Copyright © 1983, by David Brin. All rights reserved. No duplication or resale without permission.

The following article was first published in the 1980s in Analog Magazine as a popular adaptation of the much deeper and more scholarly 'classic' review of the field -- The Great Silence -- which appeared in the Quarterly Journal of Royal Astronomical Society, fall 1983, v.24, pp 283-309, which is also available on this site.

Let's go back to that expanding spacefaring species we were talking about earlier. Remember, calculations show that it might take as little as sixty million years for such a race to fill the galaxy. A question seldom asked by science fiction authors who write about colonization is, What happens to the colonized planet?

Unless the settlers leave large parts of their worlds fallow in wilderness preserves, or engage in "Uplift" bioengineering of local higher animals, their mere presence is likely to prevent the appearance of local sentient species. The cycle of production of intelligent species on a planet is probably delayed indefinitely by an active technological settlement. A world is not likely to serve as a useful nursery of intelligence so long as it is occupied by a spacefaring race.

When settlers finally do step aside -- by attrition, disaster, exodus, or whatever -- ecological recycling can resume, but recovery and regeneration of intelligence will take much more time, the longer a technological race occupied the planet.

It is generally assumed that a spacefaring race will expand into the galaxy either because of raw curiosity or population pressure. Either way, it's clear that the expansion soon becomes spherelike, with only the most recently settled worlds having much opportunity to seek new planets. For a race limited to slowboat technology, colonization will take place only' in a thin shell surrounding an older, settled region within.

If population pressure is the primary motive for expansion, we have to wonder at the fate of the long-occupied worlds in the interior of the settled sphere, especially those near the Home planet. The words population pressure themselves suggest the likely fate of these worlds.

Consider the settlement of Polynesia from roughly 1500 B.C. to about A.D. 800. The island-hopping analogy with interstellar exploration is apt up to a point. Jones borrowed growth and emigration rates for his model of interstellar settlement from Polynesian history. The intrepid Polynesian example is used as testimony to the likely success and viability of "star-hopping" colonization ventures.

Polynesia may, indeed, be representative of interstellar settlement, but not in a pleasant sense. The Hollywood image of island life is paradisiacal, but Polynesian cultures were subject to regular cycles of extreme overpopulation controlled by blood culling of the adult male population, in war or ritual. There are many stories of islands whose men were almost wiped out, sometimes by internal strife, sometimes by invading males from other islands far away.

Meanwhile, introduction of domestic animals disrupted island ecosystems. Many native species were wiped out.

The most severe example is the island of Rapa Nui, also called Isla de Pasqua, or Easter Island. Isolated thousands of miles from its nearest neighbors, it was as much like an interstellar colony as any place in human history, when it was settled around A.D. 800. Mankind may devoutly hope to do better when finally embarked to the stars.

The Pasquans utterly destroyed the virgin ecosystem of Rapa Nui in a few generations, ravaging the forest until only banana trees were left. When no wood remained for houses or boats, they had to abandon the sea and its resources, along with all possibility of escape or trade. What remained was native rock -- which they carved into hauntingly desolate images -- and warfare.

When Europeans arrived, the natives of Rapa Nui had just about destroyed themselves.

Assume a settled sphere of expansion by an extraterrestrial intelligent species. What of the inner systems, within the sphere? The Polynesian example suggests a dismal image of increasing competition for dwindling resources with no escape valve for excess population, since all surrounding systems are in similar straits.

What happens to these inner worlds? They probably don't go looking to conquer their neighbors. Interstellar warfare seems to be a frightfully expensive proposition. conflict arising from population pressure is far more likely to be local, consisting of struggles for resources within each planetary system.

In an old settled system all available asteroids would long have been turned into habitats. Safe inner orbits with unhindered access to solar power would be at a premium.

Even the most efficient space structures will require frequent replenishment of volatile substances -- gases such as oxygen, hydrogen, and nitrogen. Comets might supply part of this need, but terrestroid planets would be closer and rich in the desired light elements.

One might expect to see a profound cultural split between those living on planetary surfaces and those in space. Competition and misunderstandings might tempt the space-dwellers to take advantage of their superior position to dominate their planet-bound cousins. It would be simple to bombard the cities on a planet's surface with redirected asteroids until civilization there was obliterated. Factor L clearly falls in such a case.

(The space-born, long divorced from any attachment to planetary life, might even see a terrestroid planet as a likely source of building materials! It wouldn't be beyond their ability to pulverize a world such as the Earth by arranging planetary collisions. This would certainly affect not only L but also n(e), the number of planets on which life can evolve!)

In any event, the innocent higher animals suffer in the crossfire.

