Figures to be added shortly
SUMMARY OF THE LIFE ON EARTH
PALEOZOIC LIFE
MESOZOIC LIFE
CENOZOIC LIFE
THE PRECAMBRIAN
EVENTS:
*Archean: Earth forms 4.5 b.y.
*Crust - mantle - core separation ±4 b.y.
*Meteorite bombardment up to 4 b.y.
*Ocean & atmosphere form by 3.9 b.y.
*Life begins? (oldest fossil) 3.5 b.y.
Proterozoic (beginning 2.5 b.y. ago):
*Continents formed
*Plate tectonics, modern rock types
*Free oxygen in atmosphere
*Glaciation
*End of Precambrian (base of Cambrian) marked by fossils with
hard parts .545 b.y.
Most of the earth's history took place in the Precambrian. In fact, 88% of geologic time is assigned to this vast subdivision of the geologic time scale. It began with the origin of the planet about 4.5 billion years ago (That is 4,500 million years ago or 4,500,000,000 years ago). It ended when creatures developed hard parts (such as shells) about .545 billion years ago.
Despite the vast amount of time involved, geologists know less about the Precambrian than they do about the paltry 545 million years that followed. There are several reasons for this. (1)Early-formed rocks have been destroyed or obliterated by tectonic processes as well as by the surficial processes of weathering and erosion. (2)Precambrian rocks are largely buried beneath younger rocks and therefore inaccessible to geologists' direct observation. (3)The Precambrian fossil record is very skimpy.
Nevertheless, geologists have groped through the fog of time and pieced together a plausible history of the Precambrian. Precambrian time is subdivided into two vast eons, the Archean and the Proterozoic. The boundary between the two is placed at 2.5 b.y. (billion years ago).
ARCHEAN
Earth's Origin.
See the box at the end of the chapter on geologic time in Plummer, McGeary, Carlson or McGeary, Plummer, Carlson. According to astronomers, our solar system was originally a huge spinning cloud of gas and dust. Most of this material accumulated into our sun. The relatively minor remaining material formed the planets. Dust and gas accreted into asteroids. Gravitational attraction caused the asteroids, in turn, to accrete into larger bodies--our planets and their moons.
As the earth grew, it heated up. Part of the heat was due to the continuing collision of asteroids. Part was the compressing of material in the earth's interior. Part was due to heat given off from radioactive decay. There was considerably more radioactivity then. For example, there would have been twice as much U238 when the earth formed 4.5 b.y. as its half-life is close to 4.5 b.y. Other radioactive elements with shorter half-lives would have to have been more than twice as abundant as at present.
The earth became hot enough to partially melt. This allowed distribution of material into separate layers. The present internal layers (core, mantle, crust) were formed by about 4 b.y. Most of the iron collected to form the core. Less dense oxygen, silicon and other elements accumulated mostly into the mantle.
Also, until around 4 b.y., the earth suffered an extended period of meteorite bombardment. We have no direct evidence for this because the crust of that time has been destroyed by erosion and plate consumption. The moon, Mercury and Mars, however, are extensively cratered from meteorite impact. We have been able to radiometrically date the moon's period of heavy meteorite bombardment as having concluded around 4 b.y. ago. It has been calculated that, due to its larger size, the earth would have been hit by 16 times as many meteorites as was the moon.
Origin of the atmosphere and oceans.
Our oceans and original atmosphere resulted from degassing of the mantle by volcanic eruptions. Water vapors condensed to form oceans. The early atmosphere was quite different from today's. Being oxygen free, it would not have supported human or other air-breathing life. It was largely methane ("swamp gas") and ammonia along with water vapor. By 4 b.y., the atmosphere had evolved into one largely made up of nitrogen, water vapor and carbon dioxide. Not until after Archean time would oxygen become a significant component of the atmosphere.
Archean Rocks
The oldest rock yet found is a gneiss, metamorphosed from granite, from northern Canada, radiometrically dated at 4.1 billion years (b.y.). The next oldest rock is from Greenland and is about 3.8 b.y.. Significantly, it was a sedimentary rock before metamorphism. To produce the sedimentary rock there had to have been weathering and erosion as well as deposition in a marine environment. Thus the atmosphere and ocean must have been developed on earth by 3.8 b.y.
