The origin of tetrapods is a complex question that webs together genetic,
paleontological, developmental and physical facts. Basically, the
development of embryos is described by a complex mix of mechanical movements
and biochemical inductions of genetic origin. It is difficult to sort out in
this scientific question what are the fundamental features imposed by
conservation laws of physics, and by force equilibria, and what can be
ascribed to successive, very specific, stop-and-go inductions of genetic
nature. A posteriori, evolution selects the parameters of this process as found in the
observed species. Whether there is a general law to animal formation seems
out of the question. However, several concepts developed in biology, like
the concept of “organizer” seem questionable from a physics point of view,
since the entire deformation and force field should be the “organizer” of
development, and one can hardly ascribe such a role to a single small area
of the embryo body. In the same spirit, the concept of “positional
information” encapsulated in concentration of chemicals seems questionable
since the deformation and force fields in embryonic tissues are tensors.
Finally, the concept of a development organized in space along three
orthogonal (“Cartesian”) axes associated to chemical gradients seems also
questionable, since early embryo development is driven by complex vortex
fields, with hyperbolic trajectories which span the entire embryo. Such
hyperbolic trajectories are best understood by a description in terms of
dipolar components of the morphogenetic forces, whose projections along
orthogonal axe have no specific meaning except as a mathematical tool. I
review here the present state of description of several aspects of tetrapods
morphogenesis and evolution, from the point of view of physics. It is
getting clear that several basic features of tetrapods body are a direct
consequences of fundamental laws of physics. Several lines of work reviewed
here show that the topology of the tetrapods may be directly related to the
structure of the earliest movements in embryos. The bio-mechanical approach
leads to important consequences for the constraints on evolution of the
craniates. Such consequences have received a controversial welcome in the
last decade, although they may encapsulate the true origin of craniates,
esp. simians, and eventually homo.