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
Relative Dates by Chemistry
Absolute Dating by Radiometric Decay
Dating by Typology
Other Dating Methods
Rules for Interpretation of the Fossil
The Early Geological History of the
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.
age -> epoch -> period -> era
substage -> stage -> series -> system -> erathem
faunal complex -> stage -> zone
bed -> 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.
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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 Typology
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)
Other Dating Methods
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 force
scientists 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 dinosaurs
6. 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)
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