fabian s. klinke

How Animals Got Butts

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How Animals Got Butts

Source: How Animals Got Butts, PBS Eons, 14:19, uploaded 2024-11-19, category Science, playlist index 418.

In 2016, scientists watched a video of a comb jelly and saw something that should have been ordinary. The translucent animal had eaten gene-edited fish that glowed in the dark. When digestion finished, two openings appeared at its rear and luminous waste came out. The observation unsettled the accepted story about when animals evolved anuses, because comb jellies belong to an ancient lineage that researchers had assumed used one opening for both eating and excretion.

A comb jelly with an exit

Comb jellies are gelatinous marine animals from the phylum Ctenophora. The source places their lineage among the oldest animal groups, with relatives likely drifting through the oceans at least 600 million years ago. Researchers had studied them for centuries and had treated their digestive system as a sac with one opening. Food entered, digestion took place, and the remains left through the same route.

That arrangement appears in jellyfish, sea anemones, and some flatworms. Adult giant tube worms take a different route altogether: bacteria living inside their bodies supply their nutrients, so the worms have no digestive tract. A sack-like gut remains useful for a small animal with modest energy needs, although it creates a queue. Each meal has to finish before another one can enter, and one chamber cannot assign different stages of digestion to separate regions with their own microbes and enzymes.

A through-gut solves those limits with a tube. Food enters at one end, nutrients move through several sections, and waste leaves at the other. The source treats this separation as an important condition for animals that grow larger and handle more energy. It also gives the arrangement its common name: the birth of the butthole.

The old question is the order of events. Scientists still have to work out when the through-gut appeared and which anatomical change produced it. The clues come from embryology, living animals, rare fossils, and the cells and genes that build digestive openings.

The embryological story

The first major attempt came from a German zoologist in the late nineteenth century. His theory of recapitulation held that an animal’s embryonic development replayed the stages of its evolutionary history. Embryos could therefore reveal the past. Later researchers failed to reproduce his drawings, which weakened the story without settling the underlying question.

Scientists then used the velvet worm as a model. Its embryo begins as a rounded body with a first opening in the middle, called the blastopore, although the source notes that researchers disagree about whether the opening qualifies as a true blastopore. As the embryo lengthens, the opening becomes a groove. One end draws in water and microscopic food, while the other expels it. The groove pinches in the middle and becomes two holes. The gut folds inward at the pinch and turns into a tube.

This model made the anus a new opening produced by a division in an older one. The source places the first examples somewhere between 600 and 540 million years ago, before the emergence of bilaterians with distinct left, right, top, and bottom sides. A 1908 observation complicated the model. In several animals, the blastopore stays as one opening during development. It becomes the mouth in some lineages and the anus in others, while a second opening grows until it meets the first.

That embryological difference gave scientists two categories. A protostome develops the mouth first. A deuterostome develops the second mouth, meaning the anus, first. The video captions reverse two examples, and the description corrects them: earthworms are protostomes, whilst starfish are deuterostomes. Humans are deuterostomes as well.

The categories encouraged a flexible account of the anus. Evolution could produce an opening when a lineage benefited from one and remove it when its way of life made the opening less useful. The marine worm Ramisyllis multicaudata offers an extreme example. It has a branched gut with hundreds of openings along its body, a structure the source links to its life strategy. An acoel flatworm has lost its anus even though some of its ancestors had one.

Other hypotheses moved the starting point elsewhere. A German scientist in the 1940s proposed that the digestive tract connected to a genital opening that already existed. An Austrian zoologist suggested that the sac-like gut once also held ovaries, with the through-gut and genital tract developing together as the ovaries became more complex. These ideas help account for the cloaca in birds and reptiles, where reproduction and waste use one opening. They also leave the main question open: did one hole split into two, or did one hole emerge after the other?

A linear history breaks open

Despite their differences, the main hypotheses shared a sequence. Animals began with a sac-like gut. More complex animals evolved a tube. The through-gut then helped animals grow, diversify, and develop more demanding bodies. The account made complexity look like a direction in evolutionary time.

The comb jelly observation breaks that sequence. An animal from one of the oldest lineages apparently had a through-gut already. Its presumed simplicity had hidden an arrangement that scientists associated with later, more complex animals. A biologist had suggested in 1850 that comb jellies might have a through-gut, yet the idea lost support because no one saw waste leave through anus-like pores.

The earlier observations may have shown a different process. When comb jellies expelled material through the same opening used for food, researchers assumed that the animal was excreting. The source says the jellies were probably vomiting, perhaps because experiments fed them too much or gave them food they did not accept. The fish experiment showed a separate event: the animal digested the food and then expelled waste from two openings.

This puts pressure on the claim that anuses evolved when animals became larger and more sophisticated. It also weakens the idea that the anus itself enabled the later expansion of animal bodies. The old model had treated evolution as a climb from simple to complex, so it missed an organ that appeared in an animal placed near the beginning of that climb.

Many ways to have a butthole

The source proposes a different starting point for the investigation. Through-guts may have been the simplest digestive arrangement available to some early animals, whilst sea anemones and jellyfish may have closed an opening later when it stopped helping their survival. Separate lineages may also have evolved anuses independently, through different developmental routes and at different times.

The living examples make the category unstable. Some animals form the mouth and anus together. Some form the mouth first, and some form the anus first. Ramisyllis has hundreds of openings. Other animals have two, one, or none. An anus may therefore be a shared anatomical solution, a family of solutions, or a label applied to structures that only partly share an origin.

Molecular evidence may begin to sort those possibilities. Researchers can now compare the cells and genes that form digestive openings in different animals. The episode says that they still do not know whether comb jellies activate the same genes used by other animals when they develop an anus. A different genetic programme could produce a structure that performs the same job without sharing the same evolutionary history.

The fossil record offers little help for the oldest part of the story because gelatinous animals fossilise poorly. Modern microscopy, developmental biology, and molecular comparison can fill some of that gap, although the source leaves the results unfinished. Its conclusion is a working principle rather than a final origin story: evolution finds arrangements that suit particular animals and their lives. The history of the butthole may contain several beginnings, and its definition may depend on anatomy, development, and ancestry at once.

Limits of the source

This note follows PBS Eons’ synthesis and keeps its dates, examples, and hypotheses attached to the episode. The video’s description supplies a correction for the protostome and deuterostome examples and links a references document containing the primary study, researcher interviews, and further sources. I have not independently checked each paper or reconstructed the developmental history from those sources. Claims about the exact origin of the anus, the genes used by comb jellies, and the evolutionary status of particular openings remain open within the episode’s account.

Further reading / references

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