Nearly every animal you know shares one ancient body plan
Bilateria includes over ninety-eight percent of known animal species—everything from worms to whales built around a head, tail, and left-right symmetry. Yet scientists still cannot agree whether the first bilaterian was a bottom-dwelling worm, a planula-like larva, or a segmented ancestor with a mouth at one end.
Bilaterians develop with bilateral symmetry: a front and back, left and right, belly and dorsal surface. Nearly all maintain that symmetry as adults; echinoderms are the famous exception, pentaradial as adults but bilateral as embryos. Most have three germ layers—endoderm, mesoderm, and ectoderm—and a through-gut with separate mouth and anus. Cephalization concentrates sense organs and nerve ganglia at the head end, an advantage for animals that move directionally.
The hypothetical urbilaterian ancestor remains disputed. One camp argues acoelomate worms came first and body cavities arose later; the Archicoelomata hypothesis, first put forward by A. T. Masterman in 1899, instead proposes an original coelom, with flatworms secondarily losing theirs. Competing reconstructions include a Xenoturbella-like bottom dweller, a cnidarian planula larva, or a segmented form—supported by shared segment-building mechanisms in vertebrates and arthropods. Bilaterians share more Hox developmental genes than cnidarians or acoelomorphs.
Fossil evidence is sparse and contested. Trace fossils appear in Ediacaran sediments; Kimberella at 555 million years ago is the first accepted bilaterian body fossil. Vernanimalcula at 580 million years may be earlier—or merely an infilled bubble. Traditionally bilaterians split into protostomes (mouth first) and deuterostomes (anus first), but molecular studies upended this: acoelomorphs were reclassified as Xenacoelomorpha, sister to all other bilaterians. Karl Grobben renamed Hatschek's Zygoneura to Protostomia in 1910 and grouped deuterostomes separately. As of 2024, whether Xenambulacraria or the classic dichotomy is correct remains unresolved—with support values too low for certainty. Nervous systems vary just as much: some bilaterians keep loose nerve nets much like a jellyfish's, others run a nerve cord along the belly or the back, and hemichordates carry both.
Source: Bilateria
