David Brin's excellent science-fact article "Just How Dangerous is the Galaxy?" (ANALOG, July 1985) was a very comprehensive survey of possible explanations for The Great Silence, "the strange and apparently longstanding absence of intelligent extraterrestrial life". By any standard this article is a contender for best-of-the-year science fact article in any SF magazine. But in David's extensive list of explanations, I notice that one possible explanation of the Great Silence is missing ... mine. 

To get to where I am aiming I will need to link up three seemingly unrelated ideas: (1) Punctuated Equilibrium; (2) The Cretaceous Catastrophe; and (3) The 3:1 Resonance.

Punctuated equilibrium. Classical Darwinian evolution theory describes progress in an evolving animal species as a slow but continuous process: small beneficial mutations appear, give the mutantants a slight survival advantage, and gradually spread over the gene pool of the species until they become the standard basis for further beneficial mutations and more evolutionary progress. This is called gradualism. Recently some paleontologists have mounted a challenge to gradualism, which they find to be inconsistent with the fossil record made as species evolve. They interpret the fossil story as one of long periods with little or no change in the characteristics of a species, punctuated with (geologically) short periods of violent change in these characteristics. They are led to a revised model of evolution called punctuated equlibrium which suggests that most of the changes which together we call evolution occur mostly during these geologically short crisis periods.

It has been suggested that what constitute a crisis period may be periods when there is some severe environmental change or times of depleted population when the gene pool is small enough for rapid genetic drift to occur. Ice ages and other climate changes, new diseases, new predators, volcanic activity, changes in solar activity, and even nearby supernovas are among the suggestions for possible triggers.  In the long inter-crisis eras all of the ecological niches become filled and change is unwelcome to the secure occupant of a niche. During a crisis change is required because the niches are altered or vacated, and only a species which is genetically quick footed can respond to the altered conditions. Evolution seems to be "pumped" by these cycles of crisis and stability.

The cretaceous catastrophe,  a vivid example of a crisis period. A few years ago a group scientists at UC Berkeley led by physicist Luis Alvarez (Nobel laureate for the discovery of the antiproton) and his geologist son Walter Alvarez discovered that there is a large concentration of the element iridium in a 65 million year old layer of sedimentary rock. This clay stripe in the geological strata is the boundary marking the end of the Cretaceous period, the last age of dinosaurs, and the beginning of the Tertiary period, the first age of mammals. Since chondrite meteors are very rich in iridium, the Alvarez group interpreted the iridium enhancement as evidence that a large chondritic meteor (perhaps 10 km in diameter) collided with the earth. They showed that the dust from such a meteor strike should blanket the planet for a period of months or years, blocking sunlight, inhibiting photosynthesis, and killing a sizable fraction of the species in the oceans and on the land. That's how the dinosaurs died.

This theory, while still controversial, has received growing support from many quarters and is becoming well established. There is even speculation that Iceland, which developed from sub-ocean volcanic activity starting about 65 million years ago, may have risen from the hole punched in the Earth's crust by the cretaceous meteor. Archaeologists have searched the fossil record for evidence of other similar die-offs of many species and have produced a spiky curve of extinguished species, with the cretaceous extinction showing up as the largest among many such spikes. Some have claimed to see a periodic pattern in this curve and have interpreted it as evidence that perhaps ten such catastrophies have occurred regularly every 26 million years (fortunately with 13 million years since the last one). This has led to searches for a hypothetical "Death Star" (already named Nemesis) or a rogue planet (Shiva) which dumps debris from the cometary belt into the inner Solar System regularly every 26 million years because of the characteristics of its orbit.

As a person who has to look at poor-statistics data more often than I would like, I can't really buy these Death Star theories. It's easy to see patterns where they don't exist, as has been proved over and over in the scientific literature. The data looks to me like a random process with an average time between big crises of about 20-30 million years. This is consistent with a calculation by the Alvarez group which estimated from meteor size-frequency distributions that a big meteor (say 5-15 km in diameter) should hit the earth about every 100 million years on the average, with smaller meteor hits more probable. Thus a regular but random bombardment of the Earth by large chondritic meteors could account for the mass extinctions observed and for at least some of the punctuated equilibrium in the evolution of the surviving species.

