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Placozoo: A strange pancake-shaped animal creature shows the origin of thought | Science

Movement of a placozoan observed with a microscope.
Movement of a placozoan observed with a microscope.CRG

A multitude of religions from all over the planet agree on the same story of the creation of the human being, in which an all-powerful god creates women and men from nothing with a brain already well furnished. The reality is much less lyrical, as they suggest the placozoans, animal creatures measuring less than a millimeter that float in seawater like grains of sand. They go unnoticed, but they are extraordinary. Under the microscope, they are like tiny pancakes, without any organs, much less a brain. However, these strange animals are capable of coordinating to attack in group to their prey. The Spanish biologist’s team Arnau Sebé Pedros reveals this Tuesday that in the cells of these unusual beings the origin of the neurons, responsible for human thought, can already be sensed.

Placozoans have just 50,000 cells, but their abilities are surprising. They are immortal, because they can multiply indefinitely asexually. A piece of placozoan will form another placozoan. They can also be reconfigured and take other shapes other than a pancake, such as a donut or a whip. A person is made up of 30 billion of cells, with a multitude of well-differentiated types, such as neurons in the brain or red blood cells. Placozoans, on the other hand, are two simple layers of similar cells, stuck together like two slices of cheese. Its simplicity can help us understand how single-cell organisms came together and created increasingly sophisticated multicellular beings.

A placozoan observed under a microscope.Bernd Schierwater

Arnau Sebé Pedrós’ group cares for thousands of placozoans in their laboratory at the Center for Genomic Regulation, in Barcelona. According to his calculations, multicellular animals emerged about 850 million years ago. Very soon, 800 million ago, the paths of the families that gave rise to placozoans and humans diverged. “People tend to see living fossils In nature. We don’t know if our common ancestor looked like a placozoan, but some aspects of its biology were already there,” says the biologist, born in the Lleida town of La Fuliola 37 years ago.

The team has studied the four known species of placozoans cell by cell, including the first discovered, found in 1883 by the German zoologist Franz Eilhard Schulze in a marine fish aquarium in Graz (Austria). Schulze baptized that mysterious creature with the scientific name Trichoplax adhaerensfrom Greek tricho (hair and plax (plate) and from Latin adhaerens (sticky): sticky hairy plate, due to its tendency to land on algae to devour them. They are the simplest animals on the planet, except for myxozoans, parasites with few cells that cannot live on their own.

The Argentine researcher Sebastián Najle, co-author of the work, manages placozoans at the Center for Genomic Regulation, in Barcelona.
The Argentine researcher Sebastián Najle, co-author of the work, manages placozoans at the Center for Genomic Regulation, in Barcelona.Omar Jamshed / CRG

Sebé Pedrós and his colleagues have observed that placozoans are capable of coordinating their 50,000 cells thanks to the fact that some of them can send messages to each other using molecules called neuropeptides, like the neurons in our brain do. “We have found a rudiment of certain aspects of our nervous system,” celebrates the biologist. These secretory cells still lack the processes that transmit nerve impulses (axons) and the processes that receive them (dendrites) in human neurons.

The new results, published this Tuesday in the specialized magazine cellsupport the chemical brain hypothesispostulated by the Hungarian biologist GásPAR Jékely, from the University of Heidelberg, in Germany. “This study reveals deep molecular similarities between the neurons of placozoans and the neurons of bilateral animals (symmetrical ones, like humans),” says Jékely. “These similarities, together with the fact that placozoan neurons only communicate through chemical signals, support the idea that nervous systems may have first evolved as a collection of diverse, chemically connected cells, before developing specialized processes (axons and dendrites) and synapses,” says the Hungarian researcher, who has not participated in this study.

Sebé Pedrós emphasizes this idea. “Probably, to coordinate the cells of a small organism, with only two layers, it was enough to have cells secreting chemical signals. However, the moment you get big and start to gain three-dimensionality, you also need to emit electrical signals and have cell-to-cell communication interfaces, which are synapses,” explains the Spanish biologist. The neurons of a tall person, with extensions that allow a stimulus to be immediately felt in the foot, can measure more than two meters.

Image of a placozoan, obtained with a confocal microscope.
Image of a placozoan, obtained with a confocal microscope.CRG

Arnau Sebé Pedrós’ group believes that the first modern neuron did not appear until about 650 million years ago, in the common ancestor of the jellyfish group —the cnidarians—and that of human beings. The great mystery is what happened then to another branch of animals that set out on its own path 850 million years ago: the ctenophores, organisms similar to jellyfish and that also have neurons, although different. “It is still too early to say that ctenophores invented neurons independently, but I think there is more and more evidence that this could be the case,” explains Sebé Pedrós.

The German Bernd Schierwater He has been studying placozoans for decades. The Hannover Veterinary Faculty appears on its website: stuck in the sea with a tie and sunglasses, in the shallow waters where these animals usually appear. In his opinion, the new study, in which he has collaborated, confirms his “new placula hypothesis”, according to which the hypothetical last common ancestor of all animals was a two-layered disk, like the placozoans. “The original hypothesis comes from a German zoologist, Otto Bütschli, more than a century ago. Later, for political reasons and for fame and money, trees of life were published that placed the ctenophores at the base. This is absolute rubbish, but the researchers put money in their pockets,” says Schierwater.

The American scientist Carolyn Smith also has investigated placozoans, at the National Institute of Neurological Disorders and Stroke, in Bethesda (USA). Smith declares herself “excited” about the results of the Barcelona team. “It is strong support for the idea that the evolutionary precursor of neurons could have resembled the peptidergic secretory cells found in placozoans,” she celebrates.

Arnau Sebé Pedrós team (in the center, seated, with a gray shirt), at the Center for Genomic Regulation, in Barcelona.
Arnau Sebé Pedrós team (in the center, seated, with a gray shirt), at the Center for Genomic Regulation, in Barcelona.CRG

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