It may arise due to genetic and/or physical factors. The most common mechanism of this phenomenon would be genetic, where the genes responsible for a previously occurring, ancestral phenotypic feature are found to be expressed in the cell, either due to a loss-of-function mutation in its repressing gene, or due to a gain-of-function mutation in that gene itself. Such an expression of a latent gene, would produce the ancestral phenotype in the individual. The same result may also be seen if the period of fetal development of an individual is cut short or prolonged. The genetic mechanism is possible due to the fact that despite the non-expression of a gene, the sequence is retained in the DNA, although in an inactive form. Hence even if the trait is not exhibited in a species, there are high chances that the organism still retains a copy of the gene in its genome. The other method of altering the developmental period of the organism also achieves a similar effect. This is possible due to the fact that in the very early stages of development, the fetus looks more or less the same across all higher mammals. This can be explained by reasoning that all animals share at least a few ancestors along the evolutionary timeline. As the time progresses, these similar fetuses start developing and undergo rapid modifications to evolve into similar organisms as their parent organisms (Ernst Haeckel's theory of recapitulation). Hence if an animal's development is halted at a certain stage, it may still exhibit ancestral traits. A prolongation, in theory, would afford more time for the fetus to stay in the womb and help it in developing an ability. Both these explanations are still speculative, as a definite mechanism explaining this phenomenon has not yet been fully worked out.

╬ Hind limb on whales ╬ Hind fins on dolphins ╬ Hind limbs on snakes ╬ Extra toe on horses ╬ Reemergence of sexual reproduction in Hieracium pilosella and the Crotoniidae family of mites ╬ Teeth-like structures on the beaks of chickens ╬ Hatchling feathers on newborn birds

╬ Coccygeal projection in the form of a non-functional tail ╬ Accessory breasts, i.e. presence of extra nipples ╬ Large and long canine teeth ╬ Snake heart, i.e. myocardial structure is altered such that it resembles a reptilian heart ╬ Movable pinna