The new thinking about evolution and how it took placeRoundup
tags: evolution, Darwin, science relevant to history
... Darwin was a pioneering geologist as well as the greatest of all biologists, and his insights were backgrounded by the key concept of uniformitarianism, as advocated by Charles Lyell, his friend and mentor. The previously regnant paradigm among geologists had been “catastrophism”—cue the Noachian flood. The fossil record was therefore seen as reflecting the creation and extinction of new species by an array of dramatic and “unnatural” dei ex machina. Uniformitarianism, by contrast, presumed that the world—organic as well as inorganic—proceeded via processes currently operating. This idea better accounted for the geological record but also required thinking in terms of vast stretches of time and very gradual change. Uniformitarianism eventually won out, providing enough time for evolution by natural selection—the gradual accumulation of fitness-enhancing mutations, accumulating slowly but inexorably to “create” the organic world.
Of late, however, uniformitarianism has been on a losing streak. Catastrophism is back, with a bang . . . or a flood, or a burst of extraterrestrial radiation, or an onslaught of unpleasant, previously submerged chemicals. Thus we learn from the authors that our ancient ancestors had to deal with “fire, ice, hammer blows from space, poison gas, the fangs of predators, pitiless competition, lethal radiation, starvation, enormous changes of habitat, and episodes of war and conquest amid a relentless colonization of every habitable corner of this planet—each an episode that left its mark in the total sum of DNA now extant.” The result is a new understanding indeed, not so much of the generation of organic novelty as of events leading to species extinction, once thought to be a slow and unlikely process and now viewed increasingly as rapid and, on occasion, catastrophic.
The authors make a more-than-plausible case, for example, that our planet endured two distinct episodes of dramatic cooling before the great Cambrian elaboration of life forms, from almost nothing to an extraordinary array of species (an event, incidentally, that Gould brilliantly described in his book “Wonderful Life”). The earth “cooled so much that . . . the oceans began to freeze, starting at high latitudes, but continuing toward lower and lower latitudes until the entire ocean from pole to equator was ice-covered. The first time that singular event caused a great revolution in the history of life by leading to an oxygen-rich atmosphere. The second time, the Proterozoic snowball Earth also produced momentous if very different effects. This time the snowballs led to animals—but not without danger to all life on Earth. Once again life was in the balance.” The key, as the authors convincingly show, is that dramatic changes in the Earth’s chemical atmosphere led to equally dramatic changes in its biosphere, which, in turn, fed back into that atmosphere, as well as—notably—the oceans.
This emphasis on catastrophes is the first of a triad of novelties on which “A New History of Life” is based. The second involves an enhanced role for some common but insufficiently appreciated inorganic molecules, notably carbon dioxide, oxygen and hydrogen sulfide. Messrs. Ward and Kirschvink do a fine job of showing how these humble, inorganic chemicals have profoundly affected life on Earth. The authors write, for example, of “great bubbles of highly poisonous H2S gas rising into the atmosphere,” a “new entry into planetary killing that provides a link from the marine to the terrestrial extinctions.” Later they show how an exceptionally high level of atmospheric oxygen was likely the key factor in permitting the evolution of dragonflies with colossal 30-inch wingspans.
Here and elsewhere, “A New History of Life” deserves kudos for infectious élan, impressive scholarship and a plausible accounting of life’s herky-jerky, hurry-up-and-wait tribulations. Life didn’t so much unfold smoothly over hundreds of millions of years as lurch chaotically in response to diverse crises and opportunities: too much oxygen, too little carbon dioxide, too little oxygen, too much carbon dioxide, too hot, too cold—only rarely, it seems, a Goldilocksian “just right.”
So far, so good, except that in their eagerness to emphasize what is new and different, the authors teeter on the verge of the same trap as Gould: exaggerating the novelty of their own ideas. Unfolding planetary catastrophes and equally dramatic chemical irruptions doubtless deserve the attention urged by Messrs. Ward and Kirschvink. But these merely help describe the environments within which evolution by natural selection has proceeded; they aren’t anything approaching a brand-new mechanism. Evolution itself remains driven by that same doughty process—the differential reproduction of certain genetic constituents over others—that evolutionary biologists have long recognized as fundamental....
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