How
did the world of dinosaurs differ from our own? Since we live in a miniscule
snapshot in time, most people can’t relate to a thousand years,
let alone millions, or billions of years. So how do we get our minds
wrapped around Mesozoic timescapes? And once we’re there, how
do we then recreate the world of dinosaurs?
AN
ECO-EVOLUTIONARY DANCE THROUGH DEEP TIME
[6.17.04]
A Talk with Scott Sampson
Introduction
Over
the past few years, Edge has published several pieces
on Stewart Brand's "long now" idea, the most
recent concerning the installation Danny Hillis' 10,000-year
clock on a mountain in Nevada. If Brand is identified
with the "long now", then Sampson, a University
of Utah paleontologist, is the champion of the "long
then". His work on dinosaurs is concerned with new
ideas involved in negotiating the "eco-evolutionary
dance through deep time".
Sampson, the host of Discovery Channel’s "Dinosaur
Planet", is "most fascinated by the Late Cretaceous,
in particular the last 15 million years of the Mesozoic
(80-65 million years ago), just before a giant asteroid
(or whatever it was) slammed into the planet. We know
more about dinosaurs from this time than from any other.
Similarly, the place I’m most interested in is western
North America, because we know more about the dinosaurs
from this region than from any other. Now we can begin
to consider questions like, what role did dinosaurs play
in their ecosystems? How did they relate to their environments,
and what were these environments like? With often gigantic
sizes, dinosaurs pushed the envelope of what it is to
be a land-living animal; how were they able to do that?
Perhaps most importantly, how did evolution and ecology
converge to drive the various dinosaur radiations, and
why were these oversized reptiles so successful for so
long? In short, how did evolutionary and ecological processes
combine to drive changes in dinosaurs? Paleontologists
are only beginning to take this eco-evolutionary perspective,
with important new insights.”
—JB
SCOTT SAMPSON is a paleontologist with a dual position
at the University of Utah as Chief Curator at the Utah
Museum of Natural History and Associate Professor in the
Department of Geology and Geophysics. He is also host
of "Dinosaur Planet," a recent series of four
animated television shows on the Discovery Channel.
Scott
Sampson's Edge Bio
Page
AN
ECO-EVOLUTIONARY DANCE THROUGH DEEP TIME
(SCOTT
SAMPSON):
Like many kids, I was into dinosaurs at a young age. Unlike most kids,
I never grew out of it. It’s been very interesting watching
the evolution of dinosaur paleontology. When I was a kid, dinosaurs
were generally regarded as sprawled, sluggish, dim-witted, and lizard-like—in
short, not very interesting. That view held sway for most of the last
century. Then, in the late 1960s, Yale paleontologist John Ostrom
proposed that dinosaurs were very bird-like and that birds may be
the direct descendants of dinosaurs. Although the bones on museum
shelves remained unchanged, paleontologists suddenly began to look
at dinosaurs differently. Virtually overnight, these prehistoric beasts
became supercharged— smarter, faster, warm-blooded, and more
complex. Now more like birds than lizards, evidence was found for
such behaviors as parental care and cooperative hunting. Ultimately,
we started to question whether this new view was real. Nevertheless,
all of this dynamism within the science sparked a whole new generation
of children (of all ages) with a passion for dinosaurs.
People often forget that paleontologists are extremely limited
in the data they have access to. For the most part, we only
have bones and teeth to work with, so it’s really the
ideas that drive the science. And the ideas, of course, are
driven by the biases of that particular moment. Dinosaur paleontology
went from a lizard bias to a bird bias, and now the pendulum
is swinging back toward the middle. For example, the top speeds
of many dinosaurs have been slowed somewhat, and the assumption
of warm-bloodedness has shifted toward talk of a range of
metabolic strategies concentrated between those of reptiles
on the one hand and birds and mammals on the other.
