The animal kingdom is one of life’s great success stories — a collection of millions of species that swim, burrow, run and fly across the planet. All that diversity, from ladybugs to killer whales, evolved from a common ancestor that likely lived over 650 million years ago.

No one has found a fossil of the ur-animal, so we can’t say for sure what it looked like. But two scientists in Britain have done the next best thing. They’ve reconstructed its genome.

Their study, published in Nature Communications, offers an important clue to how the animal kingdom arose: with an evolutionary burst of new genes. These may have played a crucial part in transforming our single-celled ancestors into creatures with complex bodies made of many kinds of cells.

The new genes also proved to be remarkably durable. Of all the genes in the human genome, 55 percent were already present in the first animal.