Pikaia gracilens is the earliest known example of the phylum chordata, from whence sprang
all vertebrates, including us. Found in the Burgess Shale, Pikaia gracilens
was a worm-like fish measuring 3-4 centimeters that lived roughly 500 million years
ago. Like all chordates, it had a dorsal nerve cord supported by a cartilage-like
notochord that ran the length of its body (it was the notochord that later would form
into the line of vertebrae that separate us vertebrates from our chordata cousins such
as the hagfish or the sea squirt).
Though it is the earliest known chordate, there is no reason to believe that Pikaia
is a direct ancestor of humanity. It was not a vertebrate, and is actually grouped
as being an early member of the cephalochordata, a modern family of eel-like animals
that includes lancelets and amphioxus. With fewer than 60 specimens identified and
most of those in poor condition opinion is still mixed on Pikaia's exact place in
the tree of life, though.