iii
Missing Links
Evolutionary Concepts & Transitions through Time
by
Robert A. Martin
The McDonald & Woodward Publishing Company
Granville, Ohio
Missing Links
iv
The McDonald & Woodward Publishing Company
Granville, Ohio
www.mwpubco.com
Missing Links
Evolutionary Concepts & Transitions through Time
Text © 2014 by Robert A. Martin
All images not credited to other sources © Robert A. Martin
All rights reserved; first printing February 2015
Printed in the United States of America
by McNaughton & Gunn, Inc., Saline, Michigan,
on paper that meets the minimum requirements of permanence
for printed library materials.
10 9 8 7 6 5 4 3 2 1
21 20 19 18 17 16 15
Library of Congress Cataloging-in-Publication Data
Martin, Robert A. (Robert Allen), author.
Missing links : evolutionary concepts & transitions through time / by Robert
A. Martin. — Second edition.
pages cm
Includes bibliographical references and index.
ISBN 978-1-935778-28-8 (pbk. : alk. paper)
1. Vertebrates—Evolution. 2. Vertebrates, Fossil. 3. Evolution (Biology)
I. Title.
QL607.5.M36 2014
596—dc23
2014027532
Reproduction or translation of any part of this work, except for
short extracts used in reviews, without the written permission of
the copyright owner is unlawful. Requests for permission to
reproduce parts of this work, or for additional information, should
be addressed to the publisher.
v
Table of Contents
Preface: Preface to the Second Edition
ix
Introduction: What Is a Missing Link?
3
Section I
A Primer of Evolutionary Science
Chapter 1: Sources of Knowledge and Earth History
9
1.1 There Are No False Clues; Science As a Way of Knowing
9
1.2 An Ancient and Dynamic Earth
16
1.3 Time Enough Indeed
23
Chapter 2: Classification and Ecology: Making Sense of Nature 35
2.1 Classification and Phylogeny
35
2.2 The Ecological Context
45
Chapter 3: The Origin and Evolution of Species
67
3.1 Microevolutionary Patterns and Processes
68
3.2 Macroevolutionary Patterns and Processes
104
Section II
Case Histories
Chapter 4: Origins
161
4.1 The Origin of Life
161
4.2 The Fossil Record of Early Life
168
Chapter 5: Fishes with Fingers?
