Percentage of amino acids in hemoglobin that are different between species 500 Time (millions of years ago) Box 24.1, Figure 1

The origin of HIV In 2000, researchers at the Los Alamos National Laboratory in New Mexico used a molecular clock to date the origin of HIV infections in humans We know HIV descended from related viruses tha affect chimpanzees and another primate, the sooty mangabey The researchers sampled HIV from patients infected with HIV just a few years apart

The accuracy of molecular clocks Molecular clocks can help assess the relative chronology of branching in a phylogeny However, no genes mark time with precise accuracy in their rates of nucleotide changes (mutations) Molecular clocks are therefore accurate only in the statistical sense of a fairly smooth average rate of change –Over time, there may still be chance deviations above and below that average rate

The geologic time scale The earth’s history is divided into four eras: –Precambrian - beginning when earth was formed (4.6 billion years ago) and ending 600 million years ago –Paleozoic - from 600 million years ago to 245 million years ago –Mesozoic - from 245 million years ago to 65 million years ago –Cenozoic - from 65 million years ago to now