Paleontology
(essay)
Paleontology is the study of ancient life forms — plant, animal,
bacterial, and others – by means of the fossil record they have left
behind. Paleontologists search for, unearth, and examine fossils to
determine every aspect of these ancient life forms, including their body
structure, geographic distribution, adaptation to environment,
interaction with other species and other members of their own species,
taxonomic relationship with ancient and modern life forms, and
behavioral traits. The term paleontology is a combination of three
ancient Greek words, “paleo,” “ontos,” and “logos,” which mean ancient,
being, and knowledge respectively.
Paleontology is closely related to geology, the study of the structure
of the Earth. Indeed, the work of paleontologists often informs that of
historical geologists, as fossils provide critical information for the
understanding of the structure and age of the Earth’s crust. More
specifically, paleontological finds have been critical to the geology
sub-discipline of stratigraphy, or the study of how stratification or
layering occurs in the Earth’s crust. Aside from geology, paleontology
has also provided key evidence for the theory of evolution. While
largely an academic discipline, paleontology has its practical side too,
as the distribution of various types of fossils have proven, in some
cases, to be useful guides to the discovery of hydrocarbon reserves such
as oil and natural gas, which are, essentially, the compressed remains
of the ancient life forms studied by paleontologists.
Paleontology is subdivided into various disciplines depending on the
life forms being studied. These include paleo-zoology (the study of
ancient animals, itself divided into vertebrate paleozoology and
invertebrate paleo-zoology), paleo-botany (plants), micropaleontology
(bacteria and other microscopic life forms), palynology (pollen and
spores), and paleo-anthropology (humans), among others. (While this
article will touch on this last discipline, readers can find fuller
coverage in the article: “Humanity, Origins of”.) Other sub-disciplines
of paleontology, including paleo-ecology, paleo-geography, and
paleo-climatology, focus on the environment in which ancient life forms
lived and how ancient life forms affected that environment. A new and
burgeoning sub-discipline is paleo-biology, which applies the findings
of modern biology, particularly those concerning the genetic makeup of
life, to the study of ancient life forms.
The discipline of paleontology is one of the oldest within the natural
sciences, dating back in Europe to the seventeenth century, and among
the most controversial, as its basic suppositions about the great age of
life on Earth and the changes in life forms over time appear to
contradict biblical and other religious accounts of creation.
Historians often refer to the general period in European history in
which paleontology was born as the “age of reason,” a time when thinkers
began to explore the world around them and move beyond theological
explanations of natural phenomena. Among the first things that caught
the attention of these early naturalists were fossils, many of which
bore very little resemblance to existing life forms. By the turn of the
nineteenth century, scientists—most notably the French naturalist,
Georges Cuvier–were hypothesizing that the fossils were, in fact,
evidence of extinct forms of life and, as such, pointed to a much more
complex and lengthy history of the Earth than that offered in the
biblical account of creation. The work of English naturalists Charles
Darwin and Alfred Wallace in the middle years of the nineteenth century
provided, with the idea of natural selection, the theoretical framework
for the understanding of how species adaptation and extinction occurred.
Key discoveries of the twentieth century that have informed the work of
paleontologists have included the asteroid theory of mass extinction,
and plate tectonics, or the theory of continental drift. Key twentieth
century technologies aiding paleontologists include radiometric dating,
which allows precise dating of fossils based on the radioactive decay of
the elements of which they are composed, and DNA analysis, which allows
scientists to trace the evolution of fossilized life forms at the
molecular level.
Science and Methodology
Paleontologists largely work with several types of evidence. The first
are the imprints life forms have left in rock, usually by means of the
sedimentation process though, occasionally, through volcanic activity as
well. Such imprints are not fossils in the technical sense, though they
constitute such in the popular mind. The second form of evidence used by
paleontologists are true fossils, that is, the remains of life forms or,
more typically, the hard parts of life forms, such as teeth and bones,
in which the organic molecules have been replaced by minerals. A
different process of fossilization occurs with soft tissue when
mineral-rich water fills in the spaces normally occupied by liquids or
gases. This mineralization process can occur even at the cellular level,
leaving behind incredibly detailed fossils. Both the mineralization and
imprint processes can take thousands and even millions of years to
occur.
Another form of evidence utilized by paleontologists is preserved
organic tissue, usually from small invertebrates such as insects,
trapped in fossilized plant resin, or amber, though the organic remains
of some more recently extinct species, such as mammoths, have been found
in glaciers and bogs. Finally, some paleontologists work with existing
life forms. Popularly referred to as “living fossils,” such species —
among the best known is the ancient fish species, coelacanth–have
existed for up to hundreds of millions of years and are believed to
resemble long extinct life forms. (For simplicity sake, all but the
latter form of paleontological evidence will be referred to as fossils
in this discussion.)
The first step in analyzing fossils is to find them unless, of course,
the paleontologist chooses to examine fossils that have already been
collected. Fossils of all types are relatively rare. That is because the
conditions for fossilization depend on many factors coming together. For
mineralization, there has to be just the right combination of minerals
and groundwater, while, for the process that leaves imprinted fossils,
just the right geological processes have to be at work soon after the
organism dies. Thus, paleontologists look for telltale geological
formations to guide them to fossil remains. Examination of such
formations, known as topology, can also allow paleontologists to date
the fossils. This method—now outdated–is known as “relative dating”
because it was best for determining the order in which fossils were
created and not their precise ages.
