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
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—Henry David Thoreau (1817–1862)
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“The real, then, is that which, sooner or later, information and reasoning would finally result in, and which is therefore independent of the vagaries of me and you. Thus, the very origin of the conception of reality shows that this conception essentially involves the notion of a COMMUNITY, without definite limits, and capable of a definite increase of knowledge.”
—Charles Sanders Peirce (1839–1914)