Precambrian (Archean and Proterozoic) rocks are found in cratons, stable interiors of continents. The large central area of the United States and Canada between the Rocky Mountains and the Appalachian mountains is a craton. Most of the craton in the United States is covered by layers of Paleozoic and younger rock, but in much of Canada, the Precambrian rocks are exposed in a shield. A shield is a large region where the exposed bedrock is Precambrian igneous and metamorphic rocks. (See M,P, & C, chapter 5.)
Banded iron formations
Banded iron formations are characteristic of the Proterozoic but virtually nonexistent in younger rock. A banded iron formation is a thinly laminated rock of alternating chert (silica) and iron-rich layers. These are extensively mined, notably in Minnesota, for iron. The iron in the banded iron formations is oxidized (as in rust). They are generally regarded as indicating that oxygen was becoming part of the atmosphere. Without free oxygen in the air or dissolved in sea water, iron remained dissolved in the ocean. When sufficient oxygen became available the iron became oxidized and precipitated as the sediment of banded iron formations.
Proterozoic Movement of the continents. See Box "Dance of the Continents" in the Mountain chapter (chapter 20) of the textbook.
Glaciation
In the later part of the Proterozoic, glaciation, perhaps the most extensive in Earth's history, took place. This is indicated by tillites, lithified till, found on parts of all present major continents.
PRECAMBRIAN LIFE
The only fossils from all but the last 100 million years or so of Precambrian time represent single-celled life forms. The oldest undisputed fossils are bacteria and blue-green algae found in northwestern Australia. They are in 3.5 b.y. rocks. These rocks also contain the earliest stromatolites. Stromatolites are layered mounds formed of mats of algae and fine-grained sediment. They probably formed in the intertidal zone. Stromatolite became fairly abundant in the Proterozoic, particularly around 2.2 b.y. Later in the Proterozoic, about 1.4 b.y., large single cells with a nucleus developed. Multi-celled algae formed about 1.3 b.y. Rocks 1 b.y. contain tracks and trails of bottom crawling creatures, indicating that multi-celled animals had evolved. Rocks from the last 130 million years of the Precambrian contain fossils resembling jellyfish and segmented worms. (But shells and hard parts do not appear in the fossil record until the Paleozoic.)
Origin of Life
Living organisms are made of a large variety of organic compounds. Organic compounds contain carbon, with other element, arranged in large molecules as chains and rings. In order for life to be self-replicating, DNA and RNA are needed. These are composed of proteins, built of amino acids. Experiments begun in the 1940s proved that amino acids could have been created from the early atmosphere. Water, methane, hydrogen and ammonia (all likely ingredients of the early Archean atmosphere) were placed in a jar and subjected to electrical sparks (simulating lightning). This resulted in amino acids and other simple organic compounds.
Another possible source of amino acids is extraterrestrial. Amino acids have been found in some meteorites, so it is quite possible that amino acids were brought to earth during the meteorite bombardments early in Earth`s history.
A third possibility is that organic compounds and primitive life evolved in submarine hot springs, such as we find along present mid-oceanic ridges. The required gasses would be given off with the hot water permitting the formation of organic compounds.
There are several hypotheses regarding how the simple organic compounds became long chains of organically complex compounds. Some experiments indicate that when amino acid is splashed onto clays, more complex forms of protein develop.
It is paradoxical that free oxygen would not have permitted the early organisms to form. Thus, there would not be life on earth if the original atmosphere contained free oxygen. The build-up of oxygen in the atmosphere began as the Archean was ending 2.5 b.y. ago. This was probably due to the increasing abundance of algae. Algae, like plants, lives through photosynthesis. That is, it takes carbon dioxide (CO2) from the atmosphere and releases oxygen (O2) back into the atmosphere. The oxygen level in the early Proterozoic atmosphere was about 1% that of the present atmosphere. With oxygen available, fauna more like present life forms could begin evolving. Another important aspect of the increasing build-up of oxygen was that ozone (O3) would form in the upper atmosphere and filter out deadly ultraviolet rays, thus permit life on land beginning in the Paleozoic.
