To most people, “evolution” often means progress. A particularly sensitive or thoughtful person, for example, is “evolved.” We imagine natural selection as a gradual progression from amoebas to humans. But biologists have long known that organisms can just as easily lose features and complexity over time.

Now, research led by Prof. Dorothée Huchon of Tel Aviv University has found

that a close cousin of the jellyfish has evolved into a microscopic parasite. Although “regressive” evolution is well-known to science — certain species of cave-dwelling fish lost eyesight and snakes are believed to have once possessed feet — “the myxozoa [as these parasites are known] are indeed an extreme case,” Israeli geneticist Eva Jablonka of Tel Aviv University told The Times of Israel.

Huchon and her international team sequenced the genome of myxozoans, microscopic parasites that infect fish. They discovered that the organisms are highly degenerated cnidarians — the phylum that includes jellyfish, corals and sea anemones.

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Huchon said that the idea that myxozoans had a jellyfish ancestor was hypothesized before but that her team has proven it. In other words, this parasite — which is unicellular for much of its life — lost a head, nervous system and cell communication in its evolutionary journey. She added that most life on earth is unicellular and that it has “only happened several times in evolution that organisms became multicellular.” For multicellular organisms to evolve into unicellular organisms is even rarer.

The closest relative of the myxozoans is the Polypodium hydriforme, a parasite that infects caviar eggs, Huchon said. The organism goes out and lives on its own then returns to infect an egg. She believes that the myxozoans were also such parasitical jellyfish that over time evolved to remain close to their host. Parasites, she noted, tend to be more efficient if they have a small genome, so they lose features that are not important.

Jablonka, who specializes in genetics and the philosophy of biology, said Huchon’s research is interesting because myxozoans contain only a few nerve cells and no muscles.

“Usually nerves and muscles go together, but the myxozoans lost their muscles. This is very interesting to biologists,” she said.

In fact, Jablonka added, there is a claim that nerves evolved not just once but on two different occasions. Some creatures, like sponges, are believed to have once had a brain and lost it, she pointed out.

In terms of the idea that evolution is a progressive process, Jablonka called this a “naive intuition” with origins in the 18th century.

“The idea that we are improving over time and that the meaning of improving is growing in complexity is naive. Anyone who has thought about this systematically since Darwin and Lamarck has understood that this is not true,” she said. “When you look at the evolutionary tree, you see all kinds of directions. You see regression, you see progression, you see slight progression, you see leaps.”