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. Recently, 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. 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, said evolutionary biologist Paul Burkhardt — a landmark paper analyzing chromosome organization found that ctenophores, not sponges, are the sister group, though this is yet to be fully settled. 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. 🪼 Explore the full gallery: