Recent research suggests that the period prior to the Cambrian explosion saw the gradual evolution of a "genetic tool kit" of genes that govern developmental processes. Once assembled, this genetic tool kit enabled an unprecedented period of evolutionary experimentation -- and competition. Many forms seen in the fossil record of the Cambrian disappeared without trace. Once the body plans that proved most successful came to dominate the biosphere, evolution never had such a free hand again, and evolutionary change was limited to relatively minor tinkering with the body plans that already existed.
The Ediacaran animals disappear from the fossil record at the end of the Vendian (544 million years ago). In their place we find representatives of almost all the modern phyla recognized today: sponges, jellyfish and corals, flatworms, mollusks, annelid worms, insects, echinoderms and chordates, plus many "lesser" phyla such as nemertean worms. These "modern" organisms appear relatively quickly in the geological time scale, and their abrupt appearance is often described as the "Cambrian explosion" however, bear in mind that the fossil record of the "explosion" is spread over about 30 million years.
A team of scientists from Spain and the UK has determined that a certain curiously primitive group of flatworms are the oldest living ancestors to all "bilateral" animals-that is, those with a right and left side. These worms were previously thought to belong to a much younger group of organisms, and their newfound identity also implies that bilateral organisms began making their debut on Earth earlier than previously thought. The finding was reported in the March 1999 issue of Science. The study by Jaume BaguÃ±Ã , of the University of Barcelona, and his colleagues supports this theory by identifying a group of contemporary flatworms called the Acoela as the living descendants of an early lineage from this pre-Cambrian time. (Flatworms, several of which are parasites, are only distantly related to, and much simpler than, the familiar earthworms.)