The secret behind humans having four limbs is hidden in their belly!
If we believe researchers at University of Vienna and the Konrad Lorenz Institute for Evolution and Cognition Research, the paired appendages in backboned animals (with jaws) have modified dramatically in the course of evolution - into fins, legs, arms, flippers and wings - because of belly.
"The vertebrates known as gnathostomes does not include the living lampreys and hagfishes which have neither jaws nor paired fins," said Brian Metscher from department of theoretical biology at University of Vienna.
"We have provided 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," added Laura Nuño de la Rosa, lead author and post-doctoral fellow at the Konrad Lorenz Institute.
In its earliest stages of development, an embryo segregates into three main layers of tissue - an outer one that will form the skin and nervous system, an inner layer that becomes the digestive tract and an in-between layer that eventually forms muscles, bones and other organs.
The in-between layer (mesoderm) splits into two layers that line the inside of the body cavity and the outside of the gut.
According to new research, fins or limbs begin to form only at the places where those two layers are sufficiently separated and interact favourably - at the two ends of the forming gut, said the study published in the journal Evolution & Development.
"You could say that the reason we have four limbs is because we have a belly," added Laura Nuño de la Rosa.
"The most important function of a model like this is to provide a coherent framework for formulating specific hypotheses, which can be tested with molecular and other laboratory methods," said Brian Metscher, senior scientist in University of Vienna.
