Note: These materials are intended as supplements for students in Ant. 301. These pages are in development and will contain errors.
Relative Dates by Superposition
Relative Dates by Biostratigraphy
Absolute Dating by Radiometric Decay
Rules for Interpretation of the Fossil
Record
The Early Geological History of the
Earth
Geochronology
Superposition refers to sediments
in a basin accumulating on top of each other. Thus the most recent
additions are at the surface and deeper sediments are progressively
older. Most deposits exhibit variations of color or character
that give a hillside or an excavation wall a layered appearance.
Discernible layers are called strata, the study of which
is stratigraphy. A particular stratum, defined as a biostratigraphic
unit, is recognized by the fossil fauna and flora
that it contains. Strata in other localities that have similar
or identical fossil assemblages are presumed to be from comparable
time periods, allowing a stratigrapher to make stratigraphic correlations
between widely separated localities. In a local area, biostratigraphic
units may have subunits that are recognized by variations in physical
and chemical properties even though they share similar fossil
assemblages. These stratigraphic units are referred to as lithostratigraphic
units. Geologists usually call the smallest recognizable lithostratigraphic
unit a bed. A set of related beds can be designated as
a larger unit, a member. Likewise, a set of members are
classified into a formation, and formations into a group.
A relative chronology is compiled by excavating
numerous sites and then, based on stratigraphic correlation, reconstructing
a regional history. Local rock units can be combined into biostratigraphic
units that define a series of related faunal assemblages. These
biostratigraphic units may extend over larger geographic regions
and combine the knowledge gleaned from numerous lithostratigraphic
features. As regional histories are developed, it is possible
to construct larger chronostratigraphic units and ultimately,
a geologic time scale, a chart that summarizes this information
into a calendar of earth history. Since this calendar (and its
supporting elements in biostratigraphy) developed prior to radiometric
dating, there was no way to establish exact dates or time frames
for the events they contained. This framework came into practice
as measures of rock time, or geologic time. Only after the development
of modern dating techniques, especially radiometric measurements,
were scientists able to match calendar dates with geologic time.
geologic timeage -> epoch -> period -> era
chronostratigraphysubstage -> stage -> series -> system -> erathem
biostratigraphyfaunal complex -> stage -> zone
lithostratigraphybed -> member -> formation -> group
A vital component of this method is stratigraphic
context. If sound reconstructions are to be possible, materials
must be in primary context, that is, the stratum
must be undisturbed, and its contents must not have been lost
or contaminated by materials from other strata. The phrase "secondary
context" labels situations in which items from older
strata have been redeposited into a younger stratum. Insertion
of extraneous objects into an older stratum is called an intrusion.
When knowledge about stratigraphic provenience is lost, the fossils
or artifacts are described as having no context. Archaeologists
use assemblages of human artifacts to define cultural-stratigraphic
units in a fashion analogous to biostratigraphy. (return
to outline)
Relative Dates
by Superposition
The logic of superposition allows
a scientist to establish relationships between strata. These relationships
are either older, younger, similar, or uncertain. A calendar date
(or absolute date) is not produced by these methods. However,
if a datable object or feature (such as a coin) is recovered in
primary context, the object supplies dating information. The stratum
should not be younger than the object. (return
to outline)
Relative Dates
by Biostratigraphy
Once regional histories or chronostratigraphic
units are well described, that information can be used to date
newly discovered lithostratigraphic units. Charts of the relative
time ranges of fossil species provide a convenient guide for relative
dating . For example, a site whose fauna contains the elephant
Elephas recki brumpi and the suid Kolpochoerus afarensis clearly
dates to the late Pliocene. Faunal assembles from single stratum
at a site should represent a section or snapshot in time of the
regional faunal history. Index fossils, fossil species that have
been documented to exist only for short periods, can serve as
convenient guides to relative age.
(return to outline)
Relative
Dates by Chemistry
When a cadaver is left in soil for a long
period of time, the remnants of skeletal tissue acquire fluorine
and uranium from ground water and the surrounding soil. The decaying
organic components yield their nitrogen to soil until stability
is reached in the immediate cadaver environment. Thus several
cadavers from the same locality should exhibit similar contents
for fluorine, uranium, and nitrogen, hence the acronym "FUN
test." Substantial differences between specimens imply that
they have been interred for differing periods of time. The ability
to distinguish between specimens of different time provenience
is limited in two ways. Specimens to be compared must be from
the same soil and ground water environment, effectively limiting
comparisons to materials from the same localities. Second, the
techniques do not distinguish between materials of different antiquity
that have been interred together long enough to reach equilibrium
with the soil. (return to outline)
Absolute Dating
by Radiometric Decay
A half-life is the time interval required for half of the isotope to fission into its decay products. Some naturally occurring isotopes are common enough to serve as yardsticks for adding calendar dates to the relative chronology generated by biostratigraphy. If an object contains a radioactive isotope, the half-life of that isotope will allow us to compute how much elapsed time is represented by the ratio of isotope to decay product. Elapsed time may also be indicated if there is some other record (such as fission track scars, thermoluminescence) of fission events.
