Amphibians and mammals both have five-fingered hands, supposedly
homologous structures indicating common descent. However, they develop in
completely different ways. In humans, the limb tip (called the apical
ectodermal ridge, AER) thickens, and then programmed cell death divides
the AER into five regions that develop into digits. In frogs, the digits
grow outward from the beginning.
Frogs and mammals (and other tetrapods) use the same mechanisms of limb
develpment, including the same HOX genes and the same molecules. All
(except a few highly derived frog species) have an AER. The difference
between humans and frog is that frogs have webbed feet. Human-like
programmed cell death would destroy the webbing.