181
5.1 What Is a Fish?
184
5.2 What Is a Tetrapod?
186
5.3 Eusthenopteron, Panderichthys, and Tiktaalik: Links to
Tetrapods
189
5.4 Experiments with Limbs
192
Missing Links
vi
Chapter 6: Eye of the Flounder
201
6.1 Flatfish Biology
202
6.2 Development
204
6.3 Fossil Record
205
6.4 Interpretation and Phylogeny
206
Chapter 7: The Thanksgiving Day Dinosaur
209
7.1 Characteristics of Modern Birds
215
7.2 Genealogy of Dinosaurs and Birds
216
7.3 The Amazing Archaeopteryx
217
7.4 Reptiles, Specifically Dinosaurs, As Avian Ancestors
221
7.5 Origin of Flight
226
7.6 Frauds and Feathered Dinosaurs
231
7.7 Beyond Archaeopteryx
235
Chapter 8: Snakes with Limbs
241
8.1 Fossil Snakes with Limbs
241
8.2 How Snakes Lost Their Feet
243
Chapter 9: On Hearing and Hinges
247
9
.1 The Reptile–Mammal Transition
248
9.2 Evolution of the Hearing Apparatus and the Earliest
Mammals
253
Chapter 10: Pedestrian Whales
257
10.1 Characteristics of Modern Whales
258
10.2 Early Whales: Transition from Land
263
10.3 Whales Take to the Sea
267
Chapter 11: The Kentucky Derby in Deep Time: The
Cenozoic Evolution of Horses
275
11.1 What Are Horses?
276
11.2 Orthogenesis and the Bushy History of Horses
278
Chapter 12: Evolving Voles and Muskrats
287
12.1 North American Voles
293
12.2 Anatomy of a Speciation Event: European Water Voles 296
12.3 The Muskrat Ramble
302
Chapter 13: History of the Caddyshack Rodent in
Southwestern Kansas
311
13.1 The Replacement Pattern
312
13.2 Size and Morphological Change, and the Discovery
of an Intermediate
314
vii
Chapter 14: A History of Upstanding Primates
319
14.1 Before Adam: Living and Fossil Monkeys and Apes
322
14.2 Earliest Hominids
329
14.3 Australopithecus africanus (2.8-2.5 Ma),
Australopithecus garhi (2.7 Ma), and
Australopithecus sediba (1.98 Ma)
340
14.4 The Robust Australopithecines (2.7-1.3 Ma)
344
14.5 The Earliest Humans (2.4-1.6 Ma): Homo habilis and
Homo rudolfensis
348
14.6 Advanced Fossil Hominins and the Origin of
Modern Humans
350
14.7 Putting It All Together: A Summary of the Fossil
Evidence for Human Evolution
368
Chapter 15: Yesterday, Today, and Tomorrow
375
15.1 A Snail’s Pace Is Not Necessarily Slow
376
15.2 Evolutionary Change among Living Animals
377
Epilogue: Charles Darwin and the Fossil “Problem”
409
Glossary
415
Index
427
ix
Preface
Preface to the Second Edition
Missing Links has a new publisher, McDonald & Woodward,
specialty publishers in natural and cultural history. My original
intent for Missing Links was for it to be as readily available to the
general public as to college students, and I feel comfortable that in its
new home its first audience will now be as well served as the second.
Some things change a lot, some a little. Since the first edition
of Missing Links was published in 2004, the stories of life’s ori-
gins, early tetrapods, dinosaurs, and hominids has changed signifi-
cantly, sometimes with fascinating new discoveries such as the
ancient tetrapod Tiktaalik, a bevy of feathered dinosaurs, and the
enigmatic hominin Australopithecus sediba, and sometimes with
new interpretations, such as the modern focus on deep sea hydro-
thermal vents as the likely spawning ground for early life. The
terrestrial origin of whales is more firmly documented by skel-
etons of a new Eocene protocetid with a pregnant female showing
a near-term fetus in head-down position, as in terrestrial mammals
but not aquatic whales. Although the fossil history of voles has not
been wracked with new discoveries that shake its foundations, the
very density of fossils in this rapidly evolving group provides us
with novel insights into the timing of anatomical change and the
possibility of multiple origins of modern groups from separate an-
cestors. A new chapter documents the complex history of pocket
gophers, burrowing rodents from the Meade Basin of southwest-
ern Kansas, and another chapter considers the origin of snakes from
both evidence in the fossil record and our new understanding of
the role of Hox genes in limb formation. Finally, as additional in-
formation from long-term studies of extant animals continues to
Missing Links
x
accumulate, the more we realize the potential for rapid change
within species during the lifetime of a single observer, as with the
potential speciation of mosquitoes in the London underground rail-
way system since the 1940s.
The reader will notice the absence of a chapter on plants. In
the original manuscript I made the “executive decision” to limit
the example chapters to animals, in part to maintain the contents
within certain page limits and partly because the fossil history of
plants is not as dense as that for animals. Besides, plants seem to
hybridize so easily that determining fossil histories at the species
level is almost impossible. The absence of a chapter on fossil in-
vertebrates is for a similar reason; they are so developmentally and
anatomically flexible that identification of fossil species and inter-
pretation of morphological changes are problematic. The remain-
der of the Preface is similar to that in the first edition, with the
obvious addition of the new gopher and snake chapters and a vari-
ety of comments that refine the original chapter descriptions to
reflect new information and fossil material. I have also rearranged
the case histories in roughly phylogenetic order.
When Darwin published his Origin of the Species in 1859
very little fossil material was known. Darwin himself collected
many fossil specimens on his voyage around the world on H.M.S.
