Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body (31 page)

Just as the space program changed the way we look at the moon, paleontology and genetics are changing the way we view ourselves. As we learn more, what once seemed distant and unattainable comes within our comprehension and our grasp. We live in an age of discovery, when science is revealing the inner workings of creatures as different as jellyfish, worms, and mice. We are now seeing the glimmer of a solution to one of the greatest mysteries of science—the genetic differences that make humans distinct from other living creatures. Couple these powerful new insights with the fact that some of the most important discoveries in paleontology—new fossils and new tools to analyze them—have come to light in the past twenty years, and we are seeing the truths of our history with ever-increasing precision. Looking back through billions of years of change, everything innovative or apparently unique in the history of life is really just old stuff that has been recycled, recombined, repurposed, or otherwise modified for new uses. This is the story of every part of us, from our sense organs to our heads, indeed our entire body plan.

What do billions of years of history mean for our lives today? Answers to fundamental questions we face—about the inner workings of our organs and our place in nature—will come from understanding how our bodies and minds have emerged from parts common to other living creatures. I can imagine few things more beautiful or intellectually profound than finding the basis for our humanity, and remedies for many of the ills we suffer, nestled inside some of the most humble creatures that have ever lived on our planet.

NOTES, REFERENCES, AND FURTHER READING

CHAPTER ONE FINDING AN INNER FISH

I have included a mix of primary and secondary sources for those interested in pursuing the topics in the book further. For accounts that use exploratory paleontological expeditions as a vehicle to discuss major questions in biology and geology, see Mike Novacek’s
Dinosaurs of the Flaming Cliffs
(New York: Anchor, 1997), Andrew Knoll’s
Life on a Young Planet
(Princeton: Princeton University Press, 2002), and John Long’s
Swimming in Stone
(Melbourne: Freemantle Press, 2006). All balance scientific analysis with descriptions of discovery in the field.

The comparative methods that I discuss, including the methods used in our walk through the zoo, are the methods of cladistics. A superb overview is Henry Gee’s
In Search of Deep Time
(New York: Free Press, 1999). Basically, I present a version of the three-taxon statement, the starting point for cladistic comparisons. A good treatment with background sources is found in Richard Forey et al., “The Lungfish, the Coelacanth and the Cow Revisited,” in H.-P. Schultze and L. Trueb, eds.,
Origin of the Higher Groups of Tetrapods
(Ithaca, N.Y.: Cornell University Press, 1991).

The correlation between the fossil record and our “walk through the zoo” is discussed in several papers. A sampling: Benton, M. J., and Hitchin, R. (1997) Congruence between phylogenetic and stratigraphic data in the history of life,
Proceedings of the Royal Society of London,
B 264:885–890; Norell, M. A., and Novacek, M. J. (1992) Congruence between superpositional and phylogenetic patterns: Comparing cladistic patterns with fossil records,
Cladistics
8:319–337; Wagner, P. J., and Sidor, C. (2000) Age rank/clade rank metrics—sampling, taxonomy, and the meaning of “stratigraphic consistency,”
Systematic Biology
49:463–479.

The layers of the rock column and the fossils contained therein are beautifully and comprehensively discussed in Richard Fortey’s
Life: A Natural History of the First Four Billion Years of Life on Earth
(New York: Knopf, 1998). Resources for vertebrate paleontology include R. Carroll,
Vertebrate Paleontology and Evolution
(San Francisco: W. H. Freeman, 1987), and M. J. Benton,
Vertebrate Paleontology
(London: Blackwell, 2004).

For the origin of tetrapods: Carl Zimmer reviewed the state of the art in the field in his highly readable and well-researched
At the Water’s Edge
(New York: Free Press, 1998). Jenny Clack has written the definitive text on the whole transition,
Gaining Ground
(Bloomington: Indiana University Press, 2002). The bible of this transition, Clack’s book will bring a novice to expert status quickly.

