Fossil Record

Rock layers stack up over time, so deeper fossils are older — and read from bottom to top, they record life changing in sequence, never out of order (no rabbits in the age of dinosaurs).^[1] The layers give the order; radiometric dating of volcanic ash beds turns that order into actual ages in years. The most striking entries are transitional fossils that bridge major groups: Tiktaalik between fish and four-limbed animals, feathered dinosaurs shading into birds, and a chain of whale ancestors that walked on land before returning to the sea — complete with shrinking vestigial hind legs. Our own lineage leaves the same kind of graded trail.^[2]