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Evolution of Evolution — Text-only | Flash Special Report
Life, Climate and the Disguise of Change


Darwin’s ideas apply to evolution of climate too

By Gerilyn Soreghan

The concept of recent glaciation over parts of the planet now ice free so pervades common knowledge as to inform pop culture and art—witness the animated “Ice Age” movies, for example. But this took time. Although arguments on the concept of a recent “ice age” date to the 1830s, and Louis Agassiz’s Etude sur Glaciers appeared in 1840, the idea of Pleistocene glaciation, or widespread ice as recently as 10-20,000 years ago, was not widely accepted until the late 19th century. Agassiz’s insights proved serendipitous to Charles Darwin, who drew upon them to explain the geographical ranges of species. They revealed to him the possibility that changes in climate could have been profound, and the impact on life similarly profound.

In turn, Darwin’s acceptance of the possibility of large changes in Earth’s climate system was prescient. Now, a century and a half after the landmark publications of both Agassiz and Darwin, we continue to grapple with the concept of major, even otherworldly, climate extremes.

Murmurings of the possibility of global ice in Earth’s past, for example, began surfacing as early as the mid 20th century. Predating the paradigm shift of plate tectonics, these ideas were truly radical, ultimately leading to the hotly contested hypothesis of the so-called “snowball” earth—episodes 600-800 million years ago when Earth’s surface may have been nearly consumed by ice. The impacts on the biosphere would have been severe, perhaps even presaging the so-called Cambrian Explosion of life—a sudden radiation of complex multicellular life approximately 540 million years ago, as evidenced by their seemingly rapid appearance in the fossil record.

Oddly, initial suggestions of another great glaciation appeared in scientific literature about the same time as publication of “On the Origin of Species,” in 1859. Although absolute dates as well as the extent of this glaciation initially remained unknown, the existence of this “Late Paleozoic Ice Age” is now indisputable, with evidence of ice sheets throughout the “Gondwanan,” or southern polar continents.

Ice confined to relatively high latitudes is an acceptable concept, owing to its analogy with the behavior of the recent planet. Ice in the low-elevation tropics, however, presents a dilemma. Allowances could perhaps be granted for the “snowball” time, a truly dark age of Earth’s history. The late Paleozoic, however, archives a more familiar interval a mere 300 million years distant; a planet with a fully developed terrestrial biosphere, a favorite of museum dioramas populated with ferns, über-insects and mammal-like reptiles. Could it, too, have hosted tropical ice?

Still controversial, emerging evidence has come in the same form as that illustrated by Agassiz so long ago: an ancient landscape, hypothetically carved by ice. In this case, however, within a place formerly situated squarely upon the equator.

Darwin’s admonition in “On the Origin of Species” that “there is reason to believe that in the course of time the effects [of changed conditions] have been greater than can be proved by clear evidence” applies to climate as to life. Ice, like life, rarely leaves a clear record. The fossil evidence is elusive; the soft parts vanish, gaps fill the archives. Bits and pieces of fossil landscapes, scattered, ambiguous debris, or mysterious shifts of sea level form the auxiliary clues. We’ve learned the lessons of change many times in many forms. Yet even now we struggle to imagine the vastly different types of climate that could have prevailed in Earth’s past. Darwin’s ability to imagine the unimaginable and to conceive of life as having evolved so fundamentally and profoundly over the course of geologic time continues to inspire us to imagine how climate could have similarly evolved so profoundly.

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Gerilyn (Lynn) Soreghan is Brandt Professorin the Conoco Phillips School of Geology and Geophysics at the University of Oklahoma in Norman. She has published widely on climatic and tectonic issues of the Late Paleozoic, based primarily on research in the western United States. The National Science Foundation supports her research on the paleoclimate of western equatorial Pangaea.

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Please see the Resources section for the Bibliography/Additional Reading list for this essay.