Let's return briefly to the episode about sixty-five million years ago known as the Cretaceous Tertiary Catastrophe. There were, at that time, many advanced species of reptiles. The best candidate among those for a species possibly ripe for development toward tool-using might have been Saurornithoides, a mid-sized bipedal carnivore with the highest brain-to-body mass of any reptile, approximately matching that of modern baboons. While there is no reason to think that this creature was particularly intelligent, he filled an ecological niche that might have been rigorous enough to encourage his glimmering abilities.

But Saurornithoides died out along with virtually all of the other great reptiles during a relatively brief period by geological standards.

If the demise of the dinosaurs puzzles paleontologists, the problem has been even worse for the marine biologists. The dinosaurs, at least, took as long as a few million years to die out. The tiny sea microorganisms experienced a greater catastrophe. Over half of the species of phytoplankton went extinct within about one year.

The latter mystery, at least, now appears solved. Recent deep-core drillings have uncovered thin layers of clay rich in exotic elements, including iridium (up to 25 times normal abundance of some isotopes), at sedimentary levels associated with the end of the Cretaceous. Discoveries in locations as diverse as Italy and New Mexico all seem to correlate a sudden invasion of strange dust with the equally sudden disappearance of many classes of oceanic microorganisms. Scientists now conclude that a major meteorite impact kicked up a great pall of dust that severely altered weather, resulting in mass extinction by starvation when photosynthesis was interrupted.

For the marine creatures this seems sufficient, but don't forget that the dinosaurs were already dying out before this bombardment, starting with the greatest behemoths and so on down to the smaller herd animals. Their die-back was a lot like what we see happening to the wild animals of Africa at the hands of white and black "intelligent" beings. The meteorite seems to have been only one of the last straws for the great reptiles.

Might the demise of the dinosaurs, then, be part of a hidden pattern? Is it possible that an alien colony began a process of extinction that was by the meteorite (or meteorites) only finished?

A natural planetfall can't be distinguished from one targeted against ground settlements of a technological species. Is it possible that the dinosaurs were innocent bystanders in a genocidal war among alien settlers in the solar system?

The bombardment might only have been the last act in a more gradual ecological catastrophe that began half a million years before, when settlement of the planet resulted in extinction of species after species.

The introduction, about this time, of flowering plants, is another environmental perturbation that had profound ecological effects. It's not absurd to imagine this fitting into an overall pattern of outside intervention.

The settlement of Earth by a spacefaring race about seventy million years ago, then, offers one more (admittedly tenuous) explanation for the destruction of the higher terrestrial life-forms over a brief period.

If we make this hypothesis, however, where are the traces of this earlier technological occupancy? Over sixty million years of oxidation will destroy many artifacts, but certainly some might survive.

Who can say? The cities we look for may lie beneath astroblemes. A look at a geological map of the Earth shows that continental plate boundaries have proved to be choice living sites. These plate-edge regions have suffered pronounced geological changes that could have erased most traces of alien settlement.

The final test of this hypothesis would be found among the planetoids of the solar system. The asteroids might hold remnants of visits to our star by extraterrestrials... perhaps whole cities, the leftovers of great populations killed off, perhaps, by biological warfare in desperate retaliation by the Earthbound cousins they had annihilated.

This hypothetical explanation for the Cretaceous Mystery merely should take its place in a catalog of possibilities, perhaps near the bottom. Still, it's interesting to note that the period since that catastrophe -- an interval that culminated in the development of Homo sapiens -- is the same sixty million years suggested by Jones and others for an optimum minimal galaxy-filling by space traveling species.

If the ecological holocaust of the Cretaceous was a local manifestation of the death spasm of a prior spacefaring race, whose overpopulated sphere of settlement spoiled and self-destructed as the shell of colonization passed outward, then we humans may have come into being almost too late. Any longer, and the next wave -- the expanding shell of still another spreading technological race -- might have washed over Earth before we had the ability to assert property rights... assuming we have that ability now.

We may wonder if the Earth is the first Nursery World to have recovered sufficiently, since the last wave of "civilization" passed this way, to develop a species with intelligence. Whether or not the end of the Cretaceous corresponded to the agony of dying starfarers, it may well be that colonizing cultures inevitably leave behind them wastelands empty of intelligence and living voices.

If we humans initiate an era of interstellar travel of our own, we may find all around us the blasted remains of an earlier epoch. Would we then learn a lesson? Perhaps. But with the ever present opportunities for expansion, those humans who exercise self-restraint and environmental sensitivity toward their adopted worlds will not be able to force this tradition upon those who travel far away to establish newer colonies. A nucleus of selfishness is likely to expand more rapidly than a center of more rational colonization. While there may be zones where settlers preserve and protect the local ecospheres, cognizant of their long-range potential, others may be rapacious.

Certainly our environmental record here on Earth is a test. The list of extinct species, some of which might one day have become starfarers, is long and growing longer.

The Great Silence may be the sound of sands drifting up against monuments. It may be quiet testament to the fate of species which allow "population pressure" to be their motivation for the stars.

Next... so what are the seven possibilities?