Is complex life rare in the universe? Peter D. Ward and Donald Brownlee seem to think so. In fact, their book, Rare Earth, was written solely to discredit the idea that life may be a commonplace occurrence around other stars and in other galaxies. Unfortunately, the two make some grievous errors in reasoning while explaining their case. One is circular reasoning, demonstrated here by the propensity for first assuming that Earth was made hospitable for complex life by certain criteria, and then using these same criteria to show how life is likely here and on other planets is not very likely at all. Another error made by the duo is that Earth was an ideal location for life to start and develop. And a third error is the usage of science that is not testable — a hypothesis-type of science that no one can either support or refute based on information available to us today. With all of these problems in thinking, it is difficult to take Rare Earth very seriously at all.
Earth has developed a diverse collection of life, including animals, plants, fungi, and the most abundant organisms on the planet, unicellular bacteria and archaea. Ward and Brownlee do not argue that extremophiles (archaea that have adapted to living in extremely warm (40-200 degrees Celsius) or pretty cool (lower than 0 degrees Celsius) surroundings) could be very common throughout the universe (6, lecture). But they deny that complex, multi–cellular animals would have the chance to develop most places. The two authors base this belief on the fact that Earth appears to be located perfectly and contains many properties that seem to be required for the existence of advanced life.
There are many criteria that Ward and Brownlee state are necessary for complex life that appear to be uncommon on their own. Planets that are located too close to or too far away from the center of the universe will not develop complex life; the former will lack it because of a lack of heavy metals (they need more star development in the area to produce heavy metals), the latter because there are too many stellar objects producing too much radiation for organisms to withstand (Ward et al, 28). Plate tectonics are necessary for life because the process helps regulate temperature, produces land masses, and helps with diversification of creatures by separating them when continents split (Ward et al, 194). A large moon at the correct distance is essential because it keeps the planet’s tilt stabilized, ensuring that seasons are not too severe (Ward et al, 224).
And all of these ideas could be necessary for life. What are Ward and Brownlee making their judgments on the possibility of all life based on? One sample. Just one sample! The judgments on all possibility of life in the universe is being based, rashly, on one example of how life could develop and diversify. Just because it happened here does not mean that it is an essential element in making complex organisms. For example, if you had only had one cookie in your life, and it was an odd (yet very good and satisfactory) cookie, like pineapple-mint with a rice-based dough, you might assume that, in order to be a good, satisfactory cookie, it must contain pineapple, mint, and rice. Now, those of us who have experienced more than one cookie could tell you those criteria aren’t necessary for a good cookie. This is something that Ward and Brownlee don’t really consider; they assume that complex life has to proceed in a set way and with quite a few rare requirements. Perhaps life can develop in ways other than how it did here, and it’s irresponsible to take one example and assume that all instances must be exactly alike and draw conclusions that could cause a loss of funds for extraterrestrial life research.
The second assumption that Ward and Brownlee make is that Earth is “just right” for complex life. Not only are all these criteria needed, but they’re all present and working well for Earth. Earth has a self-regulating thermostat based on the weathering of rocks (Ward et al, 210). Earth is a very large planet that is functioning in a beneficial manner by “purging the inner solar system of bodies left over from planet formation” and prevents most large objects from impacting Earth by catching them first (Ward et al, 238, 240). We have a magnetic field that is capable of protecting the Earth’s inhabitants from cosmic rays (Ward et al, 213). Earth has liquid water, a proper balance of metals, and a quick development of the one-celled archaea. It seems that everything is coming up roses for the development of complex life here.
But is it really the perfect area for such development? Earth is in a good location in the Solar System, but not even that kept the planet from repeated bombardments during its early history. It is even thought that these impacts sterilized the surface of the Earth, vaporized the oceans, and the only life that could have survived would have been those extremophiles deep within the crust. Even after the period of heavy bombardment stopped, several instances of large objects hitting the Earth caused mass extinctions. Now, it is possible that the later impacts have encouraged diversification, but the early events could have done nothing but retard growth. Let us assume that these mass extinctions that occur on the planet have all caused greater diversity among living things on Earth. Then why is the Moon so necessary? Part of the Moon’s job, in this scenario, is to keep Earth’s tilt stable. Apparently it will move if not regulated by such a body, causing problems with the Earth’s climate as it slowly shifts. Instead of causing no animals to develop, is it not possible that animals would adapt or die out when faced with changing climate? I think that is just as likely as it being a permanently prohibitive property of a planet. Perhaps Earth has a lot of unnecessary properties, and maybe it also has properties that were seriously detrimental to the development of complex life.
The third major problem with Rare Earth is that Ward and Brownlee have given us a hypothesis that humans have no hope of proving or disproving in the foreseeable future. We have been given the ideas of what two men, who are basing their ideas on a sampling of one, believe may or may not be responsible for the origin of life on Earth. Ward and Brownlee suggest that the “[o]bservation of ozone, CO2, and H2O in the atmosphere of an unknown planet strongly suggests habitable conditions and the presence of life” and that “the most peculiar aspect of the atmosphere is the abundance of free oxygen . . . it is a highly reactive gas that would exist only at trace levels in the atmosphere of a terrestrial planet devoid of life” (247, 245). This is based on the assumption that other planets would develop to exploit the same types of chemicals that we have. Maybe, on a planet that has larger reserves of lighter materials, hydrogen would be exploited as the main energy source — “[p]ound for pound, hydrogen packs more chemical energy than any other known fuel” (Lemley, 55). Or perhaps some other element or compound would be exploited. The fact that both men would automatically assume that other organisms would have the same setup as organisms on this planet is terribly biased.
The other option for testing the hypothesis is the search for intelligent extraterrestrial life, which, I agree, is not likely to succeed. It involves looking for radio waves that are emitted from other intelligent species, and it is not likely to work well even if it does succeed. If another civilization is located so far away that responding to them would take thousands, if not millions, of years, it is not very practical to carry the search out. But maybe it would be comforting to know that we aren’t alone here, even if we can never meet our counterparts out in space.
The book did have some interesting ideas, such as the concept that Jupiter has helped protect Earth from impacts with large objects. However, the hypothesis should not have been put forth since there is not a lot of information to base the conclusion on. We only have knowledge of a few planets outside our solar system, and they are all gas giants that are thought to be incapable of supporting life. Science simply does not have enough information on how and why life develops to answer a question of this magnitude. As Lawrence Krauss stated after reading this book, “we are probably not yet well equipped, in our knowledge of the relevant variables, to perform a proper maximum-likelihood analysis of life in the universe.” Nor should we fool ourselves into thinking otherwise.