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Evidences from Geology and Paleontology
Studies of the earth and its content and of remains of plants and animals that lived in the remote past have revealed much in support of organic evolution.
A fossil is any physical evidence of life in the geological past. The term fossil is commonly applied to bones, teeth or shells of animals that have been long-dead. However, footprints and other such impressions are also part of the fossilised remains of life that once existed. The impressions of soft bodied animals, such as jellyfish, in mud which later hardens into rock are also fossils. Whole mammoths have been preserved in frozen ice. Large parts of animals, dried skin and hair, tendons and dried excrement have also been discovered with their bones. At least ninety nine per cent of the animal fossils consist of preserved parts and these give us valuable information about life in the past.
Fossilisation of a dead organism usually begins when the dead organism is buried before extensive decay sets in. The organism sinks into a bog or marsh or to the bottom of a lake, sea, or river. In some cases it is buried in sand. Even after the burial, decay occurs and the soft parts of the body are generally destroyed. The hard parts, however, survive as fossils. Water, mud or sand hardens to rock and the organic remains are safely preserved. Fossilisation is a random process, only those organisms which happen to die in a spot where they can be buried before other organisms destroy them are fossilized.
Fossils are exposed by natural erosion or excavation. Fossil bearing rocks become exposed by the actions of wind, rain or rivers, or through excavations by scientists. Once exposed, they are studied in detail in terms of their antiquity and characteristics. Such study of fossils is called paleontology.
Fossil Record
When fossils are discovered, they are removed and studied as remains of the past life, their age and their place in the historical sequence of life are determined and fossils from various parts of the world are compared. Then only can a fossil record be constructed.
Many hundreds and thousands of fossils have been discovered and studied. However, these form a minute fraction of the total number of animals and plants that lived on the earth.
By accurate studies of the strata of the earth, scientists divided the earth's history into eras. Each era has been divided into periods which are subdivided into epochs. Each division has specific characteristics, namely, ages, durations and types of life that were common during those times.
Some of the features of the fossil record can best be represented in a tabular form. The most recent fossils are found in the upper strata of the earth and the oldest ones towards the lower strata, the most ancient ones being at the bottom.

Dating of Rocks

The relative lengths of the eras and periods may be calculated in two ways. The age can be approximately determined by the thickness of the sedimentary rocks formed during each period. A definite time is required to form a certain thickness of sedimentary rock of a particular type.
The other method is the radioactive disintegration method. Radioactive elements such as uranium 238, uranium 235 and thorium are slowly changed into lead. An analysis of the ratio between the lead and the uranium (or thorium) content in a rock gives its approximate age. The rates at which uranium (and thorium) are converted to lead can be determined approximately as follows: 
Radioactive carbon (carbon14) which loses one half of its radioactivity in about 5,760 years has also been used in dating certain specimens. When bones are formed, small amounts of carbon 14 are incorporated and upon death of the organism, the radioactivity is gradually lost. Determination of the amount of residual radioactivity in the bone makes it possible to approximately determine the time of death of the organism.
Evolution by Stages
Evolution of organisms involves a change in the developmental program, a change in a series of developmental processes. We often refer to evolution as "descent with modification" and the modification we often notice first is the overall appearance of the organism. This appearance is a result of the development of the organism, thus evolution is intricately involved with development.
Embryology played a major role in evolutionary theory. The geneticists were concerned with the rules of transmission of genetic material between generations and the develop mentalists were concerned with cellular changes that led to the transformation of an egg into an adult organism. Mutations in adult phenotype were readily available for the study of genetics, but there were precious few "developmental mutants" that bridged the gap between development and genetics.
The general approach is the same as we have taken with the evolution of other traits: development has a genetic basis, if there is genetic variation for the developmental program then development can evolve.
Early embryologists noticed similarities between ontogeny and phylogeny. They held that descendants, during their ontogeny, passed through stages that resembled the adults of their ancestors. Before this, it was held that there were four major classes of organisms: vertebrates, mollusks, articulates and radiates. They noticed that there was nothing in the ontogeny of a vertebrate that resembled the adult stages of, say, a mollusk. This is because evolution is a bush or a tree not a "ladder" of the great chain of beings.
Observations about ontogeny and phylogeny that seem obvious to us today were made, but they are important in development and evolution as they run counter to recapitulation:
  • More general characters appear early in development
  • Less general forms develop from the more general forms
  • Embryos do not pass through other forms they diverge from them
  • Embryos of higher forms only resemble embryos of other forms
Putting these two views together, we see that there can be a sort of recapitulation within a lineage but there are many examples that refute the notion that phylogeny is reviewed during ontogeny.

Nature and Kinds of Fossils

The ways in which animals and plants of the past have become fossilised are as follows:
  1. by actual preservation of the organism, 
  2. by preservation of the skeletal structures, 
  3. by natural molds or incrustations, 
  4. by petrification, and
  5. by leaving trails and impressions.
Actual Preservation
May occur by freezing and preserving in ice or soil. An example is the mammoth discovered in Siberia and plants of the same period which frozen with it. When the skeletal structure of an organism is preserved, it remains almost in its original conditions. Several skeletons of ancient mastodons have been found in different states of preservation. In natural molds or incrustations neither the minute structures nor the materials of the original organism are preserved, but merely the general outlines of form and shape are recorded. In petrification, the original material of the organism undergoes a certain amount of mineralisation. In this case lime, silica, iron oxides, iron pyrites and other substances replace the original material of the organism, sometimes very accurately, retaining its original shape, size and even minute details. Trails and impressions (imprints) may be called ‘fossils of living organisms’, whereas other records are of dead organisms. Animals may leave their trails and imprints such a foot prints. A leaf may leave an impression in soft mud which later hardens. These types of fossils usually do not give much information concerning internal details.

