Paleontology or palaeontology () is the scientific
study of life that existed prior to, and sometimes
including, the start of the Holocene Epoch
(roughly 11,700 years before present).
It includes the study of fossils to determine
organisms' evolution and interactions with
each other and their environments (their paleoecology).
Paleontological observations have been documented
as far back as the 5th century BC.
The science became established in the 18th
century as a result of Georges Cuvier's work
on comparative anatomy, and developed rapidly
in the 19th century.
The term itself originates from Greek παλαιός,
palaios, "old, ancient", ὄν, on (gen. ontos),
"being, creature" and λόγος, logos, "speech,
thought, study".Paleontology lies on the border
between biology and geology, but differs from
archaeology in that it excludes the study
of anatomically modern humans.
It now uses techniques drawn from a wide range
of sciences, including biochemistry, mathematics,
and engineering.
Use of all these techniques has enabled paleontologists
to discover much of the evolutionary history
of life, almost all the way back to when Earth
became capable of supporting life, about 3.8
billion years ago.
As knowledge has increased, paleontology has
developed specialised sub-divisions, some
of which focus on different types of fossil
organisms while others study ecology and environmental
history, such as ancient climates.
Body fossils and trace fossils are the principal
types of evidence about ancient life, and
geochemical evidence has helped to decipher
the evolution of life before there were organisms
large enough to leave body fossils.
Estimating the dates of these remains is essential
but difficult: sometimes adjacent rock layers
allow radiometric dating, which provides absolute
dates that are accurate to within 0.5%, but
more often paleontologists have to rely on
relative dating by solving the "jigsaw puzzles"
of biostratigraphy.
Classifying ancient organisms is also difficult,
as many do not fit well into the Linnaean
taxonomy that is commonly used for classifying
living organisms, and paleontologists more
often use cladistics to draw up evolutionary
"family trees".
The final quarter of the 20th century saw
the development of molecular phylogenetics,
which investigates how closely organisms are
related by measuring how similar the DNA is
in their genomes.
Molecular phylogenetics has also been used
to estimate the dates when species diverged,
but there is controversy about the reliability
of the molecular clock on which such estimates
depend.
== Overview ==
The simplest definition is "the study of ancient
life".
Paleontology seeks information about several
aspects of past organisms: "their identity
and origin, their environment and evolution,
and what they can tell us about the Earth's
organic and inorganic past".
=== A historical science ===
Paleontology is one of the historical sciences,
along with archaeology, geology, astronomy,
cosmology, philology and history itself.
This means that it aims to describe phenomena
of the past and reconstruct their causes.
Hence it has three main elements: description
of the phenomena; developing a general theory
about the causes of various types of change;
and applying those theories to specific facts.
When trying to explain past phenomena, paleontologists
and other historical scientists often construct
a set of hypotheses about the causes and then
look for a smoking gun, a piece of evidence
that indicates that one hypothesis is a better
explanation than others.
Sometimes the smoking gun is discovered by
a fortunate accident during other research.
For example, the discovery by Luis Alvarez
and Walter Alvarez of an iridium-rich layer
at the Cretaceous–Tertiary boundary made
asteroid impact and volcanism the most favored
explanations for the Cretaceous–Paleogene
extinction event.The other main type of science
is experimental science, which is often said
to work by conducting experiments to disprove
hypotheses about the workings and causes of
natural phenomena – note that this approach
cannot confirm a hypothesis is correct, since
some later experiment may disprove it.
However, when confronted with totally unexpected
phenomena, such as the first evidence for
invisible radiation, experimental scientists
often use the same approach as historical
scientists: construct a set of hypotheses
about the causes and then look for a "smoking
gun".
=== Related sciences ===
Paleontology lies on the boundary between
biology and geology since paleontology focuses
on the record of past life but its main source
of evidence is fossils, which are found in
rocks.
For historical reasons paleontology is part
of the geology departments of many universities,
because in the 19th century and early 20th
century geology departments found paleontological
evidence important for estimating the ages
of rocks while biology departments showed
little interest.Paleontology also has some
overlap with archaeology, which primarily
works with objects made by humans and with
human remains, while paleontologists are interested
in the characteristics and evolution of humans
as organisms.
