Fossil, Morphological, and Geological Evidence

Rock forms in layers, with the oldest on the bottom and the youngest on top, so the order fossils appear in tells you the order species existed. Simple life shows up low in the record, complex life higher up, and that sequence alone is strong evidence of change over time, independent of knowing any exact dates. To get an actual age, scientists use radiometric dating. Carbon-14 is unstable and decays into nitrogen-14 at a fixed rate, measured as a half-life: the time it takes for half of the remaining carbon-14 to decay. Every living thing takes in carbon-14 while alive, and once it dies, the clock starts. By measuring how much carbon-14 is left compared to the original amount, you can calculate how many half-lives have passed and multiply by the half-life length to get an age.

Morphological evidence tells the same story from a different angle. A whale flipper, a bat wing, and a human hand all contain the same arrangement of bones: one upper arm bone, two forearm bones, then wrist and finger bones, just reshaped for swimming, flying, or grasping. These are homologous structures, inherited from a common ancestor and modified by natural selection for different jobs. Vestigial structures, like the human appendix or whale pelvic bones, are the leftover evidence of that same process: a trait that lost its original function in one lineage but still hints at what an ancestor needed.

These three lines of evidence, layering, dating, and anatomy, reinforce each other, and when they don't quite line up, that's useful too. A fossil appearing in an unexpected layer doesn't overturn evolution; it usually means erosion, geological disturbance, or an incomplete sampling of the record, pushing scientists to refine the timeline rather than discard it.