The Fossil Record

Scientists use two primary methods to date fossils. Stratigraphy examines rock layers, or strata, where newer layers form above older ones, allowing relative age determination. Radiometric dating measures the ratio of carbon-14 to carbon-12 in fossils to calculate actual age using the known half-life of radioactive isotopes, providing precise chronological data.

Most organisms never fossilize because decomposers and scavengers destroy tissues before fossilization occurs. Soft tissues like muscles and feathers typically disappear, while hard tissues like bones and teeth remain. Short-lived, scarce, and soft-bodied species are poorly represented, making fossils represent only a small fraction of all species that ever inhabited Earth.

Fossils preserve physical characteristics, behavior, and age of organisms. Scientists infer an organism's shape, size, and dietary habits from bones, teeth, and other hard tissues. Rare intact fossils like mammoths in ice or spiders in amber provide extremely valuable glimpses of early life-forms, revealing details about extinct species' anatomy and ecology.

The fossil record captures major evolutionary transitions through sedimentary rock layers accumulated over time. Whale evolution exemplifies this, showing ancestral tetrapod organisms transitioning to semi-aquatic and fully aquatic forms, with forelimbs evolving into flippers and hindlimbs disappearing. The evidence for evolution and common ancestor is documented across geological eras, revealing how species adapted to new environments.

The fossil record reveals five major extinction events where over 75% of species vanished. After each mass extinction, radiation of diverse species with a common ancestor occurred. The late Paleozoic extinction led to the age of dinosaurs lasting 180 million years, followed by another extinction event that initiated the age of mammals, which continues today.

Sediment such as sand or mud buries organisms or their tracks, then pressure and heat transform the layers into rock, creating fossils. Sedimentary rocks yield the most fossils, particularly as fragments rather than complete organisms. These rock layers, or strata, accumulate fossils over time, allowing scientists to study preserved remains and reconstruct accounts of life on Earth.

Long-lived, abundant, hard-bodied organisms dominate the fossil record because hard tissues like bones, teeth, and shells resist decomposition better than soft tissues. Sedimentary rock fossils of these organisms provide valuable information about physical form, behavior, and age. Soft-bodied species are poorly represented, making the fossil record biased toward organisms with durable skeletal structures.