SNOWBRAWLS - Snowball Earth - page 87

and the bacteria were still alive three years later when the camera was brought back to Earth. They had
managed to survive without food, water or even air. Hot springs are often brilliant with living colour.
The steaming, acidic pools of Yellowstone National Park, for instance, contain vivid bacterial patches
of orange, red and green despite their boiling temperatures. Life has a habit of finding its way, no mat-
ter what.
The biologists pointed out, however, that many of these resilient creatures are weirdly adapted to
their extreme conditions, whereas most of the ones that survived the Snowball were apparently more
normal in their requirements, particularly in their need for sunlight. There had to be sunlight. There
had, the biologists said firmly, to be holes.
So Paul and Dan changed tack. They obviously needed to provide some refuges for life within the
frozen seas. What kind of openings might there have been? Well, any hot spring or volcano on a shal-
low enough ocean floor would have created at least a small hole in the ice above it. Also, the Snowball
was not uniformly cold. Though global temperatures would initially have plunged to around minus
40 degrees C, they would gradually have risen as carbon dioxide built up in the air. And the equator
would always be warmer than this bleak global average. Soon the ice at the equator would grow thin-
ner, perhaps even thin enough to crack periodically.
Thinking the question through further, Paul and Dan also realized that the Snowball's stronger sea-
sons would also have helped living things cope. Even if winter temperatures near the equator were
30 degrees below zero, summers could have crept above freezing for a few days each year. In melted
puddles and ice cracks, living things could then have grabbed their chance to make and store food, as
they do in Antarctica today. And even in winter there could well have been other patches of open water
among the ice. In today's frozen oceans, odd currents keep certain places—called polynyas—ice-free
throughout the year. Whales trapped in the pack ice use these open patches as breathing holes while
they wait for spring to return and release them.
For the biologists, this line of thinking was much more encouraging. But were there enough
refuges? Could each individual species huddle together in a big enough group to survive until the
Snowball finally melted?
To find out, Dan Schrag called a friend, another hot young scientist, Doug Erwin, from the Na-
tional Museum of Natural History in Washington, D.C. Doug is an expert on ancient life, and he also
knows a fair amount about ecology in the modern world. To protect an endangered species, Doug said,
you have to maintain its genetic variety. The genetic material that passes from one generation to the
next is constantly changing—and not always for the better. In an isolated group—a herd of elephants
in a national park, say—dangerous mutations can spread quickly. For the species as a whole to sur-
vive, any one group must contain enough individuals, enough variety, to dilute this danger. And there
must be enough separate groups that if a few of them fail, the rest will still pull through.
Doug realized that the same would apply to the Snowball's inhabitants. He made a list of all the
different species that needed to make it through the Snowball. Then he used conservation models to
calculate two numbers: how many individuals of each Snowball species you'd need in a given refuge,
and how many refuges you'd need overall.
The answer astonished both Doug and Dan. It was far easier than they'd expected. To get virtually
all of the species through the Snowball, you only needed something like one thousand different
refuges. And each refuge only needed to house around one thousand individuals. What's more, the
Snowball creatures were no elephants. “Do you know how much open water you'd have needed to
support one thousand of these individuals?” Dan demanded of me as we sat in a café. “This much.”
He spread his hands apart until they outlined a region of air the size and shape of a dinner plate.