Rare Earth: Why Complex Life is Uncommon in the Universe. By Peter D. Ward and Donald Brownlee. Copernicus, New York, 1999.

Review by Mark Wolverton

Skeptical Inquirer, November/December 2000




It's all the fault of Copernicus, Galileo and Darwin. Once they proved that we humans aren't the center of the cosmos, they made it possible for the idea of extraterrestrial intelligence to become an article of pop-cultural faith. The argument usually boils down to a probability game: the basic elements of life are common in the universe and easily form organic molecules, and with so many stars and so many galaxies, the odds in favor of ET life seem overwhelming. And not just life: intelligent life. It couldn't be just us in all that vastness, could it? Of course not. The idea is beneath consideration.

But geologist Peter Ward and astronomer Donald Brownlee do consider it in their book Rare Earth, raising a host of troubling questions that even the most zealous ETI proponent will find difficult to ignore. After forty years of intermittent SETI searches with nary a peep from the cosmos, coupled with paradigm-shattering discoveries in evolutionary biology and geology, the scientific consensus is changing. It is a game of chance--but the odds for ETI are worse than we think.

It's not an easy idea to accept. More examples of "extremophiles," organisms existing under the most seemingly hostile conditions, turn up in the journals every year. If life can exist and thrive in frozen Antarctic lakes, deep-ocean volcanic vents, even inside nuclear reactors, why can't it exist on other planets?

The reason, as Brownlee and Ward explain, is that there's a huge difference between simple life and complex life. The gulf between the simplest self-replicating organic molecules that could be called alive, and the immense complexity of metazoans (i.e., multicellular animals) is vast. It's this gulf that forms the basis of the "Rare Earth Hypothesis": "the paradox that life may be nearly everywhere but complex life almost nowhere." By tracing the best (and so far, only) example we have--the evolution of life on Earth--Brownlee and Ward show that the path from simplicity to complexity is not a straight line, not an inevitable journey, and fraught with so many dangers and dead ends that it's a marvel that human beings exist to wonder at the process.

Rare Earth lays out a series of factors which appear to be crucial to the development of complex life. Some have been known for years, such as the necessity of a planet being the right distance from its sun (like Goldilocks's soup, not too hot and not too cold), a stable planetary environment, and the absence of any threatening nearby astronomical phenomena to scald the planet with radiation. Others, however, have only been recently recognized. The spectacular 1994 impact of Comet Shoemaker-Levy-9 with Jupiter made abundantly clear the role that Jupiter plays in protecting us from cosmic catastrophe: the planet acts as an gravitational vacuum cleaner, sucking up incoming debris and preventing it from reaching the inner Solar System. Without Jupiter, life on Earth might have been obliterated long before ever gaining a secure foothold.

Some factors are more local in scope. The presence of a large moon helps to stabilize the Earth's axial tilt and slow its rotation, keeping climatic variations in check. Geology is also a crucial consideration. One of Rare Earth's most fascinating ideas is the importance that plate tectonics might have played in the evolution of animals. By creating mountains, deserts, lakes, and all the other myriad varieties of microenvironments on Earth, plate tectonics encouraged the process of speciation--the development of different types of organisms capable of surviving in a greater range of conditions. The advantage? More chance of at least some form of complex life surviving later planetary catastrophes. It's already happened: the dinosaurs weren't able to survive the conditions after the impact event that occurred sixty million years ago--but if the small mammalian creatures arising at that time hadn't been able to adapt, we wouldn't be here today. The heat and movement of the planetary core that drive plate tectonics also create Earth's magnetic field, shielding its surface from lethal radiation.

What are the chances of all the necessary factors coming together to allow the emergence of complex organisms? Not good, unfortunately. As far as we know, it's only happened once. And planetary catastrophes capable of wiping out life happen fairly frequently (in a geological sense at least). Brownlee and Ward describe ten separate mass extinction events that have been recorded on Earth, and many more may have occurred. Each time, some remnant of life has been able to hang on and keep the story going, but there are no guarantees. The continued existence and evolution of complex life is a fine and delicate daisy chain, capable of being broken at any point, and the longer it goes on, the greater the odds against it become.

Frankly, for those of us who have dreamed of great interstellar civilizations and encounters with other intelligences, it's all pretty depressing. But Rare Earth isn't trying to maliciously spoil our party. It's just trying to bring our expectations more in line with what increasingly seems to be the way things are. Life: yes. In fact, Brownlee and Ward argue that simple life may be even more common in the universe than is currently believed. But a galaxy brimming with intelligent life: probably not. They may be out there, somewhere; if not in this galaxy, perhaps others. If Brownlee and Ward are right, however, we will probably never be able to meet them or communicate with them.

Yet there may be a sort of inadvertent benefit. Humanity may be more special than it seems. As Brownlee and Ward put it: "We are not the center of the universe, and we never will be. But we are not so ordinary as Western science has made us out to be for two millennia. Our global inferiority complex may be unwarranted."

I still hope they're out there somewhere, and I still run "SETI@home" on my computer, and I still get excited whenever there's talk of life on Mars or Titan. But if we really are alone, maybe it won't be so bad if that realization helps us to truly value and cherish our own precious existence in the Universe.

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(C) 2000 Mark Wolverton. All rights reserved.

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