Life was already billions of years old before the Cambrian explosion that occurred roughly 540 million years or so ago. For all the time prior to this unprecedented surge in diversity and complexity, life was mostly relegated to simple, single-celled organisms. But then in the blink of an eye on a geological timescale, the first major animal phyla appeared, including arthropods, mollusks, echinoderms, and early vertebrates. This was a period of intense evolutionary experimentation. Strange and wacky creatures struggled to survive, and competition was fierce. As the first animals evolved, some …