Endless Forms Most Beautiful: The New Science of Evo Devo and the Making
of the Animal Kingdom by Sean B. Carroll. W.W. Norton, $25.95, 350 pages.
The News & Observer
December 25, 2005
How the mindless processes of evolution create life forms brimming with intelligence
By PHILLIP MANNING
Shortly after the publication of "On the Origin of Species," Charles
Darwin received a letter from the Reverend Adam Sedgwick, who said he found
many of Darwin's conclusions "utterly false and grievously mischievous."
The hostility toward Darwin's theory of evolution continues a century and a
half later through the sophisticated descendent of Creationism -- intelligent
design or I.D. I.D. rejects the Creationists' belief in the biblical story that
God created all the species in their current form in a busy six days about 10,000
years ago. However, it also holds that organisms are too complex and too varied
to have been produced by Darwin's mindless process of evolution. Such diverse
forms, I.D. advocates claim, must have had a designer, a very intelligent designer.
If polls and headlines are a gauge of public opinion, then anti-evolution sentiment
is sweeping the country. Only one-third of the respondents in a recent NBC poll
thought evolution could account for the origin of human life, while 44 percent
believed that God created the world (and humans) in six days. In another poll,
more than 60 percent of the responders favored teaching creationism alongside
evolution in public schools.
Of course, there is no scientific evidence for Creationism or I.D. The two movements
attempt to cast doubt on evolution -- then suggest that their claims must be
true if there are any chinks in Darwin's armor. Yes, Darwinism is a theory --
so is Newton's law of gravity. But both are so thoroughly tested and well-established
that they are almost unanimously accepted by scientists.
Oddly enough, all this public questioning of Darwinism is happening just as
biologists are making tremendous strides in explaining the mechanics of evolution.
The new insights are coming from a marriage between evolutionary and developmental
biology. Called evo devo for short, it adds to the mountains of proof of evolution,
further undermining I.D. and Creationism.
In his important new book, "Endless Forms Most Beautiful," Sean Carroll
tells how evo devo developed and how it is changing evolutionary biology. Carroll
is the right man to write this book. A professor of genetics at the University
of Wisconsin-Madison, where he manages a cutting-edge evo devo research program,
he has an easygoing writing style that helps the reader grasp some of the book's
complicated arguments. Even so, it will not be an easy read for those without
some background in biology.
The central question addressed by evo devo is the one posed by opponents of
evolution: How could random mutations in DNA produce the incredible complexity
and diversity of living organisms? I.D.'s answer is, they can't; living organisms
are too intricate to have been created by a mindless process. From this assertion,
I.D. makes a staggering leap, claiming that a supreme being must have designed
life.
Carroll rejects that conclusion as nonsense. The key to understanding biological
diversity, he explains, lies in figuring out how organisms develop. According
to evo devo, animals are made from sets of the same, or very similar, genes.
What we look like -- be it fruit fly, snail or human -- is determined by how
and when those genes are expressed in the developing embryo. Although the end
products are quite different, the building blocks are the same. Think of it
this way: Toy models of the White House and the Washington Monument do not resemble
each another from a distance, but close inspection reveals that they are both
made of interchangeable parts -- Legos.
Carroll calls these building-block genes "tool kit genes." Their discovery
came from scientists' work with mutant fruit flies, called Frankenflies. These
are flies, Carroll writes, "with legs coming out of their head, or with
extra pairs of wings, or feet in the place of mouthparts." The surprising
thing about these weirdies is that they were not created by changing many genes
but a single one. How could one gene affect an animal's form so dramatically?
In the early 1980s, scientists determined that a cluster of genes called Hox
genes shapes the bodies of fruit flies, specifying to the developing embryo
what appendages go where. A Hox gene in the embryonic segment that is to become
the fly's head tells that segment to grow an antenna. Another Hox gene causes
wings to form here and legs to go there. Fiddling around with a single Hox gene
can make flies with legs where antennae should be.
The discovery of Hox genes led to a big question: How could just one gene making
one protein have such an enormous effect on a developing embryo? The answer,
it turns out, lies in the way an organism expresses its genetic code. Nearly
every cell in our bodies has a complete set of genes. In a developing embryo,
liver genes are switched on (or expressed) to make a liver, brain genes to make
a brain and so on. Tool kit genes, such as Hox genes, produce proteins that
toggle the switches of other genes off and on. Thus, disabling or adding a single
Hox gene can have major ramifications, such as building a leg instead of an
antenna.
Hox genes, scientists soon discovered, are not confined to fruit flies; they
are found in almost every animal, from houseflies to house sparrows to humans.
In addition to being widespread, they have also been around for a while (or,
as biologists say, are deeply conserved). Scientists now believe that Hox genes
(and other tool kit genes) were present in organisms 500 million years ago,
perhaps even before the Cambrian explosion during which multicellular animals
appeared.
The presence of tool kit genes in ancient life forms suggests an elegant mechanism
for evolution. Evolutionary change from one form to another does not require
a huge number of random mutations to create bucketfuls of new genes and new
proteins. Instead, it relies on the simpler process of changing where and when
old genes are expressed in the developing embryo. Carroll summarizes this idea
and its implication: "[D]ifferences in form arise from changing the way
they [tool kit genes] are used. The principle of descent by modification ...
is clear."
As cheerleader and spokesman for evo devo, Carroll firmly believes that it refutes
the arguments of I.D. advocates. No need to call on a Creator or an intelligent
designer, evo devo can account for the complexity of organisms and the vast
differences between them. "Developmental genetics," he writes, "has
been shedding new light on the making of complexity and the evolution of diversity
for twenty years. Creationists just plain refuse to see it."
Nevertheless, the conflict continues. On Tuesday, a Pennsylvania judge ruled
that I.D. could not be mentioned in biology classes taught in a public school
district. Most scientists are weary of this battle; they believe they won it
long ago. Still, they press the point -- in book after book, article after article
-- that Creationism and I.D. are not testable theories and therefore are not
sciences but matters of faith. Carroll sums up why scientists must continue
this seemingly endless struggle with a quote from Henry David Thoreau: "You
can hardly convince a man of an error in a life-time, but must content yourself
with the reflection that the progress of science is slow. If he is not convinced,
his grandchildren may be."