Microbic existence on Europa or DNA-strands etched into Martian fossils would be fascinating and profound in their implications. But the question that makes everyone tingle is the one about other civilization-building species.
Statistically, extraterrestrial life is almost a certainty. With upwards of 100 billion stars in each of upwards of 100 billion galaxies, the odds of life-bearing planets out there has a lot of 9’s in it; the question is about those that might host higher forms of auto-ambulatory life. It’s a question of time … waiting for an opposable thumb (or its analog) and the ability to solve abstract problems to arise in the same creature.
The oldest direct evidence we have for life on Earth consists of fossilized bacteria in 3½ billion-year-old rocks from Western Australia; these organisms were already quite advanced and must themselves have had a rich evolutionary history. Well, the Earth is just 4.6 billion years (GY) old, so, apparently, life established itself almost as soon as conditions permitted. That suggests that a biosphere is a relatively easy step for nature to take. If so, that would suggest the probability, rather than mere possibility, of life existing on planets exhibiting earthlike qualities in a benign relationship to a sun-like star. So we are, again, waiting for a thumb-using thinker.
Life appeared ~3.5GY ago, but multi-celled animal life didn’t appear until ~700 million years (MY) ago. This time lag (≤3GY) would suggest that the evolution of anything more complicated than a single cell is, if not unlikely, apparently difficult. If so, we can rule out planetary systems much younger than 4GY. Even if complex life always arises from simple life, given enough time, there is still no guarantee that intelligence and the ability to grasp tools will arise, let alone in the same species. Dinosaurs dominated the Earth for 140MY yet never exhibited either. It took us just 14MY to go from opposable thumbs to walking on the moon. Regardless of the inevitability of complex life forms on other planets, intelligent, tool-making life forms apparently are a chancy proposition.
Our galaxy is 100,000 light-years (LY) across, and contains ~100 billion stars. Let’s say that another advanced civilization is located 1,000LY from Earth (just 1% of the galactic diameter away), and is engaged in a search for other civilizations in the galaxy. Their view of Earth would be 1,000 years old. No RF or mWave emissions, no industrial pollutants in the atmosphere, no IR hot-spots (e.g., cities), no sign of technologically advanced civilizations because there are none. The seasons would reveal the presence of flora. We would be tagged as a life-bearing world with no signs of technological life forms. To us, such a find would be a paradigm-bending discovery, but for a more advanced society, it may be yet another “life, but no civilization” discovery – added to a list of places to visit if ever in that area. Looking again at our own experience, the photosynthetic age lasted billions of years before fauna of any type appeared. It then took hundreds of millions of years to produce species with appendages that could grasp tools. Another few millions of years to structure-builders, and 10,000 years for the structure-builders to achieve a technological civilization. From 1,000LY away, Earth would look the same in 1000AD as it did in 1000BC, or 600,000,000BC, and no way to know which.
Our star is main-sequence of average-to-small size, is 5GY old, and has about 7GY of burnable fuel left. This, in a 13GY-old galaxy. We can rule out any first generation stars as having sired civilizations because until the first supernova, there was nothing heavier than hydrogen and trace amounts of helium free of stellar cores with which to build planets. It would take many, many novae to produce enough heavy ejecta to allow for metallic, rocky planets, supporting a rich, complex chemistry on their surface. Since only ~10% of main-sequence stars, and ~60% of the 30% of all stars that are non-main-sequence, are large enough to nova upon dying, we can probably eliminate the first 5GY or so of star formation as candidates for planets containing the right elements to permit complex chemistry. So the “life permitting” galaxy is ~7GY old. That potentially gives some other stars around a 2GY head start on us. Would that make any life on these planets two billion years more advanced than we are? We just don’t know. We don’t know how long it takes to go from CO2-breathers to complex organisms that metabolize oxygen, only how long it took on Earth. As I said earlier, the step from single-celled to multi-celled seems to be the one that nature doesn’t take easily. Or if she does, and it, for whatever reason, took much longer on Earth, other civilizations may be considerably more than 2GY advanced. We just don’t know.
Our first use of wireless telegraphy (i.e., electromagnetic broadcast of an organized signal) was in 1894, meaning that Earth isn’t really technologically interesting until one gets inside of 115LY of us (at which point we would become a weak pulsar – atypical behavior for a planet – as the signal emanating from New York City sweeps past the observer). As an observer moves closer, signal sources begin popping up from multiple locations, strength increases and signal complexity begins increasing. Inside about 70LY, mWave signals join the RF as sporadic television transmissions begin, again increasing in sources, strength and complexity as one nears Earth. At some point around 80LY out, it would become obvious that these signals originate from an advanced civilization.
SETI (the Search for Extra-Terrestrial Intelligence) has scanned for any emission signatures of a signal ranging from 1027W (the total power output of a sunlike star) to that of a galaxy (~1038W) out to ~40 million LY. Nothing. Civilizations having our broadcast ability have been excluded out to ~800LY. What this means is that any source we’re looking at lacked a broadcasting technology a number of years ago equal to their distance in light years. In other words, a star system 100LY away did not host a broadcasting civilization 100 years ago, else we could detect those signals now. This tells us that any civilization at least as advanced as we are is further away than it is advanced – that is, they are not both sufficiently advanced and close enough to be communicable.
If speed-of-light communication is barren for 800LY, and increasingly impractical from there out, physical contact is even more improbable. To replicate the frequency of “credible” UFO reports, one of three conditions must be met: either a society sent out scores of waves of explorers to Earth without hearing back from the first wave; or scores of civilizations have somewhat simultaneously achieved the capability to visit Earth, and are somewhat simultaneously doing so; or, an exponentially advanced culture has mastered post-photo velocities and is able to get here without hitting anything (a feat that not even light itself can muster). Possible? Yes, but like a monkey typing out Shakespeare by random key-strokes, not in the least probable. All of this without even discussing the enormous economic commitment.
In my view, ET is out there, but, like us, is lacking the means to contact other advanced civilizations. Because the available breeding grounds for civilization are bunched into a relatively narrow band of time and conditions, I believe the progress of those resultant civilizations to be relatively parallel. We have as good a statistical chance of being someone else’s ET as someone else has to become ours.
This conversation is probably occurring in every galaxy.
 Newly united Britain is Saxon (the Normans haven’t invaded yet), but a united Europe did not survive Charlemagne, and is reverting to localized duchies (what will become France and Germany). The Abbasid dynasty out of Baghdad has just begun the Muslimizing of Punjab. The Mayan Empire is in full bloom.
 Arecibo’s potential 1011W signal is the best we’ve got.