## Solution Earth Has an Optimal Pump of Evolution

When resonance occurs, a small input force can produce large deflections in a system.

### Report on the collapse of the Tacoma Narrows Bridge

Jupiter plays a key role in another proposed resolution of the Fermi paradox — one that elaborates on an idea mentioned in the previous section. The suggestion is due to the physicist John Cramer.197

Large meteors sometimes hit Earth; but where do they come from? One idea is that they fall toward Earth from the Asteroid Belt — but for this idea to work, large numbers of asteroids must be perturbed from their stable orbits and then fall toward the inner part of the Solar System. Why should asteroids be pushed away from their stable orbits? No mechanism was known that could do this; then, in 1985, George Wetherill highlighted the importance of the gap in the Asteroid Belt at a distance of 2.5 AU.198

The rings of Saturn and the Kirkwood gaps in the Asteroid Belt were already well known. The gaps occur because of resonance effects. in the case of the gap at 2.5 AU, the resonance occurs because any asteroid at that distance orbits in precisely 3 of the time Jupiter takes to orbit the Sun. Therefore, every third occasion a 2.5-AU asteroid reaches a particular position, Jupiter is in the same relative position. The gravitational nudge that Jupiter gives the asteroid is always in the same direction, and the effect is cumulative. It is like pushing a swing at precisely the right frequency: the effects build up, and the amplitude of the swing increases. over time, therefore, the orbit of an asteroid at 2.5 AU becomes unstable, and it moves away — and the Asteroid Belt is eventually cleared of objects in this region. (Any asteroid wandering into this region from elsewhere is eventually ejected by the same mechanism.) The Kirkwood gap at 2.5 AU is due to a 3:1 resonance; other gaps, based on other resonances with Jupiter, also exist.

Where do the asteroids go when they are ejected from the Kirkwood gap at 2.5 AU? Calculations show there is a high probability of their orbits crossing the orbit of Earth. In other words, there is a chance that these asteroids hit Earth — with catastrophic consequences.

However, although the effects of an asteroid impact can be disastrous for any creatures that happen to be around, in the long run the impacts may be beneficial. After all, if the meteor impact of 65 million years ago had not happened, then Earth might still be home to dinosaurs, and mammals might still be scraping a living at the margins of a lizard-dominated world. Cramer points out that there may be geological periods when nothing much happens to species; evolution appears to take the commonsense attitude of "if it ain't broke, don't fix it." It is primarily at crises points, when for some reason the environment changes, that evolution works quickly and new species arise to take advantage of altered conditions. Evolution, in Cramer's words, seems to be "pumped" by cycles of crises and stability. And, he suggests, an ideal pump is one that drives evolution through major crises every 20 to 30 million years. Asteroids from the 3:1 Kirkwood gap may provide a pump at exactly the right rate.

u figure 49 A montage of images of Eros; the images were taken over three weeks as the near spacecraft approached the asteroid. Near-Earth asteroids like Eros are relatively few in number. Most asteroids are in the "main belt," orbiting the Sun in a torus between Mars and Jupiter. It is these "belt" asteroids that can be perturbed from their orbits by the gravitational influence of Jupiter — with potentially devastating results.

If Cramer's idea is correct — and he would be the first to admit that the idea is speculative — it constitutes another reason why life on Earth might be special. Not only might life require an Earth-like environment, the environment might need to occur in a system with planetary masses and orbits that produce a resonance in an Asteroid Belt at just the right rate. If the "pump of evolution" runs too fast — and asteroids hit a life-bearing planet too often — then life never has a chance to evolve intelligence. If the pump runs too slow — and asteroids hit a life-bearing planet too rarely — then life becomes stuck in a rut. The result is a planet full of trilobites or cockroaches or dinosaurs (or, more likely, creatures differing from terrestrial creatures in a myriad of fascinating ways). As long as these creatures were successful, in an unchanging environment there would be no "need" for them to adopt new modes of behavior, and no "need" for them to develop intelligence and thence radio telescopes or starships.

The existence of the Asteroid Belt is due to Jupiter: the Belt is the remnants of a protoplanet whose formation was aborted because of Jupiter's own formation. And the 3:1 resonance in the Belt is also due to Jupiter. If there is such a thing as a "pump of evolution," and if it is tuned to the right level in our planetary system, then we have Jupiter to thank for it.

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