Dollo's law

The principle is associated with Belgian paleontologist Louis Dollo, who argued in the late nineteenth century that an organism does not return exactly to an earlier state, even if it encounters similar conditions. The term ‘Dollo's law’ later came to describe evolutionary irreversibility. In its strongest popular form, it is sometimes phrased as saying that a lost structure can never re-evolve. A more careful interpretation focuses on the improbability of retracing the same sequence of genetic, developmental and historical changes in reverse.

When a trait is lost, mutations and changes in regulation may accumulate in the genes and developmental networks that once produced it. Over time, rebuilding the original feature through precisely the same underlying processes can become increasingly unlikely. Yet the relevant genetic material may not disappear completely: genes often contribute to multiple traits, and some developmental pathways remain active for other functions. If parts of a pathway persist, evolution may be able to reuse them, allowing a lost-looking feature to reappear or a similar function to evolve by a different route.

The principle does not mean that no trait can ever resemble an ancestral trait again. A 2007 study of oribatid mites used phylogenetic analyses and ancestral-state reconstruction to infer that sexual reproduction re-evolved from parthenogenetic ancestors in a lineage of Crotoniidae; the authors described it as a rare reversal and proposed that ancestral genetic capacity had persisted. A 2025 review of reported structural reversals argues that reappearance may be possible when homologous structures or their developmental pathways remain in an organism, while stressing that evidence and interpretation differ among cases. Resemblance alone does not establish a literal return to the same ancestral state: researchers must account for shared ancestry, retained features, convergent evolution and how traits are defined.

Dollo's law is best used as a hypothesis about the difficulty of exact evolutionary reversal, not as a universal rule that forbids all reversals. Scientists assess proposed cases using fossils, comparative anatomy, developmental biology and phylogenetic models. Conclusions depend on the resolution of the evidence: an apparently absent trait may have persisted in a reduced or modified form, and a newly similar feature may have arisen independently. Thus, evolutionary history can make some routes improbable without making every form of reappearance impossible.