Like their object of study, dinosaur paleontologists have
undergone a major transformation in recent decades. In the
1800s, paleontologists were trained largely as biologists,
with a strong grounding in anatomy. During most of the 20th
century, there was a shift in emphasis and most practitioners
were trained as geologists. This subject duality reflects
the schizophrenic nature of the field, sitting on the cusp
of two major disciplines: biology and geology. With a geologic
focus, many paleontologists rarely considered the biology
of their study organisms. That situation is changing, moving
back toward the biological end of the spectrum.
These days, there’s a variety of new, often high-tech
tools being applied to paleontology. Let’s say you find
a virtually complete, intact dinosaur skull. Of course, there
is much to be seen from the outside, yet a tremendous amount
of anatomical data is locked within, obscured by the rocky
matrix. Typically, this sediment can’t be removed, because
the process would damage the specimen. However, we can run
such specimens through a high resolution CT scanner that enables
us to look inside the skull (or whatever) and reconstruct
important features like brain size and shape. We can estimate
which portions of the brain were most developed, which in
turn permits hypotheses about sensory abilities, standard
head position in life, and even aspects of behavior.
Another very different tool, yet one also dependent upon technology,
is a widespread method of reconstructing the historical relationships
of organisms. Known as phylogenetic systematics, or cladistics,
this technique enables biologists to assess the distribution
of shared, specialized features within a group of study organisms.
Large datasets, sometimes involving dozens of different groups
and hundreds of characters, are processed using high-speed
computers, which can sift through hundreds of thousands of
branching alternatives in search of the simplest one(s), requiring
the fewest number of evolutionary steps. Generally depicted
as branching diagrams, or trees, these hypotheses are effectively
estimates of evolutionary patterns of descent. Cladistics
has become standard operating procedure throughout paleontology,
and biology generally, revolutionizing our ability to determine
organismal relationships.
An understanding of these relationships turns out to be prerequisite
to most other kinds of studies within evolutionary biology.
For example, there are well over 100 shared, specialized morphological
characters now that link dinosaurs to birds. This is strong
evidence supporting the notion of a close evolutionary bond
between these groups. Within dinosaurs, the evidence indicates
that birds show the greatest affinity with small, carnivorous
dinosaurs, informally known as “raptors.” Until
recently, feathers were the quintessential feature of avians,
associated only with flight. Now, thanks to an amazing fossil
locality near Liaoning, China, we have specimens of feathered
raptor dinosaurs that clearly did not fly. This tells us that
feathers evolved prior to flight, meaning that they must have
first evolved for some other reason—perhaps for controlling
body temperature, or for use in display. The point is that,
until we understood the evolutionary relationships, we couldn’t
make an argument as to the progression, or evolution, of feathers.
These are the types of hypotheses currently being tested that
would have been impossible 15 or 20 years ago, because we
simply didn’t have the computing power to assess alternative
hypotheses.
Another growing trend within paleontology (actually a re-emergence
of earlier methods) is to use detailed studies of living animals
in order to investigate the physiology, anatomy, and behavior
of dinosaurs and other extinct organisms. Much of this work
is directed toward reconstructing soft tissues, since bones
are in many ways simply the framework used by vertebrates
to anchor their other varied tissue types. So let’s
say you’re interested in assessing the maximum running
speed of Tyrannosaurus rex, the largest carnivore ever to
walk the Earth. It’s been argued that this predatory
behemoth, weighing in at about 6,000 kg, was capable of running
at jeep-chasing speeds in excess of 40 mph. Others have claimed
such excessive speeds to be nonsense.
How can we test this idea? Given that you can’t observe
the behavior of T. rex directly, you might create a biomechanical
model based on engineering principles. Yet any such models
would be limited by the accuracy of its parameters. So there
is a need for relatively high-resolution biological data.
Since bones are the only tissue type for which we have good
information, you might look at the cross-sectional properties
of the hind limb elements. Yet even this is not sufficient,
since we need some idea of the muscles involved. For muscles,
we can turn to closely-related living animals, such as birds
and crocodiles. If a particular soft tissue is found in one
or both groups, its presence can be inferred with some confidence
in the common ancestor, and thus in dinosaurs as well. This
“phylogenetic bracket” method, pioneered by Larry
Witmer, allows us to reconstruct not only muscles, but, at
least in some cases, blood vessels, nerves, and other structures.
