Fossils show a sequence of human change

1. A fossil is the preserved remains, impression or trace of an organism that lived in the past.
2. Scientists date fossils and compare their age, skull shape, tooth structure, brain-case volume, pelvis and limb bones.
3. A sequence from older to younger rocks reveals changes over time, while similarities show that extinct hominins and modern humans share ancestry.
4. Ardi, about 4.4 million years old, had a small brain and features suited to climbing, but the pelvis and foot provide evidence relevant to upright movement.
5. Lucy, about 3.2 million years old, had a small brain but a pelvis, knee and leg bones adapted for regular walking on two legs.
6. Fossils discovered by Richard Leakey's team from about 1.6 million years ago had a larger brain case and a body shape closer to later humans.

Evidence is strongest when features and dates agree

1. The age of each fossil places it in time, and anatomical features show which traits appeared earlier or later.
2. Older fossils with a mixture of ape-like and human-like features support gradual evolutionary change rather than a sudden appearance of modern humans.
3. The fossil record is incomplete because fossilisation is rare, remains can be destroyed, and many fossils have not been found.
4. A new discovery can change the proposed family tree, so scientific explanations are revised when better evidence appears.