An overarching theme in the story of evolution, at least over the past half billion years or so, is rising complexity. There are other themes, of course, but life has undoubtedly become more complicated since its origin. Early cells globbed together to form multicellular coalitions. Those developed more complex bodies and lifestyles as the millennia passed, finding ever more varied ways to make a living. You might expect that as bodies became more complex, genomes did as well.

But a recent study appearing in Nature Ecology & Evolution shows that not to be the case — at least for jellyfish, humble organisms that evolved at a crucial juncture in animal history. They did not need more genes — or even notably different ones — to power their giant leap in complexity. This new study adds to a growing body of work that casts doubt on finding straightforward genomic signatures of the evolution of complexity.

Jellyfish sit alongside sea anemones, coral and hydra on the Cnidarian branch of the animal tree of life. The fork leading to cnidarians represents the final turnoff before animals become bilaterally symmetric, which makes them an interesting group to study because of the greater complexity that came with that later innovation. Jellyfish begin their lives much like their cousins, as sessile polyps anchored to the seafloor, scrounging for food in the passing currents. Unlike their cousins, they eventually break free and transform into a free-swimming form known as a medusa, “what we normally think of when we think of jellyfish,” said David Gold, a biologist at the University of California, Davis, who led the study.

Gold explained that the medusa stage represents a quantum leap in complexity. Medusas actively hunt plankton and navigate the water column with neural sensory structures that detect light and orientation. To go from being a stationary polyp to a floating medusa is almost akin to humans evolving the ability to swim through the air and capture birds with springy, netlike appendages.

The danger when gauging animal complexity, however, is that it’s hard to be objective about it. Mansi Srivastava, a biologist at Harvard University who studies animal complexity and was not involved in this study, cautions that how we view complexity can have more to do with us than with what we’re trying to define. Because we’re so different from sponges and jellyfish, we can miss complex metabolic pathways or other features of “simpler” animal life, leading us to conclude simplicity where there is nuance.

Still, Srivastava and Gold agree that if you tie complexity to life history, jellyfish are more complex than their cnidarian kin. But how they made this jump was unclear. “We just had no idea of what sort of genetic changes were needed to go from this more simple lifestyle to this more complex lifestyle,” said Gold. To find out, the researchers decided to sequence the genome of Aurelia, the moon jellyfish, and then compare it to those of cnidarians without medusas.

If a radical shift in life history requires a big boost in gene content, the Aurelia genome should be riddled with novel genes unique to jellyfish. Instead, Gold found that, broadly speaking, “there really isn’t a whole lot of difference between Aurelia and their relatives with simpler lifestyles.” There were some new genes, but no more than you might expect from any distinct group.

This finding was not terribly surprising to Gold because the genomes of other, much more disparate species also look fairly similar if you squint at them. Gold already had a more nuanced hypothesis: When it comes to building a body, it’s not just what genes exist that matters but also when they’re used. If you’re looking for genes that build complexity in jellyfish, then it makes sense to look for them when that complexity is manifesting, during medusa development. Gold thought that genes unique to jellyfish would be active during the transformation from polyp to medusa.

But to his surprise, that’s not what he found. New genes unique to jellyfish were no more likely to be expressed in the medusa stage, or any stage of development, than other, older genes were. “At the broad genetic level, it doesn’t seem like you need major changes in the genome to make these big changes in your life history,” Gold said.