by JOHN HEWITT
A fundamental question in the evolution of animal body plans, is where did the head come from? In animals with a clear axis of right-left symmetry, the bilaterians, the head is where the brain is, at the anterior pole of the body. Little is known about the possible ancestor of bilaterians. Fortunately their sister group from that same progenitor, the cnidarians, can be studied in parallel today to give some clues. Cnidarians are creatures like jellyfish, hydra, and sea anemone which possess rudimentary nerve nets, but no clear brain. They all have just a single orifice to the external world, which basically does it all. In a recent paper published in PLOS Biology, researchers from the University of Bergen in Norway compared gene expression patterns in sea anemone (Nematostella vectensis, Nv) with that from a variety of bilaterian animals. They found that the head-forming region of bilaterians is actually derived from the aboral, the opposite-oral, side of the ancestral body plan.
Pioneering developmental biologist Lewis Wolpert, is often credited with having observed: “It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life.” Almost all animals undergo a similar gastrulation process early in their development. The point where the cells first invaginate during gastrulation, the blastopore, uniquely defines an embryonic axis. After this stage however, all bets are off—attempts to define phyla according to hardline criteria, like blastopore = anus, are invariably met by counterexample where it instead becomes the mouth. Gene expression, while not always constrained into single contiguous areas, therefore provides a baggage-free way to assign homology across species.
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