Evolution Through Various Ages

Evolution Through Various Ages

Fossils give definite proof of organic evolution. The study of these fossils shows that evolution has taken place for long period and in different steps. Fossils are remains or traces of living organisms preserved in any natural formation. We now define fossil as a material which provides evidence of existence of some organism in a particular era or period. Fossils may be

(i) Actual remains (unaltered fossils) in which case the entire animal or plant remains with all original substances intact. Ex. The remains of mammoth and wooly rhinoceros of Siberia and Alaska.

(ii) Petrified fossils (altered fossils) in which case original body is replaced particle by particle with some minerals. Ex. Bones of Mesozoic Dinosarurs.

(iii) Casts and impression on the imprint fossils which the original organisms form in the sediment. Ex. Moulds and casts of Ichthyosaurus.

The study of fossils has given rise to an important branch of biology, i.e., paleontology.

Age Determination of Fossils

Fossils are most found in stratified or sedimentary rocks which contain various radio-active elements. Geologists take help of these radio-active elements to calculate the age of fossils as age of rocks in which they are found. These radio-active elements (radium, uranium etc) gradually and spontaneously disintegrate into simpler and stable elements. For example C¹⁴ (isotope of carbon) disintegrate into C¹² (normal carbon) at a constant rate. This rate is not influenced by changes in temperature, pressure and chemical conditions. There are several methods based on this principle like lead-uranium ratio method (Boltwood, 1907). The most recent method is the Potassium-argon Method which is as follows-

Ordinary potassium contains 0.01% of radio-active isotopes which gradually disintegrates to form calcium and argon. The half-life period of radio-active potassium is 1,300,000,000 years. For dating a fossil, the amount of radio-active potassium (potassium 40) present in the sample is measured and compared with the same present in most living specimens.

Palaeontology has provided us many conclusive evidences in favour of organic evolution. It has showed us how ancestors of present day animals were and how diversity started in them. But still the fossil history of many animals is incomplete. But still scientists are trying to discover new fossils and they depend on the significance of any fossil to infer phyletic relation between animals and plants. We depend on fossils to know the changes in animals from one geological era to the next, connecting link between two groups, distribution of plants and animals on land and water and to know past climatic conditions. So the short definition of fossils is-it is a strong evidence of past life.

EVOLUTION THROUGH VARIOUS AGES

We know that evolution took place slowly in different steps and the evidences come from fossils discovered during different times.

Geological Disturbances- Extensive geological disturbances has taken place during main eras. These disturbances are known as geological revolutions.

These resulted in raisings or lowering the earth surface, many seas either formed or disappeared. Drastic changes took place in the distribution of land and aquatic animals, many species of animals disappeared from one region or became extinct and some new forms appeared.

Geological Eras (Time Scale)

These studies with rocks and fossils with radioactive elements have enabled geologists to prepare the geological time scale with greater accuracy. Geologists have divided the earth’s past history into six eras, which are further divided into periods and periods into epochs. The sis eras of the time scale are-

1. Azoic and Archeozoic era (initial life)

2. Proterozoic era (earliest life)

3. Paleozoic era (ancient life)

4. Mesozoic era (middle life)

5. Cenozoic era (recent life)

6. Psychozoic era (era of civilization and mental life)

(Azoic era is a legendary era of time scale. In this era gaseous earth began to cool and solid earth formed)

1. Archeozoic era

This era is generally known as Age of Unicellular life. This duration of this era is about 2,00,00,00,000 years. This oldest era began not from the origin of earth but from the time of earth crust formation. The fossils of this era are yet almost unknown but it is speculated that lowly organized protozoa and protophyta were the representatives fauna and flora. Volcanic activity, extensive erosion and beginning of sedimentation process are the characteristic of this era which lead to the extinction of many preorganisms.

2. Proterozoic era

It is the next era which started about 200 crore years ago and lasted for 1600 million years was characterized by sedimentation, volcanic activities, extensive erosion and repeated glaciations. The evolutionary trend from unicellular to multicellular organism was evident during this era. The fossils of this era not only show the presence of animals but also indicate their evolution up to the end of the era. Animals were represented by sponges (with spicules), jelly-fish, brachiopods, arthroipods, segmented worms, tube-annelids and corals. In 1947 many above mentioned animals were recorded from rock-strata of this era. The faster half of this era is known as Age of Primitive Marine Invertebrates. There are clear evidences of the presence of primitive aquatic large and fungi.

