HASlet’s admit it straight away: we had never heard of placozoans. Despite dozens of chronicles devoted to animal life, neither the name nor the very existence of this creature was known to us. Had we even encountered it, we would most likely have rejected it from the animal kingdom. Imagine: here is a creature measuring 1 to 3 millimeters, exclusively marine, which lives clinging to rocks or on the surface of algae. Rounded or elongated, it has no defined shape, except that it is flat. If we had to compare it to a known living being, we would happily lean towards an amoeba.
Except that it is indeed an animal. Far from the previously named unicellular, placozoans harbor thousands of cells. Like all animals, they have epithelial cells – those of our epidermis – and move actively, using cilia. No respiratory or digestive system, however. To feed, they stick to pieces of algae, releasing enzymes which will degrade their target. Then they absorb it by phagocytosis. In other words, they digest before swallowing.
The simplest animal in existence, in truth. And one of the most poorly known. Discovered in 1883 at the Graz Aquarium, Austria, Trichoplax adhaerens was until recent years considered the only species of the phylum Placozoa. The branching, however, is the first level of separation of the tree of life after the kingdom. Each branch therefore has hundreds, thousands, sometimes millions of species. There was only one there. Over the past ten years, three new ones have been described and a few others are in scientists’ pipeline. With one last particularity that placozoans only share with the sponge group: the absence of a nervous system.
A very particular cell type
But here it is: in an article published on September 19 in the prestigious review cell, a team of Spanish and German scientists has just announced that the tiny pancake, or rather its ancestor, from which we separated around 800 million years ago, could have provided us with the components of our future neurons. To provide this result, Arnau Sebé-Pedros’ team, at the Genomic Regulation Center of the Barcelona Institute of Science and Technology, carried out particularly meticulous analysis work: not just sequencing the genome, but characterizing the expression of all genes in each cell. She thus managed to highlight nine major cell types. “An impressive work, both in the quantity and quality of the data generated and the cutting-edge techniques usedgreets Michaël Manuel, professor of animal biology and evolution at Sorbonne University. And then there is their discovery. »
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