Scientists have studied fossils of tiny (about 1mm long) creatures from rocks deposits, around 540 million years old, from central China they have named Saccorhytus coronarious (https://www.theguardian.com/science/2017/jan/30/huge-mouth-and-no-anus-earliest-known-ancestor-saccorhytus-coronarious-evolution). Although the beasts were small, they appear to have had a disproportionately large mouth but no obvious anus (it is suggested that they swallowed their prey and had vents, comparable to gill slits, to remove excess water that they must have ingested in that process). These beasts are thought to be the first Deuterostomes (although that group gets its name from the fact that its members generally convert their first embryonic opening into an anus). Current Deuterostomes include Echinoderms (starfish and sea-urchins) as well as the Chordates (sea squirts and vertebrates). It's pushing the description a bit but they have been described as our earliest known ancestors.
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