The well-documented carbon-14 cycle is useful
for determining the ages of strata younger than 40,000 years.
Carbon-14 (6C14) is produced from nitrogen
in the upper atmosphere when nitrogen (7N14)
is bombarded cosmic radiation. Carbon-14 is an unstable isotope
that decays to nitrogen with a half life of 5,730 years. As plants
convert carbon dioxide in the atmosphere to carbohydrates during
photosynthesis, 6C14enters the planetary
food chain. Once a plant or animal dies the 6C14in
its tissue is no longer renewed and begins to diminish. The ratio
of 6C14to carbon-12 (6C12),
the stable isotope of carbon in the atmosphere, allows an estimate
of elapsed time. The length of the half-life means that after
about 30,000 years, the level of 6C14is
too small for our current technology to measure accurately.
Fortunately there are other radioactive
isotopes with longer half-lives that are fairly common ingeological
strata. One of the most useful is potassium-40 (19K40)
which decays to argon-40 (18Ar40) and calcium-40
(20Ca40) in an argon-40:calcium-40 ratio
of 1:7 with a half-life of 1.3 billion years. In theory either
the 19K40:18Ar40 or
the 19K40:20Ca40 ratio
could be used to estimate elapsed time, but the abundance of 20Ca40
"masks" the decay products. However rocks that have
been heated to a very high temperature tend to lose any previously
acquired argon gas, and if the rock matrix has characteristics
that will trap argon produced by decay, a 19K40:18Ar40
ratio can be measured. This technique is generally useful only
to date volcanic lava and ash falls. Although the long half-life
allows theoretically permits the dating of the age of the earth,
it is not useful for specimens younger than a few hundred thousand
years...
The rationale behind fission-track dating
is different phenomenon. Uranium-238 (92U238)
atomsoccurring in glass or crystals leave minute scars in the
glass matrix when particles are emitted during fission events.
Fission tracks are usually enlarged by acid etching and counted
under a microscope, giving a estimate of the fission events that
have occurred on that surface. The crystal is then heated, obliterating
the tracks as the matrix anneals. Then the surface is exposed
to neutron irradiation to induce fission of surviving uranium
atoms. The ratio of natural fission scars to induced fission scars
permits a computation of the age of the crystal or the time elapsed
since the last heating of that glass surface. Uranium decays through
a series of intermediate decay products to lead. Naturally occurring
uranium contains both uranium-238, which decays to lead-206 with
a half-life of 5.4 billion years, and uranium-235, which decays
to lead-207 with a half-life of 713 million years. Minerals containing
uranium also contain radioactive thorium-232 which decays to lead-208.
Most of Earth's radioactive elements are descendants (by decay)
from three progenitors: uranium 238 (92U238);
thorium 232 (90Th232); and uranium 235 (92U235).
The most important radio-element outside these three families
is potassium 40 (19K40).
Since uranium occurs in many rocks and minerals,
especially granitic rocks, it is often useful as anindex of age.
Since it is soluble, it is a contaminant of ground water and is
easily removed from sediments, remaining in solution in sea-water
or lakes. On the other hand, some of its decay products, Th230
and Pa231 tend to precipitate and be incorporated in sea floors
and lake sediments. Shells formed of carbonates usually contain
some dissolved uranium, but little of the relatively insoluble
Th230 or Pa231 . Thus ratios of uranium and its decay products
may serve as a way to estimate shell age. Similar rationale can
be applied to certain inorganic carbonate deposits in caves -
stalagmites, travertine, etc. The procedure is confounded in most
archaeological or fossil sites since they are contaminated by
uranium in ground water. (return
to outline)
Thermoluminescence is a technique that measures
electrons trapped in the crystal structure of objects irradiated
by radioisotopes within the objects and in the surrounding soil.
Heating frees the trapped electrons, and an accurate internal
and external dose rate can be computed. A measurement of the quantity
of trapped electrons provides a estimate of elapsed time since
the object was last heated. (return
to outline)
The earth's magnetic field has fluctuated
in intensity and polarity in the past. Although these changes
are thought to be worldwide events controlled by convection currents
within the earth's molten core, they are poorly understood. Long
intervals that are predominantly the same polarity are termed
polarity chrons; short changes are termed polarity subchrons.