Once fossils have been found they can be analyzed using a variety of
methods. The most obvious and earliest of these methods is simple visual
observation of the remains. Such observation can help the paleontologist
classify the life form. For more complex life forms, such as
vertebrates, paleontologist use visual observation to assemble the
various parts to recreate the whole organism.
Analytical tools developed over the past 60 years have moved
paleontologists far beyond simple visual observation and comparison of
fossils. Perhaps the most important has been radiometric dating, that
is, the analysis of the radioactive decay that naturally occurs, to one
degree or another, in all elements or, more specifically, within the
radioactive isotopes present in elements. Because radioactive isotopes
break down at a specific rate—their so-called half-lives—scientists can
note the amount of a radioactive isotope in a given element and know
when it was created. Since carbon forms the basis of all life,
scientists in the mid-twentieth century first focused on the decay of
the isotope carbon-14. But carbon-14 proved a useful indicator for
relatively short periods of time only—roughly good for about 40,000
years—making it helpful in the study of human remains but largely
useless for paleontologists who work in time frames of hundred of
thousands to hundreds of millions of years. Scientists soon discovered
that potassium-40, a radioactive isotope of potassium, a metal found in
all life on Earth—breaks down into the inert gas argon over a period of
roughly 1.3 billion years, making it an ideal radiometric marker for
paleontologists.
With the discovery of the structure of DNA in the 1950s, paleontologists
were offered a new avenue for the analysis of fossils, at the molecular
level, though it took several decades for the tools to be developed to
make sense of fossilized DNA, usually found in life forms persevered in
amber. Changes in the structure of the DNA molecules found in fossils
allow paleontologists to examine very specific evolutionary changes
within extinct species as well as the physical and even behavioral
traits of those species in a way simple visual and even chemical
analysis is incapable of. DNA analysis also provides key insights to
evolutionary biologists, that is, scientists who examine the biology of
adaptation and extinction.
Paleontologists divide Earth history into eons, eras, periods, and
epochs. Eons cover billions of years, eras cover hundreds of millions of
years, periods are usually in the tens of millions of years, and epochs,
the shortest of these periods, is measured in millions or hundreds of
thousands of years. The time spans in these eons, eras, periods, and
epochs vary greatly, as they do not signify specific time periods, such
as years or millennia. Instead, they are marked by great changes in the
fossil records.
Table about here
Earth is believed to be about 4.5 billion years old while life as we
know it emerged about 3.7 billion years ago. During that period, carbon
atoms were gradually transformed into complex carbon-based, or organic,
compounds, and eventually, organelles, the basic components of living
cells, such as mitochondria. The roughly 3.2 billion years that
followed—known to paleontologists as the Archaeon and Proterozoic
Eons—saw the emergence first of anaerobic and then oxygen-breathing life
forms. But these early life forms were simple and largely microscopic,
leaving virtually no fossil record behind.
It was not until the beginning of the Paleozoic era, around 540 million
years ago, that more complex plant and animal forms began to appear and
leave a fossil record. The earliest period of the Paleozoic era is known
as the Cambrian; thus, most paleontologists refer to the time span
before complex life forms began to emerge as pre-Cambrian time. For the
most part, paleontologists are forced by a lack of a physical record to
study life forms from the Cambrian period forward.
History of the Discipline
Among the key issues paleontologists grapple with is how life has
evolved on Earth. In that sense, they are examining two key questions
that have exercised the human imagination for millennia: why is there
such a diversity of life and where did it all come from? Virtually all
cultures have myths and stories to answer these questions. To Western
readers, the most familiar is that in the book of Genesis—six days in
which God first created the physical universe, the Earth, and then
populated the latter with animals, plants, and finally human beings. The
Book of Genesis also spoke of a planet-wide flood ten generations after
Adam and Eve—generations that lasted hundreds of years each–but it
noted that all of Earth’s creatures were saved by Noah in his ark:
“every animal, every creeping thing, and every bird, everything that
moves on the earth, went out of the ark” after the flood.
This biblical explanation, of course, left no room for fossils. Among
the earliest thinkers to wonder about this natural phenomenon was the
Greek philosopher Xenophanes in the sixth century BCE. Examining the
fossils of shellfish, Xenophanes assumed they were the remains of
existing species though he was required to come up with an explanation
for why they were found so far from the sea. Xenophanes hypothesized
that land forms shift. The eleventh century CE Chinese scientist Shen
Kuo explained the presence of bamboo fossils in dry climates incapable
of supporting that particular species by a theory of climate change over
time.
But not all scholars concurred with these findings. As late as the
sixteenth century, most European thinkers questioned whether fossils
were even evidence of life at all, assuming that fossils, though
lifelike in appearance, were simply odd-looking stones. Indeed, the
original Latin meaning of the word fossil was simply “something dug from
the Earth,” with no implicit meaning that the things being dug up had
once been life forms. Ancient and medieval Chinese came to a different
conclusion about the dinosaur bones that they found, explaining them
away as evidence of the mythical creatures, dragons, which, they
believed, still existed in faraway places.
