An atavism is the reappearance of a trait in an organism that has not been present in its recent ancestors. It occurs when a genetic variation that was present in a distant ancestor reappears in a later generation. Atavisms can be physical or behavioral, and can range from minor variations in appearance to completely new traits. They can be caused by mutations, genetic recombination, or epigenetic changes.

One well-known example of an atavism is the appearance of extra fingers or toes, known as polydactyly. This trait is thought to be a throwback to our early ancestors who had more than five digits on each limb. Other examples of atavisms include the appearance of body hair in humans, which is thought to be a throwback to our ape-like ancestors, and the appearance of stripes in zebras, which is thought to be a throwback to their horse-like ancestors.

Atavisms can be interesting and fascinating, but they can also be medically significant. In some cases, atavisms can be a sign of an underlying genetic disorder, and it is important to seek medical advice if you or your child develops any unusual traits.