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262 Mind-Stretcher. Although our radio and television era may be short, there may be future eras that will send other kinds of signals. Just as Enlightenment scholars could not have imagined radio, television, and radar, so we may be unable to imagine the evidence of our presence that technologically advanced aliens might detect 200 years from now. Earth has sent out two spectacular signals of life, explained the Clarks. The more recent is the sudden burst of radio waves that started 50-60 years ago. The first signal, which began about 2 billion years ago, was the change in the composition of our atmosphere with the rise of photosynthesizing plants, producing a dramatic increase in the level of oxygen.’ Alien scien- tists may be far more likely to discover the second. Their human counter- parts will be searching for similar signals as they study extrasolar planets. The other presumed telltales of our existence are even less detectable. Our existing technology could not identify the pulse from a nuclear weapon at the distance of the nearest star. Tests of such weapons now are conducted underground, making detection even more difficult. Our spacecraft and their physical effects are tiny in an interstellar context and might be detect- able only if an alien observer were located in our own solar system. Agism Focusing on our own time ignores the possible age of other civilizations. Ulmschneider estimated that, as the first population I stars appeared about 10 billion years ago and assuming that the development time for human-type intelligence is around 4.6 billion years, the first intelligent societies could have appeared 5 billion years in our past. They would be older than our planet. Our solar system has only existed for about half the time of the longest life-favorable systems, Edelson argued. If the length of time that it took us to evolve is typical, there could have been civilizations 6 billion years ago."° If such ancients exist, they are the continuity of awareness in the universe. They may have evolved far beyond our concepts of intelligence and civilization. They may be the most potent forces evolution has pro- duced, with powers to create and shatter worlds, to manipulate cosmic forces, and to determine the fate of lesser minds. Would they be mindful of us? The civilizations that we expect to encounter may have begun their searches for others millions or even billions of years ago—if they had the interest and the technological means.'' Such a civilization might have Before Contact