When something is lost in evolution, it is rarely gained again. The trend of was first noted in 1893 by the paleontologist Luis Dollo. One famous example of this trend, often called Dollo’s law, is the number of digits on the hands and feet of vertebrates. Some of the earliest fossil amphibians have eight or more toes, but that number was quickly reduced to five:
For the last 300 million years, all animals, from humans to whales, dinosaurs to birds, have had five digits or less on their hands and feet. But a good rule of thumb is that any law in biology is going to have exceptions. Anyone who knows enough about domestic dogs and cats knows that some breeds are found with multiple toes. Here is the paw of a Norwegian Lundehund, six toes and all:
What explains this violation of Dollo’s law? What explains Dollo’s law in the first place? For the next few posts, I’m going to look at the science behind evolutionary constraint, what causes it, and what happens when it’s broken.
Works Cited
Boisvert CA, Mark-Kurik E, & Ahlberg PE (2008). The pectoral fin of Panderichthys and the origin of digits. Nature, 456 (7222), 636-8 PMID: 18806778
Dollo, L (1893). Les lois de l'évolution. Bull. Soc. Belge Geol. Pal. Hydr, VII:164-166.