These soft tissues often leave bony marks, or “osteological
correlates”, in the form of scars, holes, grooves, and
the like.
An exceptional young scientist named John Hutchinson, then
a graduate student at UC Berkeley, recently employed just
this combination of methods—an engineering model and
muscle reconstructions—in order to test the locomotory
abilities of Tyrannosaurus rex. John was able to model a bipedal,
six tonne dinosaur predator and determine how much muscle
mass would be necessary in the hind limbs to propel the animal
at a speed of 40 mph. He concluded that such high running
speeds would have required that T. rex devote something on
the order of 80% of its body mass to hind limb muscles! Clearly
this was not the case, so we’ve slowed those animals
down somewhat. And lest you jump to the conclusion that the
dinosaurian tyrant king must therefore have been a lowly scavenger
(as has been argued), it is important to add that its likely
prey were likely still significantly slower than this giant
meat-eater.
Reconstructing soft tissues works best if we can use living
close living relatives to reconstruct the anatomy of extinct
forms. But what if the structure in question isn’t present
among extant relatives? For example, let’s say you’ve
got an eight-foot long skull of a Triceratops-like dinosaur
adorned with horns over the nose and eyes, and an elongate
bony frill sticking out behind. Traditionally, it was thought
that such bizarre structures functioned first and foremost
in defense against predators. Alternatively, others have suggested
that these bony bells and whistles functioned in control of
body temperature, in part by increasing the animal’s
overall surface area. More recently, the “in vogue”
hypothesis has been mate competition, with the horns and related
features used either to attract members of the opposite sex
or to intimidate same sex rivals. How could you assess these
alternatives? In this case, you might turn not to close living
relatives, but to extant analogues—that is, animals
with similar kinds of structures. This is exactly what I did
in a previous research project.
Many living animals have horns or hornlike organs; the list
includes antelope, deer, chameleons, birds, and even ants.
In virtually all these instances, these features function
primarily in the competition for mates, either in display
or in actual physical encounters. Importantly, species within
these groups tend to be distinguished mostly or solely on
the basis of these same characteristics. In other words, we
are distinguishing species based on the same features that
they themselves are using. This pattern holds for the dinosaurs
as well. Whether it’s horned dinosaurs, crested duck-billed
dinosaurs, plated stegosaurs, or dome-headed dinosaurs, paleontologists
identify different species largely by these bizarre structures.
Moreover, in both the living and the fossil examples, these
structures tend(ed) to develop fully only as the animal’s
approach(ed) sexual maturity and adult size. Together, this
and other evidence strongly supports the mate competition
hypothesis. It also underlines the importance of using living
animals—for which we can examine more anatomy and observe
actual behaviors—to assess the biology of extinct animals.
Certainly much of the best paleontology done today synthesizes
data from the modern and fossil realms.
Nevertheless, despite all the new perspectives, innovative
technological applications, and revealing comparisons with
living forms, it’s my concerted opinion that dinosaur
paleontology (and indeed evolutionary biology generally) is
currently sitting on the cusp of an entirely new era of discovery,
one focused on connections. The great majority of current
work within paleontology, and vertebrate paleontology in particular,
is devoted to investigating patterns—for example, determining
which animals lived where and when, as well as their interrelationships.
While such work is obviously critical, it falls within the
realm of alpha level science. Moreover, the underlying paradigm
guiding this work emphasizes unique, historical events rather
than common processes, let alone laws.
As many readers of Edge are aware, there is a strong trend
within the physical, natural, and social sciences away from
the traditional reductionist paradigm that has reigned over
science for centuries. The new paradigm looks instead at the
bigger picture of interrelationships among systems. Places
like the Santa Fe Institute in New Mexico encourage scientists
of various ilk to come together, learn to speak a common language,
and concoct new ways of thinking about the world. This trend
is just beginning to trickle down evolutionary biology, with
increasing movement toward cross-disciplinary research programs.
Consequently, the field is becoming much more interesting
and dynamic, with collaborations bringing together, for example,
paleontologists, ecologists, paleoclimatologists, and geologists.