The Paleozoic was a time of evolving organisms. First, evolution took place exclusively in the seas; subsequently, life forms branched out into terrestrial plants and animals. The sudden abundance of fossils in the Cambrian suggests an explosion of life. No doubt, the beginning of the Paleozoic was a time of rapid evolution and proliferation. However, we must realize that by "rapid" we are still talking about millions of years. The abundance of fossils in rocks is indicative of the advent of hard parts (e.g. shells) which are much more likely to get preserved as fossils.
Overview of Evolution
Evolution is organic change in the lineage of plants or animals, it involves development of new characteristics. If, over many generations, changes result in significantly different features from ancestors, new species develop.
To understand evolution, you must understand that: (1) You are dealing with geologic time, not just a few generations of reproduction. (2) Traits are passed on to successive generations through genes. Genes result in transmitting the characteristics of parents. However, reproduction never results in exact duplication. No two organisms are exactly alike (even identical twins). Therefore, there will always be change through the generations. Changes will not necessarily be toward making more perfect organisms. However, the changes which result in more success at surviving and reproducing in a particular environment (or ecological niche) are more likely to pass on their genes. This means offspring will have more likelihood at success and reproduction.
Natural selection means reproductive success. It can take numerous paths. If more eggs are hatched or babies born, more are likely to survive. Survival may also be enhanced through development of camouflage or a protective shell or being poisonous or distasteful to predators. An organism may develop a taste for and ability to acquire another readily available organism. One species may be able to adapt to changing physical conditions (e.g. colder water) better than its competitors in the ecological niche.
When species fail to adapt to changing conditions, they become extinct. Extinction is a common and natural phenomenon. Frequently, when one species becomes extinct, another organism moves into its ecological niche.
Cambrian Life
The record of transition from Precambrian to Paleozoic time is dramatic because of the abundant and widespread presence of fossils in Cambrian (545-505 m.y.) rocks. Life flourished in the shallow warm seas over the continents. During the first few million years of the Cambrian, there was an evolutionary "explosion" of life forms--perhaps because of the warm, nutritional seas, perhaps because of availability of previously unoccupied ecological niches. By the end of Cambrian time almost all of the major groups of fauna were established.
Stromatolites (the most common indicator of Precambrian life) continued to be important in Cambrian seas. New faunal groups included snails, Bs, primitive echinoderms (relatives of the sea urchin and starfish), clams, pelecypods (squid-like creatures) and brachiopods (bivalves, like clams, but the top and bottom shells are not mirror images of each other) (see image of brachiopod). Coral-like reef builders called archeocyanthids were important in the Cambrian, but soon became extinct.
The most abundant fossil, almost a symbol of the Cambrian, is the trilobite. Trilobites were arthropods (like present day lobsters and insects). Many species of trilobites appeared and disappeared throughout the Paleozoic era. They were at their greatest number during the Cambrian; but most species died out toward the end of the Cambrian. See image of trilobite
One of the most important fossil locations in the world is in the Canadian Rocky Mountains. A small quarry in the Burgess shale in British Columbia has yielded an incredible array of Cambrian fossils. It contains numerous trilobites and other shelled fossils; but its importance is in the well-preserved, soft-bodied fossils found only in this site. Some of these creatures are "weird wonders" and cannot be correlated to any of the major present-day faunal groups. They represent phyla that probably evolved into fairly complex life forms during the first few million years of Cambrian times, then became extinct. Stephen Jay Gould's best selling book Wonderful Life, the Burgess Shale and the Nature of History, (1990, Norton Press) tells the story of the Burgess shale and its implications. Gould points out that if some of the extinct phyla had survived and some of the present day phyla had become extinct, the present earth would be populated with a totally different bestiary and we humans would not be here.
