Summary
Around 700 million years ago, a group of organisms resembling little more than glowing, gelatinous blobs split off from the rest of the animals, forming possibly the earliest branching animal lineage. Nearly 200 species of ctenophores, commonly known as comb jellies (but unrelated to jellyfish), live today in environments ranging from the cold depths of the sea to warm coastal surface waters. Their magic isn’t just in their persistence or iridescence; it’s in their DNA.
Over the past decade, ctenophores have helped answer long-standing questions about fundamental biology, from how early nervous systems evolved to the origins of the mesmerizing phenomenon of bioluminescence.
Having access to closely related species across such variable environments “lets you ask questions about how certain things evolved,” such as adaptation to high pressure or light-sensing genes, said Steven Haddock, a marine biologist who studies ctenophores at the Monterey Bay Aquarium Research Institute.
“That’s one of the reasons why we work with ctenophores,” said Pawel Burkhardt, an evolutionary biologist at the University of Bergen who studies the origins and evolution of neurons and nervous systems. “They’re very exciting to work with, and they’re also extremely beautiful organisms.”
For more than a century, scientists thought that sponges, or porifera, were the first to branch off — the sister group to all other animals. But over the past two decades, evidence has emerged that ctenophores were earlier. In 2023 — after years of a “ping-pong game” between labs debating which group came first, Burkhardt said — a landmark paper analyzing chromosome organization found that ctenophores, not sponges, are the sister group, though this is yet to be fully settled.
How the phylogenetic tree looks under the Ctenophora-sister hypothesis.
Kristina Armitage/Quanta Magazine
What makes this all the more surprising is that sponges lack muscles and neurons, while ctenophores have muscles and exhibit evidence of a simple nervous system. “If you think about the earliest branching animal lineage, you would expect less complexity,” Burkhardt said. “That changes a lot of the assumptions [about] how the very first animal may have looked.”
The more researchers investigate comb jellies, the more complex they appear and the more we learn about the origins of animal life. Some species have recently been observed reversing their development from adult to larval stages. Others have special types of lipids that help them withstand extreme pressure in the deep sea. They hold clues to the evolution of more and more complex body shapes.
It’s really important to study organisms that might seem strange or weird because they can tell us a lot about the physical, chemical, and biological principles of life, which can then be applied to ourselves, said Itay Budin, a biophysicist who studies cell membranes at the University of California, San Diego. “We are as distantly related to a ctenophore as a ctenophore is to a jellyfish.”
Nervous systems are typically defined as networks of neurons that communicate across synapses. But Burkhardt’s lab found something unique in ctenophores: Beneath the animal’s outer surface is a nerve net (pink mesh in this 3D reconstruction) whose neurons are connected by continuous cytoplasm — but with no synapses between them. “I don’t think any other animal has a nervous system like that,” Burkhardt said. Because ctenophores are one of the earliest branches of the animal family tree, the finding indicates that evolution may have crafted nervous systems twice: in ctenophores, and separately in jellyfish and all other animals.
Pawel Burkhardt
During embryonic development, cells arrange themselves into distinct morphologies due to a blastopore: an indentation that ultimately becomes the anus or mouth (central area pointing upward in this image of M. leidyi). The same developmental process for embryos in ctenophores is also found in bilaterians, the large group of animals that share our bilateral symmetry. This indicates that the blastopore organizer — which turns a mere ball of cells into a complex, multicellular embryo — is conserved across species. “Understanding where the organizer came from tells us which parts of our development are ancient and robust, and which are recent inventions,” said Andreas Hejnol, an evolutionary biologist at Friedrich Schiller University Jena in Germany. Human embryos use the same signaling pathways that Hejnol and his colleagues found in ctenophores.
Lisa-Marie Barf