Evolutionary biology: The butt boggles the mind: Nature explores evolution of the anus

Darkfrog submitted, created time 2 months 4 days (www.nature.com)

It might not make for stellar dinner conversation, but it's a relevant question. When did multicellular organisms develop an anus? The first organisms to use guts--like modern cnidarians (formerly called coelenterates, meaning "one hollow cavity")--got by with just a mouth. (Feeling like you need to barf? They did too!) However, as the guts get longer, one hole becomes impractical.

"Just punch another hole in it" might seem like an obvious answer to anyone who's survived a kindergarten art class, but, from an evolutionary standpoint, it's pretty darn unlikely. There are several hypotheses:

1. The mouth got wider and eventually split into two. When the guts got longer, one mouth migrated.

2. Researchers from the University of Hawaii have found that certain genes expressed in the "hindguts" of anus-achieving worms are also expressed in the rear portions of unpunched flatworms, suggesting that the anus is one of those happy (but in this case not very appetizing) accidents of evolution.

Keywords: evolutionary biology anus gut