Charles Darwin · Science
Darwin’s claim that every living thing - bacterium, beetle, whale and human - descends from a common ancestor, branch by branch from a single root.
Natural selection explains how one species changes. Darwin’s second, bolder claim was about all species at once: that they are not separately created kinds but the branching descendants of common ancestors. Trace any two creatures back far enough - a sparrow and an oak, a person and a mushroom - and their lineages meet. The only diagram in the entire Origin of Species is a branching tree, and Darwin meant it almost literally.
Three kinds of evidence betray shared ancestry. Homology: the same bones - upper arm, forearm, wrist, five digits - are reshuffled inside a human hand, a bat’s wing, a whale’s flipper and a mole’s digging paw, though each does a different job. A designer would start each from scratch; descent reuses an inherited plan. Embryology: the embryos of fish, chicken and human look startlingly alike early on, betraying a common blueprint. Vestiges: useless leftovers - the human tailbone, the hind-leg bones buried in a whale, eyes on blind cave fish - make sense only as inheritances from ancestors that used them.
Some of Darwin’s most decisive evidence for common descent was not anatomical but geographical - the strange, lawful way living things are distributed across the planet. Why do oceanic islands, like the Galápagos or the Azores, teem with peculiar birds, insects and plants found nowhere else, yet almost never have native land mammals, amphibians, or freshwater fish? Why do the species on such islands resemble, above all, the species of the nearest mainland, however different the island’s climate? Why does volcanic Galápagos, with a climate like parts of Africa, hold creatures akin to South American ones rather than African?
Separate creation has no answer: a designer stocking each island for its conditions would have put mammals on the Galápagos and matched species to climate, not to the accident of which coast lies nearest. Common descent explains it exactly. Islands are colonised by whatever can cross water - birds, flying insects, drifting seeds, a lizard on a raft - which is why mammals and frogs, poor ocean-crossers, are absent until humans bring them. The colonists come from the nearest land, then diverge in isolation into new species, which is why island life resembles its nearest mainland ancestor and yet is found nowhere else. Geography records the history of descent and migration; the map of life is a map of where ancestors could travel.
This is the opening of the lesson. The rest — the dialogue, the primary source, and the recall — is in the app.
You learned that classification, embryos and vestigial organs all point to common descent. Pick one of these and explain how it argues for a shared ancestor, in your own words.
Leads to Francis Crick.
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