Our original papers describing
Tiktaalik
appeared in the April 6, 2006, issue of
Nature.
The references are: Daeschler et al. (2006) A Devonian tetrapod-like fish and the origin of the tetrapod body plan,
Nature
757:757–763; Shubin et al. (2006) The pectoral fin of
Tiktaalik roseae
and the origin of the tetrapod limb,
Nature
757:764–771. Jenny Clack and Per Ahlberg had a very readable and comprehensive commentary piece in the same issue (
Nature
757:747–749).

Everything about our past is relative, even the structure of this book. I could have called this book “Our Inner Human” and written it from a fish’s point of view. The structure of that book would have been strangely similar: a focus on the history humans and fish share in bodies, brains, and cells. As we’ve seen, all life shares a deep part of its history with other species, while another part of its history is unique.

CHAPTER TWO GETTING A GRIP

Owen was by no means the first person to see the pattern of one bone–two bones–lotsa blobs–digits. Vicq-d’Azyr in the 1600s and Geoffroy St. Hilaire (1812) also made this pattern part of their worldviews. What distinguished Owen was his concept of the archetype. This was a transcendental organization of the body, reflecting the design of the Creator. St. Hilaire was searching less for an archetypical pattern hidden in all structure than for “laws of form” that govern the formation of bodies. A nice treatment of these issues is in T. Appel,
The Cuvier-Geoffroy Debate: French Biology in the Decades Before Darwin
(New York: Oxford University Press, 1987), and E. S. Russell,
Form and Function: A Contribution to the History of Morphology
(Chicago: University of Chicago Press, 1982).

A recent volume edited by Brian Hall is one-stop shopping for information on limb diversity and development and contains a number of important papers on different kinds of limbs: Brian K. Hall, ed.,
Fins into Limbs: Evolution, Development, and Transformation
(Chicago: University of Chicago Press, 2007). Useful references for exploring the shift from fins and limbs in more detail include Shubin et al. (2006) The pectoral fin of
Tiktaalik roseae
and the origin of the tetrapod limb,
Nature
757:764–771; Coates, M. I., Jeffery, J. E., and Ruta, M. (2002) Fins to limbs: what the fossils say,
Evolution and Development
4:390–412.

CHAPTER THREE HANDY GENES

The developmental biology of limb diversity has seen a number of reviews and primary papers. For a review of the classic literature see Shubin, N., and Alberch, P. (1986) A morphogenetic approach to the origin and basic organization of the tetrapod limb,
Evolutionary Biology
20:319–387; and Hinchliffe, J. R., and Griffiths, P., “The Pre-chondrogenic Patterns in Tetrapod Limb Develoment and Their Phylogenetic Significance,” in B. Goodwin, N. Holder, and C. Wylie, eds.,
Development and Evolution
(Cambridge, Eng.: Cambridge University Press, 1983), pp. 99–121. Saunders’s and Zwilling’s experiments are now classic, so some of the best accounts are now seen in the major textbooks in developmental biology. These include S. Gilbert,
Developmental Biology,
8th ed. (Sunderland, Mass.: Sinauer Associates, 2006); L. Wolpert, J. Smith, T. Jessell, F. Lawrence, E. Robertson, and E. Meyerowitz,
Principles of Development
(Oxford, Eng.: Oxford University Press, 2006).

For the first paper describing
Sonic hedgehog
’s role in limb patterning, go to Riddle, R., Johnson, R. L., Laufer, E., Tabin, C. (1993)
Sonic hedgehog
mediates the polarizing activity of the ZPA,
Cell
75:1401–1416.

Randy’s work on
Sonic
signaling in shark and skate fins is in Dahn, R., Davis, M., Pappano, W., Shubin, N. (2007)
Sonic hedgehog
function in chondrichthyan fins and the evolution of appendage patterning,
Nature
445:311–314. Subsequent work from the lab on the origin of limbs, at least from a genetic perspective, is contained in Davis, M., Dahn, R., and Shubin, N. (2007) A limb autopodial-like pattern of
Hox
expression in a basal actinopterygian fish,
Nature
447:473–476.

