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Cretaceous Period。 But only in the last decade has the magnitude of these extinctions bee clear； and the possibility raised that in our ignorance of the interrelations of life on Earth we may be endangering our own future。
Look at the dates on this list and consider the range of new technologies currently under development。 Is it not likely that other dangers of our own making are yet to be discovered； some perhaps even more serious？
In the littered field of discredited self…congratulatory chauvinisms； there is only one that seems to hold up； one sense in which we are special： Due to our own actions or inactions； and the misuse of our technology； we live at an extraordinary moment； for the Earth at least—the first time that a species has bee able to wipe itself out。 But this is also； we may note； the first time that a species has bee able to journey to the planets and the stars。 The two times； brought about by the same technology； coincide—a few centuries in the history of a 4。5…billion…year…old planet。 If you were somehow dropped down on the Earth randomly at any moment in the past （or future）； the chance of arriving at this critical moment would be less than 1 in 10 million。 Our leverage on the future is high just now。
It might be a familiar progression； transpiring on many worlds—a planet； newly formed； placidly revolves around its star； life slowly forms； a kaleidoscopic procession of creatures evolves； intelligence emerges which； at least up to a point； confers enormous survival value； and then technology is invented。 It dawns on them that there are such things as laws of Nature； that these laws can be revealed by experiment； and that knowledge of these laws can be made both to save and to take lives； both on unprecedented scales。 Science； they recognize； grants immense powers。 In a flash； they create world…altering contrivances。 Some planetary civilizations see their way through； place limits on what may and what must not be done； and safely pass through the time of perils。 Others are not so lucky or so prudent； perish。
Since； in the long run； every planetary society will be endangered by impacts from space； every surviving civilization is obliged to bee spacefaring—…not because of exploratory or romantic zeal； but for the most practical reason imaginable： staying alive。 And once you're out there in space for centuries and millennia； moving little worlds around and engineering planets； your species has been pried loose from its cradle。 If they exist； many other civilizations will eventually venture far from home。*
* Might a planetary civilization which has survived its adolescence wish to encourage others struggling with their emerging technologies？ Perhaps they would make special efforts to broadcast news of their existence； the triumphant announcement that it's possible to avoid self…annihilation。 Or would they at first be very cautious？ Having avoided catastrophes of their own making； perhaps they would fear giving away knowledge of their existence； lest some other； unknown； aggrandizing civilization out there in the dark is looking for Lebensraum or slavering to put down the potential petition。 That might be a reason for us to explore neighboring star systems； but discreetly。
Maybe they would be silent for another reason： because broadcasting the existence of an advanced civilization might encourage emerging civilizations to do less than their best efforts to safeguard their future— hoping instead that someone will e out of the dark and save them from themselves。
A MEANS HAS BEEN OFFERED of estimating how precarious our circumstances are—remarkably； without in any way addressing the nature of the hazards。 J。 Richard Gott III is an astrophysicist at Princeton University。 He asks us to adopt a generalized Copernican principle； something I've described elsewhere as the Principle of Mediocrity。 Chances are that we do not live in a truly extraordinary time。 Hardly anyone ever did。 The probability is high that we're born； live out our days； and die somewhere in the broad middle range of the lifetime of our species （or civilization； or nation）。 Almost certainly； Gott says； we do not live in first or last times。 So if your species is very young； it follows that it's unlikely to last long—because if it were to last long； you （and the rest of us alive today） would be extraordinary in living； proportionally speaking； so near the beginning。
What then is the projected longevity of our species？ Gott concludes； at the 97。5 percent confidence level； that there will be humans for no more than 8 million years。 That's his upper limit； about the same as the average lifetime of many mammalian species。 In that case； our technology neither harms nor helps。 But Gott's lower limit； with the same claimed reliability； is only 12 years。 He will not give you 40…to…1 odds that humans will still be around by the time babies now alive bee teenagers。 In everyday life we try very hard not to take risks so large； not to board airplanes； say； with 1 chance in 40 of crashing。 We will agree to surgery in which 95 percent of patients survive only if our disease has a greater than 5 percent chance of killing us。 Mere 40…to…1 odds on our species surviving another 12 years Would be； if valid； a cause for supreme concern。 If Gott is right； not only may we never be out among the stars； there's a fair chance we may not be around long enough even to make the first footfall on another planet。
To me； this argument has a strange； vaporish quality。 Knowing nothing about our species except how old it is； we make numerical estimates； claimed to be highly reliable； about its future prospects。 How？ We go with the winners。 Those who have been around are likely to stay around。 Newers tend to disappear。 The only assumption is the quite plausible one that there is nothing special about the moment at which we inquire into the matter。 So why is the argument unsatisfying？ Is it just that we are appalled by its implications？
Something like the Principle of Mediocrity must have very broad applicability。 But we are not so ignorant as to imagine that everything is mediocre。 There is something special about our time—not just the temporal c