The first evidence of Bilateria in the fossil record comes from trace fossils in Ediacaran sediments, and the first bona fide bilaterian fossil is Kimberella, dating to the 550 million year mark. This ancient creature represents a pivotal moment in evolutionary history, marking the transition from simple radial symmetry to a complex body plan that would eventually dominate the animal kingdom. Nearly all bilaterians maintain a bilaterally symmetrical body as adults, with a distinct front end, or head, and a rear end, or tail, alongside a left-right symmetrical belly and back. This arrangement allows for directional movement, where the front end encounters stimuli such as food, favoring the development of cephalization. Cephalization is a characteristic feature among most bilaterians, where sense organs and central nerve ganglia become concentrated at the front end of the animal. The hypothetical most recent common ancestor of all Bilateria is termed the urbilaterian, and the nature of this first bilaterian remains a matter of intense debate among scientists. One side suggests that acoelomates gave rise to the other groups, meaning the urbilaterian had a solid body, and all body cavities therefore secondarily arose later in different groups. The other side poses that the urbilaterian had a coelom, meaning that the main acoelomate phyla have secondarily lost their body cavities. This debate centers on whether the original bilaterian was a marine worm somewhat like Xenoturbella or if it resembled the planula larvae of some cnidarians. Fossil embryos are known from around the time of Vernanimalcula, but none of these have bilaterian affinities, leaving the exact identity of the urbilaterian shrouded in mystery.
The Great Divergence
Bilaterians constitute one of the five main lineages of animals, the other four being Porifera, Cnidaria, Ctenophora, and Placozoa. They rapidly diversified in the late Ediacaran and the Cambrian, and are now by far the most successful animal lineage, with over 98% of known animal species. This success is largely due to the triploblastic nature of bilaterian embryos, which have three germ layers: endoderm, mesoderm, and ectoderm. With few exceptions, bilaterian embryos are triploblastic, having three germ layers, and have complete digestive tracts with a separate mouth and anus. Some bilaterians lack body cavities, while others have a primary body cavity derived from the blastocoel, or a secondary cavity, the coelom. The traditional division of Bilateria into Deuterostomia and Protostomia was based on whether the blastopore becomes the anus or mouth. In protostomes, the first opening of the embryo becomes the mouth, while in deuterostomes, it becomes the anus. The deuterostomes traditionally include the echinoderms, hemichordates, chordates, and the extinct Vetulicolia. The protostomes include most of the rest, such as arthropods, annelids, molluscs, and flatworms. Many taxonomists now recognize at least two more superphyla among the protostomes, Ecdysozoa and Spiralia. The arrow worms have proven difficult to classify, with studies published in 2004 and 2017 placing them in the Gnathifera. This complex web of relationships highlights the dynamic nature of bilaterian evolution, where new evidence constantly reshapes our understanding of the tree of life.