3. Paleozoic era

This era is about 600 million years old and its duration is estimated to be 370 million years. Mountains ranges, seas and forests developed in this era. Evolution of terrestrial plants (gymnosperms and Pterydophtes) and animals took place. This era is characterized in the absensce of birds and mammals. This era is divided into the following seven periods-

(i) Cambrian- this is the oldest period of this era whose duration as for 100 million years. The climatic condition was normal and all the main invertebrate phyla had been developed. Rock-strata of this period was very rich in fossils. The animals of this periods were diverse and complex indicating their evolution mostly from existing animals of proterozoic era although few originated from archeozoic zncestor. All plants and animals were still marine. Up to this period land was almost without animals and ewre inhabited by some animals resembling shrimps, crustaceans and arachnids whose few descendants are still living without undergoing any evolutionary change. Sea bottom was filled with simple sponges, corals, echinoderms, numerous brachiopods, cephalopods and abundant trilobites. Among plants unicellular and multicellular algae were present.

Many interesting fossils of annelids, arthropods and connecting link between annelid and arthropod, the Peripatus, of this period were obtained from mountains of British Columbia.

No new body forms were formed by evolution. Evolution took place only in different branches of pre-existing animals establishing best adapted animals for the climate or in other words no new phylum evolved. All these show that animals of this period reached such a stage of adaptation that they were at an advantageous stage than any other new adapted animals.

(ii) Ordovician period- This period started 500 million years ago and continued for 75 million years. The climatic condition in this period was warm even in arctic region. Major part of land gradually submerged in water. Conditions were more favourable for marine invertebrates. The fossils of this period show the origin of armoured fishes (placoderms) and predominance of invertebrates. This period saw the rise of progressive cephalopods, corals, gastropods and nautiloids. Sea urchins, sea lilies and new forms of trilobites were found. The bryozoans (colonial), unknown in Cambrian, appeared in early Ordovician and increased in number. The appearance of first chordate and woody plants are the important features of this period. There was abundance of marine algae and it is presumed that terrestrial plants originated during this period.

(iii) Silurian period- This period is about 425 million years old and its duration was nearly 20 million years. Deep seas and high lands were formed. Aridity of climate increased as land rose but streams and lakes started to dry up. This lead to the evolution of air-brathing arachnids (like modern scorpions), millepedes, wingless insects and double-breathing fishes like lung-fishes of to-day. This shows beginning of migration of marine animals on land. The chief invertebrates of the Silurian were graptolites, chain corals, brachiopods, bryozoans, mollusks and echinoderms. Trilobites began to decline. The fossils of ostracoderms (armoured fishes) were recorded from Silurian rocks of North-Western Europe. Land plants first appeared which were more fern-like than mosses.

(iv) Devonian period- It is about 405 million years old and its duration was 60 million years. The climate was arid. Lands rose and small inland seas developed. Many kinds of fishes developed from ostracoderms and so this period is called ‘Age of fishes’. The first with jaws and paired fish was called placoderm or spiny shark. They existed abundantly in fresh water. The true shark appeared in this period in fresh-waters but started migrating to sea. Ancestors of bony-fishes also appeared in fresh water-during this period and they developed into three groups-(i) lung fishes, (ii) lobefinned fishes, and (iii) rayfinned fishes. All these fishes had lungs and bony scales covering body. They gave rise to different fishes during Mesozoic era which resulted in the appearance of true bony fishes of present day.

The jawless ostracoderms (first vertebrate to become extinct), antiarchs (spiny sharks) and arthrodires (joint-necked fish) became extinct at the close of Devonian. Lobefinned fishes, which is considered the ancestor of land vertebrates, also disappeared. It was thought that they completely disappeared at the end of Mesozoic era but in 1939 some lobefinned fishes about 2 meters in length wre caught in S. Africa. There were coelacanths (Latiumeria) and are called living fossils. First land vertebrate appeared during late Devonian. It was an amphibian called Stegocephalia which resembled in many characters with lobefinned fishes but in place of fins limbs were prepsent. This was the first period when true jungles appeared in which ferns, club mosses, horse tails and primitive gymnosperms were present.

