I’m guessing you have never stopped to think about how cool it is that you have a butthole, but it is one of the most important developments to ever happen in the evolution of animals. Before butts came to be, animals used one hole for both eating and defecating.
Most animals with multi-purpose mouths went extinct eons ago. Creatures with a more efficient through-gut, just like our own, out-competed them.
A through-gut provides two major advantages over a primitive one-orifice gut. The first is an animal can eat while digesting. The other, perhaps more obvious one, is it removes the risk of contaminating your mouth with fecal matter.
One clade of animals has survived, and even thrived, despite not having a butt. This clade of animals, known to biologists as cnidarians, includes simple, soft-bodied creatures such as sea anemones and jellyfish.
Another clade of animals that evolved before the cnidarians was also thought, until recently, to have a single-orifice gut. These animals, known as comb jellies, or ctenophores, closely resemble jellyfish, but only superficially.
Genetic tests have shown comb jellies originated early in the evolution of animals, even before other simple creatures with one opening to their digestive tract.
In 1880 a German scientist noticed comb jellies have a pair of pores opposite their mouth, but he had also observed comb jellies defecating from their mouths — an observation also made by scientists in 1997.
In order to demonstrate without a doubt whether the comb jellies’ mysterious pores are involved in digestion, a team of researchers made a high-tech video of the animals eating, digesting and doing their business.
Like many of their jelly-like relatives, comb jellies are transparent and the workings of the insides can be easily observed.
Researchers at the University of Miami took advantage of this by feeding the jellies zebrafish and crustaceans that were genetically engineered to glow red.
They could then watch as the glowing food particles traveled through the comb jellies’ gut.
What they saw was astonishing. Food passed straight through the comb jelly and exited through the mysterious pores at the far end. The researchers took their movies to the annual Ctenopolooza, conference for comb jelly biologists, and shocked their audience.
While watching poop exit a comb jelly’s butt may not sound very exciting, this finding completely shatters the current understanding of animal evolution.
Until now, it was thought the through-gut developed once, sometime after Ctenophores and Cnidarians showed up to the evolutionary scene.
Now, evolutionary biologists have presented two competing hypotheses: the first is the through-gut evolved not once, but twice in the evolution of animals. The other is it still evolved once, but eons earlier than expected, and jellyfish and sea anemones lost this trait later on.
Evolutionary biologists sometimes try to force a ladder-like view onto the tree of life, wherein new and improved innovations come into being one-by-one. These fascinating new results challenge that view and remind us of evolution’s ability to reinvent life over and over again.
Next time you sit down on the john, take a moment to appreciate the fact that we take care of business in the most efficient way biology could offer.
jaykgold@indiana.edu
@JayKeche