The resulting questions, and thus the answers, tend to differ
under this complexity-based paradigm. How did the world of
dinosaurs differ from our own? Since we live in a miniscule
snapshot in time, most people can’t relate to a thousand
years, let alone millions, or billions of years. So how do
we get our minds wrapped around Mesozoic timescapes? And once
we’re there, how do we then recreate the world of dinosaurs?
What role did dinosaurs play in their ecosystems? How did
they relate to their environments, and what were these environments
like? With often gigantic sizes, dinosaurs pushed the envelope
of what it is to be a land-living animal; how were they able
to do that? Perhaps most importantly, how did evolution and
ecology converge to drive the various dinosaur radiations,
and why were these oversized reptiles so successful for so
long? In short, how did evolutionary and ecological processes
combine to drive changes in dinosaurs? Paleontologists are
only beginning to take this eco-evolutionary perspective,
with important new insights.
It turns out that the Mesozoic Earth was both very different
from and extremely similar to the world we know today. This
was a time lacking in polar caps, tropical rain forests, and
grasslands. Yet habitats functioned in exactly the same way
as those we are familiar with. Nutrients and chemicals cycled
through ecosystems. There were primary producers in the form
of plants and bacteria. There was a diverse array of consumers,
both herbivores and carnivores, and this of course is where
the dinosaurs came in. Completing the cycle were numerous
decomposing organisms. For example, paleobiologist Karen Chin
has described evidence from the fossilized feces of dinosaurs
demonstrating that dung beetles existed during the Mesozoic.
As with their living descendants, these dung beetles metabolized
fecal material, recycling components so they could be reused
by other organisms. Another recent discovery has been the
influence of bacteria on fossilization. It appears that many
remarkable kinds of fossils, including the rare examples with
preserved soft tissues such as skin and feathers, are due
in large part to bacterial activity. Indeed it may be that
bacteria are pivotal to the formation of fossils in general,
something we hadn’t really thought much about previously.
Once we better understand the ecological role of a given morphological
structure, we can then contemplate its evolutionary implications.
For example, I noted above that closely related species of
various dinosaur groups are often distinguished solely on
the basis of structures interpreted as mate signals. It is
remarkable how conservative these animals are in other aspects
of their anatomy, including the teeth, limbs, and vertebral
column. It’s as if these groups settled on a successful
design and evolution then tinkered with the window-dressing.
We see the same thing today in birds, fishes, and other groups.
There’s the oft-cited example of cichlid fishes in the
East African great lakes, one of the greatest vertebrate radiations
of all time. These animals identify members of their own species
largely based on color patterns. These designs enable them
to determine, “you’re one of mine and you’re
not. I can mate with you but not you.” Recently, deforestation
and subsequent erosion have been rampant along the margins
of these lakes. Erosion transports abundant soil into the
lakes. The water becomes murkier, and the fish can no longer
see each other as well as they could before; certainly they
can’t discern colors nearly as well. All of a sudden,
they start to mate with individuals from different species
because they can’t recognize members of their own kind
any more. The species boundaries turn out to be quite fragile,
with the cross-species unions actually generating viable offspring.
This pattern underlines the importance of mating signals,
which are often the very first things to change when new species
form.
Increasingly, biologists point to two distinct factors necessary
for the origin of species in macro-sized vertebrates like
dinosaurs. First, there must be persistent isolation of sub-populations
of a given species, so that interbreeding cannot occur, or
at least is severely limited. Second, the genetic make-up
of those sub-populations must differentiate to the point that
individuals of one group can no longer reproduce successfully
with those of the other. Recognizing these minimal requirements,
we can explore the process of evolutionary radiations rather
differently. Let’s take an example from dinosaurs.
I’m most fascinated by the Late Cretaceous, in particular
the last 15 million years of the Mesozoic (80-65 million years
ago), just before a giant asteroid (or whatever it was) slammed
into the planet. We know more about dinosaurs from this time
than from any other. Similarly, the place I’m most interested
in is western North America, because we know more about the
dinosaurs from this region than from any other.
