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Title[Sean B. Carroll] Endless Forms Most Beautiful Th(Bookos.org)
Tags Vertebral Column Evolutionary Developmental Biology Homology (Biology) Evolutionary Biology
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Endless Forms Most Beautiful
The New Science of Evo Devo

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arthropods, but even within them. The role of shifting Hox zones in evolution was beautifully demonstrated by Michalis
Averof and Nipam Patel, who collected and examined embryos from a wide variety of crustaceans (the group of arthropods
that includes shrimps, barnacles, crabs, and lobsters). One of the prominent differences between groups was in the number
of maxillipeds, the feeding appendages at the front end of the thorax that have been modified from limbs. Brine shrimp
do not have maxillipeds, nor did primitive crustaceans. Other crustaceans, however, have one, two, or (as in lobsters) as
many as three pairs of maxillipeds. Small changes in embryo geography underlie these important differences in
crustaceans. Averof and Patel found that the zones of expression of two Hox proteins (numbers 8 and 9) were shifted
backward by 1, 2, or 3 segments in these crustaceans, respectively, relative to those crustaceans without maxillipeds
(figure 6.7). The extent of the shift corre-

lates perfectly with the number of maxillipeds. Furthermore, these shifts and maxillipeds appeared to have evolved several
times independently in Crustacea, suggesting that similar functional adaptations were achieved by similar mechanisms in
different animals. (I'll say more about the significance of repeated instances of similar changes in the next chapter.)

Shifting Hox zones have sculpted the prominent differences along the main body axes of living arthropod groups such as
spiders, crustaceans, centipedes, and insects. It is a very reasonable extrapolation to assert that this was also the story in
the Cambrian, where body regionaliza-tion and appendage specialization are evident in all fossil arthropods. The blocks of
similar segments in fossil taxa were certainly the zones of particular Hox genes (figure 6.7). The increase in the number of
different appendage and segment types in arthropod evolution is the product of generating a greater number of unique
zones in the embryo in which specific individual or combinations of Hox genes are expressed. This relative shifting of
Hox zones is therefore one of the mechanisms underpinning Williston's Law—the specialization of repetitive parts
requires that the different parts fall into different Hox zones.

Shifting Hox zones is not just an arthropod phenomenon—this same primary mechanism underlies major features of the
anatomical diversity of our own phylum, the vertebrates.

The Making of the Vertebrates: More Genes and Many Shifts

Our family line is also traceable back as far as the Cambrian. We are vertebrates, part of a larger group of animals known
as chordates that possess a notochord. The chordates also include tunicates (such as sea squirts) and cephalochordates such
as the lancelet. Chordates are part of the deuterostome branch of the animal tree (figure 6.8). The Burgess fossil Pikaia
was for a long while the best-known ancient chordate but

Fig. 6.8 The chordate evolutionary tree and the expansion of clusters in vertebrate evolution. The common
ancestor of all chordates had one cluster, as do living tunicates and cephalochor-dates. Cluster duplication has happened

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several times since, on the line to jawless fish, on the line to cartilaginous fish (sharks), and again in lampreys. Because
is a Cambrian vertebrate whose evolutionary relationships are not certain, on the tree it branches out at the

same (unresolved) time as hagfish, lamprey, and cartilaginous fish, drawing by josh klaiss

spectacular recent finds in the Chengjiang have pushed back the earliest appearance of vertebrates to about 520 million
years ago and a treasure trove of some species has revealed details of a surprisingly complex anatomy for vertebrates at
this time.

Specimens of the fossil jawless fish reveal the presence of a head lobe with eyes, possibly
nasal sacs, ten or more separated vertebral elements, gills, a dorsal fin, and a ventral fin. This anatomy is more complex
than the later and indicates that the evolution of the vertebrate body was well advanced by the Early Cambrian.
These recent discoveries underscore the tremendous importance of the fossil record and of ongoing and new excavations.
The

date of the first appearance of groups and physical features is always tentative because subsequent finds can always push it
back—in this case a critical 15 million years earlier. Furthermore, while vertebrates were not the most numerous group in
the Early and Middle Cambrian, their discovery in Chengjiang puts these predatory firmly in the picture of
Cambrian ecosystems.

The invention and modification of many structures marked the origin of the vertebrates, including much more complex
brains, sensory structures, cartilage, the body skeleton, and skull. Many subsequent innovations led to the amphibians,
reptiles, birds, and mammals we know so well. Just as for the lobopodians and arthropods, we'd like to know whether the
early evolution of the vertebrates in the Cambrian depended upon a very similar tool kit of developmental genes as that
then possessed by other groups, or whether some changes in the tool kit might have played a role in the origin of
ancestral vertebrates.

