Scientists at the University College London have identified a key mechanism of embryonic development that helps in differentiating highly evolved species, like the humans, from less evolved species such as fish.
Early on in development, the mass of undifferentiated cells that make up the embryo take the first steps in deciding how to arrange themselves into component parts to eventually go on to form a fully developed body.
During this process, known as 'gastrulation', the cells group into three layers, the 'ectoderm', which then in turn generates the 'mesoderm' and 'endoderm' layers.
In higher vertebrates, such as mammals and birds, the mesoderm and endoderm are generated from an axis running through the centre of the embryo.
However, in lower vertebrates, such as amphibians and fish, the two layers are generated around the edge of the embryo.
Using chicken eggs and a state-of-the-art imaging device which can reveal how cells move in three dimensions, the researchers demonstrated a key difference in the way gastrulation occurs between higher vertebrate species and less evolutionarily advanced animals.
They found that the reason why higher vertebrates form their axis at the midline of the embryo is because during evolution they acquire a new mechanism of "cell intercalation" which positions the axis at the midline.
During the course of their research, the team also discovered the molecules used by the embryo to control these cell movements.
Lead researcher Prof. Claudio Stern said, in humans, this process occurs during the third week of embryonic development.
The research appears online in the Oct 11 issue of the journal Nature.