Ancient viruses that infected vertebrates hundreds of millions of years ago played a pivotal role in the evolution of our advanced brains and large bodies.

Next, they searched for RetroMyelin-like sequences in the genomes of other species, finding similar code in jawed vertebrates – fellow mammals, birds, fish, reptiles and amphibians – but not in jawless vertebrates or invertebrates.

This led them to believe the sequence appeared in the tree of life around the same time as jaws, which first evolved around 360 million years ago in the Devonian period, called the Age of Fishes.