The 3:1 resonance. Where do these chondritic meteors come from? One possible source is the asteroid belt between Mars and Jupiter. But this would require that meteors in large numbers must be dumped out of stable orbits in the asteriod belt. Now such a mechanism seems to have been found. At a distance of about 2.5 times Earth's distance from the sun there is an empty band in the asteriod belt. In this empty band of orbits an object would circle the sun in a time which is just 1/3 of the 11.9 year orbital period of Jupiter. Thus every third time an object in this orbit comes back to the same position, Jupiter is also in the same position and kicks the same way with its gravitational field. These repeated kicks in the same direction accumulate and make the orbit unstable. This is called the 3:1 resonance mechanism. It is a phenomenon which is also observed in the rings of Saturn.

Objects in the 3:1 zone have unstable or "chaotic" orbits which depend very strongly on minute details of position and velocity. Such a chaotic orbit, while it can in principle be calculated from Newtons's laws, is essentially random and unpredictable because it changes violently with small changes in position. Treating chaotic orbits statistically shows a significant probability that "Apollo-type" earth-crossing orbits are produced. Asteroids in such earth-crossing orbits have a high probability of eventual collision with the Earth. Simulations with this mechanism account for about the number and direction of chondrite meteorites observed to fall on the earth. The 3:1 zone is a sort of celestial shotgun with a good supply of ammunition provided by the asteriod belt. Even quite large asteriods can wander into this 3:1 zone and be propelled in the direction of the earth, with about the probability calculated by the Alvarez group.

Now I would like to put these ideas together. The punctuated equilibrium theory indicates that there has been a certain rhythm to the evolution of life. I would like to suggest that the principal "pump" driving that rhythm is the occurrence of large and medium size meteor strikes from the 3:1 zone of the asteriod belt. Such meteor strikes, we suggest, produce the majority of extinctions and "punctuations" shown by the fossils. If this crisis pump cycles too fast the catastrophes occur too often and are too wasteful of life, too likely to extinguish promising species, and too close for the species to fully adapt to the reshuffled ecological niches before the next cycle, and so evolution proceeds more slowly. If the pump cycles too slowly the rate of crisis-stimulated evolution will also be slowed, and species may become so firmly ensconced in their niches that they will die rather than change when displaced. 

So there must be an ideal crisis/catastrophe rate which speeds evolution along at an optimal speed by blasting species out of their niches and weeding nature's garden at just the right time to promote improvement. I would like to suggest (without any evidence whatsoever except the Great Silence and the existence of intelligent life on Earth) that the average of 20-30 million years or so between big catastrophies is near this ideal. The pump of evolution on Earth must have a resonance, an optimum speed, at nearly that rate.

When a random solar system forms the circumstances would have to be very special to reproduce both (a) an earthlike environment and (b) an asteroid belt which provides just the right bombardment rate for a proper pumping cycle. The mass of the sun, the orbital radius of the earthlike planet, the mass and orbit of a giant jovian planet, the presence of an asteroid belt with just the right range of orbits and densities, all of these must be tuned just right to get the pump running at the right speed. It happened this way in our system but would seem to be extremely unlikely for an average life-bearing planet around an average star.

So that's my explanation of The Great Silence: we haven't been contacted by an Elder Race because we are the Elder Race. We happen to have evolved on a planet where evolution is pumped to progress faster than almost any other place in the universe. This places an awesome burden on our shoulders. We can't just sit around waiting for superior aliens to show up to teach us the right path and reform our character. Instead, it is our duty and responsibility to reform ourselves and then to go out and seed the universe with intelligent life. I believe that we're up to the challenge. Do you?