Yeah, the Drake Equation is useless because we don't know the variables, but it's still fun to play around with...
Same with the Rare Earth equation, though Rare Earth is pretty biopessimistic and actually based on assumptions that are becoming outdated. Ward himself is a paleontologist, and his co-author for Rare Earth is an astronomer and astrobiologist- even the qualifications of the latter are somewhat off the mark, as fields more closely pertaining to things such as Rare Earth would probably be fields such as evolutionary biology, astrophysics and planetary science...
In addition, I have a slight feeling Ward dislikes multicellular life in general- see the Medea hypothesis, of which the included mass extinctions have nothing to do with complex life, in some cases even happening long before the Cambrian explosion.
Amusingly, I still got more than 840 000 planets with complex life out of the Rare Earth equation, using values one might consider realistic.
I created my own version of the equation (tossing out some of the more rare-Earth-y aspects), and used more inclusive (but still relatively realistic) values, and got a value of around 1.3 billion planets with complex life. Whether my version of the equation has any validity is another story, considering the fact that I have the mathamatical proficiency of a goldfish.
I get quite annoyed when people cite the Fermi paradox as why intelligent extraterrestrial species cannot exist, or even why no extraterrestrial complex life can exist, either because;
1. They're not blaring radio waves at us at frequencies we're listening in on.
2. They don't posess thousands of Dyson spheres spread around the galaxy in a monumental civilisation.
In light of these, it is important to realise;
- There is no "manifest destiny" for life to evolve into intelligent species. For most of Earth's history, the planet has been devoid of intelligent life and only one evolutionary path has so far lead to a sapient organism.
- Complex life on another planet might actually be prevented from evolving sapience due to it's biological or physiological traits. Plants and fungi on Earth are a good example of this; they are multicellular life, but they lack nervous systems- thus entirely precluding the evolution of intelligence. Just as there is no absolute destiny for life to evolve into intelligent species, there is no certainty that it will evolve into the kingdoms of life that are so important on Earth. At best, extraterrestrial kingdoms will only resemble their terrestrial counterparts vaguely. A planet dominated by photosynthetic slime and football sized mobile sponges would have as much potential to generate a sapient organism as a planet of plants.
- Sapient life could be inhibited from developing an industrial society (which potentially leads to a more advanced society through accelerating returns, etc) by geographic, climatic, and environmental factors. This is based on human experience on Earth- Europe became industrial, and the whole of Eurasia saw plenty of technological and cultural advancement, whereas populations in Africa, the Americas, Australia and elsewhere did not. This is not at all a racial argument, but rather a geographic one, as proposed by Jared Diamond. Specific circumstances in Eurasia, specifically Europe, spurred on cultural, political and technological advancement. For example, Greek and Roman civilisations could make use of the Medeterranean sea for transport, in cases when transport over land was often harder. A mild climate fostered the early development of agriculture, and the east-west layout of the continent meant food crops could easily be adapted to new areas. Europe's broken-up geography led to a variety of different nations, with numerous conflicts, also driving development. In desert regions, conditions were too harsh and resources too scarce to often facilitate large, permanent settlements. In tropical regions the climate often meant there wasn't a need to store food for a harsher season.
On another planet, an intelligent species might have things worse. Those on a planet with less water and small, scattered seas would have access to fewer resources, in a generally bland environment, and sea travel would be difficult. Those on a planet with a single, super-continent would be faced with a harsh, bland continental interior, with a continuous coastline. Those on a planet with much water and only small, scattered landmasses would have a hard time grouping together or forming large societies, and effective travel for thousands of kilometers would be difficult. On a planet with a warm, wet climate (not unlike those Earth has experienced during it's "greenhouse" periods) resources may actually be too abundant to warrant civilisation or large-scale agriculture. On a planet with climatic extremes or harsh seasons (think high eccentricity, etc), sapient organisms would have too much trouble just eking out an existence to worry about civilisation.
- Sapient life could be inhibited from developing an industrial society due to their biology or physiology. Dolphins are a good example, as they are what one might consider a "candidate" to evolve into a sapient organism. They are highly intelligent, but lack any effective manipulators (i.e. hands), so constructing anything but the simplest of tools would thus be extremely difficult for them. Elephants are another good example, as they are also highly intelligent. They posess a singular manipulator (their trunk, obviously), but already have numerous evolutionary adaptations that make many instances of tool use superfluous (such as tusks to strip bark off of trees, for example), and are also very bulky, which makes constructing shelter or attaining large amounts of food difficult. There are also various intelligent birds- namely certain parrots and corvids (the group including ravens and crows), that exhibit striking intelligence and tool-use capabilities. However, a sapient bird would have trouble engaging in activities requiring great strength, or lifting heavy objects.
- Sapient life could be inhibited from developing an industrial society due to resource availability concerns in their environment. Many developments in human history have depended on products from particular organisms (or things originally created from organisms in specific processes, such as oil or coal). Sapient organisms on a planet where "trees" have "wood" based on silicon dioxide (like a diatom shell) would have it worse off in terms of building materials; likewise, they would not have a lot of energy in fossil fuel reserves, their silacious version of "coal" being considerably less energy dense.
- Any sane advanced society would regulate their population, and thus population growth should not occur at the rates proposed by some. This does not mean killing people, or sterilising people, or enforcing draconian and unnatural birth control measures- just encouraging family planning. Even if the population grows and recedes over a period of time (i.e. birth control relaxed, population grows, family planning encouraged, birth rate drops), as long as it says within the means of the civilisation it is healthy.
One might think of population growth to 9-14 billion, over the course of this century, to be bad. But the problem is, it gets even worse as time goes on. In 2000 there was a global population of around 6 billion, and if we assume the growth rate in that year (1.2%) stays steady (it isn't- the growth rate is dropping), we get around 11 billion by 2050. By 2100, we get already nearly 20 billion, and by 2250, nearly 120 billion. By 2500, over 230 billion, and by 3000, a global population of over 90 trillion. Overpopulation is not a minor ecological problem, but a serious one for humanity; even 20 billion, is already getting too large for the planet to support, and over 90 trillion would probably be difficult to fit on the planet, even if everyone was physically pressed together in kilometer high skyscrapers covering the planet.
- Individual star systems can probably hold far more individuals than many think. One author suggested that the solar system could support a population of 10e16 people...
- Stars for colonisation need not be physically close; for example, I would much rather turn my attention to Delta Pavonis or Gliese 581 for colonisation, than Ross 248, even though the latter is physically closer. Due to this, "interstellar empires" might be more spread out and thus harder to detect than one might think.
- Activities of advanced civilisations might not be detectable as one might otherwise think. For example, you can still collect exawatts of stellar energy with relatively small structures, and a "solid shell" Dyson sphere makes sense more in science fiction than science fact (though any considerable Dyson sphere would probably be detectable from a distance, even if composed of smaller, individual power collectors).
- Civilisations may not bother communicating with others.
- Civilisations may not wish to communicate with others, even if they know of them.
- Civilisations might attempt to communicate via means we do not search for (or search for frequently), for example they could use optical means rather than radio ones, or radio frequencies that we don't scan on.
Thus, one might be able to conclude that there are very few advanced civilisations, less few primitive civilisations, even less few subsistance-dwelling sapient organisms, and far less few planets with some sort of complex life.
Complex life might be common, but it's pretty silly to assume that it's so rare as to make Earth one of a handful, or even make Earth special and unique in the entire universe...