( Adaptive radiation is the evolution from an unspecialised ancestral stock of many different species that adapt to a variety of ecological roles (see books)).Mammals and subsequently humanity may not have become dominant on earth had it not been for the global catastrophe that led to the extinction of the dinosaurs 65 million years ago.
Very simply, there are five basic principles to this theory:  Variation has a genetic origin (i.e.
is heritable) and arises through processes such as mutation and recombination.
 Natural selection, gene flow and random genetic drift (the random change in gene combinations with each generation) drive evolution, resulting in a change in gene frequencies within populations .
 Adaptation is genetically based and so leads to phenotypic changes that accumulate over time;  Speciation (species formation) evolves through reproductive isolation and (genetic) divergence of populations;  Genetic changes through natural selection lead to new species and eventually new taxa .
Once multicellular organisation became possible as atmospheric oxygen levels rose, the early multicellular organisms rapidly diverged into many adaptive forms.
Mass extinction events and an organism's unique adaptations mark the history of evolution, both of which may lead to adaptive radiation.
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I have integrated this web page on human evolution into an electronic text, " Nature's Holism - Holism.
Scientists now have accurate methods (see below ) for dating fossils.