Sponges are considered some of the oldest animals on Earth, yet researchers have struggled to pin down when they first appeared. Studies of DNA from modern sponges, together with chemical clues preserved in ancient rocks, point to an origin at least 650 million years ago. The research was recently published in Science Advances.

That early date has sparked debate because it comes at least 100 million years before the earliest known sponge fossils. An international research team led by Dr. M. Eleonora Rossi of the University of Bristol’s School of Biological Sciences set out to resolve this mismatch by focusing on how sponge skeletons evolved.