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.
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. NSES Content Standard D
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