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
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
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
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.
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Rudwick, Martin J.S. The Meaning of Fossils:
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