Two University of Hawai`i-Manoa scientists have contributed to research that helps answer an evolutionary question about how early organisms developed a "through-gut," with separate orifices for a mouth and an anus.

Mark Q. Martindale and Andreas Hejnol work at the Kewalo Marine Lab in the Pacific Biosciences Research Center. The subjects of their research were acoel flatworms, which are easy to collect and raise at Kewalo, and which have been determined to be an evolutionary stepping stone in the development sequence for bilateral animals – animals with a definite left and right side and a definite top and bottom – such as humans.

According to a UH news release explaining their findings, early animals had a single opening or mouth through which food entered, and out of which undigested food was excreted. Conventional wisdom in the field – and the accepted premise in every zoology text about evolution – is that at some point in evolutionary history, the mouth opening split into two openings (the separate mouth and anus) and thus, bilateral symmetry could not occur without having two openings to the gut. Hejnol and Martindale were able to determine that the anus developed independently as animal bodies grew in size.

"A long gut makes sorting food and waste through a single opening inefficient," Hejno said in a released statement, "so they needed to evolve an anus."

Hejnol adds, "Our ancestor was likely a very small, soft-bodied animal that lived between the sand grains in the ocean, similar to the life-style of most acoel species. We are sure that our ongoing studies of the nervous system of these worms will yield to similar important insights into the evolutionary