Minor deviations in the direction of magnetic poles reflect movements
of continents rather than shifts in polarity. (return
to outline)
With the passage of substantial periods
of time, living creatures accumulate mutations. Once demes are
isolated, the DNA represented in their gene pool diverges. DNA
sequences accumulate point mutations. If the degree of DNA divergence
is a function of time, then it is also a measure of the time elapsed
since divergence.
Sarich and Wilson (1967) argued that the
rate of divergence for the protein albumin among primate species
has been consistent and used the molecular information to revise
the time scale for human evolution. If the rate of divergence
is fairly consistent and reliable dates are available for some
events, then remaining events can be scaled proportionally. Their
estimate of an 8 million year separation between humans and African
apes produced a revolution in human paleontology.
Problems due to natural selection with one
protein are somewhat mitigated by examining the results from several
different proteins. Although the accuracy of molecular clocks
are often questioned, they clearly have phylogenetic implications.
Molecular techniques include the study of electrophoresis, immunodiffusion,
microcomplement fixation, DNA hybridization, amino acid sequences,
DNA fingerprinting, mitochondrial DNA, nuclear DNA, and chromosomes.
The results from some of these studies is discussed in chapter
14. (return to outline)
Dating by identification of "types"
is extremely useful but, like any other dating process it must
be used thoughtfully. There can be no assumption that "types"
evolved from simple to complex or that artfully made items are
inherently more recent than objects that exhibit little skill.
Indeed, many simple manufactured objects, such as a crude stone
knife, a smooth stone for use as a hammer, or a chisel-like burin
flake for incising bone are found almost everywhere there are
evidences of human habitation. But a 1958 Buick automobile can
only exist after the 20th century. Complex artifacts and complex
art styles known to date from a particular time and place are
extremely valuable clues to age. (return
to outline)
Living tissues contain proteins composed
of amino acids that begin to deteriorate with death. Thoughrate
of breakdown depends upon temperature and moisture, amino acids
are persistent in some conditions. As amino acids deteriorate
they convert from the molecular form found in living tissue (L-amino
acids) to another molecular state (D-amino acids), a process called
racemization. Attempts to use this process for estimating dates
have produced inconsistent results.
Seasonal changes in rainfall and temperature
produce growth rings in some species of trees and perhaps some
shell-producing organisms. Annual freezing and thawing of glacial
ice produces varves, bands of sediments, in glacial lakes. Both
dendrochronology and varve analysis provide limited but useful
dates for the past 12,000 years.
Information about fluctuating climates can
be used to build a chronology of cold-warm oscillations. Once
established, that chronology can assist in the dating of sites.
An important technique for reconstructing climate is based upon
the ratio of 8O18 to 8O16. The slightly heavier 8O18 evaporates
less readily. Thus when large amounts of precipitation remain
locked in glacial ice, ocean water is richer in 8O18. Foraminifera
and other sea-dwelling organisms incorporate oxygen in shells
that settle to the sea floor. The ratio of 8O18 to 8O16 in their
shell documents growth and decline of polar and glacial ice sheets.
Ancient chert or obsidian objects have weathered
surfaces. Some materials, especially obsidian, absorb water to
form a crust over the surface of the material. Though processes
like obsidian hydration are influenced by environment, they can
be of great assistance, especially in recognition of objects of
recent manufacture. (return to
outline)
Rules for Interpretation of the Fossil Record
1. All descendants have ancestors. The inhabitants of a geological epoch are descended from populations that existed in the previous epoch. It is not reasonable to omit vast periods of time when reconstructing family trees. Likewise, "special creations" are not acceptable to a scientific view of the fossil record.
2. Place more importance upon fossils that are anatomically complete. Incomplete specimens forcescientists to reconstruct the relevant portions from their imagination to their theoretical biases.
3. Place more importance upon specimens that are dated by a technique that meets geological criteria for accuracy and reliability. Materials that are dated typologically or by guesswork often result in circular logic and errors of interpretation.
4. Place more importance upon fossils that are found in primary context. The context (stratigraphy, associated artifacts, other fossils, structures, features...) provide much of the information that is needed to interpret fossils. A specimen without context is of little value.
5. Place more importance upon fossils that are represented by numerous discoveries and specimens. When one deals only with a single specimen, one has little data about many relevant questions. Is the specimen male or female? How much variation did the species exhibit? What is its time and geographic range?
6. Remember that the less data one has, the easier it is to fit the data into contrasting or conflicting theoretical models. The real world tends to be complex and our theories tend to be simplistic. The better and more extensive the data, the more likely the resulting theoretical models reflect actual events.
