Atavisms are ancestral traits that resurface after silent genes switch back on
Babies are occasionally born with a small tail, wingless stick insects sometimes sprout wings again, and chicken embryos have been coaxed into growing teeth. These throwbacks, called atavisms, happen because genes for vanished features can linger unused in DNA for generations, waiting for a slip in the controls that keep them quiet.
The word comes from the Latin atavus, a great-great-great-grandfather or, more loosely, any ancestor. In biology it names the return of a trait that evolution had lost. A lost feature need not vanish from the genome: the gene sequence may survive intact but inactive. If a mutation disables the genes responsible for the newer trait, or lets the older one dominate, the ancestral form can reappear. Changing how long a feature develops in the embryo, either cutting that window short or stretching it, can have the same effect.
Examples span the living world. Limbs have returned in limbless vertebrates, wings in wingless stick insects, leaf insects and earwigs, and sexual reproduction in mites and a flowering plant that had shifted to cloning themselves. In people, reports include tails, extra nipples, unusually large primate-like teeth and even a heart whose circulation resembled a reptile's. Researchers have also triggered dormant dinosaur features, including teeth, in bird embryos. Such cases are often cited as evidence for evolution.
Before modern genetics took shape in the early 1900s, atavism explained a trait skipping several generations, a so-called throwback. Breeders hoped such traits could be concentrated deliberately, which produced Heck cattle, an attempt to revive the extinct aurochs from old breeds with primitive features.
The idea was also badly misused. Social Darwinists claimed some races showed atavistic traits, and in the 1870s the criminologist Cesare Lombroso labelled physical features of criminals as primitive throwbacks, work that science has long abandoned along with eugenics. The term travelled into social thought too: Joseph Schumpeter blamed the First World War on atavistic, ageing empires dragging liberal, trading Europe into conflict.
Source: Atavism