It turns out that there was a great deal of environmental
change going on in North America during the Late Cretaceous.
Increased plate tectonic activity translated into rampant
volcanism, which in turn pumped abundant CO2 into the atmosphere.
Global climates responded with increased warming and higher
sea levels, which in turn resulted in flooding of most major
continents. The climate even at high latitudes during much
of this period was warm and equable year-round, described
by one investigator as “wall-to-wall Jamaica.”
One of these continental seaways extended from the today’s
Arctic Ocean to the Gulf of Mexico, splitting North America
in two. Exquisite beachfront property could have been had
at this time in Colorado, Montana, or Utah. The adjacent seaway
wasn’t static but rather expanded and contracted over
time. During times of expansion, or transgression, the animals
living on the western North American landmass were sandwiched
between the seaway to east and a rising mountain chain, the
Cordilleran thrust belt, to the west. The flowering plants,
or angiosperms, literally blossomed during this interval,
forming dense, closed canopy forests.
Amidst this dynamic environmental backdrop, various groups
of dinosaurs underwent dramatic radiations, with apparently
rapid rates of both speciation and extinction. Now, it may
be coincidence, or it may be that the environmental changes
were key factors driving this evolutionary change. Certainly
the transgressing seaway would have reduced available habitat
on land, likely fragmenting populations. Another factor in
this regard may have been the increased abundance of angiosperm
forests. Once populations became fragmented and isolated,
evolution apparently targeted mating signals, likely driven,
at least in part, by sexual selection.
Ecologically speaking, once two closely related species differing
only in reproductive structures (e.g., horns, frills, crests,
etc.) come back into contact, it’s unlikely that both
will persist for very long, since they will be doing the same
thing to make a living. The geologic record of North American
dinosaurs appears to support this pattern—that is, one
of the daughter lineages lives on while the other goes extinct.
So here we have the makings of an evolutionary scenario that
combines a dense fossil record with physical and biological
environmental changes to postulate an integrated hypothesis
of change over time.
A similar eco-evolutionary problem I have been pursuing, one
that also involves Late Cretaceous dinosaurs from North America,
is the evolution of gigantism in large carnivores. How does
evolution generate a 6,000 kg carnivore like T. rex? Few other
predatory dinosaurs approached such incredible masses, and
no other group of terrestrial carnivores has come close before
or since. In general, when confronted by such problems, the
tendency within evolutionary biology has been to focus on
single-cause explanations. But Nature is rarely so simple;
typically it’s necessary to consider multiple causal
factors. So I got together with some paleontological colleagues
of varying expertise— Mark Loewen, Jim Farlow, and Matt
Carrano—to tackle the question of giant dinosaur carnivores.
We set out to consider not only those forces that might drive
animals to gigantic sizes, but, equally important, forces
that would limit the attainment of such sizes.
Ultimately, we outlined several factors that, depending on
their timing and combination, could limit or promote gigantism
in terrestrial carnivores. First, we argued that the largest
carnivorous dinosaurs likely had intermediate metabolic rates—that
is, metabolic requirements higher than those of ectothermic
(cold-blooded) lizards but significantly below those typical
of endothermic (warm-blooded) mammals and birds. A low maintenance
metabolism appears necessary, since, in order to keep its
hot-blooded furnaces stoked, a lion-sized endotherm must consume
several times more food than an ectothermic lizard of the
same body mass. Calculations of estimated daily caloric intake
suggest that a T. rex-sized endothermic carnivore is highly
improbable, since it is very unlikely that it could have consumed
enough food to maintain its six tonne body mass.
Yet a low maintenance metabolism was not enough to result
in the evolution of gigantism. Over the entire 160-million-year
duration of dinosaurs, it was only during the Late Jurassic,
and particularly the Cretaceous, that Mesozoic ecosystems
were inhabited by truly giant meat-eating dinosaurs. So what
was going on earlier? We postulate that geography plays an
important role. Gary Burness, Jared Diamond and co-authors
have argued that geographic area dictates maximal body sizes
in terrestrial vertebrates. They were able to establish that
the larger the land area, the larger the maximal body mass.
