Evidence of evolution from embryology

The argument

Embryology offers some of the most striking evidence that living things share common ancestors, because the way organisms develop before birth reveals relationships their adult forms conceal. In their early embryonic stages, vertebrates as different as fish, reptiles, birds and mammals look remarkably alike: each develops a similar body plan, with a notochord, a tail, and a series of structures in the neck region known as pharyngeal arches. In fish these arches develop into gills; in humans and other mammals they are repurposed into structures of the jaw, ear and throat. That a human embryo should pass through a stage bearing features inherited from fish-like ancestors is, on this argument, exactly what common descent predicts and what separate creation would not. The argument extends to the developmental quirks that only ancestry explains. Human embryos transiently grow a tail that is later mostly reabsorbed; whale embryos begin to form hind-limb buds and even tooth buds that then disappear, echoes of land-dwelling, toothed ancestors. These fleeting structures serve no purpose in the adult and make sense only as developmental legacies of evolutionary history — the organism briefly retracing the modifications of its lineage as its genetic instructions, built up over evolutionary time, play out. Darwin himself regarded embryological evidence as among the strongest support for his theory, precisely because these hidden similarities and vestiges are so hard to account for on any other view. The shared patterns of early development, and the appearance and disappearance of ancestral features, trace the branching relationships of life. Because embryology provides this evidence for common descent, this argument holds, the theory of evolution by natural selection is correct.

Premises

Counter-arguments

Critics — including within science — caution that the embryological argument has historically been overstated. Ernst Haeckel's celebrated drawings exaggerated, and in places falsified, how alike vertebrate embryos look, and the strict 'recapitulation' idea that an embryo replays its ancestors' adult forms is rejected by modern biology. Early vertebrate embryos are in fact not identical, differing markedly at the earliest and latest stages, and a transient 'tail' or set of pharyngeal arches are developmental structures, not proof an embryo is momentarily fish-like. Defenders answer that common descent rests on many independent lines — genetics, fossils, biogeography — so embryology is corroborating rather than load-bearing; but as a standalone proof it is weaker than the argument implies.

Rejecting the premises

[Rejecting P1] Early vertebrate embryos are not in fact identical; they diverge at the earliest and latest stages, and the 'identical embryos' picture traces to Haeckel's exaggerated and partly falsified drawings. [Rejecting P2] Shared developmental structures like pharyngeal arches do not mean an embryo passes through its ancestors' adult forms; the strict recapitulation reading is rejected by modern biology.

Further reading

https://www.nap.edu/read/6024/chapter/4