 Fossils of chordates , the phylum to which humans belong , are very rare in Cambrian sediments . Conway Morris classified the Burgess Shale fossil <unk> as a chordate because it had a rudimentary notochord , the rod of cartilage that evolved into the backbone of vertebrates . Doubts have been raised about this , because most of the important features are not quite like those of chordates : it has repeated blocks of muscle along its sides but they are not chevron @-@ shaped ; there is no clear evidence of anything like gills ; and its throat appears to be in the upper part of its body rather than the lower . It also has " tentacles " on the front of its head , unlike living chordates . At best it may be a stem group chordate , in other words an evolutionary " aunt " of living chordates . <unk> , also found in the Burgess Shale but even rarer , may be a chordate , if the repeated chevron @-@ shaped structures along its sides represent muscle blocks . While <unk> was celebrated in the mid @-@ 1970s as the earliest known chordate , three jawless fish have since been found among the Chengjiang fossils , which are about 17 million years older than the Burgess Shale . 
