A hot-off-the-presses article in Science describes the discovery of bilaterian fossils in the Doushantuo Formation, from roughly 570 million years ago.

Ten phosphatized specimens of a small (<180 µm) animal displaying clear bilaterian features have been recovered from the Doushantuo Formation, China, 40 to 55 million years before the Cambrian. Seen in sections, this animal (Vernanimalcula guizhouena gen. et sp. nov.) had paired coeloms extending the length of the gut; paired external pits that could be sense organs; bilateral, anterior-posterior organization; a ventrally directed anterior mouth with thick walled pharynx; and a triploblastic structure. The structural complexity is that of an adult rather than larval form. These fossils provide the first evidence confirming the phylogenetic inference that Bilateria arose well before the Cambrian.

This is exciting news, not because it revolutionizes our understanding of evolutionary history, but precisely because it is nothing surprising at all—we expect, from molecular/phylogenetic evidence, that complex animal life arose long before the Cambrian 'explosion', and what these fossils represent is a satisfying confirmation of that expectation (and they neatly fit predictions about bilaterian evolution that Erwin and Davidson made in 2002). It is actually expected, though, that bilaterian coelomates are even older than the 570 million years of the Doushantuo Formation; the last common ancestor of protostomes (arthropods and others) and deuterostomes (vertebrates and others) is estimated to have lived somewhere between 600 and 1200 million years ago.

Here's a section through the little guy:

Image of fairly well-preserved specimen of the bilaterally organized fossil animal Vernanimalcula guizhouena. Left panel shows digitally recorded, transmitted light image of a section about 50 μm thick, which had been ground from larger rock samples, mounted on a slide, and viewed through a light microscope. Right panel shows a color-coded representation of the image on left. These were prepared by digital image overlay: yellow, external ectodermal layer; ochre, coelomic mesodermal layer; red, surface pits; mauve, pharynx; light tan, endodermal wall of gut; gray-green, lumen of mouth; dark gray, paired coelomic cavities; lighter gray, lumen of gut. The scale bar represents 40 μm in A. Holotype specimen, X00305: slightly tilted, almost complete ventral level coronal section, passing through the ventrally located mouth.

Note the size. This organism was only between a tenth and two-tenths of a millimeter across. And here's a reconstructed model of what this tiny little dab of an animal looked like:

Three-dimensional reconstruction of Vernanimalcula. The model was constructed using FormZ, version 4.0, a software from Auto-Des-Sys.Inc, and using proportions derived from the holotype specimen in Fig. 1 A. A and B display computed external views of the reconstruction, C-F, computed sections. (A) Perspective view indicating planes of section shown in (C-F), as indicated by the colored dots. (B) Dorsal view. (C) Perfect coronal section. (D) Slightly tilted coronal section similar to the image shown in Fig. 1 A. (E) Transverse section. (F) Sagital section. (C-F): Yellow indicates ectoderm; ochre, coelomic mesoderm; and light tan, endoderm.

It doesn't look like much, does it? There are several important things to notice in the fossil, though. One is the bilateral organization: the mouth is on one end, and the creature definitely has a front and back. This is no jellyfish. It's also a triploblast, which means it primitively possesses three body layers, an ectoderm (skin and nervous system) and endoderm (gut), with mesoderm (muscle and connective tissue) sandwiched in between. We can also tell it's significantly more complex than a jellyfish because of those dark gray spaces bounded by orange cells in the diagrams above. That's a coelom. Coeloms are a very big deal—they are one of the major defining characteristics of us complex metazoans, and they represented a huge breakthrough in animal evolution.

A coelom is simply a fluid-filled cavity that forms inside the layer of mesoderm. We have one, too: it's the space in your torso within which your organs are sloshing around. The mass of mesoderm in our embryos splits, part wrapping around the gut to form the muscular and connective tissue layers of those organs, and part forming the muscles and connective tissue of the body wall, with thin membranes bridging the two, the mesenteries.

We tend to take our coelom for granted—it's just a space, after all—but in many animals, that space is their skeleton. Think about it. It's a fluid-filled space surrounded by muscle, and when the muscles contract, it can push the fluids around...if it squeezes in one place, the fluid has to go somewhere else, where it can push and distend the body. This is the basis for a hydrostatic skeleton, just the thing for small animals, like earthworms or Vernanimalcula, to move around and burrow using internal hydraulics. Trace fossils, or fossilized animal trails and burrows, are known from the Vendian, so this is the kind of animal that could have made them.

Another notable feature Chen et al. emphasize is the presence of symmetrically-paired surface pits, which are interpreted to be sensory organs. This is another level of sophistication, that the organism has specialized tissues to sense its environment.

The important point is that this animal possesses the rudiments of morphological characters that are going to erupt into a wide range of diverse specializations in the Cambrian, and it has them roughly 50 million years before the Cambrian 'explosion'. The phyletic innovations we have first seen so clearly in the Cambrian did not come out of nowhere, but have a solid evolutionary foundation in simpler animals.

Chen et al.'s summary of their paper:

The morphology of Vernanimalcula demonstrates that the evolutionary appearance of developmental programs required to generate a multilayered bilaterian body plan preceded the entrainment of the growth programs required for macroscopic body size. Furthermore, the organization of these fossils, taken together with their provenance, indicates that the genetic toolkit and pattern formation mechanisms required for bilaterian development had already evolved by Doushantuo times, long before the Cambrian. Therefore, the diversification of body plans in the Early Cambrian followed from the varied deployment of these mechanisms once conditions permitted, not from their sudden appearance at or just before the Cambrian boundary.

Chen J-Y, Bottjer DJ, Oliveri P, Dornbos SQ, Gao F, Ruffins S, Chi H, Li C-W, Davidson EH (2004) Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian. Science, published online 3 June 2004; 10.1126/science.1099213

Erwin DH, Davidson EH (2002) The last common bilaterian ancestor. Development 129:3021-3032.