7. Remember that science is conjecture. Our models improve as we learn more. Previous ideas and theories are usually modified as better or additional information are acquired. Consequently, details of evolutionary models are updated as our knowledge increases. Such changes are desirable and should be viewed as an essential component of scientific progress. Unfortunately, those who do not understand science may view changes negatively. (return to outline)
Almost every human culture has its origin
myth, and origin myths are commonplace elements ininstitutions
within societies. Scientific disciplines debate the merits of
various "founders." Businesses, states, universities,
and departments proudly repeat the stories of their beginnings.
The scientific origin myth about the beginnings of our universe
pushes our knowledge of physics, geology, and astronomy to their
limits as new ideas and new information continually force updates.
The basic outline has changed little in the twentieth century,
a fact that reflects a richness of the evidence and robustness
of the methods of science. Some features of the planetary origin
myth are so well documented that we place great confidence in
their accuracy. Other features are fabricated out of fragile hypotheses
that represent the best that has been produced at this moment,
but which may be exploded by contradictory evidence or better
models in the future.
There are several popular theories about
the origins of the universe. One theory, popularized byGeorge
Gamow () in 1948, is "the big bang," a tremendous explosion
that occurred about 15 billion years ago and hurled all known
matter into space from a concentrated source. This source was
a dense concentration of the matter compressed to extremely high
temperatures and broken down into its elementary building blocks.
In the early moments of the expansion that followed the explosion,
chemical elements formed as protons, neutrons, and electrons found
themselves at temperatures and pressures low enough to form atoms.
If this transition was rapid, then the composition of this cloud
of material would have been primarily hydrogen. The slower the
rate of expansion of the initial mass, the greater proportion
of heavier isotopes since there would be time to form more complex
atomic units. This initial explosion propelled a cloud of matter
and radiant energy outward from the location of the initial mass.
The universe at this stage was like a balloon being inflated as
matter sped outward from the point of origin. As this enormous
cloud of matter dispersed, gravity began to coalesce the dispersing
matter into clouds and then to gaseous galaxies that continued
their rapid outward flight to create the expanding universe known
to us today. This model fits most of the observable features of
the known universe.
Another origin model is the "steady-state
universe." It hypothesizes an ageless universe in whichmatter
(in the form of hydrogen) is continually replenished, forms rotating
gaseous clouds that condense into galaxies, and is consumed by
conversion of matter to energy in the atomic reactions of stars.
In the steady-state-universe, production of heavier atomic elements
would occur in the nuclear reactions of stars, especially the
nova or supernova event that occurs when a star exhausts its hydrogen,
collapses, and then explodes in a cataclysmic dispersal of matter
and energy. This model of an ageless universe has little current
scientific support.
A modified steady-state model that incorporates
the big bang proposes that if the average density ofmatter is
large enough and the rate of cosmic expansion falls below escape
velocity, then gravitation will eventually dominate and the universe
will eventually cease expansion and collapse back to the concentrated
state that preceded the big bang. This hypothesized collapse is
usually called the "big-crunch." If the mass of the
universe is insufficient to slow expansion, then there is an open
and infinite universe that is speeding toward dispersion. The
big-bang seems well supported by observable evidence. The big
crunch is, at best, doubtful.
In either model, our galaxy, seen in the
sky as the Milky Way, had its origins in a condensing gascloud,
and the observable galaxies are moving away from each other at
great speed. Our sun, like other stars, began as an eddy in a
rotating cloud of gas. Compressed by gravitational attraction,
a whirlpool of gas solidifies. Larger masses attract smaller ones.
The quantity of mass in these objects produces temperatures and
pressures great enough to stimulate nuclear reactions. The ignition
of fusion in the sun is called the T-Tauri phase of solar history,
a model of solar history that predicts that the early sun was
much less bright than today. The reaction that fuels the fire
of sunlight is thought to be conversion of hydrogen into helium
through the catalytic action of carbon and nitrogen.
Nuclear reactions of stars inject heavier
elements into galaxies as they pass through their cycles offormation,
exhaustion, collapse, and nova. Our solar system, a star and its
orbiting debris, took shape within our galaxy about 5 billion
years ago. This star, our sun, coalesced out of dust and gases
expelled from older stars that exploded as supernovas. Perhaps
the expanding shock wave of a nova helped compress our dust cloud
toward consolidation. The elements that make up planet earth,
including many molecules important to organic chemistry (Table
11-3), are already present in the cosmic dust and debris that
formed our solar system.
Theories about the origins of planets fall
into two classes, "second-body" and "single-body."
Georges de Buffon (1808-1877) proposed the first of the second-body
theories in 1750. He suggested that the earth may have been torn
from the sun by a passing body such as a comet. In 1905, T.C.