When dealing with evidence about humans, archaeologists
and paleontologists may work together – for
example paleontologists might identify animal
or plant fossils around an archaeological
site, to discover what the people who lived
there ate; or they might analyze the climate
at the time when the site was inhabited by
humans.In addition paleontology often uses
techniques derived from other sciences, including
biology, osteology, ecology, chemistry, physics
and mathematics.
For example, geochemical signatures from rocks
may help to discover when life first arose
on Earth, and analyses of carbon isotope ratios
may help to identify climate changes and even
to explain major transitions such as the Permian–Triassic
extinction event.
A relatively recent discipline, molecular
phylogenetics, often helps by using comparisons
of different modern organisms' DNA and RNA
to re-construct evolutionary "family trees";
it has also been used to estimate the dates
of important evolutionary developments, although
this approach is controversial because of
doubts about the reliability of the "molecular
clock".
Techniques developed in engineering have been
used to analyse how ancient organisms might
have worked, for example how fast Tyrannosaurus
could move and how powerful its bite was.
It is relatively commonplace to study fossils
using X-ray microtomography A combination
of paleontology, biology, and archaeology,
paleoneurobiology is the study of endocranial
casts (or endocasts) of species related to
humans to learn about the evolution of human
brains.Paleontology even contributes to astrobiology,
the investigation of possible life on other
planets, by developing models of how life
may have arisen and by providing techniques
for detecting evidence of life.
=== Subdivisions ===
As knowledge has increased, paleontology has
developed specialised subdivisions.
Vertebrate paleontology concentrates on fossils
of vertebrates, from the earliest fish to
the immediate ancestors of modern mammals.
Invertebrate paleontology deals with fossils
of invertebrates such as molluscs, arthropods,
annelid worms and echinoderms.
Paleobotany focuses on the study of fossil
plants, but traditionally includes the study
of fossil algae and fungi.
Palynology, the study of pollen and spores
produced by land plants and protists, straddles
the border between paleontology and botany,
as it deals with both living and fossil organisms.
Micropaleontology deals with all microscopic
fossil organisms, regardless of the group
to which they belong.
Instead of focusing on individual organisms,
paleoecology examines the interactions between
different organisms, such as their places
in food chains, and the two-way interaction
between organisms and their environment.
One example is the development of oxygenic
photosynthesis by bacteria, which hugely increased
the productivity and diversity of ecosystems.
This also caused the oxygenation of the atmosphere.
Together, these were a prerequisite for the
evolution of the most complex eukaryotic cells,
from which all multicellular organisms are
built.Paleoclimatology, although sometimes
treated as part of paleoecology, focuses more
on the history of Earth's climate and the
mechanisms that have changed it – which
have sometimes included evolutionary developments,
for example the rapid expansion of land plants
in the Devonian period removed more carbon
dioxide from the atmosphere, reducing the
greenhouse effect and thus helping to cause
an ice age in the Carboniferous period.Biostratigraphy,
the use of fossils to work out the chronological
order in which rocks were formed, is useful
to both paleontologists and geologists.
Biogeography studies the spatial distribution
of organisms, and is also linked to geology,
which explains how Earth's geography has changed
over time.
== Sources of evidence ==
=== Body fossils ===
Fossils of organisms' bodies are usually the
most informative type of evidence.
The most common types are wood, bones, and
shells.
Fossilisation is a rare event, and most fossils
are destroyed by erosion or metamorphism before
they can be observed.
Hence the fossil record is very incomplete,
increasingly so further back in time.
Despite this, it is often adequate to illustrate
the broader patterns of life's history.
There are also biases in the fossil record:
different environments are more favorable
to the preservation of different types of
organism or parts of organisms.
Further, only the parts of organisms that
were already mineralised are usually preserved,
such as the shells of molluscs.
Since most animal species are soft-bodied,
they decay before they can become fossilised.
As a result, although there are 30-plus phyla
of living animals, two-thirds have never been
found as fossils.Occasionally, unusual environments
may preserve soft tissues.
These lagerstätten allow paleontologists
to examine the internal anatomy of animals
that in other sediments are represented only
by shells, spines, claws, etc. – if they
are preserved at all.
However, even lagerstätten present an incomplete
picture of life at the time.
