What I find fascinating about this is that humans and many other branches of the tree are Deuterostomes[1], which means that when the cells first divide the first 'hole' goes on to form the anus [2], rather than the mouth. This is in contrast to the Protostomes where the first hole forms the mouth.
I mean obviously it makes sense that a single-holed animal used it as a mouth - the anus has no use without the mouth - but (in my naive understanding) it means that somewhere beyond this ancestor, certain groups of vertibrates split and went in opposite directions despite both being (presumably) decended from this thing.
It fascinates me that there was some mechanism to cause this, rather than all animals forming in the same order, if we did indeed share a common ancestor that only had one entry/exit hole.
I mean obviously it makes sense that a single-holed animal used it as a mouth - the anus has no use without the mouth - but (in my naive understanding) it means that somewhere beyond this ancestor, certain groups of vertibrates split and went in opposite directions despite both being (presumably) decended from this thing.
It fascinates me that there was some mechanism to cause this, rather than all animals forming in the same order, if we did indeed share a common ancestor that only had one entry/exit hole.
[1]: https://en.wikipedia.org/wiki/Deuterostome
[2]: The clever observer will note that this means that technically, very early on, each of us is little more than an arsehole ;)