Chamberlain (1843-1928) and F.R. Moulton () proposed that close
passage by a second star pulled masses of the sun's gasses away
and supplied the inertia that put them into orbits around the
sun. Planets are condensations of this orbiting gas that grew
by sweeping up smaller masses in their orbital paths. There have
been numerous variations on this model in subsequent years, even
the suggestion that our solar system once had a second sun that
was swept way by a third body collision or supernova event. Part
of the excitement of twentieth century exploration of our solar
system is that data is accumulating relevant to understanding
the origins and histories of the various objects in our solar
system. Most of these speculations have little modern scientific
following but they are frequently "rediscovered" and
touted in our more sensational magazines, but there is little
modern evidence to support second-body theories.
In 1755, Immanuel Kant (1724-1804), the
German philosopher, proposed that planets were solidified masses
of gas from a rotating nebula. Pierre Laplace (1749-1827),a French
mathematician, computed that a rotating gas cloud would spin faster
as it contracted, leaving rings of gases behind that could form
planets. Modern astronomers recognize that the Laplace model was
flawed, but the discovery that nebula clouds are comprised of
a mixture of dust and gas revitalized the Kant-Laplace model.
The more modern version of the nebular theory explains many of
the motions and angular momentum observable in our solar system.
The planets of our solar system, including
Earth, consolidated about 4.5 billion years ago. Age estimates
based upon ratios radiometric isotopes in the earth's mantle yield
estimates between 4.65 billion years and 4.43 billion years before
present. Because the oldest known rocks are about 3.8 billion
years old, there is approximately 0.7 billion years between initial
planetary consolidation and formation of the oldest known rocks.
An early event in earth history is a hypothesized
iron catastrophe. The bulk of the planet is composed of iron which
is heavy and has a relatively low melting point. When planetary
temperature reached the melting point of iron, the planet melted,
allowing the heavier iron to displace lighter materials from the
planet's center. Even today the earth retains only a thin crust,
the lithosphere, around a molten and turbulent interior. This
fragile crust is continually shifted and distorted by currents
underlying it. (return to outline)
The first eon of earth history is known
as the Hadean , but most scientists ignore both the Archean (or
Archaeozoic) and Proterozoic terminology and refer to the entire
period of planetary history prior to the Cambrian period as the
Precambrian. The early earth atmosphere was unlike the one familiar
to us today. Atmospheric gases released from within the earth's
crust (largely by volcanic action), are trapped by planetary gravity.
Modern volcanic gases consist mainly of carbon dioxide, water,
nitrogen, and sulfur. Modern volcanoes emit enough new water each
year to replace the existing earth oceans in about 30 million
years. Ice meteorites still enter Earth's upper atmosphere, injecting
tons of water each year. The earth is bombarded with cosmic rays,
subatomic particles from beyond our solar system and from the
sun. Earth's magnetic fields deflect most cosmic particles. Some
charged cosmic particles are trapped in doughnut-shaped zones
outside earth's atmosphere called the Van Allen radiation belts.
Some particles penetrate the magnetic shield near the planetary
magnetic poles where they strike the atmosphere and produce the
auroral lights. The little cosmic radiation which strikes the
ground is generally focused by planetary magnetism to the polar
regions. Earth is warmed by radiation from the sun. It is likely
that there was little or no free oxygen in the primitive earth
atmosphere, and without an ozone barrier, Earth's surface would
have been exposed to high levels of ultraviolet radiation. Other
atmospheric gases in the upper atmosphere absorbed x-rays from
the sun. Water in the upper atmosphere could have been a small
source of oxygen as photolysis split water into hydrogen, which
could escape into space, and oxygen, which entered the atmosphere
to be consumed in neutralizing hydrogen, carbon monoxide, ammonia,
hydrogen sulfide, or reacting with iron or sulfur in earth seas
or continents.
The early planet was severely bombarded
by other bodies and debris as it swept through the remains of
the dust cloud that gave rise to our solar system. Some larger
bodies released great energy on impact, converting kinetic energy
to heat. A large impact at sea could convert oceans to steam.
Craters from this bombardment are long since obliterated but life
evolved on earth during this period. Perhaps energy and temperatures
generated at meteor impact contributed to production of complex
organic molecules and helped transform cosmic organic molecules
that were precursors of life to replicating organic systems.
Earth's first half billion years may eventually
be deduced from geochemistry and by examination of objects in
our solar system such as moon rocks. Continental evolution also
began as early continental masses were bombarded by large bodies
from space, fracturing and melting holes in the upper mantle from
impact heat. Large impact craters filled with water released from
volcanic vents and circulation patterns were established in the
molten iron core below a thin outer silicate mantle. Up welling
of the hot mantle along fracture or fault systems in the lithosphere
produced rifts that broke up continents and injected magma to
form new crustal materials. Dramatic cycles in climate were expressed
as periodic ice ages, when great glacial ice sheets obscured large
continental regions.