The majority of organisms living at the time
are probably not represented because lagerstätten
are restricted to a narrow range of environments,
e.g. where soft-bodied organisms can be preserved
very quickly by events such as mudslides;
and the exceptional events that cause quick
burial make it difficult to study the normal
environments of the animals.
The sparseness of the fossil record means
that organisms are expected to exist long
before and after they are found in the fossil
record – this is known as the Signor–Lipps
effect.
=== Trace fossils ===
Trace fossils consist mainly of tracks and
burrows, but also include coprolites (fossil
feces) and marks left by feeding.
Trace fossils are particularly significant
because they represent a data source that
is not limited to animals with easily fossilised
hard parts, and they reflect organisms' behaviours.
Also many traces date from significantly earlier
than the body fossils of animals that are
thought to have been capable of making them.
Whilst exact assignment of trace fossils to
their makers is generally impossible, traces
may for example provide the earliest physical
evidence of the appearance of moderately complex
animals (comparable to earthworms).
=== Geochemical observations ===
Geochemical observations may help to deduce
the global level of biological activity at
a certain period, or the affinity of certain
fossils.
For example, geochemical features of rocks
may reveal when life first arose on Earth,
and may provide evidence of the presence of
eukaryotic cells, the type from which all
multicellular organisms are built.
Analyses of carbon isotope ratios may help
to explain major transitions such as the Permian–Triassic
extinction event.
== Classifying ancient organisms ==
Naming groups of organisms in a way that is
clear and widely agreed is important, as some
disputes in paleontology have been based just
on misunderstandings over names.
Linnaean taxonomy is commonly used for classifying
living organisms, but runs into difficulties
when dealing with newly discovered organisms
that are significantly different from known
ones.
For example: it is hard to decide at what
level to place a new higher-level grouping,
e.g. genus or family or order; this is important
since the Linnaean rules for naming groups
are tied to their levels, and hence if a group
is moved to a different level it must be renamed.Paleontologists
generally use approaches based on cladistics,
a technique for working out the evolutionary
"family tree" of a set of organisms.
It works by the logic that, if groups B and
C have more similarities to each other than
either has to group A, then B and C are more
closely related to each other than either
is to A. Characters that are compared may
be anatomical, such as the presence of a notochord,
or molecular, by comparing sequences of DNA
or proteins.
The result of a successful analysis is a hierarchy
of clades – groups that share a common ancestor.
Ideally the "family tree" has only two branches
leading from each node ("junction"), but sometimes
there is too little information to achieve
this and paleontologists have to make do with
junctions that have several branches.
The cladistic technique is sometimes fallible,
as some features, such as wings or camera
eyes, evolved more than once, convergently
– this must be taken into account in analyses.Evolutionary
developmental biology, commonly abbreviated
to "Evo Devo", also helps paleontologists
to produce "family trees", and understand
fossils.
For example, the embryological development
of some modern brachiopods suggests that brachiopods
may be descendants of the halkieriids, which
became extinct in the Cambrian period.
== Estimating the dates of organisms ==
Paleontology seeks to map out how living things
have changed through time.
A substantial hurdle to this aim is the difficulty
of working out how old fossils are.
Beds that preserve fossils typically lack
the radioactive elements needed for radiometric
dating.
This technique is our only means of giving
rocks greater than about 50 million years
old an absolute age, and can be accurate to
within 0.5% or better.
Although radiometric dating requires very
careful laboratory work, its basic principle
is simple: the rates at which various radioactive
elements decay are known, and so the ratio
of the radioactive element to the element
into which it decays shows how long ago the
radioactive element was incorporated into
the rock.
Radioactive elements are common only in rocks
with a volcanic origin, and so the only fossil-bearing
rocks that can be dated radiometrically are
a few volcanic ash layers.Consequently, paleontologists
must usually rely on stratigraphy to date
fossils.
Stratigraphy is the science of deciphering
the "layer-cake" that is the sedimentary record,
and has been compared to a jigsaw puzzle.
Rocks normally form relatively horizontal
layers, with each layer younger than the one
underneath it.
If a fossil is found between two layers whose
ages are known, the fossil's age must lie
between the two known ages.
Because rock sequences are not continuous,
but may be broken up by faults or periods
of erosion, it is very difficult to match
up rock beds that are not directly next to
one another.