With the rise of the so-called Age of Reason and the Scientific
Revolution of the seventeenth century, many European thinkers began to
seek non-theological explanations for natural phenomena. In 1665, the
English scientist Robert Hooke, utilizing the newly invented microscope,
put forth the theory of a mineralization process to explain petrified
wood. Such a process assumed a much greater time span for life than that
offered in the Bible. Roughly a century later, French naturalist Georges
Buffon explained the existence of fossilized elephant bones in Europe by
saying that the Earth was undergoing a gradual cooling process over
time, since tropical elephants no longer lived in a temperate Europe.
The greatest breakthrough of the pre-Darwinian era in paleontology,
however, came with the findings of Cuvier. Utilizing the newly invented
species classification system of Swedish scientist Carl Linneaus, Cuvier
established that existing elephant species differed from the
elephant-like creature, which he named the mastodon, whose fossilized
bones had been discovered in North America’s Ohio Valley region. Cuvier
then hypothesized that this species was extinct, thereby undermining
both the idea that fossils were the bones of existing species and
Buffon’s cooling Earth theory. Instead, Cuvier put forth the theory of
catastrophism, that sudden geological changes explained extinction. This
undermined another existing paradigm, known as uniformitarianism, which
stated that geological change occurred gradually and uniformly through
time. These various findings have led historians of science to consider
the French naturalist the father of both comparative anatomy and
paleontology.
The next great breakthrough in paleontology came not through the result
of fossil analysis but by way of the studies of existing species. With
their theory of natural selection, Darwin and Wallace, who developed it
at roughly the same time in the mid-nineteenth century, offered an
explanation for extinction that connected geological and climatic change
with species transformation. Changes in the environment forced life
forms to adapt; those that did so effectively survived, passing on their
characteristics to new generations, while those that did not died out.
New theories from outside the discipline have also contributed to
paleontology in the twentieth century. Of these the two most important
are the geological theory of continental drift, which explains how the
major landforms on Earth have shifted over time, offering a new
understanding for the distribution of various species, existing and
extinct. The asteroid theory of extinction, also from the late twentieth
century, has offered a powerful causal factor for the various extinction
events in Earth’s history, though some paleontologists still believe
that mass volcanic activity, either independent of asteroid collisions
or connected to them, are the major cause of such events. In either
case, it is these catastrophic events that mark a number of key
divisions between eras and periods.
From within the discipline, perhaps the most important theoretical
development of the late twentieth century has been that of punctuated
equilibrium. First propounded by American paleontologists Niles Eldredge
and Stephen Jay Gould in the 1970s, this theory revisits—in biology
rather than geology–the old uniformitarianism-catastrophism debate of
the eighteenth century. What Eldredge and Gould argue is that evolution,
even in the absence of non-catastrophic events, is marked by bursts of
genetic change followed by long periods of stasis.
Twentieth century technology has also given paleontologists remarkable
new tools. Radiometry has allowed for precise and accurate dating of
fossils while DNA analysis has opened a window on changes at the
molecular level, permitting paleontologists to study precisely how
species have evolved or failed to evolve. DNA analysis has also given
scientists the ability to map the relationships, based on subtle changes
in fossilized DNA, between species with incredible precision.
While paleontology is largely seen as an interesting academic exercise
by much of the public, as well as a source of fascinating facts for
dinosaur-loving children, it may also offer lessons about humanity’s
current relationship to its environment. The current period in
paleontological history, known as the Quaternary, which began roughly
1.8 million years ago, has been marked by the rise to dominance of a
species from the hominid family of the primate order of mammals, known
as homo sapiens. With its great intelligence this species has come to
control and change its environment to an unprecedented degree and, in
paleontological terms, in a very short period of time. Like the
cataclysmic events of the past, human-wrought change to the environment
may be occurring too fast for other species to adapt. Scholars of the
environment estimate that species extinctions in the past century have
occurred at a rate anywhere between 100 to 1,000 times above the
average, or “background,” rate of extinction–a result of hunting,
pollution, habitat loss and, most recently, climate change. Thus, some
paleontologists hypothesize that the planet may be undergoing a new
extinction event, known as Holocene extinction event, after the current
epoch, which began about 10,000 years ago, produced not by asteroids or
great geological forces but by the very species that had unraveled the
story of Earth’s long history.
REFERENCES
1. L. Sprague and Catherine Crook de Camp. The Day of the Dinosaur. New
York: Bonanza Books, 1985.
2. Edwards, Wilfrid Norman. The Early History of Paleontology. London:
British Museum of Natural History, 1967.
3. Gould, Stephen Jay. Dinosaur in a Haystack: Reflections in Natural
History. New York: Harmony Books, 1995.
4. Rudwick, Martin J.S. The Meaning of Fossils: Episodes in the History
of Palaeontology. Chicago: University of Chicago Press, 1985.
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