I suppose in xenobiology, we’re going to have to come up with a taxonomic term greater than kingdom…call it “empire”…to describe all life that is phylogenetically related. We have good reason to believe that all life on earth is a member of the empire Terrae, since all earth life shares the same basic physiochemical makeup. Perhaps microbes on Mars are members of the same empire, perhaps they are not. It may be that all life we discover in this galaxy is a member of the same empire, all planets were seeded with bacteria from a common source.
But let’s stick to the hypothesis that life typically arises on a planet by abiogenesis. It seems to me that it is possible that life arose on earth more than once, which would mean that there would be several empires of organisms. But life arose on earth very quickly after the earth’s crust cooled enough to allow liquid water. The first empire to evolve would have a tremendous advantage, and would quickly invade all habitable areas. And that means it is curtains for any other abiogenesis, since the protobacteria from the first one are almost certain to eat the raw materials that might have started the second one.
It seems to me (keeping in mind we have a sample size of one) that since life arose very very quickly on earth, it is likely that it would also arise on any suitable body. Any place with liquid water, abundent carbon, hydrogen, oxygen and nitrogen, and some form of abundent energy (solar, geochemical, or other) is also likely to have life.
But what kind of life? For about 2 billion years the only life forms on earth were prokaryotes…bacteria of one form or another. Then we have another billion years of unicellular eukaryotes, one celled protozoans and algae. It is only in the last 650 million years that we have multicellular animals.
If we imagine that all planets with liquid water follow the same distribution, then about 10% will be lifeless, 50% will have bacteria only, 30% will have protozoans and algae, and 10% will have multicellular animals and plants. And of course, humans have only been around for several million years. But up until 120,000 years ago humans were just another species of large mammal. Succesful and interesting, yes, but not ecologically significant like fully modern Homo sapiens. I imagine that lots of animals could be considered about as intelligent and interesting, such as whales, dolphins, elephants, parrots, etc. Suppose that planets follow a similar distribuition, that would mean that intelligent life would be found on, say, 10 percent of those planets with metazoans, and tool using civilized life on 1% of those. That means 1% of planets with life having “intelligent” life that maybe you could learn to talk to, and 1% of those intelligent creatures being technological and civilized.
That means there’s a lot of algae out there in the universe, but not many human-like aliens.