Christian Huygens (1629–95) the Dutch mathematician, and Rene Descartes, the French philosopher/mathematician who first “correctly” explained the rainbow (1596–1650), independently solved the problem of spherical aberration in a lens, with a doublet lens arrangement; a system that now bears their names. Unbeknown to them, though, evolution had already solved this problem millions of years earlier in small ancient creatures known as trilobites. This subtle, recently decoded, palaeontological tale reveals the microevolutionary principles continuously at work and capable of providing ingenious solutions to the physical problems of vision.

Trilobites arose from the evolutionary crucible of the Cambrian explosion 535 million years ago and evolved for perhaps 300 million years, only to become extinct by the mother of all dyings—the Permian extinction, 235 million years ago. Nevertheless, much is known of these animals from the fossil record because these extinct marine arthropods had hard body parts, which included calcite lenses and allowed preservation in stone. Much remains in the fossil record to assist our understanding of the visual capabilities and experiences of these oldest eyes.

From this fossil record, we know …