Because the fossil record is incomplete, it is difficult to trace the evolution of modern animal life. There is enough information to suggest a general pattern of evolution and to reconstruct the history of groups in areas where fossils are abundant. The diagram depicts the relationships between major groups of vertebrates. Those animals of the same colour most likely evolved from common ancestors.
(EVOLUTION OF THE VERTEBRATES)
OVER 1,000,000 ANIMAL SPECIES
Mollusks :- 45,000
Chordates :- 45,000
Protozoans :- 30,000
Arthropods :- 900,000
Worm-like phyla :- 38,000
Other invertebrates :- 21,000
(Approximately :- 1,000,000)
ALL FORMS OF LIFE arose from humble beginnings three billion years ago. The major groups of plants and animals evolved from a small number of primitive forms. Living things became more complex and adapted to a variety of lifestyles. The number of different species gradually increased until they reached their current level of diversity. Paleontology (the study of fossils) traces the various paths that animals and plants took to reach their current forms. Some, such as elephants and horses, have evolved significantly over time. Others, such as the horseshoe crab and the cockroach, have remained unchanged for hundreds of millions of years. Other fossils show development paths that came to a halt. Giant Sloths, which were once common, are now only known as fossils.
GLYPTODONT, a 9-foot-tall armoured mammal from the late Cenozoic, is a spectacular and obvious adaptation. This armadillo relative was protected from carnivores and other enemies by a thick, solid, domed armour that could reach 5 feet in length in some forms. The head and tail were also armoured, and the tail of some species ended in a spiked, mace-like club. Despite, or perhaps because of, these unusual adaptations, glyptodonts went extinct.
(GLYPTODONT)
ABOUT 350,000 PLANT SPECIES
Flowering Plants :- 250,000
Ferns, Conifers, etc. :- 10,000
Mosses and Liverworts :- 23,000
Algae, Fungi, etc. :- 60,000
(About 350,000 plant species)
ADAPTATION Most plants and animals exist solely due to successful adaptation to their environments. Each distinct environment, such as a desert, pond, or mountain top, supports a population of animals and plants that is more or less distinct. Those that have become adapted to local conditions over long periods of time have survived. All of the others are now extinct. Many living things are specifically adapted to their environments. A fish's streamlined shape, as well as the structure and function of its fins and tail, are adaptations to aquatic life. Cactus fleshy stems are adaptations that conserve water in the desert. Such adaptations were successful, but the fossil record is littered with the remains of those that did not. Life's motto could be "adapt or perish."
Animal survival is dependent on adaptations as diverse and intricate as the animals themselves. Almost any structure of a plant or animal can be considered adaptive. Many animals have protective colouring, and some, like the bottom-living flounder, can change their colour to match their surroundings. Such a complex adaptation is rarely visible in fossils. If the adaptation affects bone or shell, it may be visible in the fossil record.
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