The 40 million year-long Cambrian period ended with a mass extinction. A large number of species became extinct. Trilobites, the dominant animal of the Cambrian were particularly affected as most species were wiped out. Subsequently new species of trilobites evolved and trilobites persisted throughout the Paleozoic--but all species of trilobites became extinct at the close of the Paleozoic--the mother of all mass extinctions.
Subsequent Paleozoic life
During the Ordovician, another major group of fauna came into existence. Bryozoans (see illustration)became an .important animal that lived attached to the sea floor. Brachiopods took over from trilobites as the most abundant creatures in Ordovician seas. The first corals also appeared in the Ordovician. These included colonial corals as well as a cone shaped coral known as a horn coral.
One of the most spectacular Paleozoic fossils, which dates back to Silurian time is the euripterid (see illustration), or sea scorpion, a mean-looking predator that grew up to 10 feet long. In the Silurian, coral began making reefs. This was the beginning of large barrier reefs that would be important throughout the rest of geologic time and provide an ecologic home to a wide range of organisms. Perhaps the most important aspect of the Silurian is the first certain appearance of land plants.
The earliest insects were Silurian. Eventually, they evolved into the most successful of all organisms. There are more species of insects today than of any other animal. (It is estimated that, at present, there are 30 million species of insects on earth.) During Pennsylvanian time, dragonflies having a wingspan of 3 feet flew amongst the thick vegetation of the coal swamps.
The Mississippian is sometimes called the "age of crinoids" because of the abundance of crinoid fossils (see illustration). Crinoids are animals related to starfish but look like flowers and are informally called "lily of the sea".
Land forests developed in the Devonian and probably raised the level of oxygen in the atmosphere. Some scientists feel that this paved the way for land animals; not because of the free oxygen for breathing, but because it allowed buildup of ozone (O3) in the upper atmosphere. The ozone layer absorbs ultraviolet rays that otherwise is deadly for animals not living in the seas. The vast coal deposits of the eastern United States attest to extensive swamps hosting a thick forest of vegetation. These formed during Pennsylvanian during a time of frequently repeated rise and fall of sea level. (Coal is the Unites States' most abundant energy resource.)
Vertebrates, that is animals with backbones, date back at least to the Ordovician (and perhaps late Cambrian). The first vertebrates were jawless fishes. The Devonian is sometimes called the "age of fish". During the Devonian, jawed fishes evolved and became abundant both in variety and in quantity. Two major branches of fish developed during the Devonian--the cartilaginous fish (such as sharks and rays) and the bony fish (relatives of our common fish). Some Devonian fish were as much as 30 feet long.
Amphibians branched off from fish in the Devonian. The first amphibians are known as labyrinthodonts. They evolved from a primitive lungfish. The dorsal fins of the fish evolved into the legs with sufficient strength to support the animal on land. Reptiles evolved, in turn, from the amphibians. The oldest reptile is Late Pennsylvanian and closely resembles a labyrinthodont. Reptiles became the first creatures to lay eggs on land.
Reptiles became important during the Permian as they developed strong jaws. Permian reptiles evolved into two important groups. One was the sail-backed reptiles, such as dimetrodon, a predator. (Dimetrodon is often included in sets of plastic dinosaurs. It is not a dinosaur and became extinct before the first known dinosaur evolved.) The other group is the mammal-like reptiles, therapsids. It is believed that dinosaurs as well as mammals evolved from therapsids in the Mesozoic.
Mass extinction
The largest mass extinction ever took place as the Permian ended (and, of course, the Paleozoic era came to a close). Approximately half of marine species became extinct. The earth would never again see trilobites nor lesser known families of fauna.
Perhaps contributing to the extinction was the radical changes in climates. Southern hemisphere glaciation in Pennsylvanian time was followed be a hot and dry Permian climate (at least for North America). A more likely cause is attributable to continental drift. When Pangea, the supercontinent, formed during the Permian, there was an enormous decrease in the shallow waters compared to that of the previously separated continents. This resulted in more competition for living space.
