Study the diagram of the geologic time scale.

A time scale measured in Millions of years ago or M Y A. Cenozoic Era includes Quaternary Period, 0 to 1.6 M Y A, and Tertiary Period, 1.6 to 66.4 M Y A. Mesozoic Era includes Cretaceous Period, 66.4 to 144 M Y A, Jurassic Period, 144 to 208 M Y A, Triassic Period, 208 to 245 M Y A. Paleozoic Era includes Permian Period, 245 to 286 M Y A, Carboniferous Period, 286 to 360 M Y A, Pennsylvanian Period (286 to 320 M Y A), Mississippian Period, 320 to 360 M Y A, Devonian Period, 360 to 408 M Y A, Silurian Period, 408 to 438 M Y A, Ordovician Period, 438 to 508 M Y A, Cambrian Period, 508 to 570 M Y A, 570 M Y A and older is the Precambrian Period.

Which method is most likely used to identify what happened before the start of the Paleozoic era?

Scientists study fossils from the Jurassic period.
Scientists observe index fossil records of the Mesozoic era.
Scientists examine fossil evidence during the Precambrian time.
Scientists explore the emergence of mammals and humans in the Cenozoic era.