The Archean eon saw the formation of some
of the modern continents, and more important, the appearance of
life. The oldest known earth rock unit, the Isua Supracrustal
deposits of western Greenland, 3.8 billion years old, contains
marine sediments that have been so altered by postdepositional
heat, motion, and pressure that they are unlikely to retain fossil
evidence even if organisms were present at that time. Sediments
from the Warrawoona Group of Western Australia, dating to about
3.5 billion years before present, contain fossils that indicate
the presence of many types of microorganisms. Rock units that
date between 3.5 billion years and the Cambrian Period (590 million
years ago), include numerous localities with evidence of organisms,
but the data usually are one of three types: (a) layered mound-shaped
structures in limestone and dolomites called stromatolites, (b)
sandstone impressions of large organisms, or (c) hundreds of deposits
that contain microfossils. One of the more famous sources of fossil
microbes is the 2 billion year old Gunflint Iron formation in
southern Ontario. There are thousands of fossiliferous deposits
from the Cambrian onward that provide a rich documentation of
the history of life on earth.
What were these organisms that evolved on
earth about 4 billion years ago? We assume they were primitive
self-replicating cells that fed on organic molecules of cosmic
origin in the seas of Earth. Their metabolism was probably powered
by fermentation and the scavenged cosmic organic molecules until
an organism, probably resembling the present cyanobacteria, developed
an ability to photosynthesize. Presumably the first photosynthesizers
used hydrogen sulfide in a manner similar to cyanobacteria. Some
modern cyanobacteria build layered shallow-water structures called
stromatolites. Successive generations of cyanobacteria incorporate
grains of sand at the top of their colony, producing stony domes
and columns. Fossilized stromatolites are dated to 3.5 billion
years ago. The evolution of autotrophs, organisms that manufacture
food with CO2 as its only carbon source, meant that life was no
longer just heterotrophs, organisms that forage for food upon
other organisms or upon organics of cosmic origins. Once life
itself could generate food from sunlight and nutrients, an earth-type
biosphere became possible.
A subsequent innovation of incalculable
consequence was the development of oxygen-generating photosynthesis,
the source of free oxygen in quantity in modern earth atmosphere.
As this process began to produce atmospheric oxygen, it changed
the characteristics of the atmosphere and earth's oceans. At first
oxygen could be taken up by iron dissolved in the ocean to form
iron oxides that precipitated from iron rich deposits on the ocean
floor. The excess of carbon in atmosphere and planetary surface
bound oxygen in the form of carbonates or carbon dioxide. Eventually
the oceans were cleared of dissolved iron and oxygen levels began
to rise. Oxygen levels rose as carbon was sequestered as organic
material in sediments. Microbes contributed their organic carcasses
to sediments in marine basins. These organic-rich sediments, when
sealed beneath impervious cap rocks, are sources of modern oil
fields. If lithic temperatures are high, these organics are converted
to natural gas. Oil, gas, and coal deposits represent carbon removed
from earth's atmosphere that has been replaced by oxygen. O2 levels
appear to be relatively low in earth's atmosphere until about
2.5 billion years ago. In the modern world, green plants generate
a volume of O2 equal to our planetary atmosphere every 2,000 years.
Oxygen must have been toxic to many early
life forms, perhaps destroying the early organisms that first
solved the metabolic problems of living. After a period of time,
perhaps a billion years or more, some organisms developed respiration.
Instead of using fermentation to convert sugars to energy, oxygen
is used in a more efficient way. Oxygen respiration breaks the
sugar into smaller units and releases more energy. A gram of sugar
produces 110 calories by fermentation, but the same sugar releases
about 3900 calories by respiration. Powered by the new metabolism,
respiring organisms began to diversify, experimenting with a wide
array of cellular forms.
The next innovation was the invention of
sexual reproduction, an innovation that facilitated evolutionary
changes by promoting genetic recombination. A new kind of cell
appeared, a cell which had a nucleus, a eukaryotic cell. Eukaryotic
cells appear about 1.5 billion years ago, possibly by a symbiotic
combination of older single-celled prokaryotes. A fermenting organism
combined with an oxygen-respiring bacteria. The host's genetic
material is kept separate from that of the bacteria by the nuclear
membrane, but both cells used the energy provided by the bacteria's
oxygen respiring abilities. The remnants of this ancient bacterial
DNA and energy producing mechanism are seen in the mitochondria
of the cells of modern organisms.
A second symbiotic association that probably
occurred in a similar fashion, is incorporation of a cynobacteria
into some of these cells. Their descendant cell lines could both
respire and photosynthesize and are represented in the modern
world by plants.
