The secret lay in the bacteria's ability to form spores, molding a hard calcific shell around themselves. This shell enabled the organism to survive freezing or boiling, and, if necessary, thousands of years without food. It combined all the advantages of a space suit with those of suspended animation.

There was no doubt that a spore could travel through space. But was another planet or galaxy the most likely source of contamination for the earth?

Here, the answer was no. The most likely source was the closest source—the earth itself.

The report suggested that bacteria could have left the surface of the earth eons ago, when life was just beginning to emerge from the oceans and the hot, baked continents. Such bacteria would depart before the fishes, before the primitive mammals, long before the first ape-man. The bacteria would head up into the air, and slowly ascend until they were literally in space. Once there, they might evolve into unusual forms, perhaps even learning to derive energy for life directly from the sun, instead of requiring food as an energy source. These organisms might also be capable of direct conversion of energy to matter.

Leavitt himself suggested the analogy of the upper atmosphere and the depths of the sea as equally inhospitable environments, but equally viable. In the deepest, blackest regions of the oceans, where oxygenation was poor, and where light never reached, life forms were known to exist in abundance. Why not also in the far reaches of the atmosphere? True, oxygen was scarce. True, food hardly existed. But if creatures could live miles beneath the surface, why could they not also live five miles above it?

And if there were organisms out there, and if they had departed from the baking crust of the earth long before the first men appeared, then they would be foreign to man. No immunity, no adaptation, no antibodies would have been developed. They would be primitive aliens to modern man, in the same way that the shark, a primitive fish unchanged for a hundred million years, was alien and dangerous to modern man, invading the oceans for the first time.

The third source of contamination, the third of the vectors, was at the same time the most likely and the most troublesome. This was contemporary earth organisms, taken into space by inadequately sterilized spacecraft. Once in space, the organisms would be exposed to harsh radiation, weightlessness, and other environmental forces that might exert a mutagenic effect, altering the organisms.

So that when they came down, they would be different.

Take up a harmless bacteria—such as the organism that causes pimples, or sore throats—and bring it back in a new form, virulent and unexpected. It might do anything. It might show a preference for the aqueous humor of the inner eye, and invade the eyeball. It might thrive on the acid secretions of the stomach. It might multiply on the small currents of electricity afforded by the human brain itself, drive men mad.