Output Stream of Consciousness

Every now and again, you run into a really odd fact in biology. An interesting but tame one is the fact that we also run through multiple different kinds of globin in our hemoglobin (we're done with Hba-x, or "zeta globin", for example, by the end of the first trimester), from different genes, growing from embryo to infant.

An odder interesting fact is that humans, and indeed all vertebrates, have three successive sets of kidneys.

The first and most anterior and largest to develop is the pronephric kidney or the pronephros. The second kidney to form is the mesonephros. In birds, reptiles and mammals, a third kidney develops posterior to the mesonephros, called the metanephros.

The odd thing seems to be that this would appear to be one of the closest things you get to an equivalent of Haeckel's mostly-discredited recapitulation hypothesis, where he posited that embryos are "replaying their evolutionary past" in their development.

There's a paper in Kidney International about it.

The development of the vertebrate kidney has been cited as the most compelling evidence for the validity of Haeckel's dictum1. Three successive renal structures develop during embryogenesis; these structures are the pronephros, mesonephros, and metanephros. It was thought originally that these were three distinct kidneys that succeeded one another in their "phylogeny" as well as in their "ontogeny." However, such a thought, like the "biogenetic law" itself, is a half-truth. The pronephros, mesonephros, and metanephros, all of which derive from the same band of intermediate mesoderm, are best considered as a single organ, or holonephros

Essentially, the three kidney sets develop out of the same set of tissue, starting at one end, like a slow lit fuse.

Embryos really show a lot of odd things going on that you just don't get to see in adults, many of which are seemingly redundant (and may very well be, but don't have much genetic "room" to change quickly).

Yes, yes, I'm on a biology kick these days :)