Do you want to evolve? Try to get rid of a few people

The evolution of animals has fascinated researchers in many fields since Charles Darwin proposed the theory of natural selection. In one of the passages in his book he talks about the importance of embryonic development for evolution, because during this period the structures of adult organisms are formed. What he could not explain was how the changes on which natural selection acts occur. We now know that genetic material contains instructions for organisms to form, so changes or mutations in DNA are the source of evolutionary innovations.

Intuitively it may seem that complex organisms must have more genes. It is now known that complexity does not depend so much on the number of genes as on how their functioning is regulated. What few people would suspect is that sometimes the loss of genes can involve an increase in evolutionary complexity. This is what Cristian Cañestro and his collaborators, from the evolution and development research group (Evo-Devo) of the University of Barcelona, ​​have shown. They have discovered that a series of sudden gene losses freed a group of sessile animals, which lived attached to the seabed, from this dependence, and became free-living organisms. As stated in the article that the magazine Nature he pointed out on the cover, this fact has introduced an important turn in the evolutionary vision that until now had of the origin of our lineage, and at the same time has opened the door to new biomedical research.

From an evolutionary perspective, vertebrates – which include the human species – come from a group of ancestral organisms called cephalochordates. With them we share the basic structure of the neural tube, which is the embryonic origin of the brain and spinal cord. Both cephalochordates and vertebrates are free-living. However, there is another group of animals called tunicates (because they are protected by a kind of tunic they make themselves), which share the same evolutionary ancestor with vertebrates, the cephalochordates. Tunicates and vertebrates are evolutionary brothers. However, the tunicates are sessile and live attached to the sea floor. One of the best known tunicates is the ascidians, which can be seen on the rocks of various places on the Catalan coast. When juveniles they are swimming larvae, but when they mature they cling to the ground. This suggested that, originally, tunicates must also have been free-living, such as cephalochordates and vertebrates, and that at some point they became sessile as a result of evolutionary changes.

An exceptional tunic

In this work, the authors have analyzed the genome of a tunicate that, unlike the rest, maintains free life throughout the life cycle. It’s called Oikopleura dioica and is part of marine zooplankton. When they compared its genome to that of other animals, they realized that it had far fewer genes involved in the formation of the heart and pharyngeal muscles. Somehow, natural selection has favored in this species the loss of these genes, which has affected how the heart and pharynx are formed. And, in turn, these changes have allowed theOikopleura you don’t need to live stuck to the ground when you mature and become an adult.

From these data, the authors conclude that the most primitive tunicates, contrary to what was originally thought, were not free-living, but sessile, and that the most evolved are those that have the free lifestyle throughout the life cycle. In other words, the increase in evolutionary complexity has not been due to an increase in genetic complexity but on the contrary, to a simplification. Gene loss can also be a major driver of evolutionary change. Finally, the authors point out that these genetic data involved in the construction of the heart may serve to better understand the structure of the human heart and some of the pathologies that affect it.

David Bueno is director of the UB-EDU1st Chair in Neuroeducation

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