Study this image of a geologic time scale. A 4 column table. Column 1, titled ERA, with entries: Caenozoic, Mesozoic, Paleazoic, Proterozoic, Archean, Hadean. Column 2, titled PERIOD, with entries: Quaternary, Tertiary, Cretaceous, Jurassic, Triassic, Permian, Carboniferous, Devonian, Silurian, Ordovician, Cambrian, Also known as Precambrian. Column 3, titled MILLIONS OF YEARS AGO, with entries: 1.6, 138, 240, 330, 410, 500, 3500. Column 4, titled KEY EVENTS, with entries: Humans evolve, Extinction of Dinosaurs with arrow pointing to approximately 68 M Y A; Permian mass extinction with arrow pointing just below 240 M Y A; Invertebrates become common with arrow pointing just below 500 M Y A; Earliest life with arrow pointing to 3500 M Y A. How long ago did invertebrates become common on Earth?