artists image of cell differentiation

When multicellular organisms grow, their cells can begin differentiating to form internal structures

bennu phoenix/Alamy

A tiny organism that lived a billion years ago had two different cell types, one forming its core and another its outer “skin”. It may have been one of the first life forms built that way, making it a crucial step towards modern organisms like animals that also have a skin that is distinct from the cells inside the body.

“This fossil clearly is multicellular with two different types of cell,” says Charles Wellman at the University of Sheffield, UK. While organisms made of multiple …