Multicellular organism
The first multicellular fossils have been found in 2.1 billion years old Palaeoproterozoic rocks from the Gabon in Africa. These are eukaryotic cells which live colonially. The fossils are visible to the naked eye, and hence much larger than bacteria. The authors comment that one advantage of  multicellularity is larger size, and that an adequate level of oxygen is required to support this kind of life.