However, fossils of species that survived
for a relatively short time can be used to
link up isolated rocks: this technique is
called biostratigraphy.
For instance, the conodont Eoplacognathus
pseudoplanus has a short range in the Middle
Ordovician period.
If rocks of unknown age are found to have
traces of E. pseudoplanus, they must have
a mid-Ordovician age.
Such index fossils must be distinctive, be
globally distributed and have a short time
range to be useful.
However, misleading results are produced if
the index fossils turn out to have longer
fossil ranges than first thought.
Stratigraphy and biostratigraphy can in general
provide only relative dating (A was before
B), which is often sufficient for studying
evolution.
However, this is difficult for some time periods,
because of the problems involved in matching
up rocks of the same age across different
continents.Family-tree relationships may also
help to narrow down the date when lineages
first appeared.
For instance, if fossils of B or C date to
X million years ago and the calculated "family
tree" says A was an ancestor of B and C, then
A must have evolved more than X million years
ago.
It is also possible to estimate how long ago
two living clades diverged – i.e. approximately
how long ago their last common ancestor must
have lived – by assuming that DNA mutations
accumulate at a constant rate.
These "molecular clocks", however, are fallible,
and provide only a very approximate timing:
for example, they are not sufficiently precise
and reliable for estimating when the groups
that feature in the Cambrian explosion first
evolved, and estimates produced by different
techniques may vary by a factor of two.
== Overview of paleontology's results about
the history of life ==
The evolutionary history of life stretches
back to over 3,000 million years ago, possibly
as far as 3,800 million years ago.
Earth formed about 4,570 million years ago
and, after a collision that formed the Moon
about 40 million years later, may have cooled
quickly enough to have oceans and an atmosphere
about 4,440 million years ago.
However, there is evidence on the Moon of
a Late Heavy Bombardment from 4,000 to 3,800
million years ago.
If, as seems likely, such a bombardment struck
Earth at the same time, the first atmosphere
and oceans may have been stripped away.
The oldest clear evidence of life on Earth
dates to 3,000 million years ago, although
there have been reports, often disputed, of
fossil bacteria from 3,400 million years ago
and of geochemical evidence for the presence
of life 3,800 million years ago.
Some scientists have proposed that life on
Earth was "seeded" from elsewhere, but most
research concentrates on various explanations
of how life could have arisen independently
on Earth.For about 2,000 million years microbial
mats, multi-layered colonies of different
types of bacteria, were the dominant life
on Earth.
The evolution of oxygenic photosynthesis enabled
them to play the major role in the oxygenation
of the atmosphere from about 2,400 million
years ago.
This change in the atmosphere increased their
effectiveness as nurseries of evolution.
While eukaryotes, cells with complex internal
structures, may have been present earlier,
their evolution speeded up when they acquired
the ability to transform oxygen from a poison
to a powerful source of energy in their metabolism.
This innovation may have come from primitive
eukaryotes capturing oxygen-powered bacteria
as endosymbionts and transforming them into
organelles called mitochondria.
The earliest evidence of complex eukaryotes
with organelles such as mitochondria, dates
from 1,850 million years ago.
Multicellular life is composed only of eukaryotic
cells, and the earliest evidence for it is
the Francevillian Group Fossils from 2,100
million years ago, although specialisation
of cells for different functions first appears
between 1,430 million years ago (a possible
fungus) and 1,200 million years ago (a probable
red alga).
Sexual reproduction may be a prerequisite
for specialisation of cells, as an asexual
multicellular organism might be at risk of
being taken over by rogue cells that retain
the ability to reproduce.The earliest known
animals are cnidarians from about 580 million
years ago, but these are so modern-looking
that the earliest animals must have appeared
before then.
Early fossils of animals are rare because
they did not develop mineralised hard parts
that fossilise easily until about 548 million
years ago.
The earliest modern-looking bilaterian animals
appear in the Early Cambrian, along with several
"weird wonders" that bear little obvious resemblance
to any modern animals.
There is a long-running debate about whether
this Cambrian explosion was truly a very rapid
period of evolutionary experimentation; alternative
views are that modern-looking animals began
evolving earlier but fossils of their precursors
have not yet been found, or that the "weird
wonders" are evolutionary "aunts" and "cousins"
of modern groups.
