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Human ancestors looked like this?!
Written by Seven Kings
The brain tells us that the brain is the most important organ of the human body, and that the intelligent animals on the earth have brains. But there is a class of animals that eat their brains when they become adults. Researchers believe that this brainless creature hides the secrets of all the ancestors of vertebrates, including humans.
Sea squirt clavelina moluccensis. Image source: britannica
Life on land comes from the ocean, and close relatives of human ancestors also live on the ocean floor. If you dive in the sea, you probably won't associate this creature with a brain, which is a tunicata.
The adult sea squirt looks like a "rectum" blowing water, and the sea water enters from one hole and comes out of another, and survives by filtering the organic matter in the sea water. It's hard to imagine that a chain of mallets with pestles on the ocean floor or floating in the water turned out to be the closest thing to humans among marine invertebrates: the sea squirt belonged to the urochordata, and the vertebrates shared a common ancestor.
In fact, sea squirts not only have primitive hearts and primitive blood cells, but also respond to touch. Sea squirts also have primitive muscular systems that help them control the shape of the body and switch in and out of the water. If you fish sea squirts out of the water, they will spray you in the face, earning you the realistic honor of sea squirt. However, the people of Shandong will disagree with this title, and in the context of Lu Cuisine, the hydrodynamics of the sea squirt after it comes out of the water have earned it the nickname of "sea tits".
Sea squirts water
Sea squirts are also the only animals capable of producing cellulose. Their appearance has puzzled humans for thousands of years. At first, Aristotle thought of sea squirts as mollusks similar to conchs. Later, in the 19th century, Lamarck believed that sea squirts were echinoderms, and sea cucumbers were related to them.
It was not until 1866 that the tadpole-like larvae of the sea squirt were discovered, and their association with vertebrates became apparent. It turns out that sea squirts had brains when they were young, but when they grew up, their brains degenerated.
Comparison of sea squirt larvae (bottom) and tadpoles (top). Image source: wikiwand
When the sea squirt was a child, it looked like a tadpole, could swim, did not eat, and did not fight wild. The body of the little sea squirt has a very simple dorsal nerve tube, similar to the spine. Near the head is a ganglion that functions like a brain, and the small head can sense light and gravity, which can help it find shelter and avoid predators. The small sea squirt even has eye-like eye spots.
The process of benthic sea squirt phallusia nigra from larval to adult. Image credit: centro de biologia Marinha - cebimar/usp
Usually after a few days of floating, the small sea squirt will quickly retrograde metamorphosis: within a day, the original dorsal nerve tube, eyes and most of the tail will be absorbed, most of the brain will disappear, and finally retain a ganglion that connects the mouth and the heart, and whose main function is to control eating. In this way, they went from cute people shaking their heads to headless monsters eating and pulling overnight.
It should be pointed out that there is a type of sea squirt that still maintains most of the brain and tadpole form after adulthood, and the head also has a structure that senses light and gravity, which is the tail sea squirt (appendicularia). The tail sea squirt is also known as the juvenile squirt because it resembles other sea squirts when they were young.
Because the tail sea squirt still retains its childhood form in many sea squirts, and most of the sea squirts are developing towards proctology, many scholars in the early 20th century began to speculate whether the tail sea squirt was a sea squirt that did not want to grow up. Further, could the ancestor of a vertebrate be a small sea squirt that did not want to grow up? The little sea squirt that refused to grow up insisted on keeping his brain from degenerating, and eventually became a human being from a small "tadpole", so as to be familiar with the story of nature.
One of the most famous exponents is walter garstang, a marine biologist at Lincoln College in Oxford, a hypothesis known as paedomorphosis that was written into numerous biology textbooks in the 20th century.
But then in the 1980s, new genetic evidence overturned that view. We now know that sea squirts are close relatives of vertebrates, but not ancestors.
During the same period, more alarming reversals also occurred.
Under a transmission electron microscope, sea squirts attached to the seafloor (ascidiacea) are not quite the same as vertebrates in that their eggs do not have the cortical particles that most vertebrates have, and sperm do not have the apical particles that most vertebrates have. Surprisingly, the "reluctant to grow" tail sea squirt retains these features, and they look more similar to vertebrates.
That is to say, the tail sea squirt is not that it does not want to grow up, perhaps it is a more ancient form in itself; other brainless simplified versions may be new.
And these new discoveries have made the common ancestor of vertebrates and sea squirts even more mysterious: What exactly did this ancestor look like, more like a tail sea squirt with a head and tail, or a sea squirt that other brains don't want? This mindless question may make humans unhappy for a long time, because the fossils left by boneless sea squirts are so rare.
Then again, the sea squirt's skill tree is not only brainless, but also has very special blood.
In 1911, german chemist Martin Henze discovered that certain sea squirts contain a rare metal: vanadium.
It turns out that some sea squirt blood cells contain vanabins, not iron-containing hemoglobin like we do, and this blood containing vanadium-binding protein is green.
A sea squirt enriched with vanadium in the body phallusia nigra. Image credit: inaturalist
For example, this solitary sea squirt phallusia nigra contains vanadium. Its scientific name is not only figurative, but also very insulting: phallusia comes from the Greek phallos, which means tintin; nigra means black.
The concentration of vanadium in the body of a sea squirt that can use vanadium can reach tens of millions of times the concentration of this metallic element in seawater. Vanadium compounds are highly toxic to humans and many organisms, so sea squirts may use vanadium-containing blood to make themselves unpalatable.
Sea pineapple (halocynthia roretzi) sashimi. Image source: wikipedia
Still, humans, who are good at turning evolutionary trees into recipes, have found some varieties to eat in so many unpalatable sea squirts. In South Korea and Japan, there is a sea squirt called halocynthia roretzi that is a local specialty, and there are also farms for sea pineapples.
Sea pineapple farm in South Korea
When it comes to eating, even if a piece is eaten, the sea squirt has a super resilience, and this resilience is also related to their blood.
In 1964, Gary Freeman, a marine biologist at the University of Chicago, conducted an experiment in which he gave sea squirts gamma rays that deprived their cells of the ability to divide and sprout, or repair.
He then injected the unresponsive sea squirts with the same species of sea squirt blood cells that had not been irradiated, and as a result, the irradiated sea squirts regained the ability to repair the body, proving that the strength of the sea squirt's body to repair was hidden in the blood.
Perhaps it is these strange skills that have allowed the brainless sea squirt to survive until now, and it is all over the world.
Sea Squirt: Is my head dirty? Human: Like.
Further reading:
According to the principle of probability, you can't | at first glance Bring science home
People who throw garbage without sorting are creating the "poison of the century" | bringing science home
Referees who don't understand physics have caused the classic Unjust Case of the World Cup, and physicists discovered the strange movement of the ball 50 years ago| Bring science home
The world's first international vaccination campaign relied on 62 children | who acted as containers for the virus Bring science home
Background: The article was published on February 28, 2021 on the WeChat public account Bringing science home (after growing up, it sucked out its own brain, this animal was once regarded as the primitive ancestor of human beings by mainstream textbooks). The author authorizes the reprint of The Voice of the Storm. Editor-in-Charge: Xinyue Chen