Around 700 million years ago, a group of organisms that look like glowing, gelatinous blobs split off from other animals, possibly forming the earliest branch in the animal family tree. These organisms, known as ctenophores or comb jellies, are not related to jellyfish despite their similar appearance. Today, there are nearly 200 species of ctenophores found in a wide range of environments, from the deep, cold ocean to the warm, shallow coastal waters. Their unique DNA has provided important insights into fundamental biological processes, such as the evolution of nervous systems and the ability to produce light, known as bioluminescence. By studying closely related species that live in different environments, scientists can explore how certain traits evolved, such as adaptations to high pressure or the development of light-sensing genes. “They’re very exciting to work with, and they’re also extremely beautiful organisms,” said Pawel Burkhardt, an evolutionary biologist at the University of Bergen. For over a century, scientists believed that sponges, or porifera, were the first animals to branch off from the rest of the animal kingdom. However, in the past two decades, new evidence has suggested that ctenophores may have split off earlier. In 2023, a major study analyzing the structure of chromosomes provided strong support for this idea, indicating that ctenophores, not sponges, are the earliest branching group. However, this conclusion is still being debated by scientists. What makes this surprising is that sponges lack both muscles and neurons, while ctenophores have muscles and show signs of a basic 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.” As scientists continue to study comb jellies, they are discovering more about their complexity and the origins of animal life. Some species have been observed reversing their development from adult to larval stages, a rare phenomenon in the animal kingdom. Others have special types of lipids that allow them to survive in the extreme pressure of the deep sea. These findings are helping scientists understand how more complex body structures evolved over time. “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. Understanding the biology of ctenophores can provide insights into the fundamental processes that shape all life, including humans.