ow hardy is life on Earth? Imagine a globe cased in ice: A cap a kilometer thick over land and sea, frozen solid for ten million years. The most recent Ice Age, during which Cro-Magnon’s teeth chattered and great hunks of North America and Europe were covered by glaciers, was a tropical honeymoon in comparison.

Now imagine life beneath all that ice. Not a lot of life, mind you — almost everything with a pulse is turned into a Popsicle. But in a few hidden niches, those hot springs under the ocean, say, the hardiest specimens — bacteria, archaea — survive. And, in the long run, life prospers. For when things eventually thaw, they do so in such a way that they accelerate the process of evolution as it has not been accelerated before or since.

Such a scenario, said Paul Hoffman, a professor of geology at Harvard University, is not at all far-fetched — nor is the idea new. Rather, he said, he wanted to share in his lecture “a variety of new evidence supporting an old theory.”

In 1964, Hoffman told us, British geologist Brian Harland found glacial deposits present in the ancient rock strata of every continent, even near the Equator and at sea level — evidence, Harland claimed, of the advance of great ice sheets over much of the Earth some 600 million years ago. “Harland proposed a series of extreme Ice Ages, and suggested that the amelioration of climate following these Ice Ages might have had something to do with the great burst in biological evolution that became known as the Cambrian explosion.”

Doubts were voiced. With continental drift, Harland admitted, he couldn’t be sure where the land masses had been when glaciers covered them. But the real problem was that he had no good explanation for how an ice-covered Earth could have happened. How could it get so cold? “In the absence of a theory,” Hoffman said, “no one believed him.”

ronically, Hoffman added, there was a contemporary theory that fit Harland’s evidence. A physicist named Mikhail Budyko, at the Leningrad Geophysical Observatory, had worked through a series of calculations based on the global energy balance: the fundamental principle that the heat Earth absorbs must always equal what it gives off. “This balance includes the planetary albedo, the energy reflected back to space,” the amount of which is determined largely by surface cover. Dark cover, such as trees and other vegetation, absorbs energy, while a light-colored surface — snow and ice — reflects it away.

Budyko was most interested in something called the ice-albedo feedback. (Maybe it was those long winters in Leningrad.) The ice-albedo feedback, Hoffman explained, says that for any drop in global temperature, you get an increase in surface snow and ice, which means that in turn more heat is reflected away, insuring that things will get still cooler.

What Budyko determined was what Hoffman called “an underlying instability” in the ice-albedo feedback. In short, if temperatures ever went low enough to allow that ice cover to creep to within 30 degrees of the Equator — Houston, Texas, say — “the feedback would be so strong you’d get a runaway effect. It would be unstoppable. The Earth would quickly freeze over.”

Budyko didn’t think a snowball Earth had ever actually happened, Hoffman said. If it had, he thought, life would have been completely wiped out. Then too, Budyko thought a snowball Earth, once in place, would be permanent: What could generate the enormous heat it would take to undo such a hammerlock? (In 1992, Penn State geoscientists Jim Kasting and Ken Caldeira estimated that such a reversal would require raising atmospheric CO2 to 350 times its present level.)

Since Budyko’s day, however, “a couple of things have happened,” Hoffman noted. One is the discovery of living organisms in those deep-sea vents, creatures not dependent on sunlight. “We’re not certain that these organisms could have survived — ocean chemistry would change in a snowball Earth — but it raises the possibility.” A parallel discovery, he added, was of frozen lakes in places like Victoria Land, East Antarctica, where despite mean annual temperatures in the range of –20 degrees C (–4 degrees F), “things never completely freeze. And the water under the ice is teeming with life.

he other thing Budyko didn’t know about,” Hoffman said, “was plate tectonics. Plate tectonics drives the carbon cycle, which allows Earth to be a habitable planet.”

Earth’s crust is made up of a dozen great plates, like ill-fitting puzzle pieces, that float atop the hot molten rock below. The bumping and grinding of these plates shapes Earth’s geography, raising mountains, occasioning earthquakes, breaching and redistributing continents. Pressures that build up at the heated core beneath all this activity are released via volcanoes, which belch out CO2.

In the normal course of events, Hoffman related, “Rainwater washes this CO2 out of the atmosphere as dilute carbonic acid, which falls on silicate rocks. This weathering produces alkalinity, which is washed by rivers into oceans and winds up as carbonate sediment on the sea floor.” This limestone deposit is drawn by churning and settling down to the core, where it is reheated to liquid and gas, and eventually spewed back up volcanically into the atmosphere, renewing the cycle.

A snowball Earth, however, would screw up the carbon cycle something awful. “The oceans are frozen. The air is very dry. There is no source of atmospheric moisture, no way to scrub CO2.” Meanwhile, “plate tectonics is continuing. CO2 is being emitted, but there’s no way of getting rid of it. CO2 builds up and up, drives temperatures higher and higher — the escape mechanism is inevitable. And boy, what an escape.” After about four million years, things warm to the point that dark ponds of open water appear at the equator. This sudden switch in albedo at low latitudes then kicks off wholesale melting, and from there, “Deglaciation is extremely violent. The ice will disappear in a few hundred years — much faster than you can get rid of the excess CO2.”

That thick blanket of gas means an extreme greenhouse period: “Surface temperatures at the tropics over 40 degrees C (104 degrees F), super-hurricanes, torrents of carbonic-acid rain.” And — with no ice and the maximum surface area of rock exposed — powerful carbonate weathering. This combination eventually resets the atmospheric chemistry to pre-Snowball levels.

A “freeze-fry” scenario, Hoffman called the whole process. And it fits nicely, he added, with the existing rock record. “Glacial deposits world wide are capped by carbonate sediments. This has long been a puzzle — why are warm-weather rocks sitting on top of glacial rocks? But with all this alkalinity being delivered in conditions of rapid warming, massive deposition of inorganic limestone is exactly what you would predict.

t seems pretty likely, given the evidence, that a Snowball Earth did take place, somewhere between 600 and 700 million years ago. And that likelihood brings us back to the Cambrian explosion.

The extreme environmental conditions post-Snowball, Hoffman suggested, may have ramped up the rates of evolution. “The crash in population size accompanying a global glaciation,” he has written, “would be followed by millions of years of comparative genetic isolation in high-stress environments,” conditions “favoring the emergence of new life forms.” Whether this speed-up would create new branches on the tree of life (as the molecular data would determine) as well as new body types within existing branches (as fossil evidence may show) is not clear. But changes in molecular sequence, Hoffman noted, will always show up earlier than changes visible in the fossil record. Whichever type of explosion the Cambrian was, it seems reasonable to speculate that a string of freeze-fry events could have triggered it.

And how does all this relate to astrobiology?

“We’re finding there are still many things to be discovered about the history of this planet,” Hoffman concluded, “which shed light on the probability of finding life elsewhere. If life’s expansion here depends on an event like a Snowball Earth, that’s another thing that makes the persistence and evolution of life on this planet extremely remarkable.”