At the onset of the 20th century, most scientists had accepted the great antiquity of the earth, the theory of evolution and that humanity had evolved from an ape ancestor, but by 1908 the fossil evidence of early man was scarce.Biologists included principles from genetics in evolutionary theory during the 1920s and 1930s.Scientists now have accurate methods (see below ) for dating fossils.methods rely on characteristic faunal and geological patterns to bracket the period when the fossil existed.sapiens (including Neanderthals and Cro-Magnon) (see the modern taxonomic classification of humans).This formed the foundation of the neo-Darwinian evolutionary theory.
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.
 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.