There’s an origin story about butts: Once upon a time, all animals were essentially blobs with mouths, and they had to eat and defecate through that single hole. But over the millennia, some blobs straightened out and gave way to animals with heads to ingest food on one end, anuses to defecate at the other, and guts in between.
The story is based on today’s animals. Those so odd they’re sometimes mistaken for plants—like sponges, sea anemones, and gelatinous jellies—have one hole. And evidence from DNA and fossils suggest that these creatures descend directly from the first animals that evolved more than 540 million years ago.
Other animals, including rats, flies, snails, worms, and humans, share a more recent ancestor with a mouth and an anus. The fact that there’s so many more kinds of two-holed animals today suggests this system of digestion had been a very good adaptation—one that led to diversity. One reason for the anus's success might be that it provides creatures with a way to not accidentally eat what they excrete. (Read "Why Do Animals—Including Your Dog—Eat Poop?")
It’s a scientific scenario perfectly suited for textbooks. But a new finding shows it’s crap.
Comb jellies, long thought to be part of the one-holed crowd, have pooped on tape. Now scientists must rethink the evolution of the anus, along with development of a digestive system that passes material from front to rear, also known as a through-gut.
“If you don’t see the video, it’s hard to believe,” says George Matsumoto, a marine biologist at Monterey Bay Aquarium Research Institute in California, who studies comb jellies but was not involved in the project. The movies and corresponding report will be published in an upcoming edition of the journal Current Biology.
Comb jellies are translucent ocean animals named after the pulsating iridescent combs that line their gelatinous bodies. Based on analyses comparing the genetic sequences of hundreds of animals, biologists think the comb jelly lineage evolved before other animals that still have just one hole—namely, sea anemones, corals, and jellyfish.
In the late 1800s, zoologists identified a pair of holes opposite the comb jelly mouth and named them “anal pores.” But they weren’t sure that the pores expelled digestive products. Comb jellies in the lab would ingest food and spit it back up—rarely shunting it out the other side.
In the century that followed, biologists speculated that the pores might serve another purpose, such as acting as a pressure valve underwater.
William Browne, an evolutionary biologist at the University of Miami in Florida, guesses that his predecessors had fed the delicate jellies incorrectly, and the creatures were vomiting in response. In his laboratory, he keeps comb jellies alive on a slow trickle of tiny larval fish.
After he saw his jellies pooping one day, he set up a video camera to record time-lapse footage over a few hours. (Also see "Zoo Poo and Glitter Reveal Animal Health Secrets.")
In the movies, the pale brown larvae circulate through vessels in the jellies’ see-through bodies. Like dust bunnies, their undigested scales collect near the jellies’ rear and then pop out of the anal pores. To take a better look, Browne fed his jellies fish that had been genetically engineered to glow red and filmed again. Sure enough, the red fish became red poop.
Back to Basics
The discovery that comb jellies eat through one hole and excrete through pores on the opposite side destroys the simple origin story of the butt and suggests two possible alternatives.
One is that comb jellies evolved their anus-like pores and through-guts independently of all other animals, and their digestive system just happens to look very much like our own. Or, sea anemones and their kin inherited through-guts with exit holes from an ancient ancestor but lost them over time.
To figure out which scenario holds water, biologists will re-examine animals with greater attention to the gory details. For example, if comb jellies and mice use the same genes to develop their digestive systems, that’s a point in favor of the latter theory.
Brown suspects that most scientists didn’t question what had been said about comb jelly digestion because the anus origin story was convenient in its simplicity. Casey Dunn, an evolutionary biologist at Brown University, agrees. A few biologists had observed comb jelly excretion in the past, but until now, it has never been so well documented that the dogma required correction.
“If a comb jelly poops in the ocean and no one publishes clear data on it, then it didn’t happen,” he says with a laugh.
But it’s the fundamental nature of Browne’s discovery that really makes Dunn giddy.
“It shows there is still so much basic, cool work to be done,” he says. “You don’t need a particle accelerator or a next-generation DNA sequencer to do this. You can just go out and watch animals poop.”