Embryological Evidences

Embryology, the study of development from egg to adult, shows that very different adult animals pass through strikingly similar early stages. Vertebrate heart development is a clear case: in every vertebrate the heart begins as a pair of tubular structures that later differentiate into a two-chambered heart in fishes, a three-chambered heart in amphibians and most reptiles, and a four-chambered heart in crocodiles, birds and mammals — pointing to a shared ancestry across the whole group. Building on such observations, Ernst Von Haeckel proposed the biogenetic law, or 'theory of recapitulation': the developmental history of an individual (ontogeny) briefly retraces the evolutionary history of its lineage (phylogeny) — summarised as 'ontogeny recapitulates phylogeny'. The appearance of pharyngeal gill pouches, a yolk sac and a post-anal tail in the early human embryo, mirrored in the embryos of fish, salamander, tortoise and chick (Fig. 6.4), were cited as supporting examples. The law is not treated as universally true today — later understanding holds that an embryo recapitulates the embryonic stages of its ancestors, not their adult forms. A further embryological line of evidence is that the embryos of fish, salamander, tortoise, chick and …