In the year 1893, Louis Dollo, a French born Belgian palaeontologist proposed the principle of Irreversibility. This is also called Dollo’s law of Irreversibility or Dollos law which states “An organism is unable to return, even partially, to a previous stage already realised in the ranks of its ancestors.” That is, once an animal has passed through a number of stages, a reversion, stage by stage, to the original ancestral condition does not occur. A structure that changes its form in evolution will not revert to its earlier form.
In studies of primate evolution, irreversibility is an important principle. The dentition of a given form is often crucial evidence of its ancestral or descendant status with respect to another form. Once a tooth of a particular series (incisor, canine, premolar or molar) is lost, it does not recur again in the same series in the same form. Changes in dentition are irreversible.
Organisms adapt to different environmental conditions throughout the Pleistocene, humans adapted to changing conditions in terms of certain changes in their lower jaw, dentition, erect postures and bio-cultural realms. All these changes cannot be re-enact bit by bit, part by part across the long time periods.
This does not mean that similar structures or the same adaptive patterns will not be repeated a second time in the evolutionary record. Another example is the flying reptiles. After these reptiles became extinct, wings and adaptation to an airborne way of life occurred in two other distinct lineages – the birds and mammals.
However, irreversibility is a descriptive generalization, it is not a law of nature and one must realize that it is not a property of living organisms.