These authors found somewhat different regression lines for
warm-blooded carnivores, cold-blooded carnivores, warm-blooded
herbivores, and cold-blooded herbivores. The regression lines
were highly predictive as well, such that you can accurately
estimate the shrinkage in body size that a mammoth species,
for example, would ultimately undergo if marooned on an island
of a particular size. Moreover, this and other studies show
that, in order to maintain populations sizes large enough
to stave off extinction, large warm-blooded carnivores such
as lions require vast, continent-sized species ranges. Humans,
of course, have restricted the movements of virtually all
large animals, but such extensive ranges were the norm traditionally.
Given the remarkable, virtually law-like consistency of this
relationship among living and recently extinct vertebrates,
we assumed that geography must also have been a major factor
governing dinosaur body sizes. We further postulated that
T. rex-sized carnivorous dinosaurs, whether warm- or cold-blooded,
would require vast species ranges. An examination of the fossil
record bears out this prediction; the largest carnivorous
dinosaurs occur only on continent-scale landmasses.
Yet this pattern in the fossil record raises a fundamental
question. When the dinosaurs originated in the Late Triassic,
about 230 million years ago, all of the continents were united
as the supercontinent Pangaea. Given this proposed relationship
between maximal body size and landmass area, you would expect
the largest carnivorous dinosaurs to occur when all the continental
landmasses were connected. But that’s not what we find.
It’s only during the Cretaceous, after most of the continental
fragmentation was completed, that the largest forms, such
as T. rex, existed. Clearly, then, intermediate-grade metabolic
rates combined with vast species ranges were insufficient
to provoke the evolution of meat-eating titans. We realized
that at least one other ingredient was necessary.
Next we turned to competition, a dominant concept in evolutionary
biology, yet one that has fallen out of favor somewhat in
recent years. Over the past two decades or so, paleobiological
hypotheses founded on competition have been brought into question,
and there has been much emphasis—rightfully so I think—on
the role of cooperation and symbiosis. Yet in this case we
argue for interspecific competition as a limiting factor.
When all of the continents were united as Pangaea, and even
during the initial phases of fragmentation, virtually every
terrestrial ecosystem for which we have good data indicates
the presence of multiple, perhaps two to four, kinds of large
carnivorous dinosaurs, in the range of 750-2000 kg. Given
the extensive continental connections, this was a time when
terrestrial animals were able to move around much of the planet.
It is also why we find remains of dinosaurs on every continent.
They didn’t need to fly or swim across major marine
barriers—they simply walked from landmass to landmass.
With all of this faunal mixing, it is not surprising that
we find multiple species of large carnivores in most ecosystems.
Unlike living carnivorous mammals, which often have highly
specialized teeth and jaws for particular diets (meat, bone
marrow, etc.), large carnivorous dinosaurs apparently lacked
such ecological diversity. So, given that they were all doing
pretty much the same thing to make a living, it seems reasonable
to postulate that inter-species competition would have limited
the maximal body size for any one species. It’s highly
unlikely that a given lineage could have evolved to be a giant
of five or six tonnes when several other species were in direct
competition in the same ecosystem. As the continents split
apart, dinosaurs and all other parts of the terrestrial biota
went along for the ride on these giant rafts of continental
crust, setting sail on independent different evolutionary
courses. We postulate that it was only after all the continents
broke apart that opportunities arose for a single species
to dominate an ecosystem and grow to T. rex proportions.
Indeed the evidence suggests that this is exactly what happened
with the tyrant king himself. About 75 million years ago,
when North America was divided into two landmasses by a seaway,
several smaller-bodied tyrannosaurs such as Daspletosaurus
and Gorgosaurus lived alongside one another. These animals
were large, about 1,000 to 2,000 kg, and no doubt menacing,
yet a fraction the size of their subsequent relative, Tyrannosaurus
rex, which lived about 67-65 million years ago. In contrast
to its predecessors, T. rex lacked the direct competition
with other large carnivores. For whatever reason, all other
tyrannosaur lineages died out. Almost simultaneously, the
seaway receded for good, reconnecting east and west America
for the first time in 25 million years and effectively doubling
the geographic area for North American dinosaurs. The additional
area allowed Tyrannosaurus to increase in body size while
maintaining population densities high enough to avoid extinction,
at least for awhile. Thus, according to this hypothesis, at
least three factors—intermediate metabolism, reduction
in interspecies competition, and dramatic increase in geographic
area—were necessary to allow Tyrannosaurus rex to pump
up to record-breaking body sizes. It is important to note
that integrative hypotheses like these result in testable
predictions that can be falsified or supported by future observations.
