Backboned animals, at least the ones with jaws, have four fins or limbs, one pair in front and one pair behind and these have been modified dramatically in the course of evolution, into a marvelous variety of fins, legs, arms, flippers, and wings.
Researchers in the Theoretical Biology Department at the University of Vienna and the Konrad Lorenz Institute for Evolution and Cognition Research have found that our earliest ancestors settled into such a consistent arrangement of two pairs of appendages because of the belly.
Lead author Laura Nuno said they have drawn together a large body of molecular embryology work, as well as results from paleontology and classical morphology to work out an overall explanation of how the vertebrate embryo forms pairs of appendages along each side, and only two pairs situated at front and back ends of the body cavity.
The proposed model incorporates results from much previous research, including information on gene expression and on interactions among the different tissues that make up an early vertebrate embryo.
In its earliest stages of development, an embryo segregates into three main layers of tissue: an outer one (ectoderm) that will form the skin and nervous system, an inner layer (endoderm) that becomes the digestive tract, and an in-between layer (mesoderm) that eventually forms muscles, bones, and other organs.
The early mesoderm splits into two layers that line the inside of the body cavity and the outside of the gut.
The new hypothesis proposes that fins or limbs begin to form only at the places where those two layers are sufficiently separated and interact favorably with the ectodermal tissues - namely at the two ends of the forming gut. In between, no fin/limb initiation takes place, because the two mesoderm layers maintain a narrower separation and, the authors propose, interact with the developing gut.
Behind the back end of the digestive tract (the anal opening), along the bottom of the tail, the two mesoderm layers come together as the body wall closes up, forming a single (median) fin.
Along the length of the developing gut, the body wall cannot close completely, so the conditions for initiating fins or limbs occur to the left and right of the midline, allowing the development of paired instead of median fins.
The study was published in the journal Evolution and Development.