Recent work points the finger at a major period of volcanic eruption. The largest flood basalts in the world are in Siberia. They have now been radiometrically dated and found to correspond with the end of the Paleozoic. Huge volumes of sulfur would have been released by the eruptions and these could have changed the atmosphere sufficiently to cause the mass extinction.
The Mesozoic, often referred to as the "Age of Dinosaurs", began about 245-250 million years ago and ended around 65 million years ago.
Following the great mass extinctions that took place at the close of the Paleozoic, there was a great expansion of species.Marine life was much like that in the present oceans with Mesozoic species of clams, snails, modern corals, modern echinoderms (sea urchins and star fish), fish, arthropods (crabs and lobsters). Microscopic sea life with shells (plankton) were abundant, especially during the Cretaceous, at which time thick chalk formed on shallow sea floors (perhaps the best known are those of the white cliffs of Dover in England).
Ammonites thrived during much of the Mesozoic. (Ammonites were pelecypods, similar to squids, that had coiled shells divided into chambers.) Some ammonites were only a few centimeters in diameter, whereas others had shells of a meter or more. Species are often distinguished by the pattern of their sutures (the line where adjacent chambers of the shells are joined). Early ammonites tended to have very simple sutures, whereas some of the later Mesozoic species had very ornate patterns. All ammonites became extinct by the end of the Mesozoic.
Mesozoic seas had some large creatures. Crocodiles were fast, ocean-going creatures that reached lengths of up to 50 feet in the Cretaceous. Turtles had shells up to 12 feet across. Perhaps the most spectacular marine fauna were the large marine reptiles: Plesiosaurs were long necked with paddle-like limbs. They were fish-eating and up to 10 ft. long in the Jurassic and 40 feet long in Cretaceous. Mosasaurs were short-necked and up to 50 feet long in Cretaceous. Ichthyosaurs were dolphin-like and probably fast swimmers. During the Cretaceous they reached up to 60 feet in length.
On the land, plants became more varied from those of the Paleozoic. The early Mesozoic plants included ferns, conifers, ginkgoes and other nonflowering plants. The first angiosperms, flowering and fruiting plants, evolved during the Jurassic and became abundant during the Cretaceous. (At present over 90% of plants are angiosperms.) As flowering plants became abundant, so did the insects that live with flowers, such as butterflies and bees.
Some of the familiar land vertebrates evolved during the Mesozoic,
notably frogs and turtles. The first snakes appear in the Cretaceous
fossil record. Primitive birds appear to date back to the Jurassic
(see discussion below relative to dinosaurs). Mammals evolved
from the mammal-like reptiles early in the Mesozoic and were not
very significant throughout the Mesozoic.
Dinosaurs
The creatures regarded as almost synonymous with the Mesozoic are, of course, dinosaurs. Although they came to be the dominant land animal, they did not evolve until late Triassic, after around ten million years of the Mesozoic had gone by. The dinosaurs evolved from early Triassic reptiles called thecodonts. The first dinosaurs were probably small (a foot high), agile, bipedal (walked on their hind legs), and carnivorous. By the end of the Triassic, species up to 20 feet high had evolved. Even larger species evolved later. Evolutionary radiation resulted in a wide variety of species within a geologically short period of time. Although some species would become extinct and new species evolve throughout the rest of the Mesozoic, dinosaurs, as a whole, prevailed on land for about 110 million years.
Although fossils for several hundred species of dinosaurs have been discovered and analyzed, we will restrict our descriptions here to those that are best known to the public. (For a more detailed discussion of dinosaurs, see Cowan, 1991, History of Life. Oxford Press)
The largest dinosaurs were herbivores having small heads, long necks and long tails. The longest dinosaur was Diplodocus, a Jurassic herbivore that was 95 feet long and probably weighed 13 tons. However, a newly discovered dinosaur, Seismosaurus, may have been even larger. Apatosaurus (formerly called Brontosaurus) was, at least until recently, regarded as the heaviest dinosaur, weighing in at 35 tons and having a length of 70 feet. (Its claim to the heavyweight championship is currently being challenged by some recently found partial skeletons of dinosaurs called Supersaurus and Ultrasaurus. These may have had weights of 80 tons.) Apatosaurus was also a Jurassic plant eater. It is probably the best known dinosaur as it is usually the one depicted in oil company logos, cartoons and other popular media presentations.
