The first thing I want to make perfectly clear is that I am NOT referring to Haeckel’s work nor to his long discarded theory. Ontogeny does not recapitalate Phylogeny, but there are some interesting similarities in development which I believe are best explained by evolution.
Mammal Kidneys
Mammal Embryos develop three sets of kidneys. The first, pronephros, is the same set found in primitive fish like Lampreys. After 3.5 weeks, the mammal embryo replaces it. The second set, the mesonephros, is the same set found in higher fish and amphibians. In human males it gives rise to urogenital structures, while in females the remnants are vestigial. The third set (Metanephros) is the set which develops and becomes the adults set of kidneys, and it is the same set found in mammals and birds.
The Legs of the Legless, The Hair of the Hairless
Snakes as well as Dolphins are known to develop legs as embryos, only to reabsorb them later.
Whales Develop hair as embryos, only to discard it later (except for the nosehair).
Development of the Mammalian Jawbone matches the Evolution of the Mammalian Jawbone
"From embryological studies it is known that two bones of a developing reptile eventually form the quadrate and the articular bones in the hinge of the adult reptilian jaw (first reported in 1837 by the German embryologist Karl Reichert). However, in the marsupial mammalian embryo, the same two structures develop, not into parts of the jaw, but into the anvil and hammer of the mammalian ear. This developmental information, coupled with common descent, indicates that the mammalian middle ear bones were derived and modified from the reptilian jaw bones during evolution (Gilbert 1997, pp. 894-896).
Accordingly, there is a very complete series of fossil intermediates in which these structures are clearly modified from the reptilian jaw to the mammalian ear."
Why do they do this? Why go through these stages? Why not develop their adult forms more quickly? These facts are best interpreted as developmental remains of their past.
