I've given this a lot of thought (I'm sure I'm not alone in that) and I actually have very conservative Drake Equation numbers, myself. To the point where I doubt there'd ever be more than a handful of technological civilizations ever in our galaxy. And those would be so widely spaced in terms of both time and distance that the odds of any two of them ever interacting are essentially hopeless.
Earth is the only one of dozens of worlds we've looked at that has life in evidence. Mars has enough "yes, but" cases that I hold out hope. (By this I mean numerous results that fit a biological model, but might not--numerous enough to not just write off the possibility.) And the ice worlds orbiting the gas giants may hold more than we have been able to see yet. What may be swimming in the oceans of Europa or Enceladus? Very likely nothing at all, but then again; maybe.
But taken what we do know from the geological and fossil record of Earth (the only planet we know for sure where life ever flourished) shows us that bacterial life showed up as soon as possible. Only the very oldest surviving rocks, from almost 4 billion years ago don't show evidence of bacteria. This evidence, as well as the probability that Martian bacteria is the most we can reasonably hope to see, suggests to me that microscopic life may well be unavoidable complex chemistry that will eventually happen given the correct environment and ingredients. I would almost be surprised not to find bacterial life on other worlds, perhaps around a majority of stars. Given the dramatic changes life makes to a planet's atmosphere, this question likely will be answered within our lifetimes with upcoming space telescopes.
But then it took a long time for multi-cellular life to crop up. As far as we know Mars never got this far. Or if it did, it didn't go for very long. Maybe the conditions on Mars didn't last long enough? The core cooled, the magnetosphere died and the atmosphere was lost before bacteria evolved the means to join up? On the other hand, maybe there's a specific adaptation that is required to allow this to happen and it just never did. However, with trillions of these little guys swimming around, the appropriate mutation would likely happen even if it has low odds for a given individual. But it took around 3 billion years for the higher algaes to show up. So, for about 3/4 of the history of the Earth, there were only bacterias alive. On that basis, I would guess maybe a quarter to an eighth (if we assume small worlds like Mars don't have complex life due to early cooling) of all worlds that develop life would stay amiable to life and also develop the anatomy to team up into multi-cellular life.
We then had the Pre-Cambrian Explosion, which suggests the possibility that once complex life does get going, it takes off in a big way. This was only somewhat more than half a billion years ago, roughly one sixth of Earth's history. But it does suggest that huge diversity shows up right as soon as it's even possible.
So of all the worlds out there that are even in a suitable starting position, most by far will only have bacterial life. But of the percentage that has animal life, how much of that will develop technological intelligence? Such intelligence couldn't be marine life, as you can't get the technological ball rolling with out the use of fire and we can probably agree fires under water are exceedingly rare. On Earth, land animals didn't show up really until about halfway through the history of complex life. Then give them some time to develop. Given any suitable planet where life developed at all, now we're down to maybe 5% of a chance to have any animals that are even in the running for creating tools.
Of that 5% what is the likelihood of actually getting such an animal? Well, again looking at our sample of one, of all the thousands of evolutionary lineages that developed over the eons, only homonids produced the winner. Evolution is subject to no mandates about end goals. It simply operates by random mutations and natural selection. Sharks evolved relatively early and swim the oceans today not too dissimilar from their ancestors of 450 million years ago. They are no closer to building radios than they were back then. They have been able to breed successfully enough without being that clever. And of the homonids, only Homo Sapiens had the goods to develop agriculture and technology. Other species do use tools to some extent. Certain other primates and even some birds like crows and others can fashion crude tools from sticks and stones, but that's not what we're talking about. There is evidence that Neanderthals actually invented the first industrial process, refining sap to use as adhesive in a complex process, much earlier than there even were modern humans, but they stagnated for millions of years. It took whatever special spark in our direct ancestors to develop society and technology. But if you look into the ideas for how that spark showed up, it really seems like it was just a fluke. Some grandparent of our mitochondrial Eve was just a little sharper than his or her peers and passed that to their children and on down the line to ourselves.
So the possibility of these guys even showing up out of the candidate animals seems quite slim to me. Coupled with the fact that there are periodically mass extinction events where 70 to 90 % of all animals die off in short bursts lowers those odds even more.
Then we get to the later numbers in the Drake equation, about what the cultures do when they do develop. I get more generous with these terms. I assume that once a species has the creative intellect to start doing complex things like aggregating into cities and inventing metallurgy and agriculture, that rising to modern tech levels is pretty likely. After all, the entirety of human civilization has taken place in an instant of geological time. Assuming a stone age technological culture doesn't succumb to a mass extinction event beyond their control, they will probably have their own Ptolemy and Galileo and Copernicus and Newton and Faraday and Einstein. And once they do develop their own nuclear weaponry, I bet the majority of them will keep cool enough heads not to exterminate themselves.
But the hard part is getting complex animals to begin with. And then one of those animals becoming technological. This is where I disagree the most with Drake himself who uses a value of 1 in this term. Way too generous in my opinion.
For me this also answers the Fermi Paradox handily. We don't hear from aliens because we shouldn't expect to. On the other hand, if we ever do invent starships and go exploring, I bet we do find life-bearing worlds, many with bacteria, some few with complex and fascinating animals, some incredibly rare of which do make use of simple tools, but almost none of which host technologically advanced human like cultures. (Although given the principle of convergent evolution, I wouldn't be surprised if such a creature did happen to be at least vaguely humanoid, though there would be no need for it. So who's to say?)
SHORT VERSION: Examining the history of life on Earth suggests that the development of technologically capable life may be exceedingly rare.
--Alex