In summary, three major lineages, archaebacteria,
eubacteria, and eukaryota, developed from a single progenitor.
They all share a similar genetic code and metabolism but represent
different elaborations on a fundamental biochemistry common to
all planetary life. Presently, there is no known way to discover
how many forms or lineages of living organisms appeared in earth's
early seas. We are confident only that the similarity of genetic
code indicates that all modern life descended from a single Archean
lineage. As the Precambrian era came to an end, life was flourishing
in the form of microbial faunas and floras. The first metazoans
may have appeared in the latest Proterozoic period, less than
670 MYBP. The Precambrian closes with an ice age, glaciered continents,
and widespread microbial species extinctions. (return
to outline)
This era begins with global warming and
reduction of continental glaciation. The first multicelled creatures
appear to have been soft bodied creatures, but near the beginning
of the Cambrian period creatures began to gird themselves with
shell armor, producing familiar looking brachiopods and gastropods.
Shells preserve readily as fossils, and the Cambrian explodes
with evidence of life in the sea. Fossil animals called trilobites
became numerous. Seaweeds, sponges, jellyfish, and sea anemones
provide a familiar context for various organisms unlike any known
today. By the end of the Cambrian, all major phyla of modern organisms
have appeared. Even the lineage leading to humans, the chordates,
is represented. Trilobites, at earlier times the most numerous
of Cambrian metazoan fossils, wane and brachiopods become numerous
in marine deposits.
The evolving marine life of the Ordovician
becomes even more diverse and this period records the first appearance
of vertebrates in the form of ostracoderms, jawless, backboned
fish. Simple tetraspore-bearing vascular land plants invade the
continents. Scorpion-like and millipede-like invertebrates from
the Silurian may be the first air-breathing animals, and perhaps
some of the first animals to live part of their lives on land.
The first extensive fossil evidence for plant life on land comes
from the Devonian Period. After the solution of technical problems
of living on land, reproducing without a water medium, trapping
water and oxygen, finding nourishment, securing stability, and
transporting nutrients, plant life rapidly colonized the continents.
A great variety of animal life, amphibians, insects, mites, spiders,
snails, and worms, follow the plants onto land. Amphibians, sharks,
and bony fish become common during the Mississippian. Flying insects
appear in the forests. Cockroaches, scorpions, and giant insects
are abundant in the swamp forests of the Pennsylvanian period.
Large reptile-like amphibians are common and the first reptiles
appear. The Permian period saw reptiles diversify and the first
mammal-like reptiles appear.
The end of the Paleozoic is marked by trauma.
Continental collisions produced great mountain uplifts and increased
volcanism. Paleozoic organisms experienced mass extinctions with
more than half of the existing families coming to an end. There
have been many hypotheses formulated to explain this "time
of the great dying" that occurred about 230 million years
ago. One that received much media discussion was the asteroid
hypothesis. In this model, a rocky asteroid with a diameter of
more than 3 miles struck the earth, releasing enormous kinetic
energy. If impact was on a continent, the crater would be larger
than Rhode Island, and an ocean strike would produce a shock wave
that could empty an ocean basin, swamp continental margins with
a tidal wave, and vaporize a great quantity of steam. Debris lofted
into the atmosphere was carried world wide, blocking the sun,
and cooling the surface. The planet would be plunged into the
darkness that scientists imagined would occur in a "nuclear
winter," the disruption of the usual planetary climate cycles
after a nuclear holocaust. Unfortunately for advocates of the
asteroid model, there is at present no convincing geochemical
or lithic evidence tying such an event to the Permian extinctions.
It is important to recognize that extraterrestrial
objects are not necessary for a "nuclear winter" to
occur. Although most Americans are familiar with the Mount St.
Helens explosion, it was tiny as such events are measured. Consider
the explosion of Tambora on the East Indian island of Sumbawa
in 1815. Thirty six cubic miles of mountain disappeared. The debris
circled the planet for years, lowering worldwide temperatures.
The year 1816 was called "the year without a summer."
In New England, there were severe frosts in July and August. It
is easy to imagine an even larger series of volcanic episodes
surrounding the planet in dust and ash sufficient to disrupt photosynthesis,
drop temperatures, and severely disrupt ecosystems.
A vast area of volcanic basalts (340,00
km2), the "Siberian Traps," are known from the Northern
Soviet Union. These basalts represent volcanic eruptions that
discharged more than 1.5 million times as much material as the
1981 North American eruption of Mt. St. Helens. Radiometric dating
places the formation of the Siberian Traps about 240 million years
ago - near the Permian Triassic boundary. Thus volcanism appears
to be a more likely mechanism for the Permian extinction. (return to outline)
Life on the planet rebounded from the Permian
extinction event and diversifies. Reptiles continue to evolve
during the Triassic, producing many marine forms and expanding
lineages such as dinosaurs and mammal-like reptiles. The Jurassic
is an age of gigantic marine reptiles and dinosaurs. Early mammals
are found as rather inconspicuous components of Jurassic faunas.
