Monday, February 6, 2012

Invert sea squirt to human vert

An important ancient relative: tunicates 

(Photo property of Phil Camill)

Humans have long been fascinated with the idea of where people came from.  Scientists favor the theory of evolution.  Evolution, very simply put, describes changes in groups of living organisms over generations and how these changes relate to speciation (for more information check out this, this or this).

 This theory allows us to tract how species evolved to become the current species we have today.  Based on evolution, scientists discovered that sea squirts/tunicates are vitally important to the development of human beings.  So what the heck do we have in common with this underwater dog toy?

Tunicates are small filter feeders that live in marine habitats.  They have a variable outer layer, known as a tunic, to protect their somewhat precious insides from the open water.  Their "precious" insides contain a heart, reproductive organs and a primitive digestive tract.  Most adults are sessile (ascidians) and have permanently attached themselves to rocks or other hard substrates in the ocean.  They feed by siphoning water through their body by means of an incurrent (buccal) and excurrent (atrial) siphon.  Incoming water contains plankton and other organic material that stick to internal mucus and is then digested. Siphoned water and waste exit the organism through the excurrent siphon.  They often live in colonies but can be found individually as well.  Tunicates found in a colony are much small than those found living alone. 

So both tunicates and humans have a skin-like outside, two main openings, a heart, digestive system and reproductive organs; pretty nifty for the little blobs!  But there must be some other reason I'm making such a fuss about tunicates...

The fuss is warranted because tunicates are one of the first organisms to develop a primitive central nervous system, which makes them humans closest invertebrate relative!  In their larval stage they have a notochord and nerve cord in their larval stage and resemble tiny tadpoles.  Their notochord is the pre-runner to our backbone, while the nerve cord is like our spinal cord.  These attributes contribute to the defining characteristics of the phylum Chordata, to which humans and tunicates belong.  The creature I described above has little similarity to humans because tunicates undergo a drastic metamorphosis from their larval to adult stage.  However, it is their larval stage that makes tunicates so special.  This stage represents the start of the evolution of vertebrates, which eventually leads to complex vertebrates like humans.  These simple organisms represent the start of the bridge from invertebrate to vertebrate evolution!

Although humans and tunicates are separated by about 500 million years of evolution we share about 80% of our genes and have remarkably similar stages early development. Based on these similarities,  tunicates are being used for a variety of research projects on human health, including early development of human embryos and  regenerative medicine


  1. Tunicates make cellulose, a characteristic usually associated with plants, and some researchers feel this is explained by horizontal gene transfer from bacteria, rather than the vertical gene transfer of evolution. (see

    'Hey squirt, get a backbone'. 'I don't need one now, I'm sedentary'.

  2. According to a tunicate fact sheet, they "can be eaten, but are of no particular culinary interest". One more thing in common with humans!

  3. It seems to me that humans (particularly Americans) have quite a lot in common with tunicates. We basically siphon down and inhale whatever food is available and therefore have become sessile blobs. Its the new "tuicate fad".

    1. Well, at least now we have a connection between cultural studies and the natural sciences.