Vertebrates remained an obscure group until
the first fish with jaws appeared in the Late
Ordovician.
The spread of life from water to land required
organisms to solve several problems, including
protection against drying out and supporting
themselves against gravity.
The earliest evidence of land plants and land
invertebrates date back to about 476 million
years ago and 490 million years ago respectively.
The lineage that produced land vertebrates
evolved later but very rapidly between 370
million years ago and 360 million years ago;
recent discoveries have overturned earlier
ideas about the history and driving forces
behind their evolution.
Land plants were so successful that they caused
an ecological crisis in the Late Devonian,
until the evolution and spread of fungi that
could digest dead wood.
During the Permian period synapsids, including
the ancestors of mammals, may have dominated
land environments, but the Permian–Triassic
extinction event 251 million years ago came
very close to wiping out complex life.
The extinctions were apparently fairly sudden,
at least among vertebrates.
During the slow recovery from this catastrophe
a previously obscure group, archosaurs, became
the most abundant and diverse terrestrial
vertebrates.
One archosaur group, the dinosaurs, were the
dominant land vertebrates for the rest of
the Mesozoic, and birds evolved from one group
of dinosaurs.
During this time mammals' ancestors survived
only as small, mainly nocturnal insectivores,
but this apparent set-back may have accelerated
the development of mammalian traits such as
endothermy and hair.
After the Cretaceous–Paleogene extinction
event 66 million years ago killed off the
non-avian dinosaurs – birds are the only
surviving dinosaurs – mammals increased
rapidly in size and diversity, and some took
to the air and the sea.Fossil evidence indicates
that flowering plants appeared and rapidly
diversified in the Early Cretaceous, between
130 million years ago and 90 million years
ago.
Their rapid rise to dominance of terrestrial
ecosystems is thought to have been propelled
by coevolution with pollinating insects.
Social insects appeared around the same time
and, although they account for only small
parts of the insect "family tree", now form
over 50% of the total mass of all insects.Humans
evolved from a lineage of upright-walking
apes whose earliest fossils date from over
6 million years ago.
Although early members of this lineage had
chimp-sized brains, about 25% as big as modern
humans', there are signs of a steady increase
in brain size after about 3 million years
ago.
There is a long-running debate about whether
modern humans are descendants of a single
small population in Africa, which then migrated
all over the world less than 200,000 years
ago and replaced previous hominine species,
or arose worldwide at the same time as a result
of interbreeding.
=== Mass extinctions ===
Life on earth has suffered occasional mass
extinctions at least since 542 million years
ago.
Although they are disasters at the time, mass
extinctions have sometimes accelerated the
evolution of life on earth.
When dominance of particular ecological niches
passes from one group of organisms to another,
it is rarely because the new dominant group
is "superior" to the old and usually because
an extinction event eliminates the old dominant
group and makes way for the new one.The fossil
record appears to show that the rate of extinction
is slowing down, with both the gaps between
mass extinctions becoming longer and the average
and background rates of extinction decreasing.
However, it is not certain whether the actual
rate of extinction has altered, since both
of these observations could be explained in
several ways:
The oceans may have become more hospitable
to life over the last 500 million years and
less vulnerable to mass extinctions: dissolved
oxygen became more widespread and penetrated
to greater depths; the development of life
on land reduced the run-off of nutrients and
hence the risk of eutrophication and anoxic
events; marine ecosystems became more diversified
so that food chains were less likely to be
disrupted.Reasonably complete fossils are
very rare, most extinct organisms are represented
only by partial fossils, and complete fossils
are rarest in the oldest rocks.
So paleontologists have mistakenly assigned
parts of the same organism to different genera,
which were often defined solely to accommodate
these finds – the story of Anomalocaris
is an example of this.
The risk of this mistake is higher for older
fossils because these are often unlike parts
of any living organism.
Many "superfluous" genera are represented
by fragments that are not found again, and
these "superfluous" genera appear to become
extinct very quickly.
Biodiversity in the fossil record, which is
"the number of distinct genera alive at any
given time; that is, those whose first occurrence
predates and whose last occurrence postdates
that time"shows a different trend: a fairly
swift rise from 542 to 400 million years ago,
a slight decline from 400 to 200 million years
ago, in which the devastating Permian–Triassic
extinction event is an important factor, and
a swift rise from 200 million years ago to
the present.
