Before we answer that question, we must acknowledge that any extraterrestrials we make contact with may well be thousands or millions of years more advanced than we are. Why do SETI scientists assume this? Because for our search to succeed, it needs to be true.
If the galaxy is populated only by young civilizations that have the capacity for interstellar communication for only a few decades before they destroy themselves or simply lose interest in making contact with other worlds, then we will effectively be isolated, alone in the universe. If other civilizations transmit evidence of their existence for only a few decades – the length of time that humans have been capable of interstellar communication – and then they lose the interest or ability to make contact, it's extremely unlikely that the precise time they are transmitting and the time that we are listening will coincide. On a galactic scale, where time is measured in billions of years, it is extremely unlikely that these two "blips" would happen at the same time. This would be as unlikely as two fireflies each lighting up once, at exactly the same time, during the course of a long, dark night. The chance that both would flash on simultaneously is virtually zero; it's more likely their flashes would be separated by minutes or hours. So too is it unlikely that two short-lived civilizations that had evolved independently of one another would come into being at almost precisely the same time in the fourteen billion year history of our galaxy.
If we hear from a distant civilization, on purely statistical grounds it's very likely they will be our elders.
But is this a leftover memory of the European invasion of North America? The Europeans had guns and exotic diseases and the Native Americans didn't. Hence history.
But what if SETI's extraterrestrials eventually crash land on Earth because of a mixup between two rival groups of scientists, one of which was using Imperialoid measurements and the other was using Metricosis measurements?
Hey, don't tell me it never happens.
