Annelids: the first segmented animals
It is hypothesized that the period between 600 and 1000 million years considerable erosion of the continents was producing great expanses of mud and sand adjacent to the continental shelf. This environment may have contained abundant quantities of organic material. However, in order to give protection and concealment in this environment burrowing would be a pre-requisite, and more tubular body plan would become necessary. It is possible that under such conditions the segmented worms evolved. Some of these animals became active burrowers who tunnelled through mud in search of food, whereas others lay half-buried, with their mouthparts filtering food above the sediment.