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We know that most of the so called "Advanced" organisms are deuterostomes (i.e., development of gut starts from anus). Is there any evolutionary advantage of that? If not, why and how did it evolve?

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Just a word of caution: it's very easy to invent "just-so" stories when it comes to speculating about evolutionary advantages/disadvantages. Without rigorous experiments proving the relative advantages of one trait over another, or at least providing quantifiable evidence thereof, it's impossible to do anything more than speculate. – user3934 Jul 19 '13 at 7:19
One other note: I would discourage the use of the word "advanced" here; that would imply that increasing complexity is somehow a goal of evolution that natural selection works towards. I would argue that it is a side effect. There are many extremely successful species that never evolved beyond a single cell. Thus, I would prefer to say "complex" here, which is pretty objective. – user3934 Jul 19 '13 at 13:25
Your premise is quite doubtful. While you are considering vertebrates as the pinnacle of evolution, you should note that arthropodes, nematodes, annelids and mollusc, wich contain more than 95% of known animal species are, in fact, protostomes. – Miguel Ángel Naranjo Ortiz Aug 18 '13 at 15:59
Rather than thinking about "complexity", I think a better question would be, what selective advantages do protostomes and deuterostomes have, if any, and how are these related to the environments ? – user6462 Apr 23 '14 at 17:38
Are protostomes less complex/advanced than deuterostomes? Well, it is true that the sequenced genomes of the fruit flies and Anopheles Gambiae show that these genomes are slightly shorter than the one of the puffer fish or the human. Also, I would like to remind that correlation does not mean causality. Other traits that are associated with deuterstomia/protostomia are likely to explain the observed variance. – Remi.b Apr 23 '14 at 17:59
up vote 1 down vote accepted

I would speculate that there is no direct advantage imparted by the deuterostomic development process over that of protostomes with regards to the evolution of more complex species. Instead, I would guess that sometime after that split, a series of novel alleles arose in various genes which were eventually fixed and laid the groundwork for future increased complexity. Given the trait in question (increased organismal complexity), I would be certain that these alleles were in mostly unrelated genes and were selected for some other function (in other words, there is no gene encoding a protein that imparts organismal complexity; that trait is, by definition, a complex one!). Perhaps the genes involved in the deuterostome/protostome split were part of this groundwork, given their activity at the base of development, however it would be very difficult to know at this point.

I don't know why I didn't think about this before, but the more likely explanation for the difference in complexity between deuterostomes and protostomes is actually unrelated to that difference. Most of the complex deuterostomes that you're probably thinking of are vertebrates. Vertebrates went through a couple of rounds of genome doublings (tetraploidizations) during their early divergence. The extra copies of all the genes provided fertile ground for evolutionary novelties that seems to have led to some wonderfully complex creatures. It's still possible that the tissue groundwork laid out by the deuterostome developmental process somehow contributes to that complexity, but it would be rather indirect, I think.

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