Grypania

Grypania is an early, tube-shaped fossil from the Proterozoic eon. It is regarded by some as the first known eukaryote. 
"Grypanias status as a eukaryote is not absolutely proven. It might have been a giant bacterium or bacterial colony. Its size (over one centimeter) and consistent form, suggest it is a eukaryotic alga. The oldest known "Grypania" fossils come from an iron mine near Negaunee, Michigan. The fossils were originally dated as 2100 million years ago, but later research showed the date as about 1874 million years ago. "Grypania" lasted into the Mesoproterozoic era.