The ancestors of comb jelly are earliest multicellular animals
When algae evolved into multicellular organisms, they gained sufficient nutrition and resistance to predation. This put survival pressure on the protozoans that relied on phagocytosis for survival. It is generally believed that multicellular animals appeared long after plants. The analysis of DNA sequence shows that approximately 700 to 800 million years ago, cells came together to form the first multicellular animals on Earth, which fed on algae or protozoans. Recent research indicates that the ancestors of comb jellies are the earliest multicellular animals in the animal evolutionary tree. Later, they split into two branches. One branch evolving into the current comb jellies. Almost all animals are descendants of the other branch, including sponges, which were once considered the earliest multicellular animals but now are ranked after the ancestors of comb jellies.
The early multicellular animals were not only small in size but also lacked cell walls and mineralized skeletons, leaving almost no fossil evidence. Their appearance remains unknown, but researchers can speculate their evolutionary processes from existing organisms.
Choanoflagellates and Multicellular Animals
Through genetic sequence analysis, researchers have discovered that the closest relatives of animals are choanoflagellates. They possess a long flagellum surrounded by a collar-like structure formed by sticky microvilli. When they wave their flagella, water flows through the collar, and microorganisms get trapped on the microvilli.
The first step in the evolution of multicellular animals might have involved substances that stick to cells. Modern choanoflagellates contain genes for both cadherin and collagen, which are used for cell connections, as well as genes of tyrosine kinases for cell-to-cell communication. They anchor to the seabed individually or in colonies. Choanoflagellates are on the surface of colonies, and the interior contains amoebae. They are connected by a gel-like matrix. They are considered as an intermediate between unicellular and multicellular animals. The ancestors of comb jellies probably had a similar colonial structure. Eventually the amoebae inside the colonies became a digestive cavity, and they evolved into filter-feeding soft-bodied animals seen in the Ediacaran biota.
Ediacaran Biota
The oldest fossils of multicellular animals appeared approximately 600 million years ago in the Ediacaran biota (635-538 million years ago). Most of these animals had soft bodies and lacked skeletons. They had no mouths or anuses and had flat bodies, some resembling leaves and others flat discs. It's possible that before this time, there weren't many consumers in the ecosystem; microorganisms were not only abundant in seawater but also formed thick microbial layers covering the seafloor. Therefore, these animals didn't develop muscles and nerves for movement; they simply crawled or stood upright in the shallow seafloor, much like plants, to absorb endless food. They completely gave up their ability to move and evolved into larger, flatter forms. Their wrinkled bodies increased the surface area to make them more efficient in extracting oxygen and food particles from seawater. This peaceful underwater scene, where creatures didn't compete with each other, is known as the "Ediacaran Garden."