Beagle, but they were primarily described by Richard Owen, an
English paleontologist who rejected Darwin’s mechanism of natu-
ral selection for evolutionary change. Owen was a pretty active
fellow; he also named the Dinosauria. In the United States, the
first fossils were gathered from a marsh somewhere along the Ohio
River in 1739 by a French Canadian officer, Baron Charles de
Longueuil. It is a strange and sometimes ironic aspect of evolu-
tionary science that some of those influential in the development
of the modern synthesis of evolutionary theory themselves did not
embrace evolution or its primary mechanisms, either because they
were unaware of it or simply rejected it. A few of these include:
Carolus Linnaeus, a Swedish physician and father of animal and
plant classification, who believed he was cataloguing examples of
God’s omnipotence; Baron Georges Cuvier, the father of comparative
xi
anatomy and paleontology, who was convinced that God initiated
“catastrophes” to erase entire communities of organisms; Alfred
Wallace, who developed the theory of natural selection at the same
time as Darwin and later rejected the idea that humans could have
resulted from this process; and the afore-mentioned Richard Owen,
England’s premiere paleontologist. But the one-two-three punch
provided by James Hutton’s Theory of the Earth, Charles Lyell’s
Principles of Geology, and Darwin’s Origin of Species sent
shockwaves through the scientific community and began a mod-
ern exploration for fossils and their meaning that continues to this
day. Hutton’s work, published in 1785 and popularized by Lyell in
his 1830-33 three volume tome, was particularly influential for
Darwin, as Hutton’s principle of uniformitarianism, the concept
that the same slow and methodical forces at work on the planet
today shaped the planet in the past, provided circumstantial evi-
dence for an ancient Earth. Fundamentalist Christians of Darwin’s
time fought Darwin very strongly, for a literal reading of the Chris-
tian Bible is incompatible with both an ancient Earth and the con-
cept of evolution. It is an amazing testimony to the power of reli-
gious influence that millions of Americans today still follow this
basic credo. And one of the tenets of those who support a “cre-
ationist” model for humans and the universe is that there are no
true “missing links,” extinct organisms that connect modern and
ancient life in an unbroken chain through long periods of time,
measured in the millions and billions of years. But we have come
a long way since Darwin’s day. Thousands of expeditions have
unearthed literally millions of fossils, today housed in many muse-
ums and academic institutions worldwide. Paleontologists have
described fossils 3.5 billion years old, and organisms are known
continuously from that point onward. Many “missing links” are
recognized (though, as we shall see, a fossil “missing link” is not
exactly what most creationists have in mind), and it is the primary
purpose of this book to provide a compendium of this information
for the general reader in the context of modern scientific inquiry.
Missing Links is designed to satisfy two needs. First, as repre-
sented by the case histories, it can be read purely as a compilation
Preface
Missing Links
xii
of fossil histories in support of evolutionary theory. It is my hope
and intention that this anthology be used in every conceivable cir-
cumstance where such information is needed, from a student class-
room presentation to an intellectual religious discussion. The sec-
ond usage, represented by Section I, is as a primer on evolutionary
science, particularly as it applies to fossil materials. Evolution can
no longer be expressed by the simple phrase “survival of the fit-
test.” Hundreds of scientists have worked carefully for 150 years
to establish evolution as a theory that is profound in scope and
universally supported, and more scientists are likely at work today
on evolutionary problems than were active in the first 125 years
combined. This section should also be of interest to professionals,
as I present new perspectives on concepts such as punctuated equi-
librium and species selection.
Evolutionary science encompasses many fields and can be
extremely complicated to the novice. Consequently, I have pre-
pared a synthesis in Chapters 1-3 of the major concepts, processes,
and vocabulary necessary for a reasonable understanding of evolu-
tion, particularly as it applies to interpretation of the fossil record.
Because the focus of this treatment is on transitions in the fossil
record, there is little here on evolutionary genetics as practiced
with extant organisms. For those interested in this area, I recom-
mend the sections on that topic in the texts Evolutionary Analysis
by Scott Freeman and Jon C. Herron (2007; Prentice Hall) and
Evolution (2009; Sinauer) by Douglas Futuyma. Although it is not
necessary to be familiar with the contents of Section I to consult
the case histories, it will be of considerable benefit. This section
begins with a chapter on how science operates as a method of know-
ing. Most people fail to understand that science is not limited to
chemistry, biology, physics, etc., the so-called “hard” sciences.
Everything we do as an adult is based on a series of experiments
we performed as we were growing up. We could not function if
this were not so. Hopefully, as this universal aspect of science be-
comes more widely known and appreciated perhaps science will
not seem so mysterious. Chapter 1 also documents the ancient age
of the Earth and its dynamic nature. One of the greatest and most
xiii
powerful discoveries of our time, the theory of continental drift, is
reviewed. This is followed by a more specific examination of meth-
ods for dating rocks and fossils, called chronometry.
The publication of German systematist Willi Hennig’s book
Phylogenetic Systematics in 1966 led to a major revolution in the
way classification is accomplished. Today, with the help of power-
ful computer programs such as MacClade and PAUP, we methodi-
cally examine and analyze character variation in modern and fos-
sil organisms in order to identify relations among them. The un-
derlying philosophy of phylogenetic systematics, or cladistics as it
is sometimes known, is that classification should represent phy-
logeny, or the revealed genealogy of organic life. Because evolu-
tionary processes seem to work primarily at the population level
within species, and also because species seem to have morpho-
logical boundaries (albeit sometimes fuzzy ones) and are therefore
recognizable units, I have provided in Chapter 2 a brief examina-
tion of the species concept, concluding that species are dynamic
units, full of variation in time and space. Chapter 2 also considers
the myriad influences that lead to the creation of a fossil locality
and the way scientists learn about past environments. What forces
affect the burial, preservation and distribution of fossils? In what
ways can fossils be used to interpret past climates? In addition to
these applications, fossils also can lead to an understanding about
how ecological communities change through time.
