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The beginning of the end
the beginning of the end We’ve got the origin of the anus all wrong, says Chelsea Whyte
C
OMB jellies are wondrous, magical creatures. Their translucent, spherical bodies are lined with iridescent cilia, which they use to motor around the ocean deep and suck in vast quantities of zooplankton. They are some of the most primitive animals on the planet, closely descended from the common ancestors of all animal life. The surprising thing is that they have an anus. Two, even. Together, these tiny holes are rewriting our understanding of how anuses ever came to be. This is as yet a short story: for all its essential functions, the anus is a very understudied organ. We do know there are only a few basic ways that animals defecate (see “Gut thinking”, page 52). Humans and plenty of other creatures have a through-gut, which starts at the mouth and ends at the sphincter. This cheek-to-cheek nutrient highway runs relatively straight through the body, and is generally de rigueur for creatures that have a front and back, right and left sides, and an upside down. For animals that lack this bilateral symmetry – sponges, stinging jellyfish, anemones – the digestive system is
50 | NewScientist | 17/24/31 December 2016
more like a cul-de-sac, a fitting turn of phrase for what is essentially a bag into which food flows, gets digested and then must be expelled before more can be consumed. Comb jellies used to be considered part of the second group, with just one portal for shits and giggles. And since several lines of evidence suggest they are our best representatives of the ancestor of all animals, the very first animals were also assumed to have lacked an anus. In a way, that made
“What the researchers thought was excrement turned out to be vomit” sense. On the surface at least, a through-gut seems a more advanced solution to digestion than a system in which waste comes out of the same orifice the food went in. But things are not always what they seem. In remarkable videos released last year, evolutionary biologist William Browne at the University of Miami in Florida captured the first
definitive evidence that the primitive comb jellies had a through-gut. The experiment was simple. His team fed comb jellies with zebrafish that had been genetically engineered to glow in the dark, and used time-lapse photography to see what happened next. The set-up meant the biologists could see which cells were absorbing nutrients from the zebrafish. “It just lit up the entire digestive system,” Browne says. “And we saw them poop.” The jellies had not one but two anuses. The researchers were taken aback. It just didn’t chime with the accepted wisdom, says Browne. The comb jellies should have regurgitated the zebrafish waste out of their mouths instead of excreting it through different pores. It wasn’t until they went back to research published over 150 years ago that they found accounts matching what they were seeing. In 1850, a naturalist named Louis Agassiz had published observations of comb jellies, or ctenophores, with digestive tract openings opposite their mouths. The truth had been staring biologists in the face all along. “Comb jellies are transparent,” says Browne. “If you really look at them, you see the anal >
Tom gauld
17/24/31 December 2016 | NewScientist | 51
Brandon Cole/NaturePL
jellies excrete through not just one but two anal pores. The revelation suggests that far from being a relatively recent innovation, the through-gut originated with the earliest animals. The real mystery then is why some animals lack an anus. According to Andreas Hejnol, an evolutionary biologist at the University of Bergen in Norway, those that do live in cold water, have slow metabolisms and don’t need much food to survive. Over millennia of evolution, they lost the superfluous exit to their digestive tract. Brachiopods – a sort of mostly extinct clam – are one example. The presence of thin tissue at their rear reveals they once had an opening.
One ring to rule them all
GUT thinking Broadly speaking, animals defecate in one of two ways. You have either two orifices, or just the one – in which case you are probably a sponge, a coral or a sea anemone. There are variations on these themes. In some stationary animals, such as barnacles, a twist in the gastric geometry creates a U-shaped gut with an exit ramp just next to the mouth. Snails are also afforded the luxury of a separate orifice for excrement, but dignity doesn’t ensue. Their crap chute follows the curvature of their shell, so the digestive tract deposits dung directly behind their heads. Evolution has also devised temporary anuses that lack a sphincter: instead, a flap of tissue closes the rear opening after each defecation. Tiny jaw worms have this sphincter-free opening, though researchers have yet to see it in use. Stranger still, some species of face mites don’t have an anus at all. These minuscule critters just eat and eat until they die, their bodies exploding like tiny poo grenades.
