The anus is a body part that we rarely mention. In daily life, it basically has no sense of existence, as if it is the same without it. But in fact, the role of the anus is too great. People with constipation can understand this more deeply. The pain of poor defecation is no less than that of other diseases.
But can you imagine it? When life first appeared, creatures actually did not have anus. How did the creatures at that time eliminate “stool”? How did the important part of the anus evolve?
We know that the food we eat cannot be converted into nutrients 100%. There are always some things that the body can’t use. We call these things waste. If the waste accumulates in the body and cannot be discharged, it will eventually burst the body. In the time when living things were still very small, wastes were also particles, which could be emitted to the outside world through the skin. But as living things get bigger, so does the volume of waste, and it can no longer be easily excreted, so what can we do about it? At this time, there is only one opening in the whole body of the creature, which is the mouth. Therefore, waste can only be discharged from here. We call this kind of creatures basic animals.
Sponges and sea anemones are common basal animals, and the blastopore that divides during embryonic development eventually develops into a mouth, which is also their anus. Sponges and sea anemones are not very large, they do not like to exercise, and they often bury themselves in the mud of the seabed and stay motionless all day. As the prey passed by, the sponges stretched out their poisonous tentacles to numb them and swallow them. After the food is digested, the waste goes back into the mouth again through the same passage, where it is excreted. Scientists speculate that the size of the sponges may not have grown too large in order to facilitate the transportation of waste by the same route.
‘Roughly Made’ Temporary Anal
Scientists originally thought that primitive creatures like sponges and sea anemones did not have anus, but the appearance of comb jelly completely overturned this perception. The comb jelly also belongs to the coelenterate, which is what we usually call a class of animals with a mouth but no anus. Most of the time, scientists cannot see its anus. It is unexpected that it can temporarily “create” an anus. .
In 2016, William Brown, an evolutionary biologist at the University of Miami in the United States, filmed a series of videos of ctenophores from feeding to excretion: genetically modified small shrimps, crabs and zebrafish were put into the water tank as food, The comb jellyfish devoured the meal without refusing to refuse. By tracking fluorescent proteins, scientists can easily observe the passage of food through the ctenophores. After a period of time, something that was inconsistent with the scientists’ expectations happened. Small red dots came out not from the mouth of the comb jelly, but from the small holes at the end of its body!
This small hole is thought to be the “temporary anus” of the comb jelly, an opening that only forms when a discharge is required. As waste builds up in the body, the ct’ jelly’s gut expands outward until it fuses with the epidermis, which splits open into a tiny hole it didn’t have before. When the waste is excreted through this new anus, the intestinal skin and the epidermis are separated, each returning to a separate layer, and the small holes disappear. Temporary anuses are rapidly formed and used at a high rate, with mature ctenophores producing a new anus almost every hour, while juvenile ctenophores grow an anus approximately every 10 minutes.
comb jellyfish with temporary anus
Penis worms are considered one of the oldest creatures with a formal anus
This temporary anus has the same function as the formal anus, so we know that anal-like structures evolved no later than 700 million years ago, when the ancestors of the ctenophora appeared.
After the discovery of the temporary anus in comb jellyfish, a new question emerged in the minds of scientists: According to genetic pedigree and body structure analysis, comb jellyfish are closely related to sponges, sea anemones and jellyfish, and they share a common ancestor. So why do comb jellyfish have temporary anus, but sponges and sea anemones don’t? The only plausible explanation is that 700 million years ago, the complete digestive tract and attached anus appeared in some ancient animal ancestor, and then disappeared in sea anemones and sponges, and some remnants in comb jellyfish. Later, they reappeared.
When will the anus reappear? Scientists estimate that around 500 million years ago. In Chengjiang County, Yunnan Province, China, there is a famous animal fossil group. The soft and wet geological environment has preserved all kinds of creatures in the area 530 million years ago. It was here that scientists found the earliest creatures with a formal anus. In the 1980s and 1990s, scientists found fossils of penis worms in Chengjiang, and found that it had a digestive system similar to the human body and a complete anus.
After studying the gene expression in the gut, stomach and anus of the penis worm, scientists have discovered some evolutionary patterns, the worm’s digestive system evolved in the same way as humans, fish and frogs, and the process of evolution The genes involved are also identical. Two of these genes played a crucial role in the evolution of the anus, allowing organisms to evolve a fully functional anus, while other organisms without these genes have only a “temporary” anus.
Anal changing again
But the anal weirdness doesn’t stop there. After in-depth research, the scientists found that although the worm’s anal structure looks similar to that of humans, the origin of the two is not the same. As mentioned earlier, in basal animals, blastopores develop into mouths. In the worm, the blastopore has also developed into a mouth, but at the other end of the embryo opposite the blastopore, it has an additional opening, which develops into the worm’s anus. Originally, scientists thought that the human anus was similar, but after analyzing the embryonic development, they found that the human blastopore developed into the adult anus, and the new opening at the other end developed into the mouth.
In order to distinguish these two kinds of creatures from different anal origins, scientists call worm-like creatures protostomes, and humans such as deuterostomes. In addition to humans, echinoderms such as starfish and other chordates also belong to deuterostomies. Why has the anal story changed again?
The earliest deuterostomiae found in the Chengjiang fossil group, the Cleus cyst, may be able to solve our doubts a little. It looks like a bag with only one opening, with a “big mouth” and four pairs of eyes-like cone-shaped holes above the mouth, but these are not eyes, but gill holes. prototype. You may be impatient to raise an objection. Since Schleptococcus only has one big mouth, doesn’t it mean that it has a mouth but no anus? How is it a deuterium? In fact, it is because it has those four pairs of conical holes.
Corrugated cysts shed new light on the evolution of the anus
Gill openings were important organs for gas exchange and waste excretion in early deptostomes, and when they ingested water along with food, the excess water and smaller digested waste flowed out of the gill openings. Later deuterostomes also inherited this feature. The essential structures of aquatic fish are gill openings and gill slits. Although terrestrial animals no longer rely on gills for respiration, gill slit structures still appear during embryonic development. Therefore, gill openings are considered to be the most typical feature of deutostomes, and the rudimentary worms with gill openings are classified as deuterostomes.
And the clavicle with gill openings but no anus provides us with another way of thinking. When the gill openings that can both breathe and excrete appear, the role of the anus is gradually replaced by the gill openings, so does it disappear again? And as the particles of waste became larger and larger, when the gill openings could not be discharged, the anus evolved again? These questions await further research to answer.
The anus, this small structure has undergone such a complex evolution process, until today, the secret of its evolution has not been completely solved.