杏吧原创

Shark Bay, Australia: cyanobacteria help form modern stromatolites (Image: Frans Lanting/National Geographic Creative)

Hubristic humans should heed the boom-and-bust vision of Peter Ward and Joe Kirschvink鈥檚 book, a grand synthesis of all that is known about evolution

THE most brilliant innovation in the history of our planet was also the most catastrophic. Some 2.4 billion years ago, a microbe evolved to extract energy from sunlight. That chemical reaction exploited the abundance of carbon dioxide in Earth鈥檚 early atmosphere. As greenhouse gas levels fell, our planet became a great snowball, enduring a mass extinction from which our young world barely recovered.

This cycle of boom and bust is typical of life on Earth. In fact, mass extinction is one of life鈥檚 only constants, as Peter Ward and Joe Kirschvink show in A New History of Life. The other constant, counteracting life鈥檚 destructive exuberance, is that 鈥渢imes of crisis promote new innovation鈥�. Each of us is 鈥渢he descendant of survivors tempered by catastrophe and quenched by time鈥�, they say.

聯Mass extinction is one of life鈥檚 only constants, the other is that times of crisis promote new innovation聰

The two are well qualified to make such generalisations, and to take up the broader challenge of synthesising all that has happened since the first nucleic acids began replicating 4 billion years ago. Their interests and expertise are complementary. Ward is a palaeobiologist at the University of Washington, whose research and writing focuses on mass extinction; Kirschvink is the geobiologist at the California Institute of Technology responsible for the Snowball Earth hypothesis. Moreover, a new history of life is much needed since the last grand synthesis, by palaeontologist Richard Fortey, was published in 1998.

As an update, Ward and Kirschvink鈥檚 book is stimulating. Unfortunately, it is blighted by poor organisation and sloppy writing. Some of the same basic science is explained two or three times in different sections, and the prose seems rushed, making for a less-than-pleasant read. That said, A New History of Life is fascinating when the authors write about subjects close to their hearts, such as Snowball Earth.

The oxygen habit

That deep chill lasted over 100 million years, freezing the oceans and killing most organisms. One survivor was the cyanobacteria responsible for 鈥渋nventing鈥� photosynthesis and for the 鈥済reat oxygenation event鈥� that squandered atmospheric CO₂.

While the planet was frozen, the microbes bided their time in hot springs. As volcanism slowly replenished CO₂ and the greenhouse effect thawed out the great snowball, cyanobacteria flourished again, guzzling the precious CO₂ and snowballing the world for a second time.

What extracted Earth from this vicious cycle, explain the authors, was the evolutionary enticement of oxygen: 鈥淓volution would have immensely favored any organism that could use oxygen, since no other molecule lets the chemical reactions we call life take place faster, with more precision, and liberate as much energy as those where oxygen is used.鈥�

The microbe that got oxygen metabolism right 鈥� mastering one of the most noxious of gases 鈥� became the basis of all animal life, subsisting inside every cell as mitochondria. By a similar process, cyanobacteria evolved into the photosynthetic chloroplasts inside plants. Evolution channelled crisis into opportunity, ultimately allowing for the multicellular complexity of forests and people. Snowball Earth episodes offer a perfect example of how perversely creative mass extinction can be.

Yet once we were on oxygen, there was no kicking the habit. Nearly 2 billion years after the great oxygenation event, a precipitous drop in O₂ caused the greatest extinction of all time. Colloquially known as the Great Dying, the Permian extinction eliminated 90 per cent of species on Earth. Purple sulphur bacteria suffocated the oceans, burping up toxic hydrogen sulphide gas. Atmospheric oxygen plummeted by two-thirds. Simultaneously surging greenhouse gas levels turned the climate tropical. As Ward and Kirschvink note, this rising heat increased the oxygen demands of animals at the worst possible moment. Many of the terrestrial species that didn鈥檛 die took to the cooler seas. Largely abandoned, the land was wide open for any organism that could adapt. Dinosaurs made the leap.

The first crucial change was to go bipedal, allowing the reptiles to breathe while they walked. More impressively, write Ward and Kirschvink, dinosaurs developed 鈥渢he most sophisticated and efficient set of lungs鈥� the world has ever seen鈥�. Summarising research Ward first published in his 2006 book Out of Thin Air, the authors make a compelling case that these formidable lungs were decisive in the dinosaurs鈥� ascent, at the expense of our mammalian ancestors. With a richer supply of O₂ in the low-oxygen Triassic, the reptiles simply had more energy, positioning them to dominate the Jurassic and Cretaceous.

聯Dinosaurs developed the most sophisticated and efficient set of lungs the world has ever seen聰

But the dinosaurs could not survive the aftermath of a major asteroid impact. Darkened skies killed the vegetation that fed their massive bodies, leaving the world to scrappy mammals. 鈥淪ooner or later,鈥� say the authors, 鈥渆volution, competition, and the natural changing of our Earth and sun as they age will make any body plan obsolete.鈥� No advance is absolute. Evolutionary innovation is only as good as its fitness for a niche.

There is much to learn from Ward and Kirschvink鈥檚 dinosaurs and cyanobacteria. As they note, 鈥渟cientific information from so-called deep time is the single most ignored aspect of the climate change debate鈥�. Earlier mass extinctions show us how radically the planet is altered by the balance of greenhouse gases and how sensitive life is to those changes. Yet the authors seem oblivious to deep time鈥檚 deepest lesson when they claim humans are 鈥減retty extinction-proof鈥�, that somehow we are exempted from obsolescence by our superior intelligence.

The strength of most species is also their weakness. Ours, clearly, is hubris.

This article appeared in print under the headline 鈥淭he deepest danger鈥