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Salemicus comments on What false beliefs have you held and why were you wrong? - Less Wrong Discussion
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I think this was a great idea for a post. If LessWrong rationality is worthwhile, then it ought to get lots of replies on concrete facts - not moral preferences, theology, or other unproveables.
I used to believe that embryos pass through periods of development representing earlier evolutionary stages - that there was a period when a human baby was basically a fish, then later an amphibian, and so on. I believed this because my father told me so; he was a doctor (though not an obstetrician), and the information he had given me about other subjects was highly reliable. Most knowledge is second hand - it was highly rational for me to believe him. I now know (also second hand!) that Haeckel's ideas were debunked a long time ago - although they might well have been in a textbook when my father was at medical school.
To me, the lesson is trust, but verify.
- graffiti in the men's room of the Life Sciences Building, University of California at Santa Cruz
The way you stated it is actually not wrong, especially if your use of "representing" means "looking like". What is wrong is the much stronger statement that follows, "basically a fish", as opposed to, say, "human embryos pass through a stage where they have slits in their necks resembling gills, though without the same function". I suspect that this is closer to what your father meant.
If this “looks like” this your definition of looking like is much broader than mine.
A better comparison would be to a fish embryo. I don't know if among all the wildly different kinds of fishes there are some whose embryos superficially resemble mammalian ones for a time.
I only recently realized that evolution works, for the most part, by changing the processes of embryonic development. There are some exceptions-- things like neoteny and metamorphosis-- but most changes are genetic differences leading to differences in, say, how long a process of growth is allowed to occur in the embryo.
Oh horror. I believed this until right now. Not exactly literally. I never knew Haeckels strong thesis. But from what I had been told (probably as contracted as "ontogeny recapitulates phylogeny") I still assumed the linearity part to be true. And I passed this wrong simplification on to my children (probably like your father did). So now I have some damage control to do...
Indirect quotes from the cited Wikipedia article: