Betta splendens had a long history before becoming a pet store staple. Native to the streams and rice paddies of Southeast Asia, this aggressive fish was bred by people, such as the King of Thailand, to fight for sport.

Domestication of these fish started at least four hundred years ago — much earlier than previously thought — scientists recently discovered. By comparing the genomes of wild and domestic bettas, Andrés Bendesky, MD, PhD, and a team of researchers also uncovered genes that breeding has changed over time. One gene has a small effect in determining the sex of wild betta but became a strong sex-determining gene in domestic bettas. Others contribute to the range of colors and shapes that make domestic bettas more flamboyant than their wild cousins.

Read more about this new research published today in Science Advances, and how it lays the groundwork for exploring the genetic origins of aggression, in the New York Times, The Atlantic and The Scientist.