GEOLOGICAL TIME CHART
|Era,‘Period||Age Millions||Major Biological events|
|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|
|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.|
|Permian||285||Climate cold early, but then warmed. Mammal–like
reptiles common. Widespread extinction of
|Carboniferous||375||Warm and humid with extensive coal–producing
swamp Arthropods and amphibians common. First
|Land high and climate cool. Freshwater basins
developed Fish diversified. Early amphibians
|Extensive shallow seas Warm climate. First terrestrial arthropods. First jawed fish.|
|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.|
|2,000||Multicellular organisms appeared and flourished. Many invertebrates. Eukaryotic organisms appeared (1,500
million years ago). Oxygen accumulated in the
|Archean||4,600||Prokaryotic life appeared (3,500 million years ago).
Origin of the earth (4,600 million years ago).
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
Cat Whale Bat
Fig: Homologous structures
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.
- EVIDENCE OF EVOLUTION
- FURTHER PHYLOGENETIC CONSIDERATIONS IN FISHES
- FURTHER PHYLOGENETIC CONSIDERATIONS
- MOLECULAR EVOLUTION
- Chapter 12 : THE FISHES VERTEBRATE SUCCESS IN WATER