Scientists have discovered a rare evolutionary “missing link” dating to the earliest chapter of life on Earth. It’s a microscopic, ball-shaped fossil that bridges the gap between the very first living creatures — single-celled organisms — and more complex multicellular life.

The spherical fossil contains two different types of cells: round, tightly-packed cells with very thin cell walls at the center of the ball, and a surrounding outer layer of sausage-shaped cells with thicker walls. Estimated to be 1 billion years old, this is the oldest known fossil of a multicellular organism, researchers reported in a new study.