CREATION - Snowball Earth - page 100

Kevin chose the creatures for his molecular clock carefully. He decided on echinoderms—the fam-
ily that contains urchins and sea stars. For one thing, there was a complete, well-dated set of fossils
for the ancestors of these creatures, so he could check more carefully than previous studies how well
his clock was doing as he went backwards in time. For another, these creatures had similar body size,
metabolic rate, and amount of time between one generation and the next. Kevin felt that previous
studies had erred by picking creatures that were too different in all these respects. The more alike the
creatures that he started with, Kevin realized, the more accurate the clock tracing their mutual ancestry
should be.
So he set about uncovering the genetic sequence behind seven different creatures. He calculated
how fast the DNA must have changed. He worked steadily backwards, checking his dates as he went.
Whenever he had well-dated evidence from fossils for the timing of a particular ancestor, he checked
whether the clock agreed. Each time, the clock looked good. Encouraged, Kevin projected his clock
further back in time. Now there were no fossils to check against. Now he was getting closer and closer
to the animal ancestor. And then, finally, he had his answer. The last common ancestor of all complex
animals lived . . . somewhere around . . . 700 million years ago. 18
Kevin was stunned. “I couldn't refute that diagram of Paul and Dan's,” he told me several months
later, still sounding dazed. Why not? After all, his clock didn't throw up that magic date of 590 mil-
lion years—which marks both the ending of the ice and the beginning of the first complex fossils. But
neither did it prove that animals had existed more than a billion years ago, as Kevin had expected.
Instead, its date agreed almost exactly with the end of the very first Snowball.
Remember that Paul was dealing with a series of events, not just one. And the first Snowball ended
around 700 million years ago, exactly the date Kevin's clock produces for the animal ancestor. Per-
haps complex animals were triggered by the first Snowball, and then survived through the remaining
episodes of ice. If each subsequent Snowball wiped out all but a few of those new animals, that might
explain why widespread fossils didn't appear until the ice finally receded.
It's also possible that even Kevin's careful clock overestimated the age of complex animals. Gen-
etic studies like these assume that their clocks have always ticked at the same rate. But some biologists
think that genetic changes happened more quickly in the past, and that all the clocks give older times
than they should. Kevin believes he solved many of the problems with the earlier clocks, but he says
himself that he may not have solved them all.
T HERE WILL be many more attempts to find traces of the earliest animals. Researchers are collecting
genetic material, designing newer, better molecular clocks, and scouring the world's rocks for trails
blazed by ancient life. But the more biologists try to pin down the timing of widespread complexity,
the more their results seem to point towards the Snowball.
Was this just a coincidence? “Most people I know think they're connected,” says Jim Gehling.
And Kevin has changed his mind about the timing at least, though he still wants a testable hypothesis
to explain the connection. Nick Butterfield, the algae man, says the same. He is annoyed about Paul's
airy biological assumptions. He says that Paul's paper was “cringingly awful in its biology”. He says
it's up to Paul to explain exactly how ice could have triggered life's industrial revolution.
But forget Paul for a moment. What does Nick think about the biological evidence, the new fossils
and the molecular clocks and all the other developments that are steadily moving the date of burgeon-
ing complexity closer and closer to the ice? Nick pauses. Then says this. “I think it's fascinating.”