Origin
The evolution of animal body plans became inevitable with the emergence of differentiated multicellular animal life in the Ediacaran Era, over 600 million years ago.
All land animals are descended from a common bilaterian ancestor. This animal had a "pipe" or alimentary canal body plan. It is essentially a passage having a mouth at one end, and a cloaca or anus at the other. This is common to organisms as diverse as humans and earthworms, and is derived from their shared bilaterian ancestor. The process of nutrient capture, digestion, and waste disposal is fundamental to the body plan of advanced, free-moving animals. Vertebra, limbs, even brains are supplementary to the pipe. Natural selection has spun off an enormous range of variations on this basic theme, but the pipe model itself remains. The basic symmetry and organization of this body plan apparently gave an ancient organism an enormous advantage at survival and reproduction, and it has been preserved in most animals ever since, with the notable exception of the echinoderms.
The Cambrian explosion refers to the massive increase in different body plans that took place around 530 million years ago. Fossils from this era show all of the body plans in existence today.
Read more about this topic: Body Plan
Famous quotes containing the word origin:
“There are certain books in the world which every searcher for truth must know: the Bible, the Critique of Pure Reason, the Origin of Species, and Karl Marx’s Capital.”
—W.E.B. (William Edward Burghardt)
“Art is good when it springs from necessity. This kind of origin is the guarantee of its value; there is no other.”
—Neal Cassady (1926–1968)
“Someone had literally run to earth
In an old cellar hole in a byroad
The origin of all the family there.
Thence they were sprung, so numerous a tribe
That now not all the houses left in town
Made shift to shelter them without the help
Of here and there a tent in grove and orchard.”
—Robert Frost (1874–1963)