UR ,Oh, a CHORDATE?

Surprise! Although this may seem hard to believe considering the current state of affairs, you are, in fact, among the 1% and are not of the 99%. We, the Vertebrata, along with our brothers and sisters from within Phylum Chordata, evolved the gifts of a notochord, a dorsal hollow nerve cord, pharyngeal slits, and most importantly, a post-anal tail. These endowments separate us from the rest of life on planet Earth, forming our elite 1% faction. The movers and shakers of the ‘Occupy Movement’ are our spineless relatives and most of you know this for you see them squatting within the confines of your home; especially if you live in the tropics.

So how did we get here? How did we progressively separate ourselves from the masses? How did we inherit the evolutionary trust fund?

Well it all presumably started around 550 million years ago, give or take 50-100 million years, when our ancestors fled the Protosomes to create a new community called Deuterostomes. They distinguished themselves by developing radial and indeterminate cleavage, turning their blastophore into an anus, and pilfering parts of their existing archenteron to create a coelom. Our Deuterostome forefathers clearly had issues with commitment and trouble determining the function of their orifices. Hmm, the apple doesn’t fall far from the tree.

Some of the founding Deuterostomes engaged in ciliated movement of dipleuria larvae, while others preferred tadpole like larvae that contained a notochord and a muscular tail for locomotion. This larval disjuncture created two groups within the Deuterosomes, the Ambulacraria and the Chordata. Besides sounding like a people from an 8th Kingdom of Westeros, Ambulacrarians are composed of organisms from the House Echinodermata (sea stars, sea urchins, sea cucumbers) and House Hemichordata (acorn worms). While Chordata includes organisms within Vertebrata (us, along with all the big charismatic mobile organisms in this world, with the exception of squid and octopus), Urochordata (sea squirts, also known as tunicates, and salps), and Cephlochordata (headless chordates but not of the Sleepy Hollow kind).

Until recently, it was thought that we were derived from sea squirts within our Chordata pedigree; they decided to shed the notochord after settlement but we chose to keep it to attach our muscles and house our nerves. Incredible to think that we emerged from these sessile squirts that spit like a cowboy and foul our boat hulls. However, it has recently come to pass that they may not actually be a direct relative. Through the use of phylogenetics (constructing a family tree through genes rather than the Mormon church) and evodevo (evolutionary development, rather than 80s pop culture), it has been suggested that we, Vertebrata, are no longer a part of the same Phylum as Urochordata. Instead, we are both our own Phyla and together form a sister group, called the Olfactores, which not only sounds like a 1950s doo wop group but shares a common ancestor with Cephlochordata – which, btw, has also become its own Phylum. Together, these three musketeers create the….wait for it…..wait for it…. SuperPhylum Chordata! I know, it sounds like a Sesame Street character.

So what does this mean? Well for starters, Ur, Oh, not a Chordate. Well, I take that back, you are a chordate, a Vertebrate to be specific. You just no longer need to invite Urochordata to your immediate family reunion for they are more distant cousins now. At least, that’s the word on the street, today. But then again, you never know. With additional DNA testing using more gene markers, the tree of life may take another turn and the ‘whose your daddy’ chordate catechetic inquiry may be back in the headlines. Regardless of our SuperPhylum pedigree, we still stand together as the 1%. So be thankful for the dorsal neural tube and our push to transform it into our brain. For without this 1% prestige, our species in it’s current evolutionary state of being hairless, soft skinned, and having molar teeth and poor eyesight would have never survived mother nature.

 

For a more precise scientific outlook on our ancestry check out:

Satoh N, Rokhsar D, Nishikawa T. 2014 Chordate evolution and the three-phylum system. Proc. R. Soc. B 281: 20141729. http://dx.doi.org/10.1098/rspb.2014.1729