A strange animal creature shaped like a pancake shows the origin of thought

Many religions around the world agree on the same story of the creation of the human being, in which an almighty god manufactures women and men from nothing with a brain already well furnished. The reality is much less lyrical, as suggested by placozoans, animal creatures smaller 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, let alone a brain. However, these strange animals are able to coordinate to attack their prey in a group. The team of Spanish biologist Arnau Sebé Pedrós reveals on Tuesday that in the cells of these unusual beings the origin of neurons, responsible for human thought, is already intuited.

Placozoans have only 50,000 cells, but their abilities are amazing. They are immortal, because they can multiply indefinitely asexually. A piece of placozoan will form another placozoan. They can also be reconfigured and take on shapes other than the pancake, such as a doughnut or a whip. A person is composed of 30 trillion 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, glued together like two slices of cheese. Its simplicity can help us understand how single-cell organisms came together and created increasingly sophisticated multicellular beings.

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

Arnau Sebé Pedrós' group cares for thousands of placozoans in his laboratory at the Centre for Genomic Regulation in Barcelona. According to his calculations, multicellular animals arose about 850 million years ago. Very soon, 800 million years ago, the paths of families that gave rise to placozoans and humans were separated. "People tend to see living fossils in the wild. We don't know if our common ancestor resembled 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 cell by cell the four known species of placozoans, including the first discovered, found in 1883 by German zoologist Franz Eilhard Schulze in a marine fish aquarium in Graz (Austria). Schulze baptized that mysterious creature with the scientific name Trichoplax adhaerens, from the Greek tricho (hair) and plax (plate) and from the Latin adhaerens (sticky): sticky hairy plate, for its tendency to land on algae to devour them. They are the simplest animals on the planet, with the exception of myxozoans, parasites with few cells that cannot live on their own.

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

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

The new results, published Tuesday in the journal Cell, support the chemical brain hypothesis, postulated by Hungarian biologist Gáspár Jékely of the University of Heidelberg in Germany. "This study reveals profound molecular similarities between neurons in placozoans and neurons in bilateral animals [symmetrical, such as humans]," says Jékely. "These similarities, together with the fact that placozoan neurons only communicate using chemical signals, support the idea that nervous systems could first evolve as a set of chemically connected diverse cells, before developing specialized processes (axons and dendrites) and synapses," says the researcher, who was not involved 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 the synapses," says the Spanish biologist. The neurons of a tall person, with extensions that allow you to immediately feel a stimulus in the foot, can measure more than two meters.

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 humans. The great mystery is what happened then to another branch of animals that went its own way 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 have been the case," explains Sebé Pedrós.

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