== History of paleontology ==
Although paleontology became established around
1800, earlier thinkers had noticed aspects
of the fossil record.
The ancient Greek philosopher Xenophanes (570–480
BC) concluded from fossil sea shells that
some areas of land were once under water.
During the Middle Ages the Persian naturalist
Ibn Sina, known as Avicenna in Europe, discussed
fossils and proposed a theory of petrifying
fluids on which Albert of Saxony elaborated
in the 14th century.
The Chinese naturalist Shen Kuo (1031–1095)
proposed a theory of climate change based
on the presence of petrified bamboo in regions
that in his time were too dry for bamboo.In
early modern Europe, the systematic study
of fossils emerged as an integral part of
the changes in natural philosophy that occurred
during the Age of Reason.
In the Italian Renaissance, Leonardo Da Vinci
made various significant contributions to
the field as well designed numerous fossils.
At the end of the 18th century Georges Cuvier's
work established comparative anatomy as a
scientific discipline and, by proving that
some fossil animals resembled no living ones,
demonstrated that animals could become extinct,
leading to the emergence of paleontology.
The expanding knowledge of the fossil record
also played an increasing role in the development
of geology, particularly stratigraphy.The
first half of the 19th century saw geological
and paleontological activity become increasingly
well organised with the growth of geologic
societies and museums and an increasing number
of professional geologists and fossil specialists.
Interest increased for reasons that were not
purely scientific, as geology and paleontology
helped industrialists to find and exploit
natural resources such as coal.This contributed
to a rapid increase in knowledge about the
history of life on Earth and to progress in
the definition of the geologic time scale,
largely based on fossil evidence.
In 1822 Henri Marie Ducrotay de Blanville,
editor of Journal de Physique, coined the
word "palaeontology" to refer to the study
of ancient living organisms through fossils.
As knowledge of life's history continued to
improve, it became increasingly obvious that
there had been some kind of successive order
to the development of life.
This encouraged early evolutionary theories
on the transmutation of species.
After Charles Darwin published Origin of Species
in 1859, much of the focus of paleontology
shifted to understanding evolutionary paths,
including human evolution, and evolutionary
theory.
The last half of the 19th century saw a tremendous
expansion in paleontological activity, especially
in North America.
The trend continued in the 20th century with
additional regions of the Earth being opened
to systematic fossil collection.
Fossils found in China near the end of the
20th century have been particularly important
as they have provided new information about
the earliest evolution of animals, early fish,
dinosaurs and the evolution of birds.
The last few decades of the 20th century saw
a renewed interest in mass extinctions and
their role in the evolution of life on Earth.
There was also a renewed interest in the Cambrian
explosion that apparently saw the development
of the body plans of most animal phyla.
The discovery of fossils of the Ediacaran
biota and developments in paleobiology extended
knowledge about the history of life back far
before the Cambrian.Increasing awareness of
Gregor Mendel's pioneering work in genetics
led first to the development of population
genetics and then in the mid-20th century
to the modern evolutionary synthesis, which
explains evolution as the outcome of events
such as mutations and horizontal gene transfer,
which provide genetic variation, with genetic
drift and natural selection driving changes
in this variation over time.
Within the next few years the role and operation
of DNA in genetic inheritance were discovered,
leading to what is now known as the "Central
Dogma" of molecular biology.
In the 1960s molecular phylogenetics, the
investigation of evolutionary "family trees"
by techniques derived from biochemistry, began
to make an impact, particularly when it was
proposed that the human lineage had diverged
from apes much more recently than was generally
thought at the time.
Although this early study compared proteins
from apes and humans, most molecular phylogenetics
research is now based on comparisons of RNA
and DNA.
== See also ==
== References ==
== External links ==
Smithsonian's Paleobiology website
University of California Museum of Paleontology
FAQ About Paleontology
The Paleontological Society
The Palaeontological Association
The Society of Vertebrate Paleontology
The Paleontology Portal
"Geology, Paleontology & Theories of the Earth",
a collection of more than 100 digitised landmark
and early books on Earth sciences at the Linda
Hall Library
