GEOLOGICAL TIME CHART

Era,‘Period Age Millions Major Biological events
.c.) o

N

2

SD

0

Quaternary 0.01 Subtropical forests gave way to cooler forests and grass areas.
Tertiary . 65 Modern orders of mammals evolved. Human evolved in the last 5 million years
P

5

N

o u)

a)

2

Cretaceous 135   . Continental    seas    and    swamps   spread.    Extinction

ancient birds and reptiles.

Jurassic 195 Climate warm and stable. High reptilian diversity. Birds first appeared.
Triassic 240 Climate warm. Extensive deserts. Dinosaurs replace mammal-like reptiles. First true mammals.
.0

5

N

o

a3

co

0-

Permian 285 Climate cold early,      but then warmed.        Mammallike

reptiles       common.      Widespread        extinction      of
amphibians.

Carboniferous 375 Warm    and    humid    with   extensive      coalproducing

swamp Arthropods and amphibians common.            First
reptiles appeared.

Devonian • 420

450

Land    high   and    climate    cool.   Freshwater     basins

developed      Fish     diversified.    Early       amphibians
appeared.

Extensive shallow seas Warm climate. First terrestrial arthropods. First jawed fish.
Silurian
Ordovician 520 Shallow    extensive    seas.    Climate    warmed      Many

marine invertebrates. Jawless fish widespread.

Cambrian 570 Extensive shallow seas and warm climate. Trilobites and brachiopods common. Earliest vertebrates found fate in the Cambrian.
Proterozoic

I–I

2,000 Multicellular organisms appeared and flourished. Many invertebrates. Eukaryotic organisms appeared (1,500

million    years     ago).   Oxygen   accumulated      in   the
atmosphere.

Archean 4,600 Prokaryotic life appeared     (3,500 million    years ago).

Origin of the earth (4,600 million years ago).

COMPARATIVE ANATOMY


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Similarities among the structures of animal occur due to common evolutionary origin. Comparative anatomy is branch of zoology. It is fundamentally based on relationship of structures among organisms. Comparative anatomists study the structure of fossilized living animals. They look for similarities that could indicate close relationships. They study three types of structure:

(a) Homologous structures: Structures derived from common ancestry are called homologous structure. For example, vertebrate appendages have a common arrangement of similar bones although some of them perform different functions. This similarity in appendage structure indicates that the vertebrates evolved from a common ancestor.

(b)Analogous structures: The structures having different origin but performs similar functions are tailed analogous structures. Analogous structures are produced due to convergent evolution. Convergent evolution occurs when two unrelated organisms adapt to similar conditions. It causes superficial similarities in structure. For example, the wing of a bird and the wing of an insect are both adopted for flight. But they are not homologous. These structures are analogous.

(c) Vestigial structure: The functionless structures in the body are called vestigial structures. Organisms often retain structures that have lost their usefulness. These structures are vestigial structures. They are poorly developed. For example, boa constrictors (a reptile) have minute remnants of hind limb (pelvic) bones left. Such remnants are clear indications of evolution

1

Cat                  Whale                    Bat

Fig: Homologous structures

MOLECULAR BIOLOGY

Molecular biology has provided a lot of important information about evolutionary relationships. The animals also have homologous biochemical processes like homologous structures.

Structure and function are based on the genetic DNA molecule. Related animals have similar DNA derived from their common ancestor. DNA carries the codes for proteins. Thus related animals have similar proteins. The zoologists extract aid analyze the structure of proteins from animal tissue with the help of latest laboratory techniques. They also compare the DNA of different animals. They look for dissimilarities in the structure of related proteins and DNA. It gives – constant mutation rate. In this way they estimate the time since divergence from a Common ancestral molecule.

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