This is not at all what I study, but it seems possible to me that sponges have evolved a tolerance for low oxygen conditions in the billion or so years since they first arose. It's a common mistake to think that because something is morphologically very similar to an ancient form (a so called "living fossil"), that it hasn't undergone any evolution. As long as it has been reproducing and mutating, better adapted versions have always had differential success, hence they've evolved, even if the shape hasn't changed.
For reference, the source paper: <a href="http://www.pnas.org/content/early/2014/02/13/1400547111.abstract" rel="nofollow">http://www.pnas.org/content/early/2014/02/13/1400547111.abst...</a><p>It's not a completely novel idea, but it it an interesting find which may lead to more interesting conclusions.
I tuned out when they started dating the emergence of a group of animals by working backwards based on a guess of how long they would have needed to achieve a certain level of genetic diversity.
That's interesting as conjecture, and I'll take it as such.
recently read about early life conditions on wiki [1], says pretty much the same thing. also relevant [2]<p>[1] <a href="http://en.wikipedia.org/wiki/Abiogenesis" rel="nofollow">http://en.wikipedia.org/wiki/Abiogenesis</a><p>[2] <a href="http://en.wikipedia.org/wiki/Oxygen_catastrophe" rel="nofollow">http://en.wikipedia.org/wiki/Oxygen_catastrophe</a>