Fossils — How They Form

FOSSILS are remains or traces of ancient life preserved in rock. Most organisms DON'T fossilize — they're eaten, decay, or are destroyed by weather. Fossilization requires SPECIAL CONDITIONS: rapid burial (often by sediment in water), no oxygen (slows decay), and minerals to gradually replace organic material with stone over millions of years.

Types of fossils. PERMINERALIZATION: minerals seep into bone or wood, gradually replacing organic material. The result looks like the original but is now rock. MOLDS AND CASTS: organism leaves an impression in mud (mold); later filled with sediment (cast). COMPRESSION: thin organic film (common for leaves). AMBER: insects/plants trapped in tree resin, preserved with details. PERMAFROST: frozen mammoths from the Ice Age, with hair and meat preserved! TRACE FOSSILS: footprints, burrows, dung — evidence of behavior, not bodies.

The fossil record. Despite its biases, the fossil record is one of the most important sources of evidence for evolution. Older rock layers contain simpler organisms. Newer layers contain increasing diversity. TRANSITIONAL fossils (like Tiktaalik, between fish and amphibians; Archaeopteryx, between dinosaurs and birds) document major evolutionary changes. The record has gaps but consistently supports evolutionary theory.

Each fossil is a small miracle of preservation. Most life leaves no record. Those that do — they're our window into worlds long gone.