The very first form of eukaryotes fossils that have been found date back to 2.1 billion years ago. Specifically, the acritarch represents the oldest fossil that humans have found that is evidence of early eukaryotic beings, according to the Smithsonian National Museum of Natural History. The acritarch looked like marine algae, and scientists think that it had an acid-resistant wall. In addition to acritarch fossils, scientists also found a creature called Grypania spiralis, which is a ribbon-like fossil that is only 2 mm wide.
The earliest evidence of eukaryotes suggests that eukaryotes evolved somewhere between 2.0 and 3.5 billion years ago, which is a huge range and points to the difficulty in pinpointing these ancient time frames. The earliest bacteria found are located in sedimentary rocks, in small colonies that are formed by photosynthetic bacteria. Regardless of the specific dates, Michigan State University reports that scientists place the origins of eukaryotic cells somewhere in the Precambrian Era.
Because scientists have found such a diversity of species, it makes it difficult to pinpoint the exact nature and variety of the species. However, there is a general conclusion that the majority of these creatures were sea dwellers that fed on smaller single-celled organisms. Scientists claim that these animals behaved a lot like algae and probably had an amoeba-like shape, according to the Smithsonian. Judging by the fossils, the first eukaryotic organisms were probably very small and only a few centimeters wide and long.
Identifying the first eukaryotic fossils is difficult not only because of their small size and their scattered locations, but also because scientists also disagree on what constitutes a eukaryotic fossil. Some claim that the term "eukaryote" can refer to single-celled organisms that have a complex structure, shape or cellular components. Others argue that the eukaryotes must be multicellular organisms, despite how complex a single-celled organism may be. This debate complicates the categorization of the first eukaryotic cells.