We cannot, of course, recover genes from but we can study some proxies, living species that occupy key
places on the chordate and deuterostome family trees, to allow us to infer the genetic complexity of ancestral vertebrates.
One key group is the cephalochordates. These animals lack the vertebrate features of a cranium or bony structures, but they
are the sister group to vertebrates in the same way that living Onychophorans are the sister group to arthropods. The
content of the cluster in cephalochordates reveals that cluster's content in the last common ancestor of
cephalochordates and vertebrates.

The lancelet is the one and only cephalochordate still around today. These two- to three-inch-long animals can be found in
Tampa Bay, Florida, and some other waters. When Jordi Garcia-Fernandez and Peter Holland examined its genes,
they found just a single cluster of genes. Recall that modern vertebrates such as mice and ourselves have four
clusters containing thirty-nine genes in all. The lancelet tells us that an increase in cluster number happened
sometime after the split between the vertebrate and cephalochordate lines, in the Cambrian or a bit earlier. We also know
that other

deuterostomes such as tunicates and echinoderms possess single clusters. So, while the entire diversity of tunicates
and echinoderms in the Cambrian and ever since has, like the arthropods, evolved around a cluster containing ten or so

genes, the vertebrates did expand their number of genes.

When in vertebrate evolution did the number of clusters increase? Could this increase have triggered vertebrate
evolution? To answer these questions, a whole bunch of living species representative of different groups on different
branches of the vertebrate family tree had to be examined. All mammals, birds, and certain fish, including the primitive,
deep-sea-dwelling coelacanth, have four clusters. It is safe to conclude then that there were four clusters in the
common ancestor of all of these jawed vertebrates.

But more primitive living vertebrates such as lampreys have fewer clusters. Detailed scrutiny of these clusters' genes
and comparison with those of bony fish and mammals suggests that our four clusters are the product of two rounds

Page 203

The story of the thylacine is told in D. Owen, (Crows Nest, NSW:
Allen and Unwin, 2003). Further information on species extinction is found in E. O. Wilson and F. M. Peter, eds.,

(Washington, D.C.: National Academy Press, 1988), and E. O. Wilson, (New York:
Penguin, 1992).

Huxley's address to the Royal Institution in February 1860 is quoted in A. Desmond and J. Moore,
(New York: Warner, 1991), p. 489.

Acknowledgments
As I hope is true for any author, this book has been a labor of love. But I have certainly been luckier than most because
my labors were made possible and lightened enormously by the help of my wife, Jamie Carroll. Her critical taste and
encouragement nourished the book's birth, her hard work and artistic talent fed its growing body, her patience endured
countless questions of "Honey, what do you think of this quote/paragraph/section/chapter/title/picture/etc.?," and her
honest answers spared readers much confusion and pain. No one could ever hope for a more generous partner, a warmer
home in which to create, or a better sense of humor for getting through the inevitable twists and turns.

The research for and the making of the book were aided in many ways by my wonderful family, who have trudged happily
through various jungles, swamps, muddy rivers, and innumerable museums for the love of natural history. My sons, Will
and Patrick, helped to find fossils in the field and key animals in museums and my stepson, Josh Klaiss, created several
important graphics.

I thank my sister, Nancy, with whom I have studied and discussed

the lives of Darwin, Huxley, Lyell, and their contemporaries for almost a decade, my brother Peter for always pushing for
the big picture and our many discussions on human evolution, and my brother Jim for his great encouragement.

I also thank my parents, Joan Carroll and the late J. Robert Carroll, for encouraging each of their children to pursue
whatever interested us, even when that meant keeping snakes in the house.

The artwork here was a big undertaking. Original drawings and graphics were created by Jamie, Josh, and Leanne Olds.
Leanne also composed or redrew most of the figures that originated from other sources. Steve Paddock, a longtime
member of my research group, compiled and arranged the color artwork. I am grateful for the care everyone put into each
image and I am thrilled at the results.

Much of the artwork was contributed by colleagues around the world and is the fruit of their field and laboratory research.
Albert Einstein came close to the mark when he wrote:

A hundred times every day I remind myself that my inner and outer life are based on the labors of other men, living and
dead, and that I must exert myself in order to give in the same measure as I have received and am still receiving.

—The World as I See It,

(1954)

(trans. Sonja Bargmann)

My gratitude and debts are owed to a larger and more diverse community than that in which Einstein toiled. I was in the
fortunate position to write this book because of the individual and collective efforts of a huge community of biologists,
including paleontologists, geneticists, embryologists, and evolutionary biologists. While some of the giants preceded my
time, most of the discoveries discussed in the book belong to the current generation. I thank the large number of col-

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