Chapter 3 has been completely rewritten to incorporate more
detail on almost every aspect of the evolutionary process. A revo-
lution is underway in our understanding of evolutionary change,
originating from the discovery of Hox gene complexes, conserved
gene groups shared by all metazoans (complex organisms), and
how these genes are responsible for basic body symmetry and shape.
Complex changes in body form may be the result of disproportion-
ate influence of only a few regulatory genes. Observations and pro-
cesses in the lifetime of single species (variation, natural selection,
genetic drift) are considered separately from patterns and processes
involved in the origin of new species and subsequent radiation within
clades. Can small-scale mutation in populations and intraspecific
Preface
Missing Links
xiv
natural selection explain life’s full panorama of diversity, or are
other processes, perhaps involving a selection mechanism at the
species level, necessary? How do we explain long-term morpho-
logical (anatomical) trends in species? Can trends arise by random
processes? Extinction, the flip side of origination, is also exam-
ined at different temporal and mechanistic scales. As the late
Stephen Jay Gould often noted, contingency (chance) can have a
powerful influence on the overall direction of life’s history.
The case histories begin, in Chapter 4, with a scenario for the
appearance of life on Earth (with contributions from Mars, if that
turns out to be the case). We see how Stanley Miller and others have
generated all the building blocks of complex cellular molecules, such
as DNA and ATP, in the laboratory. The mixture used by Miller may
not have been exactly appropriate, however, as other scientists con-
clude that life arose in deep sea hydrothermal vent ecosystems;
strange associations of organisms that exist without sunlight.
In Chapter 5 we examine one of the great changes in the his-
tory of vertebrate animals; namely, the transition from water to
land, or terrestrialization. Traditional ideas on the origin of limbs
in land animals, such as possessed by the modern amphibians, pro-
posed that hands and feet evolved to support the body on land. Not
so, says Jennifer Clack of Cambridge University. Her work indi-
cates that tetrapod (four-footed animal) limbs appeared first in fully
aquatic animals! If limbs are not present for support on land, then
what was their original purpose?
I almost retitled this book Eye of the Flounder, so excited
was I over recent discoveries regarding the origin of one of Earth’s
most amazing groups of animals, the “flatfish,” best represented
by flounders, halibut, sole, and the like. During development one
eye of these fishes migrates to the other side, and the adult ends up
swimming on its side, with its mouth still pointing sideways. I won’t
spoil the story by disclosing more information here, but new fossil
finds show that this condition in modern species evolved gradually
from Eocene ancestors.
Archaeopteryx needs little introduction, but again, as in
whales, there have been many new discoveries linking early birds
xv
to their modern relatives. Chapter 7 examines these new finds in the
context of a raging debate about bird ancestry. Some paleontologists,
such as Robert Bakker, Louis Chiappe, and Kevin Padian, are con-
vinced that birds arose from dinosaurs; in fact are dinosaurs, whereas
another camp, led by Alan Feduccia and the late Larry Martin, think
that birds originated from an earlier reptilian ancestor that may have
given rise to both dinosaurs and birds. Whatever the outcome, new
fossils have cemented the links between reptiles and birds. Lots of
early “toothy” and feathered relatives are now known. New discover-
ies of fossil snakes with well-preserved partial hindlimbs prompted
me to include Chapter 8 on the fossils and work by developmental
biologists that help explain how limbs were lost in serpents.
Chapter 9 reviews the fossil evidence for the transition from
reptiles to mammals. It is an interesting segue, involving not only
modifications in thermoregulation, but also in hearing and food
acquisition and processing. Were the last mammal-like reptiles
endothermic? Did they possess fur? How does one define and rec-
ognize a mammal? Examination of the fossil record will show how
this question is answered with the fossil material available.
Chapter 10 tells the magnificent story of whale evolution that
has unfolded only in the last 20 years, complete with aquatic inter-
mediates sporting tiny hooves on their feet! In Chapter 11 we look
at the vast Cenozoic panorama of horse evolution, showing the
eventual progression of changes leading from tiny forest dwelling
ancestors to the large and speedy descendants of today. Thanks
largely to the modern work of Bruce MacFadden and Richard
Hulbert, we now can see that horses were a diverse lot, with many
experiments in both size and morphology that did not survive.
