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Personally, I think it's easier to understand the deuterostomes: the "mouth" will benefit from being controlled by input from sensory organs, and those need to be on the outside of the body. Thus, a natural bending of a flattish body-shape into a horseshoe, with the mouth on the outside of the "bend."

Also, a fun fact to go with yours: vertebrates are triploblasts. That means, when we first fold over (gastrulize), our cells differentiate into three types--one on the "inner wall" of the fold, one on the inside of the body, and one on the exposed "outer wall" of the fold. In other words: we're all tacos!

Another, perhaps more-weird-than-fun fact: our DNA is basically three separate "code bases", one for each layer. It is much easier for a given feature to evolve by differentiating from similar tissue proteins in its own layer, than in another layer. (In fact, to do otherwise, the resulting proteins still have to "start off" in their ancestral layer, before "migrating" during development.)

Anyway: our brains--which you'd think of as an "internal" organ--are actually part of the ectoderm, and in lower organisms, the first thing that the ectoderm is, is skin. So, your brain (and your nerves, your spine, and most of your sensory organs) are all effectively "kinds of" skin tissue. Your nerves? Skin with capacitance. Your spine and teeth? Calcified nerves. The reward centre of your brain? Modified skin pigmentation cells. Your retina: also skin pigmentation cells. Your iris: modified retinal cells.

Basically, evolution always starts its feature branches with a bit of copy-and-paste programming. :)

So what you are saying is that nature ALSO shuns multiple inheritance? :)

Yes, but have you tried reading the codebase?

I presume that by "vertebraes" you mean "vertebrates"? All vertebrates are actually deuterostomes (see the Wikipedia page [1]), as are echinoderms (sea urchins, sea stars, etc). Insects, etc., are protostomes, meaning "mouth first" (though apparently there is some debate about that, see references at https://en.wikipedia.org/wiki/Protostome ). Vast majority of animals fall into one of these two groups, though not all, for example https://en.wikipedia.org/wiki/Cnidaria .

Anyway, quite agree this is a fascinating discovery!

Yes I did, sorry. Thanks! Edited to fix that.

I was of the understanding that some vertibrates were protostomes, but it appears I'm mistaken! Maybe this wasn't a common ancestor then, though it still begs the question how it went from one hole being a mouth/anus combo to a deuterostome.

Probably no way to know if this critter is an ancestor of ours. Though I'm not a biologist, as far as I know the question you pose remains a mystery...

> 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.

I understand the story differently: there were animals with a single opening. Then the animals with two openings evolved, but more than once, based on the animals we have today, there are at least two different evolution internal mechanisms to more than one opening: it's just how the cellular mechanisms work during the cell division (meaning the "programs" driving them are different). We can't say there were ever "anus-only" animals just as we can't say there were "mouth-only" animals. There were single-opening animals and then there were two internal mechanisms leading to the two-opening animals.

So this this headline wrong?

https://www.theguardian.com/science/2017/jan/30/huge-mouth-a...

Is it actually a huge anus with no mouth?