Amphioxus

What an interesting little creature. This little guy is believed to hold key information about the evolution vertebrates and, thus, humanity.

It remains a little incredible to me that humanity evolved from something even more insubstantial that this creature. But my opinion, at this point, is rather inconsequential, I suppose. 

These creatures, also known as lancets, are usually found buried in sand in temperate to tropical seas. They share some features with vertebrates, such as a nerve cord, a tail that goes past the anus, and similar muscular arrangement.