ago) on the surface of the Earth are almost as old as the Earth itself (4.55 Gya), yet appear to harbor fossilized cells. Unambiguous fossils of cyanobacteria, closely resembling modern species, are found from 3.5 Gya. The earliest life seems to have emerged soon (within of the order of 0.1 Gy) after the last globally-sterilizing meteoroid impact. The first eukaryotic fossils (Grypania) show up much later, about 2Gya (about the time when the atmospheric oxygen level rose substantially); widespread eukarya (acritarchs, a form of planktonic algae) do not appear until much more recently (1.5 Gya). Significant morphological diversity only began about 1 Gya, possibly paced by the emergence of sex. The Cambrian explosion, which took place over a remarkably narrow interval of time between about 0.50 and 0.55 Gya, created essentially all of the variety and complexity in body plans of modern animals. Since then there have been several mass extinctions triggered by catastrophic impacts (including possibly the huge Permo-Triassic event 0.25 Gya, and almost certainly the smaller dinosaur killer Cretaceous-Tertiary (KT) event 0.065 Gya), indicating that extraterrestrial factors are even recently at work in shaping biological history.

What is the clock that determines the roughly 4 Gy timescale from the formation of the Earth to the Cambrian explosion? If it is a purely biological clock, there is a striking coincidence between this timescale and the main-sequence lifetime of the Sun, about 10Gy. Why should Darwinian bioevolution occur on a similar timescale to stellar evolution? Why should it be that we show up when the Sun is just halfway through its lifetime? Carter (1983) considered these coincidences and proposed an anthropic explanation: if the biological clock has a very long intrinsic timescale, most systems fail to evolve significantly before their suns die; those that by chance evolve quickly enough will tend to do so ``at the last minute''. If there are a small number of rare rate-limiting steps, the coincidence can be explained.

Indeed the emerging picture of continual cosmic catastrophes affecting the biosphere and the mounting evidence for the intimate coupling of life and the global environment has started to flesh out the details of what paced evolution, and how it has been controlled or limited by astrophysical events and thereby by astrophysical timescales. In addition to asteroid and comet impacts, intimate couplings are now recognized between geophysical and biological evolution, although their relative importance is not settled.

One example is the global carbon cycle, which includes biological components (important in the precipitation of carbonates) as well as plate tectonics, vulcanism, and climate-controlled erosion; the sum of these elements may allow the planet to maintain a surface temperature which tracks the habitable zone, in spite of variations in insolation since the Sun formed of up to twenty percent (Schwartzmann and Volk 1989). The most spectacular failures of this stabilization mechanism may have led to ``Snowball Earth" events (Evans et al. 1997, Hoffman et al. 1998) where the entire surface of the planet iced over, and the subsequent superheated recovery from these events by volcanic replenishment of greenhouse gases. The most recent of these events may have triggered the Cambrian explosion. Another example is the accumulation of oxygen, a biological process partly paced by geochemistry (the global oxidation of iron) which also took place over a billion years, which certainly enabled and may have paced the explosion of complex life forms.

Direct evidence thus suggests that interdependent ``co-evolution'' accounts for the coincidence of biological and astrophysical timescales, even though the dominant couplings may not yet be known. The actual situation is subtly different from Carter's original guess; the intrinsic timescale of biological evolution, if one exists, appears to be relatively rapid, and the pace of evolution has been set by occasional rare opportunities (such as the isolation of Darwin's finches on various Galapagos islands, but on a global scale). Carter's main conclusion, that advanced life is relatively rare, is substantiated by the accumulation of evidence over the last twenty years: many fortuitous circumstances seem to have played a role in the emergence of animal life on Earth (Ward and Brownlee 1999). ⁽²⁾

² As yet another example, Gonzalez (1999) has recently pointed out that even the orbit of the solar system in the Galaxy appears to be finely tuned to reduce comet impacts: compared to other stars of the same age, the sun steers an unusually quiet path through the Galaxy - an orbit with unusally low eccentricity and small amplitude of vertical motion out of the disk. This could be explained anthropically, perhaps through the effect of Galactic tidal distortions on the Oort comet cloud which create catastrophic storms of comet impacts in the inner solar system (Heisler and Tremaine 1986, Matese and Whitmire 1996).