(v) Mississipian period- This period started 345 million years ago and its duration was nearly 25 million years. The climate was warm and humid at initial stage, later on it became cooler as land rose. Sharks of many kinds were the dominant fishes in the seas and oceans. Foraminifera, crinoids and small salamanders were in abundance.

(vi) Pennsylvanian period- This period is nearly 320 million years old and its duration is about 40 million years. Due to mild moist climate vegetation reached its maximum development in variety and size of plants. Ancient insects, (coackroach, dragon-fly) spiders, scorpions were dominant invertebrates. Fishes and amphibians continued to grow in number and form. True reptiles appeared for the first time during this period. It is also called stem reptile. The fossil of Petrolacosarus was discovered in late pennsylvnian rocks near Garnett. Kansas as considered to be the ancestor of reptiles. Great coral deposits of today are the gift of this period. So Mississipian and Pennsylvanian periods together constitute the carboniferous period.

(vii) Permian period- This period is about 280 million years old and its duration is estimated to be 50 million years. During this period there were maximum changes in land and climate. The climatic condition was characterized by increasing glaciations and aridity. The floor of continents rose high up due to which great mountain ranges appeared and river originated. Towards the end of this period there was folding of earth’s crust which is called applachian revolution. Many palezoic animals which were unable to adapt themselves in the changed climate became extinct due to app;acjian revolution. Even many marine animals became extinct.

From stem reptile originated a group of reptiles from which mammals originated. These were Pelycosaurs. There were carnivores and lizard like. Another group of reptiles from pelycosaurs evolved which had more mammalian characters. This occurred towards the end of this period. These were Therapsid. These became extinct towards the beginning of Mesozoic era.

4. Mesozoic era

This era is estimated to be 130 million years old and its duration is about 167 millions years. It is divided into 3 periods-triassic, Jurassic and cretaceous. Generally the era is called Age of Reptiles.

(i) Triassic period- This is the first period of Mesozoic era. It lasted for nearly 49 million years. Many islands appeared and due to increase aridity many deserts developed at many places. Fishes belonged to chondrostei and sharks were poorly represented. Primitive amphibians disappeared and first true frogs appeared in Madagascar. Reptiles increased. First dinosaurs originated. The number of Plesiosaurs and Ichthyosaurs increased in water. The first turtle also appeared during this era. This era was also remarkable due to the appearance of egg laying mammals and insects. Sea urchins reached to its final stage of evolution.

(ii) Jurassic period- It is 181 million years old and lasted for 46 million years. The climate was comparatively hot and arid. Teleosts and birds appeared in this period. Holostei were dominant fishes. Reptiles increased in number and dinosaurs (Brontosaurs, Diplodocus, Brachiosaurs, Stegosaurs, fish like Ichthyosaurs) increased in size. First bird with teeth appeared. The fossil of Archaeopteryx (a large, toothed bird) was recovered during this period. It connects reptiles with birds as it possesses characters of both classes. Marsupials and insectivores appeared during this period. Dicotyledon plants increased in number. In addition cycads and conifers were in abundant.

(iii) Cretaceous period- This period is 135 million years old and lasted for 72 million years. Towards the end of this period there were great crustal changes and great mountains (Andes, Alps, Himalayas) were formed and inland seas developed. This period was marked by the presence of archaic mammals. Toothed birds but dinosaurs culminated. Land reptiles showed their highest development. Living representatives on the crocodiles, alligators, snakes and lizards. Snakes were first found during this period. Towards the close of this period mammals were numerous (Platypus, Echidna etc.). marsupials and placentals originated and dominant reptiles of land and water became extinct. All these were favourable for the evolution of modern warm-blooded mammals.

5. Cenozoic era

It was 65 million years old and is commonly known as Age of Mammals. The vegetations and climate conditions were similar to those of today. Grazing increased. This era is divided into two periods:

(i) Tertiary period and

(ii) Quaternary period

(i) Tertiary period- This period is further divided into several epochs- Paleocene, Eocene, Oligocene, Miocene and Pliocene. The marine invertebrates present were Foraminifera, Gastropods, Bryozoans, Crustaceans Sea-urchins and Cephalopods (squid, cuttle-fish and octopus). Teleosts, birds and mammals progressed while reptiles reduced.

(a) Paleocene epoch-Its duration was only 5 million years. Dinosaurs were replaced by archaic mammals and fossils of 15 order of mammamls and no further important evolution took place during this epoch.