Yet why, you might be thinking, should we bother with fossils
at all, given that the record of ancient life is sporadic
and limited, whereas the modern record is so much denser?
Part of the answer is deep time. Evolution unfolds not during
human life spans, but over thousands, millions, and billions
of years. Despite what geneticists may argue, any understanding
of evolutionary mechanisms will be grossly incomplete without
a consideration of processes operating over deep time. In
other words, if your window is restricted to the present,
you will by necessity have a myopic view of life. By analogy,
how could you really understand a given person if you knew
nothing of their past? Paleobiology, by making use of the
fossil record, has the ability to gaze backward and watch
evolution play out over vast time spans.
Although there have been outstanding exceptions, like George
Gaylord Simpson and Elizabeth Vrba, most of the theory work
in paleobiology has been conducted by invertebrate, rather
than vertebrate, paleontologists. This is in part because
invertebrate sample sizes are typically so much larger than
those available to fossil vertebrate workers. Nonetheless,
because of their sheer size and propensity for fossilization,
the record of dinosaurs (and mammals) is reasonably dense.
So I think vertebrate paleontology will have much to add to
this discussion in the coming years.
There is much to learn from a connections-based perspective, both
in terms of the contingency of unique events (i.e., bifurcation points)
that history provides, and the general rules that guide long-term
change in natural systems. From this vantage point, the unfolding
of life can be viewed as a tapestry in which every new thread is contingent
upon the nature, timing, and interweaving of virtually all previous
threads. This is an extension of the idea that the origin of new life
forms is fundamentally contingent upon interactions among previous
biotas. As Stephen J. Gould described it, if one could rewind the
tape of life and let events play out again, the results would almost
certainly differ dramatically. The point of distinction here is a
deeper incorporation of the connections inherent in the web of life.
Specifically, the origin of new species is inextricably linked both
to evolutionary history and to intricate ecological relationships
with other species. Thus, speciation might be aptly termed “interdependent
origination.”
For example, it is often said that the extinction of dinosaurs
65 million years ago cleared the way for the radiation of
mammals and, ultimately, the origin of humans. Yet the degree
of life’s interconnectedness far exceeds that implied
in this statement. Dinosaurs persisted for 160 million years
prior to this mass dying, co-evolving in intricate organic
webs with plants, bacteria, fungi, and algae, as well as other
animals, including mammals. Together these Mesozoic life forms
influenced the origins and fates of one another and all species
that followed. Had the major extinction of the dinosaurs occurred
earlier or later, or had dinosaurs never evolved, subsequent
biotas would have been wholly different, and we almost certainly
wouldn’t be here to contemplate nature. An equivalent
claim could be made for any major group at any point in the
history of life.
While a few investigators, such as Stuart Kaufmann and Niles Eldredge,
have begun to work around the problem from different angles, we still
seem to be a long way off from a synthesis of ecology, which focuses
on matter-energy transfer systems, and evolution, which emphasizes
genetic (information) change over time within complex adaptive systems.
At this point, we are beginning to discern the steps in this eco-evolutionary
dance, yet the music eludes us. Ultimately, if a true synthesis is
to come, it will be accomplished only by combining insights from the
modern realm—for example, through genetics, biochemistry, microbiology,
and ecology—with those from the deep past. To my mind, this
search for law-like properties amidst the numerous patterns in Nature
is one of the principle challenges facing evolutionary biologists
and ecologists alike in this century. All indicators suggest that
we are approaching an exciting time of discovery.
|