The first true mammals, small mouse-like and shrew-like animals
from the Triassic of Europe, China, and Africa, may have been
food for the toothed birds of that time. By the Jurassic, mammals
are more numerous and more diversified but still dwarfed by giant
Mesozoic reptiles. A super continent, Pangaea, is assembled by
consolidation of previous land masses. Pangaea begins to break
up by the middle Jurassic with the opening of the Tethys sea that
divided the supercontinent from east to west along the equator.
Much of the Mesozoic is marked by a moderate climate with a great
flourishing of reptiles and tropical plants on land and reefs
and plankton in the Tethys sea. Flowering plants expanded greatly
in the Cretaceous, and both marsupial and placental mammals appear.
The giant reptiles become extinct and the landscape exhibits modern
types of insects. The great extinctions at the close of the Mesozoic
era are less severe than those of the Permian. Some of the vanishing
animals species, especially the great reptiles, are so impressive,
that their demise attracts much attention. Only two descendant
lineages from the dinosaurs survived the Mesozoic, birds and crocodilians.
What killed the dinosaurs? Popular theories include:
1. Collision of earth with a comet or asteroid that disrupted earth climate
2. Volcanic eruptions, especially those represented by Cretaceous volcanic basalts from India
3. "Smarter" mammals out-competed the dinosaurs
4. Mammals ate the dinosaur eggs
5. The evolution of angiosperms was accompanied by the evolution of new plant alkaloids that were toxic (poisonous) to dinosaurs6. Sex determination failure in a cooler environment
The close of the Mesozoic was a time of great volcanic activity. As giant reptiles vanished, Cretaceous insectivorous or rodent-like mammals become more noticeable. Both marsupial and placental mammals evolved from Jurassic antecedents. (return to outline)
The current era began with fairly warm and
moist climates that have grown colder with time. The present circumantarctic
currents, glaciered poles, warm tropics and drier temperate zones
are progressively established. The continents separate, isolating
the faunas of Australia and South America. Teleost fish become
important members of marine communities and marine mammals appear.
On land archaic mammals dominate from the Paleocene and peak in
diversity during the Pliocene. The Pleistocene Epoch witnesses
the emergence of humanity as the dominant mammal.
Our knowledge and hypotheses about earth
history generates many more questions than answers. Afeature of
large scale extinction events in the fossil record is that they
appear to be cyclic. The earliest documented fossil extinction
occurred near the end of the Cambrian (505 MYBP) and is reflected
in loss of most existing families. The second occurs at the close
of the Ordovician Period (438 MYBP) and a third marks the end
of the Devonian Period (360 MYBP). The Permian extinction (248
MYBP) was the most drastic known. A fifth extinction event terminates
the Mesozoic (65 MYBP). Smaller extinction events are scattered
throughout the record. It seems likely that Earth periodically
experiences unusual environmental transformations but parts of
its biota endure. Some species exist only for short periods, a
mere instant of geologic time. Others, such as the horseshoe crab
seem to have enjoyed earth's seas since the Paleozoic era. Even
more interesting is the possibility that some modern bacteria
might be little different from their ancestors after an elapsed
time of several billion years. (return
to outline)
Continental modification prior to the Paleozoic
is probably not particularly relevant to mammalian evolution.
The earth's basaltic crust is broken in pieces, called plates,
that are floating on the liquid inner core of the planet. The
plates are very thin and they move slowly along the earth's surface.
Much of the energy that drives plate movement is theorized to
be subcrustal heat and convection currents in the underlying liquid
planetary body. Generally movement is away from a spreading center
where new crust appears to be formed by up welling and cooling
from the liquid planetary body. Movement is toward subduction
zones where crust sinks into to the subcrust and is melted. When
a plate contacts or overrides another plate, great energies are
released. Lighter granitic continents ride on top of these plates.
Since the granitic continents are too light (buoyant) to be pulled
into subduction zones, the down welling of plates can move continental
masses together with great force. The earth's great mountain chains,
including the Alps and the Himalayas, reflect continental collisions
at subduction zones. Continental masses can come together and
move apart on the underlying movements of basaltic plates.
The construction of a supercontinent, Pangea
about 250 million years ago provided a common planetary habitat
for early reptiles and mammal-like reptiles. Subsequent movements
have played a role in the flora and fauna of earth land surfaces.
(return to outline)
Table of Contents