Four Kinds of Evidence, One Answer

How old is it, and how would you know?

Carbon-14 decays at a fixed rate: half of whatever is present decays every 5,730 years, whatever else is happening. Measure how much is left in a sample and the decay law gives its age. Nothing here is looked up — every age on this page is computed from the half-life.

Carbon-14 remaining—

Half-lives elapsed—

Age of the specimen—

Check: decay law run forward—

Where carbon-14 stops working

Isotope	Half-life	Useful range	What it dates

How radiometric dating works, and what it cannot do

Every kind of data the CED asks you to name

The next two tabs take the biochemical and the physical in turn. What matters is not any one of them but the last panel of this tool: they were produced by people using completely different methods, and they agree.

One protein, eight species, counted position by position

Cytochrome c carries electrons in the respiratory chain of every aerobic eukaryote. The sequences below are real, aligned to the same length, and compared against the human one by counting the positions where they differ. No tree was consulted.

Positions compared—

Positions that differ—

Identical to human—

Divergence—

All eight, ranked by how different they are

Organism	Differences from human	Positions compared	Identity

A molecular clock, calibrated on one point

Organism	Divergence	Clock estimate	From fossils and rocks

Two lineages, the same elapsed time, different amounts of change

This is worth dwelling on, because it is the honest version of what a molecular clock is. It is a rate that holds approximately, averaged over long spans and many sites, and not a law. That it is approximate is exactly why the agreement with the fossil dates is evidence: two methods with independent errors landing in the same place is informative, and a perfect match would be a reason to look for a mistake.

Why sequence comparison is evidence of common ancestry

Five forelimbs, one skeleton

A human arm, a cat's foreleg, a whale's flipper, a bat's wing and a bird's wing. Different sizes, different proportions, entirely different jobs — and the same bones, in the same order, connected the same way.

Organism—

Structure—

What it is used for—

Bones present—

Homologous or analogous?

Two of these pairs look alike because they do the same job. Two look alike because they were inherited from the same ancestor. The test is not what they do — it is what they are made of.

Pair	What is similar	What is different	Which is it?

Homologous, analogous, and how to tell them apart

Structures that are still there and no longer used

Structure	Why it counts as evidence

Four methods, one answer

Where did whales come from? Four groups of people answered this using methods with nothing in common. None of them could see the others' results.

Line of evidence	What was found	What it says whales came from

Why independence is the whole argument