Thomas R. Holtz, Jr.
The search for life on other worlds is one of the missions of NASA, among other scientific institutions. In fact, there has been a long history of philosophers debating the possibility of life on other planets:
With Darwin's publication of The Origin, scientists and other thinkers had a scientific context in which to consider the development and adaptation of life on planets other than Earth.
However, some of the most influential workers in terms of the scientific thinking about alien life make the tacit assumption that technologically advanced civilizations are a likely, even expected, outcome of evolution. Physicist Enrico Fermi was troubled by the fact that there did not seem to be evidence of alien contact, which formed his paradox.
Astronomer Frank Drake formulated an equation to predict the number of civilizations in the Milky Way able to communicate with us by radio. This equation starts with the number of stars in the galaxy, and then predicts in turn the number of habitable worlds, the number of those with life, the number of those with intelligence, and so on. Here is a version of Drake's Equation:
(NOTE: traditionally, the first value is R*, the rate of star formation per year, and the last value is L, the lifespan of a communicating civilization in years. I have shown the version of Drake's Equation used by Carl Sagan in his book and TV show Cosmos.)
The number of stars in the Galaxy can be fairly well estimated; the rest of the factors, however, are very poorly constrained. Let's look at two alternative scenarios:
What insights can natural historical sciences (geology, paleontology, evolutionary biology, anthropology, etc.) add to these values?
Life needs a place to originate, evolve, and survive. Life is highly entropic (it uses a lot of energy), so it almost certainly needs a world in which renewal of resources is common. This helps narrow things down a bit. For instance, we now expect that worlds towards the Galactic Core are unlikely to survive undamaged for a long enough time to develop and sustain life: too many supernovae and collisions with matter from those crowded parts of space! On the other hand, the worlds towards the Rim seem to be too metal-poor, and thus would typically lack the abundant radioactive materials which help start and drive plate tectonics. So probably only the middle third or so of the star systems are likely to have planets on which life can evolve.
The big problem we face is that we only have one instance of life known: all Terran life is descended from a common ancestor. So we have to be very theoretical for the frequency of worlds in which life evolves. Terran life is all carbon-based, but there is more complexity to our biochemistry than a macroscopic view would suggest! In terms of big things (animals, plants, fungi, protists), we have photosynthesizers that eat light, get carbon from carbon dioxide, and oxidize it with oxygen, and consumers that eat organic material, get carbon from organic material, and oxidize them with oxygen. However, there is a huge diversity of prokaryotes, some of which get their energy from hydrogen, ammonia, nitrogen dioxide, hydrogen sulfide, sulfur, and iron, get their carbon from carbon dioxide, and oxidize with nitrates or sulfates. And in principle there are several other alternative biochemistries not found on Earth that could work in principle. So lots of worlds might harbor life.
On Earth, at least, life seems to have evolved very early. Problematic traces of life go back to 3.8 billion years ago, and definite traces back to 3.4 billion years. So Earth has harbored life for about 75% of its history minimally, possibly much longer. This suggests that life is easy to evolve in conditions favorable to it. Or, to put it another way, life will be likely on worlds that could support the origination of life.
On the other hand, terrestrial life was single-celled for the vast majority of its history (87%). Animals only show up around 600 million years ago, and then only as very simple jellyfish, sponges, and worms. Animals first appear during a time of major environmental changes. It might be that life is easy to evolve, but that animal-grade life is far more difficult.
How often does intelligence evolve? As we have seen, tool-use, complex language ability, social structures, and the other hallmarks of technological human societies do not necessarily all show up in the same species. There's lots of potential out there, but even our close relatives among the hominins seemed to have been poor candidates for communicating societies: their technologies remained unchanged for tens of thousands of years, for instance. So Homo sapiens seems to have been the sole species to evolve on Earth capable of becoming a communicating civilization, and only during the last 50,000 years or so (i.e., the Great Leap Forward).
A common myth of our culture is that we (agricultural, technological societies) represent the ends to which other human societies were evolving. Instead, the evidence suggests that most human societies were relatively stable at the band-to-tribal level, but that it is the development of agriculture that allowed explosive growth of a small subset of societies. These societies have swamped the non-agricultural peoples around the world very rapidly from a geological standpoint. But only these sorts of societies would be capable of supporting the specialists required to develop a scientific, technological civilization.
What is the long-term prospect for such civilizations? (In other words, what are likely values for fL?) In the short term, civilizations face the exhausting resources (energy, food, space, etc.), catastrophic alteration of their enivornment, nuclear (or other apocalyptic) war, etc. So a couple of centuries at communicating-level technologies might be a realistic maximum. On the other hand, should sustainable technologies develop, civilizations that last tens of thousands to millions of years would face the same sorts of perils that all ecosystems due: the potential for massive ecological change and mass extinctions driven by volcanism, asteroid/cometary impacts, etc. On the upper boundary, a super-civilization that lasts hundreds of millions of years or longer will face the end of its planet's tectonics and the death of the ecosphere as its homestar continues to grow hotter (as Earth faces in about 1 billion years time). For civilizations to survive for very long times, therefore, it is best to move on to other (preferrably multiple) worlds!
How does Drake's Equation look now? We'll add a new value, fh, to reflect the fraction of stars that exist in the habitable zone of the Galaxy. Here is a possible scenario:
But that is only for communicating civilizations. We would expect about 2.667 low-technological civilizations/galaxy (assuming hunter-gatherer societies can persist for millions of years: multiply by 10 if they might be sustainable for tens of millions, and by 100 for hundreds of millions of years).
And why this obsession with communicating civilizations? (Okay, that's because they are the only ones that we'd likely know anything about in a reasonable amount of time!) The Earth did perfectly fine without behaviorally modern Homo sapiens for all but that last 0.000011% of its history. It is likely that there might be many worlds in which "slime" (i.e., microbial organisms equivalent to prokaryotes or protists) are the most complex organisms.
Ignoring intelligence, civilizations, and the like, what about the possibility of animal-grade organisms? Introducing fa (the fraction of living worlds in which animal-grade organisms evolve), and fLa (the fraction of the planet's lifespan in which animal-grade organisms are supported), we find: