Light is the subject of this final episode of Brian Cox's four-part series on the universe, and Cox treats it as evidence rather than scenery. He travels to the Burgess Shale in Canada, where fossils preserve one of the first eyes in the evolutionary record, and to the sands of what he calls the world's oldest desert, using the colors in the dunes to explain how the physical properties of light produce a spectrum. From there he builds the case that starlight carries a full record of the universe's history and composition, readable once you understand how it behaves, but that this information stayed useless until eyes evolved to receive it. The episode links two very different kinds of evidence, geological and astronomical, through the same physics: the same rules that color a rainbow or a grain of sand also tell astronomers what a distant galaxy is made of. It closes the series on the idea that human sight itself, an evolutionary accident, is what makes the universe's light legible to us at all.