Sea anemones: food goes in one hole, waste comes out… the same hole
“Face mites eat and eat until they die, their bodies exploding like tiny poo grenades” 52 | NewScientist | 17/24/31 December 2016
pores. There was just a fundamental misunderstanding of their use.” Browne says many scientists simply didn’t believe that these pores were used as the primary exit for waste, and so perpetuated the myth that ctenophores lacked a second opening. His study, published this October, finally laid that myth to rest with the observation of sphincters strikingly similar to our own. Why, you might ask, did researchers so fervently believe that comb jellies lacked an anus? For more than a hundred years, they had seen them expelling stuff through their mouth and assumed it was excrement. It turned out to be vomit. Comb jellies eat constantly and the ones grown in labs simply hoovered up whatever was in front of them, which was far more food than they could hold. “It sounds crazy, but I think they were making observations on animals that were in really bad shape,” Browne says. In March, he showed up to the Ctenopalooza conference in St Augustine, Florida, with his videos (see newscientist.com). Gasps filled the packed room of evolutionary biologists as they watched the comb
“Six hundred million years ago, many things happened in animal evolution, a lot of diversification,” says Hejnol. “We know that 500 million years ago, there were already animals that had an anus. So in this time frame, maybe one or several anuses evolved in different animals, but we don’t know how often it happened.” For all the mystery about when the anus and through-gut evolved, the why is much clearer. Larger animals need more fuel to grow and live, and beyond a certain gut length it becomes less efficient to cycle waste back up to the mouth. In humans, for instance, nutrients are mostly absorbed in the lower gut, so waiting for food to digest down there and then come all the way back up would be a waste of time and energy. Plus, if your digestive tract has a separate exit, you can eat a second meal while you’re still digesting the first, meaning more energy to fuel the body. The anus is truly an evolutionary marvel. “The brain is the organ that uses up most of our energy,” says Hejnol. “If we didn’t have such an efficient digestive system, we wouldn’t have a brain and we couldn’t even think about how the anus came about. Our anus allows us to think about us.” Ponder that the next time you’re on the toilet. n Chelsea Whyte knows her backside from her elbow
C
OMB jellies are wondrous, magical creatures. Their translucent, spherical bodies are lined with iridescent cilia, which they use to motor around the ocean deep and suck in vast quantities of zooplankton. They are some of the most primitive animals on the planet, closely descended from the common ancestors of all animal life. The surprising thing is that they have an anus. Two, even. Together, these tiny holes are rewriting our understanding of how anuses ever came to be. This is as yet a short story: for all its essential functions, the anus is a very understudied organ. We do know there are only a few basic ways that animals defecate (see “Gut thinking”, page 52). Humans and plenty of other creatures have a through-gut, which starts at the mouth and ends at the sphincter. This cheek-to-cheek nutrient highway runs relatively straight through the body, and is generally de rigueur for creatures that have a front and back, right and left sides, and an upside down. For animals that lack this bilateral symmetry – sponges, stinging jellyfish, anemones – the digestive system is
50 | NewScientist | 17/24/31 December 2016
more like a cul-de-sac, a fitting turn of phrase for what is essentially a bag into which food flows, gets digested and then must be expelled before more can be consumed. Comb jellies used to be considered part of the second group, with just one portal for shits and giggles. And since several lines of evidence suggest they are our best representatives of the ancestor of all animals, the very first animals were also assumed to have lacked an anus. In a way, that made
“What the researchers thought was excrement turned out to be vomit” sense. On the surface at least, a through-gut seems a more advanced solution to digestion than a system in which waste comes out of the same orifice the food went in. But things are not always what they seem. In remarkable videos released last year, evolutionary biologist William Browne at the University of Miami in Florida captured the first
definitive evidence that the primitive comb jellies had a through-gut. The experiment was simple. His team fed comb jellies with zebrafish that had been genetically engineered to glow in the dark, and used time-lapse photography to see what happened next. The set-up meant the biologists could see which cells were absorbing nutrients from the zebrafish. “It just lit up the entire digestive system,” Browne says. “And we saw them poop.” The jellies had not one but two anuses. The researchers were taken aback. It just didn’t chime with the accepted wisdom, says Browne. The comb jellies should have regurgitated the zebrafish waste out of their mouths instead of excreting it through different pores. It wasn’t until they went back to research published over 150 years ago that they found accounts matching what they were seeing. In 1850, a naturalist named Louis Agassiz had published observations of comb jellies, or ctenophores, with digestive tract openings opposite their mouths. The truth had been staring biologists in the face all along. “Comb jellies are transparent,” says Browne. “If you really look at them, you see the anal >
Tom gauld
17/24/31 December 2016 | NewScientist | 51
Brandon Cole/NaturePL
jellies excrete through not just one but two anal pores. The revelation suggests that far from being a relatively recent innovation, the through-gut originated with the earliest animals. The real mystery then is why some animals lack an anus. According to Andreas Hejnol, an evolutionary biologist at the University of Bergen in Norway, those that do live in cold water, have slow metabolisms and don’t need much food to survive. Over millennia of evolution, they lost the superfluous exit to their digestive tract. Brachiopods – a sort of mostly extinct clam – are one example. The presence of thin tissue at their rear reveals they once had an opening.
