All organisms are related by descent from common ancestors. Humans and other mammals descend from shrewlike creatures that lived more than 150 million years ago (Ma); mammals, birds, reptiles, amphibians, and fishes share as ancestors aquatic worms that lived 600 Ma; and all plants and animals derive from bacteria-like microorganisms that originated more than 3 billion years ago. Biological evolution is a process of descent with modification. The process consists of two components. Lineages of organisms change through the generations (anagenesis or phyletic evolution); diversity arises because the lineages that descend from common ancestors diverge through time (cladogenesis or lineage splitting, the process by which new species arise). [download]
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