Stegosaurus had large back plates and a spiked tail. It was a 20 foot long, 12 foot high herbivore that lived in the Jurassic.
Among the carnivores, the largest were Allosaurus, in the Jurassic, and Tyrannosaurus, in the Cretaceous. These had large, massive heads with long sharp teeth. They walked on their hind legs and had short front limbs. Allosaurus was the largest meat eater, being about 25 feet high (3 stories) and lived during the Jurassic. Tyrannosaurus, with its ferocious, huge jaws, lived during the Cretaceous. It was 40-50 feet high and weighed more than an elephant. A carnivorous dinosaur similar to T-Rex, but with an even bigger skull, was discovered in Morocco in 1995.
Triceratops is rhino-like and looks like a battle vehicle. This Cretaceous, herbivorous quadruped has a shield-like head with three horns.
The Cretaceous duck-billed
dinosaurs were unusual. Some of them had long crests on their
heads. Apparently they ran in herds. The crests have long hollow
tubes, believed by some dinosaur researchers to have been used
to trumpet signals to one another.
What were the dinosaurs like? They lived in many environments,
not just the tropics. Some lived near the poles in colder climates.
It used to be accepted that dinosaurs were rather sluggish, cold-blooded
creatures; but this view has recently been challenged and current
thinking regard them, or at least some of them, as warm blooded.
The following support warm-bloodedness: (1)They had primitive
hair or feathers. (2)Tracks indicate that they were fast (traveling
up to 25 mph), and that their bone structures were more like mammals
than reptiles. (3)They apparently hunted like mammals, rather
than reptiles, taking lots of prey to support themselves.
At least some dinosaurs were social animals. Tracks indicate that they traveled together with the trackways of big creatures outside and little tracks on the inside, indicating babies were shielded by adults. Further, there is evidence of maternal care in nests.
The Air
The best known Mesozoic flyers are the pterosaurs. They are closely related to dinosaurs. They, like dinosaurs, evolved during the Triassic and became extinct at the end of the Cretaceous. At least some of them fed on fish. The Cretaceous pterosaur Pterodon had a wing span of 22 feet. The largest pterodon, Quetzalcualtus, found in Texas had a wingspan of 35 feet and was the largest flying creature to ever inhabit the earth (this is larger than many fighter planes). Analysis of fossils indicate that they were capable of flapping flight, but probably spent much of their time soaring (much like buzzards do).
Pterosaurs are not related to birds. Strangely enough, dinosaurs
are (at least according to current thinking). One of the most
famous fossils in the world is Archeopteryx,
an Upper Jurassic fossil found in the Solenhofen Limestone in
Germany. Archeopteryx is preserved in detail, showing feathers
as well as teeth. The feathers on its wings and tail indicate
that it is a bird; however, its bone structure suggests a small
dinosaur. Thus, it is the link, the transitional fossil, between
dinosaurs and birds. Archeopteryx's ability to fly was probably
limited. Recently discovered in China is a sparrow-sized fossil
skeleton that is 10 to 15 million years younger than Archeopteryx.
It shows that it was adapted for sustained flight. Its claws indicate
that it probably lived in trees. Yet it still shows dinosaurian
features.
Mammals
The earliest mammal fossils indicate that they evolved around the same time as the first dinosaurs, late in the Triassic. They probably evolved from the mammal-like reptiles. Throughout the Mesozoic, mammals were small, rodent-like creatures. They were rather insignificant, probably because they had to stay out of the way of dinosaurs to survive.
END-CRETACEOUS EXTINCTION
The Mesozoic era ended with one of Earth's "great dyings".
It was exceeded only by the mass extinction that closed the Paleozoic
era. Approximately a fourth of all faunal families became extinct.
Most notable were the dinosaurs. Others that ceased to exist were
the flying reptiles, the marine reptiles, the ammonites, and many
species of single-celled plankton.
