Evidence for Evolution — Embryology and Molecular Evidence

Embryology, the study of the development of an organism from a fertilised egg through to birth or hatching, provides a further, independent line of evidence for evolution, one first noticed and emphasised by nineteenth-century biologists studying the early developmental stages of different vertebrate animals side by side. When the very earliest embryonic stages of fish, amphibians, reptiles, birds and mammals — including humans — are compared, a striking pattern emerges: these early embryos closely resemble one another across all these different vertebrate groups, far more closely than the very different adult body forms into which these same embryos eventually develop. A human embryo at an early stage of development, for instance, looks considerably more similar to a fish embryo, or a chicken embryo, at the same corresponding early stage than it does to a fully developed adult human.

The single most striking and most frequently cited shared embryonic feature is the presence, at an early developmental stage, of a series of paired slit-like structures in the throat (pharyngeal) region, often called gill slits or pharyngeal slits, which appear in the embryos of every vertebrate group without exception — fish, amphibians, reptiles, birds and mammals alike. In fish, and to some extent in amphibian larvae, these embryonic structures develop further into fully functional gills, used throughout life (in fish) or during an aquatic larval stage (in many amphibians) for extracting dissolved oxygen from water. In reptiles, birds and mammals, including humans, however, these embryonic gill-slit structures never develop into functional breathing gills at any point — land vertebrates breathe using lungs, not gills, at every stage of their life after hatching or birth — and instead, these same embryonic structures are extensively remodelled during later development into entirely different adult structures, such as parts of the jaw bones and structures of the middle ear.

The question this pattern raises is an important one: why would land vertebrates, whose adult members never breathe through gills at any point in their lives, nonetheless develop these gill-slit structures, however briefly, at an early embryonic stage, only to remodel or discard them later in development? The most economical, parsimonious explanation — and the one accepted by modern evolutionary biology — is that all vertebrates, land-dwelling and aquatic alike, share a common ancestor that was itself an aquatic organism possessing functional gills, and that the underlying developmental genetic instructions for building these embryonic gill-slit structures early in development have simply been retained, largely unchanged, across all the many descendant lineages, even in those lineages (reptiles, birds, mammals) that no longer use gills for breathing as adults and instead repurpose the same embryonic structures for other adult functions. …