The stunning genetic similarities in the development of flies, chickens, and humans is discussed in Shubin, N., Tabin, C., Carroll, S. (1997) Fossils, genes, and the evolution of animal limbs,
Nature
388:639–648; and Erwin, D. and Davidson, E. H. (2003) The last common bilaterian ancestor,
Development
129:3021–3032.

CHAPTER FOUR TEETH EVERYWHERE

The importance of teeth to an understanding of mammals is evident in the many treatments in the field. Dental structure plays a particularly important role in understanding the early record of mammals. Extensive reviews are found in Z. Kielan-Jaworowska, R. L. Cifelli, and Z. Luo,
Mammals from the Age of Dinosaurs
(New York: Columbia University Press, 2004); and J. A. Lillegraven, Z. Kielan-Jaworowska, and W. Clemens, eds.,
Mesozoic Mammals: The First Two-Thirds of Mammalian History
(Berkeley: University of California Press, 1979), p. 311.

Farish’s mammal from Arizona is analyzed in Jenkins, F. A., Jr., Crompton, A. W., Downs, W. R. (1983) Mesozoic mammals from Arizona: New evidence on mammalian evolution,
Science
222:1233–1235.

The tritheledonts we found in Nova Scotia are described in Shubin, N., Crompton, A. W., Sues, H.-D., and Olsen, P. (1991) New fossil evidence on the sister-group of mammals and early Mesozoic faunal distributions,
Science
251:1063–1065.

A recent review on the origin of teeth, bone, and skulls, in particular the new evolution gleaned from conodont animals, is found in Donoghue, P., and Sansom I. (2002) Origin and early evolution of vertebrate skeletonization,
Microscopy Research and Technique
59:352–372. A thorough review of the evolutionary relationships among conodonts and their significance is in Donoghue, P., Forey, P., and Aldridge, R. (2000) Conodont affinity and chordate phylogeny,
Biological Reviews
75:191–251.

CHAPTER FIVE GETTING AHEAD

A wonderfully comprehensive and detailed treatment of the details of skull structure, development, and evolution is found in a three-volume set:
The Skull,
James Hanken and Brian Hall, eds. (Chicago: University of Chicago Press, 1993). This is a multi-author update of one of the classic volumes on head development and structure: G. R. de Beer,
The Development of the Vertebrate Skull
(Oxford, Eng.: Oxford University Press, 1937).

Details of head development and structure in humans can be found in texts on human anatomy and embryology. For embryology, see K. Moore and T.V.N. Persaud,
The Developing Human,
7th ed. (Philadelphia: Elsevier, 2006). The companion anatomy text is K. Moore and A. F. Dalley,
Clinically Oriented Anatomy
(Philadelphia: Lippincott Williams & Wilkins, 2006).

Francis Maitland Balfour’s seminal work is encapsulated in Balfour, F. M. (1874) A preliminary account of the development of the elasmobranch fishes,
Q. J. Microsc. Sci.
14:323–364; F. M. Balfour,
A Monograph on the Development of Elasmobranch Fishes,
4 vols. (London: Macmillan & Co., 1878); F. M. Balfour,
A Treatise on Comparative Embryology,
2 vols. (London: Macmillan & Co., 1880–81); M. Foster and A. Sedgwick,
The Works of Francis Maitland Balfour,
with an introductory biographical notice by Michael Foster, 4 vols. (London: Macmillan & Co., 1885). A successor at Oxford, Edwin Goodrich, produced one of the classics of comparative anatomy,
Studies on the Structure and Development of Vertebrates
(London: Macmillan, 1930).

Balfour, Oken, Goethe, Huxley, and others were addressing the problem known as head segmentation. Just as the vertebrae differ in a regular progression from front to back, so the head has a segmental pattern. A selection of classic and recent resources (all with good bibliographies) to pursue this field further: Olsson, L., Ericsson, R., Cerny, R. (2005) Vertebrate head development: Segmentation, novelties, and homology,
Theory in Biosciences
124:145–163; Jollie, M. (1977) Segmentation of the vertebrate head,
American Zoologist
17:323–333; Graham, A. (2001) The development and evolution of the pharyngeal arches,
Journal of Anatomy
199:133–141.