(b) Eocene epoch-The duration this epoch was 22 million years and it was 58 million years old. Erosion of mountains occurred and placental mammals diverged into many branches. Carnivores, hoofed mammals odd toed forms (Eohippus form), rhinoceros, even toed hoofed animals developed. Ancestors of deer, pig and camel also appeared. Mammals were also represented by bats in the air and whales in the ocean. No significant plants appeared during this epoch.

(c) Oligocene epoch- It is about 36 million years old and continued for 11 million years. The climate was comparatively arid. Ancient mammals disappeared but arthropods flourished. Most of the ancestors of the present day mammals appeared. Opossum, moles, shrews, poncupines, mice and rats were present.

(d) Miocene epoch- It is about 25 million years old and its duration was 12 million years. Sierra, Cascade mountains were formed during this epoch. In North-Wetern America many volcanos appeared. The climate was comparatively cool and mammals reached their maximum in evolution. During this epoch the first man like Ape, larger panther, tiger-like cats, the first bear appeared.

(e) Pliocene epoch- It is about 13 million years old and its duration was 12 million years. It was a period of unrest due to the formation of new land bridges and separation of other land connections. Migration to different regions took place and so competition for survival increased. From precursor, man evolved, showing evolution of highest degree.

(ii) Quarternary period- the duration of this period is the last 10-10.5 lack years. It is divided into the following two epochs.

(a) Pleistocene epoch-It continued for 1 million years. Due to continued elevation of great continents and several glaciations this epoch was dominated by cold weather and was also named as ‘Age of Ice’. The severe cold was separated by interglacial warmer conditions. The pleistoane epoch was thus characterized by four glacial and three inter-glacial sequences. During this epoch many animals became extinct except mankind which became the dominant form of life. The glacial deposits of central and western Europe showed the presence of wooly mammoths, rhinoceros and arctic foxes. Fossils of lion, hippopotamus and hyena wre also found in the inter-glacial deposits of the same region. Social life in man was found for the first time during this epoch and herbaceous plants appeared. Due to several glaciations many species of plants and animls became extinct.

(b) Recent epoch-This epoch is xontinuing since 11 thousand years ago till today. The Age of Ice ended and climate became warmer. This is called Age of Man. No significant evolution occurred during this epoch. During this time distribution of woody plants decreased whereas herbaceous plants increased. This epoch is also called post-glacial and is included under Psychozoic era.

Geological time table

Era	Periods and epoch	Duration in millions of years		Total time(beginning to present in millions)		Advance in life		Dominant forms of Animals
Psychozoic	Recent(post glacial)	0.011		0.011		Rise of world civilization. The era of mental life		Age of man
Cenozoic	Pleistocene	1		1		Extinction of many species at large scale.(periodic glaciations)		First human social life.
	Pliocene	12		13		Origin of man(rise of mountains and volcanic activity)		Man and grazing mammals.
	Miocene	13		25		Culmination of mammals. (new mountains formed, climate cooler)		Mammals at height of evolution, first man like ape)
	Oligocene	11		36		Rise of higher mammals. Extinction of archaic mammals(lands lower, climate warmer		Dominance of browsing forms. Rise of anthropoids fore runners of most living genera of mammals.
	Eocene	22		50		Decline of archaic mammals. Terrestrial animals flourished.(climate warmer, mountains eroded)		Placentals diversified hooded mammals and established
	Paleocene	5		63		Rise of archaic mammals. Decline of dinosaurs.		Archaic mammals.
Mesozoic	Cretaceous	72	135		Extinction of dinosaur. Origin of marsupials and placentals.		Specialized reptiles toothed birds and primitive mammals.
	Jurassic	46	181		Dinosaurs increased. Flying reptiles, frogs, crocodiles and toothed birds originated		Dinosaurs increased in size
paleozoic	Triassic	49	230		Disappearance of primitive amphibians, origin of dinosaurs.		Plesiosaurs and ichthyosaurs increased in water, first dinosaurs.
	Permian	50	280		Modern amphibians and primitive reptiles originated, extinction of old amphibians		Amphibian
	Pennsylvanin	40	320		Rise of primitive reptiles and insects		Arthropods, fishes and amphibian.
	Mississippian	25	345		Rise of ancient sharks and echinoderms		Sharks, echinoderms and salmnders
	Devonian	60	405		Origion of amphibian		Age of fishes and labryinthodont amphibians.