The Evidence: Reading the Family Tree
Fossils, anatomy and DNA are three independent records — and they agree, which is the part that matters.
Builds on: 5.1 Natural Selection2.3 The Genetic Code: From Gene to Protein
Three independent lines
Any single line of evidence can be argued about. The reason common descent is not seriously disputed in biology is that several completely independent methods produce the same tree — and they had every opportunity not to.
1. Fossils
The record is patchy — fossilisation is rare and needs particular conditions — but it is ordered. Nothing appears out of sequence: no rabbits in Precambrian rock, no flowering plants beneath the first land plants. And transitional forms keep turning up where the theory says to dig. Tiktaalik was found in 2004 after a deliberate search of 375-million-year-old rock chosen because that was where a fish-to-tetrapod intermediate should be. It had gills and fins, and also a neck, a flat skull and wrist bones.
2. Anatomy
A human arm, a whale flipper, a bat wing and a horse leg do entirely different jobs, yet all contain the same bones in the same arrangement: one upper bone, two lower, a cluster of wrist bones, five digits. These are homologous structures — the same inherited plan, modified. There is no engineering reason a swimming limb and a flying limb should share a skeleton; there is an obvious historical one.
Vestigial structures make the point sharper: leftovers with no current function. Whales and pythons carry reduced hip and leg bones. Flightless birds have wings. You have an appendix, a tailbone, and muscles for moving ears you cannot move.
3. Molecules
This is the strongest line, and the newest. Compare the same protein or gene across species and count the differences. The more differences, the longer since the lineages split — and the resulting tree matches the one built from fossils and anatomy, despite being derived from completely different data.
Cytochrome c, a respiratory protein present in nearly everything, differs from the human version by 0% in chimpanzees, about 12% in horses, 21% in tuna and 45% in yeast. That ordering is exactly the fossil ordering. Nobody arranged it that way.
And recall Unit 2: the genetic code itself is shared by essentially all life. An arbitrary lookup table, identical across bacteria and oak trees and you, is very hard to explain except as inheritance from a common ancestor that already used it.
Speciation: how one lineage becomes two
Species split when populations stop interbreeding. Most commonly a physical barrier — a river changing course, a rising mountain range, a few birds blown to an island — divides them. The isolated populations then accumulate different mutations and face different selection, and after enough divergence they can no longer produce fertile offspring even if reunited. They are now two species.
Darwin’s finches are the standard example: one founding population across the Galápagos, differing beak shapes suited to different food, and no interbreeding between them today.
When protein sequencing arrived in the 1950s, it could have produced a tree contradicting a century of comparative anatomy. It did not. Every subsequent method — whole genomes, retroviral insertions, pseudogenes — could have broken the pattern, and instead reproduced it. A theory that survives that many independent chances to be wrong has earned its place.
⚗️ Lab — The Molecular Clock
One protein, cytochrome c, compared across eight species.
- Pick the chimpanzee: zero differences from a human. Then pick yeast: 45%.
- Read the branch points. Sequence difference and divergence time rise together.
- This tree was built from molecules alone — and it matches the fossil one.