How Does a Fossil Form?

Step 1 of 5

Almost everything rots away.

Scavengers, bacteria and weather destroy a body within months. For anything to survive, it has to be removed from all of that fast.

Step 2 of 5

Quick burial is the crucial step.

Mud, sand or ash covering a body seals out oxygen and scavengers. This is why most fossils are of animals that died in or near water.

Step 3 of 5

Groundwater seeps through the bone.

Bone is porous. Water carrying dissolved minerals soaks into every tiny space and slowly deposits them there.

Step 4 of 5

Mineral replaces bone, atom by atom.

Over millions of years the original material dissolves away and mineral takes its place — keeping the structure so faithfully that cells are sometimes still visible.

Step 5 of 5

Then it has to be found.

The fossil must be lifted by geology and exposed by erosion at a moment when someone is looking. Almost every creature that ever lived left nothing.

Footprints, burrows and even droppings fossilise too, and they record behaviour that bones cannot.