Biological evolution is a great inventor. Over 4 billion years, it has generated an astonishing diversity of lifeforms, from the tiniest bacteria to the tallest trees. Each new organism inherits a genetic program from its parents - a set of instructions to “build” the organism itself. Random mutations in this program can alter the organism’s traits, affecting its ability to survive in its environment. But how do these small changes combine over thousands of generations to yield the vast complexity we see in present-day lifeforms? In this talk, we discuss examples from our research, using computer simulations to model the early evolution of animals, from single-celled microbes to complex multicellular organisms. We show that evolution behaves a bit like a hacker, repurposing the programs it previously built in unexpected ways to create new functions and structures.