In Rare Earth, Ward and Brownlee make a detailed and fascinating case that life may be very common in the universe, but complex or animal or intelligent life may be very uncommon. Given naturalistic assumptions about life’s origin and evolutionary progression on Earth, they explore numerous difficulties for both attaining and maintaining life over millions or billions of years, and the likely uncommon attributes of our planet that have made this possible here.
“As early as 3.8 billion years ago… life seems to have appeared simultaneously with the cessation of the heavy bombardment.” (p.61) On progress of abiogenesis theories: “No one has yet discovered how to combine various chemicals in a test tube and arrive at a DNA molecule.” Furthermore, “with an oxygen-free atmosphere the amount of ultraviolet radiation reaching Earth’s surface would have been far higher… making delicate chemical reactions on the planet’s surface very difficult.” (p.62) The recent discoveries of “extremophiles” has suggested hydrothermal vents as a possible origin of life, and also enhanced the hopes of finding creatures in extreme conditions elsewhere in the universe. (Although hydrothermal vents are connected to plate tectonics and Earth’s global temperature system, which require a number of parameters to even exist, much less be maintained for billions of years – see below).
However, while single-celled creatures have long been common on Earth, more advanced creatures took longer to arise and may require specific characteristics for their arrival. Additionally, they have a narrower range of habitability; “Complex metazoans tolerate a far narrow range of environmental conditions than do microbes,” (ex. 0-50 degrees C compared to 100+ C) and are “far more susceptible to extinction caused by short-term environmental deterioration.”
External Threats to Maintaining Life
A planet’s star must have a fairly constant energy output, but even the best-case scenario involves a gradually increasing brightness that the planet must compensate. “On Earth, the maintenance of a relatively constant temperature has been attained through a gradual reduction in greenhouse gases as the amount of energy from the sun has increased, thus keeping temperatures in check.” (p.164)
Threats to maintaining life include asteroids, supernovae, and gamma ray bursts.
Some of these factors limit a habitable time window in the history of the universe; some dangers would have been more common in the past. Stars also would not have produced heavier elements in the first generations. On the other hand, radioactive elements, important for regulating temperature, “are produced by supernovae explosions, and their rate of formation is declining with time.” Newer stars “have less of these radioisotopes” than our sun. “It is entirely possible that any true Earth clones now forming around other stars would not have enough radioactive heat to drive plate tectonics.” (p.30)
The book discusses the importance of Jupiter. It’s large enough and close enough to limit the amount of deadly asteroids hitting Earth, but not so large or so close, or too elliptical in orbit, or having too many more similar large objects like itself and Saturn, to threaten the stable orbit of Earth itself.
Additionally, the relatively large, close moon plays an important role in stabilizing Earth’s axis tilt over long periods of time. The moon’s formation appears to have required a very precise collision with a large object at just the right time in Earth’s history for the collision to have distributed the right elements into the right places. If its formation had left it rotating in the opposite direction, its gravitational tidal effects (also important) would have slowly spun it into the Earth, instead of slowly spinning away, which also means there is a time limit to both its tidal and stabilizing properties.
Internal Requirements to Maintaining and Progressing Life
The inter-connected role of plate tectonics, water, and carbon dioxide seems crucial to maintaining complex life on planet Earth.
“For complex life to be attained (and then maintained), a planet’s water supply (1) must be large enough to sustain a sizable ocean… (2) must have migrated to the surface from the planet’s interior, (3) must not be lost to space, and (4) must exist largely in liquid form. Plate tectonics plays a role in all four of these criteria” (p.208)
There is some mystery to the source of Earth’s water, given planetary formation theories, but somehow “the volume of water was sufficiently large to buffer global temperatures, but small enough so that shallow seas could be formed by the uplifting of continents,” which are “necessary for limestone formation” and “continental weathering.” (p.264) “The violent events” of early Earth “may have determined the final abundance of water and carbon dioxide… If Earth had had just a little more water, continents would not extend above sea level. Had there been more CO2, Earth would probably have remained too hot to host life.” (p.51)
Some planets in our solar system have volcanoes but Earth is the only known with “linear mountain ranges,” caused by plate tectonics. This cycle involves subduction zones in the ocean. Due to comparative density, “continents cannot be destroyed (though they can be eroded)… Since the formation of our planet, the total area of oceanic plates has gradually diminished as the area of continental plates has grown” (p.201) Volume of continents is still increasing, but if it had been higher earlier in Earth’s history, its affects on atmospheric climate would have been more hostile to life.
“The average temperature of the Moon is -18 degree C.. because it has no appreciable atmosphere.” Without ours, Earth’s temperature “would be about the same as that of the Moon,” below the freezing point of water. (p.207) Plate tectonics maintain the “tiny fraction” of important greenhouse gases, acting as a “global thermostat” through CO2 cycles with volcanoes / weathering / limestone, that require shallow surface water and other factors to work. (p.208)
Some Curious Details Regarding Evolutionary History
Prokaryotes to eukaryotes: “It appears that attaining the eukaryotic grade was the single most important step in the evolutionary process that culminated in animals on planet Earth.” (p.88) “Eukaryotes… have repeatedly evolved multicellular forms.” (p.89) “Some species of bacteria… seem indistinguishable from fossil forms… 3 billion years old… The majority of eukaryotic species… seem to persists for… 5 million years or less.” (p.89)
“The jump from single-celled.. to organisms of multiple cells requires numerous evolutionary steps.” A “brave (or lucky) morphological change,” an organism “shed its external cell wall,” its protective “tough outer coating,” so “individual cells could begin exchanging material”. (this apparently initially harmful event happened multiple times?) (p.101)
The book discusses evidence of iron-banded formations and the “oxygen revolution”. This is one example of many in the book where there is evidence that changes happened over a long periods of time, but unknown or unconvincing explanations as to how or why these changes occurred naturally or by chance.
Cambrian Explosion: Genetics shows diversification “must” have taken place before the Cambrian explosion, but paleontologists are “stymied by an almost complete lack of fossils” (p.103) “It is clear that the evolution of animals occurred not as a gradual process but as a series of long periods of little change, punctuated by great advances.” Of the Cambrian Explosion: “In this single, approximately 40-million-year interval, all major animal phyla (all of the basic body plans found on our planet) appeared… Although the number of species… has been increasing through time, the number of higher taxa, such as phyla, has been decreasing.” (p.140-142)
Inertial interchange event: “Much of this continental drift happened during the Cambrian evolutionary explosion… no more than 10 to 15 million years. The continental shifts were quite dramatic.” (p.145) Seems to be connected to periods of Snowball Earth followed by lush green: “Both of the two great episodes of Snowball Earth nearly ended life on Earth, as we know it. But each, ultimately, may have been crucial in stimulating the great biological breakthroughs necessary for animal life: the evolution of the eukaryotic cell and the diversification of animal phyla.” (p.121) Not just oxygen but continent formations and phosphorous levels also correlate with rise of large/complex “animals.”