Of course, most species survived, including crocodiles, turtles,
birds, flowering plants and mammals (we wouldn't be here otherwise).
What caused the mass extinction? Most previous extinctions appear to have been gradual and are usually blamed on climate changes. The K-T extinction appears to have been more sudden. (K stands for Cretaceous, T for Tertiary, marking the boundary between these two periods, and, of course between the Mesozoic and Cenozoic eras as well.) In recent years, two hypotheses for the K-T extinction have been hotly debated. One is that an asteroid hit the earth; the other regards exceptional volcanic activity as the cause. (See P&M, p. 167, Astrogeology Box 8.1)
The asteroid impact hypothesis was based on the chemical analysis of a thin layer of clay marking the K-T boundary. The K-T boundary clay was found to have about 30 times the amount of a rare element iridium as is normal for crustal rocks. Iridium is relatively abundant in meteorites and other extraterrestrial objects such as comets and it was suggested that the iridium was brought in by an extraterrestrial body.
Scientists proposed a scenario in which an asteroid 10 kilometers in diameter struck the earth. This generated a gigantic dust cloud, which darkened the earth long enough to disrupt plant life growth and reproduction as well as drop the temperature of the earth worldwide. The dinosaurs perished because of the disruption in their food supply or because of their inability to withstand the sudden climate change.
The alternate hypothesis proposed by other scientists blames exceptional volcanic activity for the extinction, as well as for the high iridium in the K-T boundary layer. (Some Hawaiian eruptions have a high iridium content.) Debate between proponents of the two alternate hypotheses became heated and further evidence to support each was sought. K-T layers throughout the world were found to have grains of quartz that have been subjected to shock metamorphism, supporting the asteroid hypothesis. Concentrations of some elements common in volcanic rocks, but not in meteorites, were found in the K-T boundary layers, supporting the volcanic hypothesis. Very heavy volcanic activity in India took place during a several million year period that straddled the K-T boundary.
The asteroid hypothesis advocates predicted that a large meteorite crater should be found someplace on earth (unless the impact was at sea) that could be dated as having formed around 65 million years ago when the Mesozoic ended. In 1990, geologists reported what they feel is the very large impact crater, mostly in the Caribbean, but partially in Mexico's Yucatan peninsula.
One current view is that the extensive eruptions were causing
considerable stress on the ecosystem before the asteroid struck,
causing the final unfortunate blow for the dinosaurs. "Unfortunate"
is from the perspective of dinosaurs, not humans. The only mammals
in the Cretaceous were inconsequential, small creatures. They
survived the K-T extinction and, with dinosaurs no longer dominating
the land, evolved into the many mammal species that populate the
earth today, including humans.
CENOZOIC
The Cenozoic Era began 65 m.y. and is divided into two periods, the Tertiary and the Quaternary. These, in turn, are divided into epochs. We do not need to be concerned with the epochs of the Tertiary. However, as we live in the Quaternary, you should know that its two epochs are the Pleistocene (the epoch that included the glacial ages) and the Recent (more formally known as the Holocene).
Plants
Flowering plants took over as the prevailing type of plant. These
plants that have seed-bearing fruits make up the vast majority
of today's vegetation.
During Cenozoic times there was a gradual decrease in the world's forests, due to global cooling and drier climates. Grasses and shrubs replaced former forests.
Sea Life
Marine fauna evolved into the present array of clams, snails,
star fish, fish, etc. During early Cenozoic, some sharks were
huge. One species had jaws that would open to 7 feet. Mammal evolution
branched out from the land to the sea as whales joined the marine
ecosystem.
Mammals
Mammals are the great evolutionary success story of the Cenozoic.
From the small creatures of the Mesozoic, the mammals diversified
into numerous branches.
Modern mammals developed rapidly (geologically speaking). Some
of these branches were isolated from one another as the continents
drifted apart. The major branches that developed are:
Dogs - including large wolves
Cats - including the now-extinct saber-toothed cats
Camels, horses, deer, pigs, bears, elephants
Rodents, monkeys, apes, whales
Some of the early Cenozoic mammals were very different from what evolved later.
