Endless Forms Most Beautiful: The New Science of Evo Devo and
the Making of the Animal Kingdom. Sean B. Carroll. xiv +
350 pp. W. W. Norton, 2005. $25.95.
All plants and animals, including humans, are essentially societies
of cells that vary in configuration and complexity. As Darwin's
theory made clear, these multitudinous forms developed as a result
of small changes in offspring and natural selection of those that
were better adapted to their environment. Such variation is brought
about by alterations in genes that control how cells in the
developing embryo behave. Thus one cannot understand evolution
without understanding its fundamental relation to development of the
embryo. Yet "evo devo," as evolutionary developmental
biology is affectionately called, is a relatively new and growing field.
Sean B. Carroll, as a leading expert both in how animals develop and
in how they have evolved, is ideally placed to explain evo devo. His
new book on the subject, Endless Forms Most Beautiful: The New
Science of Evo Devo and the Making of the Animal Kingdom
(the title borrows a phrase from Darwin's On the Origin of
Species), was written, he says, with several types of readers
in mind—anyone interested in natural history, those in the
physical sciences who are interested in the origins of complexity,
students and educators (of course), and anyone who has wondered
"Where did I come from?" Carroll has brilliantly achieved
what he set out to do.
One of the most striking discoveries of the last half-century has
been that, despite the fact that animals differ greatly in
appearance, common principles control their development from a
single fertilized egg. They even have in common many master
genes—genes that control many aspects of development. One can
almost imagine Drosophila fruit flies saying to one another
that they are amazed at how similar humans are to them. Indeed, many
of the genes that have been identified as controllers of vertebrate
development were originally discovered in these flies.
It's a key point that when and where genes are expressed determines
how animals develop. The control regions of the genes (switches that
change an existing pattern of gene activity into a new pattern of
gene activity) are crucial, as Carroll makes clear, and one gene can
have many control regions. (For example, in the fruit fly, there is
a group of genes—known as the pair-rule genes—that
express proteins in seven stripes along the body axis of the embryo
[see illustration on next page]; each of these genes has seven
discrete control regions, and each region specifies one stripe.) It
is thus unsurprising that 95 percent of the genes that code for
proteins are similar in humans and mice. Evolution of control
regions has made us human—and different from our primate ancestors.
Carroll explains the basic tool kit for development that all animals
share, placing particular emphasis on Drosophila. He
introduces both Hox genes, which are considered master
genes, and widely used intercellular signaling molecules such as the
proteins specified by hedgehog genes. It is striking how
few signaling molecules animals use in development. This is because
the same molecules can be employed again and again, as cells will
respond differently according to their genetic constitution and
Carroll doesn't give much attention to the fact that a cell has a
positional identity (based on the position it occupied on the axis
of the body of the early embryo) or to how that positional identity
is acquired. Nor does he delve into how a cell senses its position
and figures out how to act according to its genetic constitution and
developmental history, thereby differentiating to give any
imaginable pattern. Consider that a change in a single Hox
master gene can convert the antenna of the fly into a leg. There is
evidence that cells in the leg and those in the antenna have the
same positional identity. It is somewhat embarrassing that we still
do not know how the change in that particular Hox gene
controls the response of all those unknown downstream genes to make
a leg rather than an antenna. And this downstream target problem is
present for all Hox genes.
Carroll emphasizes that individual animals are made up of similar
parts, such as vertebrae, bones in fingers and spots on butterfly
wings, and that modular construction played an important role in
evolution. He is a supporter of Williston's law, which states that
"in evolution . . . the parts in an organism tend toward
reduction in number, with the fewer parts greatly specialized in
function." I must confess to finding the idea of modules not
that easy to appreciate. Is, for example, the leg/antenna basic
structure a module?
The earliest complex animals, fossils of which were found in the
Burgess Shale, appear to have arisen about 500 million years ago,
over a period of some 15 million years. Evidence from evo devo shows
that all the genes for building those complex animals existed long
before that morphological explosion. The dominance of arthropods at
the time of the explosion may have been due both to Williston's law
and to the power of Hox genes to specify differences
between the body segments that formed different appendages at
specific positions along the body. But how, asks Carroll, did the
number of distinct appendage types increase? His answer is that the
relative shifting of Hox genes could have provided the
mechanism. That still leaves a big problem—how did arthropod
appendages such as limbs and wings evolve? An answer lies, he says,
in the origin and modification of the ancestral biramous (forked)
limb. But even if the origin of the limb can be explained, wings are
even more difficult. One answer is that they evolved from the gills
on the limbs of aquatic ancestors.
But this conclusion raises a key, and much neglected, problem that
even Carroll does not properly explore. If evolution proceeds in
small steps, what were the intermediate stages in the evolution of
wings from gills, and what was the selective advantage of each of
those forms? How could the intermediate structures have been an
advantage before the animal could fly? One possibility is that they
played a role in thermoregulation, but there is no good evidence for
that hypothesis. This is a general problem in evo devo, and Darwin
fully understood the difficulties it poses.
A related problem is how to explain the evolution of the
autopod—the digits—from fins. One possibility is that
the autopod is merely a distal extension of the mechanism that gives
rise to more proximal elements, such as the humerus, radius and
ulna. A much more difficult problem is raised in the evolution of
development itself. Gastrulation (during which an embryo forms its
innermost, middle and outer layers) occurs in the early development
of all animals and has evolved in a variety of ways related to later
development; it is at present not possible to account for the
intermediate forms or their advantage to the animal. Although
evolution, as François Jacob pointed out, tinkers with what
is there, rather than inventing something new, these problems remain unsolved.
A nice example of what could be considered clever tinkering is
butterfly spots. Each spot appears to evolve its shape, color and
size independently of other elements. Evolution has tinkered not
only with the qualities of each spot, but with the making of the
spot itself. Carroll's group discovered that at the center of each
spot, the gene Distal-less (a key gene controlling the
distal development of appendages such as insect limbs) is expressed
and initiates spot development.
Even the evolution of humans can be thought of as tinkering with the
genes of our primate ancestors. But this view is totally
unacceptable to religious creationists. Carroll criticizes their
views and emphasizes how important it is for evo devo to be taught
Evo devo is fundamental to understanding the biological world we
live in, including ourselves. This is a beautiful and very important book.
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