Today, Earth is home to animals of all shapes and sizes, from nearly microscopic creatures like tardigrades to 80-foot-long (25 meters) blue whales. These organisms have arisen and evolved over millions of years of evolution. But what was the first animal on the planet?
The answer to this question is heavily debated by scientists. Dozens of different studies using everything from chromosome evolution over time to ancient fossils have boiled it down to two candidates: sponges and comb jellies.
