R M Cullen MD MSc MFM BA DipStats DipProfEthics

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ontogeny, lecture one

The goal of this lecture is to present human embryological development as the embryological development of common ancestors with extra bits 'added on'.

ontogeny recapitulates phylogeny

This is an idea of much the same vintage as Darwin's theory of evolution by common descent. The idea is that as the human embryo develops it initially looks very like a fish embryo, then passes through stages where it looks like an amphibian, then a reptilian, then finally a mammalian embryo.

branchial arches

this image is taken from http://php.med.unsw.edu.au/embryology/index.php?title=Pharyngeal_arches The image is of a 32 day old human embryo. The structures labelled 1 through 4 are branchial arches. Human embryos have a sixth branchial arch but it is not visible externally. The fifth branchial arch does not normally develop in humans. The six branchial arches of fish go on to develop as gills and the associated blood vessels and nerves. In humans the arches develop as structures of the head and neck. The first develops into the maxilla and mandible. The second into the hyoid bone. The 3rd and 4th go on to form neck structures (the sixth fuses with the fourth)

kidneys

Human embryos form three sets of kidneys. the first, resembling the kidneys of embryonic jawless fish vanishes and is replaced by kidneys resembling those of embryonic reptiles. These too are resorbed and replaced by our final form kidneys.

blood vessels and heart

Our circulatory system passes through a similar set of stages. Initially it looks very like that of a fish embryo, then it passes through stages where it looks like that of an amphibian embryo, then that of a reptilian embryo, then finally that of a mammalian embryo.

recurrent laryngeal nerve

The recurrent laryngeal nerve is a branch of a cranial nerve (one of the nerves that begins inside the cranium). Its endpoint is the larynx. However, to get here it dives into the chest, under and around one of the main blood vessels connected to the heart. The total journey is about a meter rather than the 30cm direct route. In giraffes, with their longer necks the total length of the recurrent laryngeal nerve is around 4.5 meters!

The reason once again relates to the initial development of this nerve when the embryo is 'fishy', at about four weeks of age.

Ontogeny not amenable to natural selection

There is a distinct choreography to embryonic development that natural selection does not appear able to influence.

For example, the excessively long recurrent laryngeal nerve remains excessively long (and prone to injury because of this)

There would seem to be selection pressure for whales to 'redevelop' gills. Clearly, as mammals who pass through a "fishy" phase in embryology at least some of the required genes are still present. But there are no whales with gills

readings