One ring to rule them all
GUT thinking Broadly speaking, animals defecate in one of two ways. You have either two orifices, or just the one – in which case you are probably a sponge, a coral or a sea anemone. There are variations on these themes. In some stationary animals, such as barnacles, a twist in the gastric geometry creates a U-shaped gut with an exit ramp just next to the mouth. Snails are also afforded the luxury of a separate orifice for excrement, but dignity doesn’t ensue. Their crap chute follows the curvature of their shell, so the digestive tract deposits dung directly behind their heads. Evolution has also devised temporary anuses that lack a sphincter: instead, a flap of tissue closes the rear opening after each defecation. Tiny jaw worms have this sphincter-free opening, though researchers have yet to see it in use. Stranger still, some species of face mites don’t have an anus at all. These minuscule critters just eat and eat until they die, their bodies exploding like tiny poo grenades.
Sea anemones: food goes in one hole, waste comes out… the same hole
“Face mites eat and eat until they die, their bodies exploding like tiny poo grenades” 52 | NewScientist | 17/24/31 December 2016
pores. There was just a fundamental misunderstanding of their use.” Browne says many scientists simply didn’t believe that these pores were used as the primary exit for waste, and so perpetuated the myth that ctenophores lacked a second opening. His study, published this October, finally laid that myth to rest with the observation of sphincters strikingly similar to our own. Why, you might ask, did researchers so fervently believe that comb jellies lacked an anus? For more than a hundred years, they had seen them expelling stuff through their mouth and assumed it was excrement. It turned out to be vomit. Comb jellies eat constantly and the ones grown in labs simply hoovered up whatever was in front of them, which was far more food than they could hold. “It sounds crazy, but I think they were making observations on animals that were in really bad shape,” Browne says. In March, he showed up to the Ctenopalooza conference in St Augustine, Florida, with his videos (see newscientist.com). Gasps filled the packed room of evolutionary biologists as they watched the comb
“Six hundred million years ago, many things happened in animal evolution, a lot of diversification,” says Hejnol. “We know that 500 million years ago, there were already animals that had an anus. So in this time frame, maybe one or several anuses evolved in different animals, but we don’t know how often it happened.” For all the mystery about when the anus and through-gut evolved, the why is much clearer. Larger animals need more fuel to grow and live, and beyond a certain gut length it becomes less efficient to cycle waste back up to the mouth. In humans, for instance, nutrients are mostly absorbed in the lower gut, so waiting for food to digest down there and then come all the way back up would be a waste of time and energy. Plus, if your digestive tract has a separate exit, you can eat a second meal while you’re still digesting the first, meaning more energy to fuel the body. The anus is truly an evolutionary marvel. “The brain is the organ that uses up most of our energy,” says Hejnol. “If we didn’t have such an efficient digestive system, we wouldn’t have a brain and we couldn’t even think about how the anus came about. Our anus allows us to think about us.” Ponder that the next time you’re on the toilet. n Chelsea Whyte knows her backside from her elbow