Fossil Record and Geological History of the Earth


Period Epoch Time when each epoch started (in millions of years) Life and General Characteristics
1 2 3 4 5
Cenozoic Quaternary Recent 1/40 Modern man dominant, modern species of plants and animals, climate warmer; end of fourth ice age.
  Tertiary Pleistocene 1 Extinction of giant mammals and many plants, development of man; cold and mild climates; four ice ages with glaciers covering much of north America, Europe and Asia.
    Pliocene 12 Modern mammals, rise of herbaceous plants; invertebrate animals similar to modern types; dry, cool weather; volcanoes active; continents elevated.
    Miocene 28 Grazing mammals; man apes; temperate types of plants; moderate climates; grass lands and plains developed; forests diminish.
    Oligocene 40 Primitive monkeys and apes, whales; forests widely distributed temperate types of plants; mild climate; mountains formed.
    Eocene 60 Modern mammals appear; first horses; subtropical forests; heavy rainfall; North America and Europe connected by land.
    Paleocene 75 Former mammals dominant; modern birds; dinosaurs extinct; subtropical plants; temperate to subtropical climates; mountains formed.
Mesozoic Cretaceous   130 Extinction of giant reptiles and toothed birds; origin of placental mammals, flowering plants rise; gymnosperms decline; mild to cool climates; inland seas and swamps forming.
  Jurassic   180 Giant dinosaurs and marine reptiles dominant; mammals appear; first toothed birds; angiospermous plants appear; conifers dominant; shallow inland seas.
  Triassic   230 First small dinosaurs, marine reptiles; mammal like reptiles; conifers dominant; seed ferns disappear; deserts formed.
Paleozoic Permian   260 Reptiles displace amphibians; many marine invertebrate animals extinct, modern insects; evergreen plants appear; cold dry and moist climate; mountains formed.
  Pennsylvanian   310 Reptiles originate, amphibians; giant insects; spore bearing trees dominant; extensive coal-forming swamp forests; moist warm climate; shallow inland seas.
  Mississippian   350 Amphibians; winged insects; sharks and bony fishes; horse-tails and seed ferns abundant; early coal deposits; warm climate; hot swamp lands, mountains formed.
  Devonian   400 First amphibians; fresh water fishes, sharks; forest and land plants; wingless insects; corals; ferns and seed ferns; arid land.
  Silurian   430 Fishes with lower jaws; first land plants and arthropods; algae dominant; warm mild climate.
  Ordovician   475 First vertebrates; trachiopods, trilobites abundant; marine algae dominant; warm mild climate; oceans increase in size; land submerging.
  Cambrian   550 Algae and marine invertebrate animals; first abundant fossils; trilobites and brachiopods dominant; mild climate.
Proterozoic Upper Precambrian   2000 Bacteria; fossil algae; sponges; soft bodied animals, start of autotrophic nutrition; warm moist to dry, cold climate; volcanoes active; glaciers.
Archeozoic Lower Precambrian   4500 Life presumed to originate; start of heterotrophic nutrition (obtaining nourishment from organic substances); no fossils found; lava flows; granite formed.

Evolutionary Significance of Fossils

A study of animal and plant fossils is important because it often includes the ancestors of modern species. The data obtained from fossil studies, often explains the relationships among various groups of present-day animals and plants. In some instances the ancient types serve to form connections between groups of organisms that at present seem to have no direct links. These points could be illustrated by the following examples.
Evolution of the Horse
Evolution of Horse
The fossil record indicates that modern horses have developed through a series of successive changes. Much of the evolution of the horse occurred in North America and extends back to sixty million years to the eosine epoch in geological history. Fossil records indicate that the evolution of horse proceeded in a definite direction. The first ancestor of the horse is thought to be Eohippus (eocine epoch) which was about one foot high at the shoulder and grazed on forest underbrush. Its feet had four toes (and a splint bone), whereas the hind foot had three well-developed toes (and two splinters). Another type in the line of descent was Mesohippus (Oligocene epoch) which was taller and was also the browsing type. Each foot had three digits with the middle one larger and better developed.
Merychippus (Miocene epoch) was probably the direct ancestor of later horses; it was three toed with the lateral toes high above the ground. The skull was larger and the lower jaw was heavier than the ancestral forms. Merychippus was between 3 and 4 feet high at the shoulder and gave rise to several types. However, the type that persisted was pliohippus (Pliocene epoch) which was the first one-toed horse. The modern horse Equus arose from pliohippus during the Pleistocene epoch in North America from where it spread to other continents.
Fossil records of a majority of cases are incomplete. However, such complete fossil records of certain mammals like the horse, camel and elephant indicate how their evolution has occurred through geological time.
Archaeopteryx (A),
Compared with a pigeon (Columba livia) (B)

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