Every inch of our planet is coated in life. The vast majority of life by any measure is bacteria and other types of tiny single-celled organisms.
Out on one tiny branch of the tree of life are strange, highly complex, and very large types, called plants, animals, and fungi.
They discovered how to make large colonies of cells coordinate to become parts of a larger whole. The average human contains just under 40 trillion cells, who have learned to specialise into all the parts that a person needs to live, from hearts, to eyes, to neurons.