- Book information
- A New History of Life: The radical new discoveries about the origins and evolution of life on Earth by Peter Ward and Joe Kirschvink
- Published by: Bloomsbury
- Price: $30/£20
Hubristic humans should heed the boom-and-bust vision of Peter Ward and Joe Kirschvink's 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's early atmosphere. As greenhouse gas levels fell, our planet became a great snowball, enduring a mass extinction from which our young world barely recovered.
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's 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 "inventing" photosynthesis and for the "great oxygenation event" 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: "Evolution 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."
The microbe that got oxygen metabolism right – mastering one of the most noxious of gases – 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't 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 "the most sophisticated and efficient set of lungs... the world has ever seen". 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' 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.
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. "Sooner or later," say the authors, "evolution, competition, and the natural changing of our Earth and sun as they age will make any body plan obsolete." No advance is absolute. Evolutionary innovation is only as good as its fitness for a niche.
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.
No comments:
Post a Comment