“There’s a deeper history that’s been missing
from the fossil record.” —KEVIN PETERSON
Biology’s big bang had a long fuse
By Susan Milius
A new effort to date the early
history of modern animals finds
a lot of evolutionary dawdling.
The last common ancestor of
all living animals probably arose
nearly 800 million years ago,
a multidisciplinary research
team reports in the Nov. 25
Science. From that common
ancestry, various animal lineages diverged and evolved on
their own paths. Yet the major
animal groups living today
didn’t arise until roughly 200
million years later, in an exuberant burst of forms preserved
in fossils during what’s called
the Cambrian explosion.
“There’s a deeper history
that’s been missing from the
fossil record,” says study coauthor Kevin
Peterson of Dartmouth College. He and
his colleagues have been pushing back
that date for a last common ancestor,
and now, he reports, the analysis has
the broadest reach yet. “We show that
animals evolved quite a bit before they
show up in the fossil record.”
This work updates the notion of a
long evolutionary lag, when much of the
basic biological toolkit was already in
place for a later surge of new body forms,
says paleontologist and study coauthor
Douglas Erwin of the National Museum
of Natural History in Washington, D.C.,
and the Santa Fe Institute.
“The Cambrian explosion is like the
industrial revolution,” Erwin says.
Inventions that would later be important for a major shift in technology — or,
in this case, genetic novelties important
for evolution—appeared long before
they played a role in widespread changes
that had a major impact on life.
For understanding animal origins,
The evolutionary extravaganza that began more
than 500 million years ago spawned early
arthropods such as the Olenoides trilobite (left)
and the proposed spider relative Sidneyia.
the new paper “is really worthwhile as
it stands back and tries to make sense of
the whole picture,” says James Valentine
of the University of California, Berkeley,
who studies animal evolution.
Just what happened with animals
during that Cambrian explosion remains
one of the more celebrated puzzles in the
history of life. Charles Darwin mused
over how diverse animal forms appear
suddenly (geologically speaking) without
much in the way of precursors. Darwin’s
answer, as Erwin puts it, was that paleontologists just needed to look harder.
More than a century of hard looking has turned up some signs, fossils as
well as traces of biological chemistry,
of enigmatic animal life before the
Cambrian period began about 541 million years ago. Yet the relationship
to modern animals often is not clear.
Theories themselves have exuberantly
exploded in number and form.
For the new study, Erwin and
the rock side of the team updated the
scorecard on the earliest fossil occurrences with recent fossil finds and the
current thinking on dates of rock layers.
On the molecular side, Peterson and his
colleagues expanded the family tree to
cover seven genes from more than 100
different kinds of living animals. Fossils
provided dates for a scattering of branch
points in the tree, allowing researchers
to estimate when major groups arose.
Combining fossil dates and the DNA
analysis, Peterson, Erwin and their colleagues conclude that the basic genetic
tools for fancy animal bodies arose long
before a surge of evolutionary innovation around the Cambrian period gave
rise to modern animal groups.
During that 200 million-year-plus
run-up to the Cambrian explosion,
animals did evolve more diverse cell
chemistry to regulate basic genes, and
the environment changed. But Peterson
attributes much of the Cambrian rise
of modern animal forms to changes
in the interactions among organisms
themselves. “You see an evolutionary
explosion, if you will, because animals
are eating other animals for the first
time,” he says.
The paper’s discussion of toolkit genes
and the diverse cell chemistry that arose
to orchestrate them overlooks some
possibly important complexity, objects
molecular biologist Mark Q. Martindale
of the University of Hawaii. At least 30
percent of the genes of animals analyzed
so far have no recognizable similar gene
in another species. “These so-called
orphan genes could have a tremendous
amount to do with diversification of
animal lineages, but people just pooh-pooh these differences and focus on the
things that are shared,” he says.
Some of the relationships in the
evolutionary tree “have been and will
continue to be controversial,” says evolutionary biologist Casey Dunn of Brown
University in Providence, R.I., who
wasn’t involved in the research. “But the
point of the tree isn’t the relationships
themselves — it is some key dates.”
SMITHSONIAN INSTITUTION, COURTES Y OF DOUGLAS ERWIN