Thursday, March 10, 2005

318: Cycles in fossil diversity

Don't read the excerpt if you don't want to.

Cycles in fossil diversity:

The 62-Myr cycle is strong. It might be a largely biological process or a variation in the integrity of the fossil record; however, in either case it is also worth considering geophysical processes that could be driving it. We consider seven possibilities. First, periodic passage of the solar system through molecular clouds, Galactic arms or some other structure could periodically perturb the Oort cloud and cause variations in the rate of comet impacts on the Earth24. It has been argued14 that a 140-Myr period between spiral arm crossings is consistent with existing astrophysical constraints, and that such passages can affect climate by varying the cosmic ray flux. Second, laboratory simulations of mantle plumes, under idealized conditions, show relaxation oscillator modes in which plumes reach the surface at regular intervals for six to nine cycles25. Similar behaviour in the Earth could cause periodic volcanism. Third, the Sun currently oscillates up and down across the Galactic plane every 52–74 Myr (ref. 26), but plausible responses24 would seem to occur every mid-plane crossing (namely 26–37 Myr). Moreover, the period is not constant, but decreases to half when we encounter higher-density Galactic arms. Fourth, solar cycles could affect climate, but solar theory27 predicts that long-period oscillations do not occur. Fifth, Earth orbital oscillations could affect climate. Using an orbital integration package28 and nine point-mass planets, we found no significant cycles with periods of 62 Myr or 140 Myr. Changes in obliquity were not included in our calculations. Sixth, one or more companion stars to the Sun could trigger periodic comet showers. However, a 62-Myr orbit is unstable to perturbations from passing stars. The interaction of two or more short-period companions could generate a longer periodicity (for example, through beats), but our simulations suggest that mutual perturbations would probably destroy any regularity. Last, 'Planet X' is a proposed large planet that perturbs the Kuiper belt and could yield periodic comet showers on the right time scales29. No evidence for it exists.

Although no explanation exists, the 62-Myr cycle is not a subtle signal. It is evident even in the raw data (Fig. 1a), dominant in the short-lived genera (Fig. 2) and strongly confirmed by statistical analysis. We do not know whether this cycle is a variation in true diversity or only in observed diversity, but either case requires explanation and implies that an unknown periodic process has been having a significant impact on Earth's environment throughout the Phanerozoic. Most models seem to make testable predictions, so we are hopeful that the cause of this behaviour will not remain a mystery for long.
The authors demonstrate a clear 62 million year cycle in fossil diversity. They don't understand it, but macroevolutionary theory, combined with astronomy and geology, generate a number of testable hypotheses. That's science. It's good science. It doesn't answer every question, but it moves our knowledge forward.