Yours truly and others have chosen to work with the fossil
history of the innocuous but ubiquitous rodents, in the hope that
the dense fossil record of these animals will reveal the secrets of
evolution, if for no other reason than that there are so many fossils
available for study. Chapter 12 provides examples of short-term
change in species, or microevolution, as well as a likely record of an
arvicolid speciation event. In the rodents we have good fossil evi-
dence for considerable morphological and size change at different
Preface
Missing Links
xvi
chronological scales. Here we can get a close-up look at how links
operate, and we see that it is often a blurry mess of related popula-
tions originating in an almost helter-skelter fashion, each with its
own set of characteristics and evolutionary trajectories. There is a ben-
efit to working with a group that has a dense fossil record.
Chapter 13 continues the stories provided by the rodent record,
this time with subterranean rodents from southwestern Kansas
known as pocket gophers, those furry animals that made life mis-
erable for Bill Murray in the movie Caddyshack. The Meade Basin
record of these rodents is unparalleled for any vertebrate animal
group in the late Cenozoic (last 5 million years), and here we see
that despite their success today, their history is replete with species
extinctions and replacements. The Kansas record also demonstrates
a significant anatomical transition, from animals with rooted to
unrooted cheek teeth (premolars and molars), the latter which char-
acterizes all living pocket gophers.
Our own heritage is taken up in Chapter 14. As with the whales
and birds, new hominids are being reported regularly. As I began
outlining this chapter for the first time, Australopithecus
bahrelhghazali was described from three to four million year old
deposits in Chad. Soon after, the amazing, diminutive Homo
floresiensis was discovered in southeast Asia, and in 2010
Australopithecus sediba was described from South Africa. It is an
exciting time in paleoanthropology, and as one might expect, there are
probably as many theories of human relationships as there are investi-
gators. There was an old joke that suggested there are likely as many
anthropologists studying hominids as there are fossils, a past testa-
ment to a lousy fossil record, but that isn’t true any more. There are
now hundreds of specimens of Australopithecus afarensis alone.
The final chapter is one that may seem unusual in a book
on missing links, but hopefully by the time the reader has made it
this far its purpose will be obvious. In this chapter I consider the
“smoking gun” of evolution; evolution in action during the life-
time of the reader as an observer. Missing links on a generational
scale, complete with the sudden appearance of new forms, ana-
tomical change, and the potential for new species.
xvii
Acknowledgements
I must first acknowledge evolutionary scientists worldwide,
whose diligent research provided the source of most of the infor-
mation in this book. I could not cite even a small part of the tre-
mendous written database supporting Darwin’s grand theory of
evolution, and I apologize to the hundreds of scientists, past and
present, whose names do not appear here.
Many of my colleagues contributed in one way or another to the
writing of this treatment. Pablo Peláez-Campomanes and the late Jim
Honey taught me everything I know about field paleontology, from
identifying likely fossiliferous sites, to stratigraphy, to field procedures
to reclaim and preserve the fossils. Ted Daeschler, Robert Carroll, J.
W. Schopf, Jim Hopson, Michael Bell, Richard Hulbert, Jr., Hans
Thewissen, Ian Tattersall, Larry Martin, Howard Whiteman, Jennifer
Clack, Alan Feduccia, Frances James, Kevin Padian, Dave Canning,
and Ed Zimmerer either reviewed chapters or provided useful infor-
mation and opinions for both editions. Thanks are also due Ian Tatter-
sall for granting permission to use some of the many outstanding illus-
trations in his book The Fossil Trail. I tried out early versions of Miss-
ing Links in my undergraduate Evolution classes at Murray State Uni-
versity, and I appreciate the help provided by Tom Timmons in mak-
ing the various accommodations necessary for this process.
Judy Hauck, a former editor at Jones & Bartlett, was the first
individual in the publishing business to get excited about Missing Links,
and I thank her and the staff at Jones & Bartlett for publishing the first
edition. The second edition owes its existence to publication by
McDonald & Woodward, and I want to thank Jerry McDonald, whose
work in paleontology I have known about for many years, for recog-
nizing the value of Missing Links and giving me the opportunity to
produce a second edition. My extended family, Marsha, Rachel, Jer-
emy, and Alice, provide constant encouragement and support, and
I also want to acknowledge my second wife, the late Linda L. Mar-
tin, for her many kindnesses and help in myriad ways.
Preface
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