Rare Earth by Peter D. Ward and Donald Brownlee, is an examination of the many factors involved in the evolution of intelligent life on earth. The authors, a Professor of Geological Sciences and a Professor of Astronomy, tell why certain things unique to the earth were necessary for humans to evolve, such as our sun, oceans, tectonic stress, the moon, Jupiter, etc. Without the contribution of any one of those things, we just wouldn't have happened.
For instance:
- Without tectonic stress, the continents wouldn't have formed; the only land would have been mountaintop islands like Hawaii. So what? Well, water-covered planets, we are told by the authors, tend to develop atmospheres heavy in carbon dioxide. That makes for a strong greenhouse effect and resulting high temperatures that would have prevented life as we know it from evolving. What do continents have to do with lowering carbon dioxide? Well, continents are surrounded by shallow water, and shallow water is necessary for the formation of carbonate compounds such as limestone, which is the main way CO2 was reduced from the atmosphere of the young earth. Not to mention that a lot of land area is necessary for lush forests, and forests are another way to reduce CO2 levels. Finally, the waters of an all-ocean world would have a tendency to become acidic to the point of discouraging life as we know it from developing.
- What did the Moon have to do with evolution? Well, it seems that the moon is a great stabilizing influence on the earth's tilt, the tilt that causes the seasons. Without the moon, the tilt would be much greater and therefore temperature extremes would be so great as to have a huge impact on the development of complex life.
- Jupiter? Jupiter's main function, from our evolutionary point of view, is related to its size and gravity. It acted as a shield to guard us from comets incoming from the Oort belt.
- What about the sun? The sun is probably the most important of all, from its having a stable habitable zone at the right distance from it to its being rich in the heavier elements.
From their examinations of factors such as these, Ward and Brownlee arrive at the “Rare Earth Hypothesis” which says that while microbial life is probably common throughout the universe, animal life is rare. The unique factors that make up the Earth-sun-moon system are rare in the universe, so therefore complex life must be rare. Microbial life can evolve and survive under much more extreme conditions than can animal life, so it is probably common.
I've never felt comfortable with Carl Sagan's solution of the Drake equation - that there are perhaps a million planets with intelligent life in our galaxy alone. I've always asked myself, echoing the Fermi Paradox, “If there are that many, where are they? Why haven't they landed on the White House lawn?”
In fact, Ward & Brownlee give us an updated version of the Drake equation that includes their additional factors:
N* x fp x fpm x ne x ng x fi x fc x fl x fm x fj x fme
Where:
- N* = number of stars in the Milky Way Galaxy
- fp = fraction of those stars with planets
- fpm = fraction of those planets that are metal-rich
- ne = fraction of those planets that are within their stars' habitable zone.
- ng = stars in a galactic habitable zone
- fi = fraction of habitable planets where life does arise
- fc = fraction of those planets where animal life (complex metazoans) evolves
- fl = percentage of the lifetime of a planet during which complex metazoans (animals) are present.
- fm = fraction of planets with a large moon
- fj = fraction of solar systems with Jupiter-sized planets.
- fme = fraction of planets with a critically low number of mass extinction rates.
Now I personally have no problem with Ward & Brownlee's conclusion, per se. I think it is probably correct that planets with intelligent life are rare in the universe. I do, however, strongly disagree with an extreme interpretation of that conclusion as saying that “rare” means “only once”. Rare is a relative term. On the scale of the universe, a million might be rare. People who say intelligent life has happened in the universe "only once" may someday be besieged by black swans.
I think that when we look back at our own evolution on the Earth, we must avoid a tendency to think that the evolution of intelligent life couldn't have happened unless it happened under those same conditions. Those conditions may have been vital for us to evolve exactly as we did, but to generalize them into being necessary conditions for the evolution of intelligent life at all is, in my opinion, an error.
Instead, we have to ask:
- Are stars with planetary systems really so rare as Ward & Brownlee think? They title one section of their book: Are Stars With Planets Anomalous? At the time Rare Earth was written, it seemed so to many people, but thinking on the subject has changed a lot since the book was published in 2001. Many more planets around distant stars have been discovered, and as our ability to detect them gets better, we are finding that they aren't all gas giants as were the first ones discovered.
- Is a large moon the only way a planet can have a stable climate, or are there other sets of circumstances that might stabilize a planet's rotation enough to keep its climatic variation within an acceptable range?
- Although planets that are gas giants don't seem to be rare, is it really necessary to have a Jupiter-sized planet in position to protect the inner planets from cometary collisions? Is it possible that the Oort belt is the real rarity here? Perhaps many planetary systems are relatively free from all this orbital debris. Since cometary and asteroid collisions are thought to be the major cause of Earth's past extinctions, then this also relates directly to the last factor in the “Rare Earth” version of the Drake equation. Given fewer cometary impacts, there are fewer extinctions.
I heartily recommend the book "Rare Earth". Read it with these things in mind
This article was previously published in 2001. It has been edited slightly.
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Print References:
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• Rare Earth by Peter D. Ward & Donald Brownlee
• Cosmos by Carl Sagan
• Intelligent Life in the Universe by I.S. Shklovskii & Carl Sagan
• The Science of Aliens by Clifford Pickover
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