Space.com has an interesting little presentation about The Drake Equation....

video

Experts suggest that there are about 50,000 civilizations in our galaxy. But I wonder if they've taken into account the age of our galaxy, about 15 billion years old. If you assume that it was, say 5 billion years until the first stars matured and spawned life, then we've had 10 billion years of life popping up, or 200,000 years per civilization.

I assume that most of these civilizations would not co-exist simultaneously at the same technological level, since 200,000 years is literally an evolutionary timespan. On earth that is the difference between 3-foot tall early homonids, and modern man. Therefore if we are to discover life elsewhere in the galaxy we should not merely look for vastly different levels of technology, but also distinct variation in evolutionary stage. If the first civilization that arose 10 billion years ago is still lurking about the galaxy, then it will be at a more advanced stage of evolution equivalent to the difference between monkeys and man, multiplied by 50,000. That's a pretty major difference.

I would suggest that we could become more advanced by establishing a language key for every species on earth. Apes must communicate with each other, we should learn how they do it. Likewise for birds, reptiles, etc. We should become fluent in the animal-speak of millions of species. Otherwise we have no hope of understanding what is "out there".

If other species died out, and there is no "overlap", then we are alone in the galaxy. But we might look for beacons of different types. Perhaps we could look for modifications to stars themselves, persistant sunspots, jets, gas giants with moons in unusual resonant orbits, engineered perturbations in star wobble caused by planetary orbit resonance amplification. Who knows.