Geoscience Reference
In-Depth Information
T O FIND the cause of a historical event, you first need to know when to look. And until recently, most
biologists have assumed that complexity arose with an event called the Cambrian explosion. This epis-
ode has grabbed all the early-life attention for decades.
E VOLUTION'S BIG BANG ! screamed the front cover of Time magazine on 4 December 1995. “New
discoveries show that life as we know it began in an amazing biological frenzy that changed our planet
overnight.” The animals that reared up on its cover were from the beginning of the Cambrian period,
around 545 million years ago. At first sight, that seems like a serious problem for Paul. The Cambrian
explosion can't possibly have been triggered by his Snowballs. They ended around 590 million years
ago, 1 and 45 million years is far too long to sit around with a lighted fuse waiting for the bang. Even
Paul admits this.
But he also says that the Cambrian doesn't deserve quite as much attention as it receives. The be-
ginning of the Cambrian was certainly a burgeoning, inventive time for life. During this rapid burst of
new evolutionary shapes and strategies, the foundations were set for every modern family of animals.
The Cambrian fossils have been known for centuries; they mark the end of the Dark Ages without
fossils and the beginning of geological and biological enlightenment. They are Stephen Jay Gould's
“Wonderful Life”. 2 But all this fame has come to them mainly because they were easy to preserve .
They show up everywhere. At the beginning of the Cambrian, life invented skeletons: scales, shells,
spines, all the sorts of bodily supports that stick around long enough after death to turn into clear, un-
ambiguous fossils.
So the Cambrian fossils weren't the first complex animals, any more than language began with
the printing press, or with papyrus. Complex life could easily have been around for millions of years
before then, and just not left such a clear record in the rocks.
The invention of multicellularity was certainly a prerequisite for the Cambrian explosion. Some
biologists even say that it made the Cambrian explosion inevitable. Of course, life began experiment-
ing with its new toy, exploring the many new possibilities it now had for shapes and functions, tissues
and organs. With complexity already in place, the Cambrian explosion was just regular evolution in
action. 3
So forget the brash fossils of the Cambrian. To find the real moment that life learned to use many
cells instead of one, biologists need to seek out creatures that are much more mysterious. If Paul Hoff-
man is right, and the Snowball truly triggered the invention of complexity, the world's first complex
creations must have appeared shortly after the ice receded. The question is, did they?
C RUNCH ! J IM Gehling's foot lands on a coke can and squashes it into the ground. He picks up the can,
points to the misshapen circle its trace has left in the mud, and grins. “Go on,” he says. “Look at that,
and tell me what shape the can was originally, or what it was used for.”
This is one of Jim's favourite metaphors for the work he does: reconstructing some of the first
new creatures to emerge after the end of the Snowball. His task is extraordinarily hard. At least people
studying more recent fossils such as dinosaurs have something concrete to dig up—bones, scales or
shells. But the creatures that Jim studies had none of these attributes. They lived before skeletons
had been invented. Their bodies were soft, like jellyfish. And the fossils they left behind are like
the smudgy circle from the coke can—indistinct impressions, squashed into ancient mud. From these
scant clues Jim and his colleagues have been trying to figure out whether these jelly-creatures were
the world's first complex animals.
Search WWH ::




Custom Search