DO YOU KNOW THAT YOU ARE A DEUTEROSTOME?

Imagine that your father or mother had a sibling you never knew about and you suddenly found yourself becoming acquainted with numerous cousins.   That is how I felt when reading a collection of articles in Science volume 520 (April 23, 2015).  They were on the origin of vertebrates.  The articles covered the history of classifying vertebrates, the comparative anatomy involved, and the DNA sequences of genes shared in common among the vertebrates and their nearest relatives. We are vertebrates because we have a spinal column consisting of a number of vertebrae. That we all know.  But do you know that as an embryo you had gill slits like a fish?  Did you know that as an embryo you had a notochord, a cartilage-like rod that ran from your head to your tail region?  Did you know that most other phyla have a ventral (belly side) nerve cord instead of a dorsal (your back) spinal cord?  I knew that from my course in comparative anatomy that I took as an undergraduate at NYU and that got reinforced when I was a teaching assistant at IU watching students dissect and follow the organ systems of different types of animals from worms to fetal pigs. When I last took coursework in zoology as a graduate student I knew that I was a deuterostome, but didn’t reflect on its meaning. It means we have a second mouth.  The first mouth is a hole in the embryo during its formation when a hollow ball of cells gets dented at one end and what is pushed in becomes the future gut and the opening becomes a mouth.  This is the real mouth that worms, mollusks, and arthropods (crustaceans, insects, and spiders) have.  But in vertebrates like us, the mouth comes about from a second opening when a gut-like tube extends forward and downward in the future head and punches through. That makes your outer lips derived from skin (ectoderm) and the inner part of your mouth is from endoderm.  I used to tell my students that if you give a dutiful lip to lip kiss it is ectoderm to ectoderm, but when you get really passionate and enjoy deep kissing, it’s ectoderm on endoderm that thrills you. If we are deuterostomes, what are worms, insects, and clams?  They are protostomes (using the first mouth).  Who is “we”?  We deuterosomes include some strange bedfellows, including echinoderms (starfishes and sea urchins), hemichordates (acorn worms), cephalochordates (lancelets), tunicates, jawless fishes, and jawed vertebrates (that’s us).

So now you know some family secrets.   Your closest ancestors were not annelid worms, insects, or octopuses.  They were echinoderms.  That’s sand dollars, starfishes, sea urchins, and crinoids (extinct members of that phylum echindermata who are known as sea lilies or whose skeletal remains are mostly “Indian beads” found scattered in the limestone of the local quarries and lakes in southern Indiana).  The next time you stand on a sandy beach and look out to the ocean to commune, remember that your closest relatives, fellow deuterostomes like you, are the starfishes, sand dollars, and sea urchins.