NSES Content Standard C  Life Science: Biological evolution Grades 9-12, page 185 The great diversity of organisms is the result of more than 3.5 billion years of evolution that has filled every available niche with life forms.

 
Benchmark 5F The Living Environment: Evolution of Life
Grades 9-12, page 125
Life on earth is thought to have begun as simple, one-celled organisms about 4 billion years ago. During the first 2 billion years, only single-cell microorganisms existed, but once cells with nuclei developed about a billion years ago, increasingly complex multicellular organisms evolved.