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When avid science readers browse the shelves for new titles, the books that grab their attention are best described by a single adjective: thought-provoking. And no scientist/author is more provocative in his approach and innovative in his thinking than University of Washington astrobiologist Peter Ward.
Ward's previous books have challenged broadly accepted interpretations and asked questions few others would even conceive of. If his conclusions dashed cherished hopes, such as the likelihood of communicating with other intelligent civilizations in Rare Earth: Why Complex Life is Uncommon in the Universe (co-authored by Donald Brownlee), at least readers had the comfort of knowing that they live on a favored world.
But readers looking for solace will not find it in Ward's latest effort, The Medea Hypothesis: Is Life on Earth Ultimately Self-Destructive? This time Ward goes after motherhood itself--or at least the central idea of the Gaia ("good mother") hypothesis that has evolved to describe the relationship between life and the planet as a whole.
The Gaia hypothesis takes two forms, "Self-regulating Gaia," and the more extreme "Optimizing Gaia." Both argue "that Earth life in the past, present, and surely the future has had and will have the effect of maintaining planetary habitability by affecting the external environment." Optimizing Gaia adds that life "actually improves conditions by changing such factors as planetary atmosphere and oceanic chemistry, the cycling of elements through the biosphere and the availability of nutrients to levels more favorable for life." (Emphasis in original)
Ward recognizes the value of the Gaia hypothesis. It gave birth to Earth system science, which "has proven both vigorous and enormously rich... [and] has attracted some of the best brains in all of science."
Where he takes issue is with the assertion of some Gaia adherents that the hypothesis deserves the scientifically exalted title of theory. He argues that when tested, some aspects of the Gaia hypothesis have come up short.
That is his motivation for this book, which develops a coherent counter-hypothesis. His archetype of the anti-Gaia is Medea, wife of the legendary Jason. In a fit of rage against her husband, she kills all of her children. Simply stated, the Medea Hypothesis claims that instead of enhancing or evolving in concert with Earth for mutual benefit, life is predatory and ultimately self-destructive.
Ward makes a strong case that scientists have accepted the Gaia hypothesis with too little challenge. The Gaian view is cozy, but it is inconsistent with the unforgiving competition of natural selection that has given rise to every species.
In nature, a species reproduces beyond the capacity of its environment to sustain it. The inevitable result is a catastrophic decline in population. We humans are the only species in Earth's history to consider that we may be reproducing beyond our resources. But our population growth continues nonetheless. We are also the only species ever to understand our impact on the environment and the global climate. Yet we continue to test the limits of the system.
One Gaian interpretation of our current situation posits that humanity may destroy itself, but the Earth will recover from our destructiveness. Another is that we will learn our lesson in time to save our species, develop a culture of sustainability, and stabilize the world. Either way, the good Mother Earth will continue along its path of becoming a better place for living things.
But is the Gaian central assumption correct? Does the life change the planet in ways that enhance its own survival, or do those changes accelerate life's ultimate decline? To answer that question, Ward, ever the scientist, examines the evidence.
The first step is to define what it means for a species to be successful. The measure can be its total numbers, its total biomass, its geographic range, and its ability to survive planetary changes, including events that lead to mass extinctions.
Will the ultimate measure of success be the ability to keep Earth inhabitable beyond its natural lifetime? Or will humanity (or our successor species) prove its mettle by moving to other planets before inexorable changes in the Sun or the Earth itself make this planet uninhabitable?
Beyond the success of individual species, Ward asks how best to measure the success of the biosphere. Do we count the number of species or estimate the total biomass through geologic history? Do we judge by the stability of the planet's overall ecology or the ability of life to recover from cosmic or geologic calamities?
The Gaian and Medean views offer very different predictions. Thus those questions provide a means to evaluate the competing hypotheses. The evidence is in the geological record, but the conclusions are far from obvious. Unsurprisingly, Ward's reading of that evidence is distinctly Medean. According to his estimates, Earth's total biomass peaked about a billion years ago, even before the Cambrian explosion of complex plants and animals.
Most Precambrian organisms were single-celled. They were also prokaryotes, which means their cells did not contain smaller bodies called organelles. The emergence of eukaryotes, with their genetic material contained within their cellular nuclei, soon led to the development of complex plants and animals.
Prokaryotes and eukaryotes have different strategies for surviving. In response to environmental challenges, eukaryotes change their body structures and evolve new species. Prokaryotes change their internal chemistry and transform the planet in the process.
No one would argue Ward's point that prokaryotes were responsible for the development of our oxygen-rich atmosphere. But Ward extends that argument. He claims that prokaryotes contributed to and were usually instrumental to the loss of diversity in every mass extinction event. They did so by changing the greenhouse gas content of the atmosphere, thus producing large and rapid excursions in global average temperature.
The net result, according to Ward, is that Earth's diversity peaked early in the age of animals and has been declining ever since. The long-term trend has been the decline of carbon dioxide in the atmosphere, notwithstanding the current upward blip caused by humans' burning of fossil fuels. Much of that decline is attributable to biologically driven geological processes.
Ward expects that trend to lead to the demise of most plant life, followed by oxygen starvation of animals, long before changes in the Sun would doom Earth life forever. In other words, the processes of life itself will ultimately destroy life.
He is also pessimistic about our chances of colonizing other worlds in this or other solar systems. Our best hope is to adopt prokaryotic behavior supplemented by our intelligence. We have to change the natural world, with benevolence aforethought. "Only engineering will save us now," he concludes. "Time to roll up the sleeves, take out the slide rules, encourage the boffins, and get to work. All of us."
If that doesn't provoke thoughtful readers, it's hard to imagine what will.
Physicist Fred Bortz's most recent book for young readers is Astrobiology in Lerner's middle-grade "Cool Science" series.