During the ice ages, large-bodied mammals were especially important.
These included the wooly mammoth, saber toothed cats, the dire
wolf, giant sloths. Amongst the birds were giant condors. Extinction
of most of the large animals took place near the end of the ice
ages, approximately 10,000 ago.
Birds
During early Cenozoic, there were some now-extinct large, flightless
birds that were as much as 8 feet high.
Songbirds became abundant during the Cenozoic.
Insects
Insects have succeeded enormously well at evolving into many species.
There are many thousands of know species and more are being continually
discovered. It is estimated that there are 30 million species
of insects.
Humans
Humans live in an environment that is not very conducive to fossilization,
therefore there is a poor fossil record.
We are primates. (See the primate evolutionary tree)Our ancestors are not the modern ape, as some people believe, but apes and humans have a common ancestry in an ape-like family that lived in the Miocene (24-5 million years ago).
The first hominids were several species of Australopithecus.
They lived from 4.0 to 1.3 million years ago in Africa. Our most
direct ancestor was probably Australopithecus afarensis, nicknamed
"Lucy". This species existed 4.0-2,8 m.y. ago. Their
bodies were upright and they had pelvis, legs and feet like humans.
Their hands were used for grasping (not knuckle-walking). "Lucy's"
skull is ape-like and had a small brain capacity--approximately
400 cc, like a chimp's. They had low brow ridges, low forehead,
long muzzle and teeth that are intermediate between apes and humans.
The first species to appear sufficiently similar to ourselves
to be considered part of the genera Homo came on the scene about
2 million years ago. Eventually they spread from Africa to the
rest of the world.
Homo erectus fossils date back to 1.6 m.y. Their bodies were entirely human, their skulls had a large braincase (around 1300 cc.). But they still had a large brow ridge, long skull and muzzle. Their teeth were still intermediate between apes and humans. There is evidence that they made tools.
Homo sapiens, our species, evolved from H. erectus. Some transitional humans having characteristics of H. erectus and H. sapiens existed around 300,000 years ago. These are regarded as primitive H. sapiens. Their skeletons and skulls were subtly different from modern humans.
Homo sapiens neanderthalis--Neanderthal man, lived from 100,000 to 35,000 years ago. Their body is massive and short. They had a big brain, but still had a heavy brow ridge and low forehead. Their is evidence they practiced religion; bodies were buried with flower offerings, some with ritually missing skull or brain. They primative jewelry similar to that of CroMagnons.
We are Homo sapiens sapiens. The first truly modern humans were the CroMagnons. They were cave painters (the first interior designers). Some of their cave paintings depict Ice Age animals (such as the wooly mammoth) that are now extinct. They co-existed briefly with the Neanderthals. Recent evidence indicates that Neanderthals and CroMagnons traded, but did not interbread with each other.
What happened to the Neanderthals? Two possibilities are: (1)They
were outcompeted for food by CroMagnons. (2)They were killed by
CroMagnons.
North America has had people here since about 11,000 years ago.
(Some scientists now think that people where here much earlier.)
They came from Asia by the Bering land bridge during the lower
sea level of the last Ice Age.
ACKNOWLEDGMENTS
Copyright for most of the above belongs to McGraw-Hill. Used by
permission.
This historical geology summary evolved from my class notes prepared
for Geology 1 at CSUS. Those notes, in turn, evolved from notes
used by my predecessors who taught Geology 1: David McGeary, Judi
Kusnick, Marlon Nance, Brian Hausback.
Figures 1, 2, 4A-D appeared in Peterson, M.S. and Rigby, J.K.,
1994, Interpreting Earth History: A manual in Historical Geology,
5th Ed. Dubuque, Iowa, Wm C. Brown Publisher.
Figures 3, 4E-F, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18,
19, 20 appeared in Lemon, R.R., 1993, Vanished Worlds: An introduction
to Historical Geology. Dubuque, Iowa, Wm C. Brown Publisher.
