The Oldest Ciliate Fossils Ever

Protozoans are some of the strangest organisms on the planet. The first ever member of this ancient domain of life were likely to have been very simple, not much more than a single lipid shell containing the bare essentials needed for a genome that encodes a life form. Today protozoans are the one of the most common creatures on Earth, with every square centimetre of soil containing potentially hundreds of thousands. They occupy a bewildering number of forms and are startlingly complex for such small beings.

One important group are the ciliates. While they are not the most diverse, with only about 7000 species, they are some of the most common marine micro-organisms. This immediately creates a problem, as when they die, they simply dissolve into their environment, leaving no fossil traces, making them a very rare fossil group and incredibly hard to assign a date of origin to. However ciliate fossils have been discovered, and are among the most ancient on Earth, which push back the date for the origins of truly complex life itself.

The remarkable, 635 to 715 million year old, goblet
shaped, tintinnid ciliate protozoan fossils

The fossils were first discovered during excavations in 2008 conducted by a team from MIT and Harvard University at the Tsagaan Oloom Formation in Upper Mongolia. The rocks there are some of the few to preserve the traces of the most devastating climatic event in Earth history, Snowball Earth, a global ice age lasting for over 100 million years which destroyed life almost completely. Rock samples were collected by Francis Macdonald, an assistant professor of geology at Harvard.

They were taken back to the lap, broken down with acid, and the residue examined under a microscope for micro fossils. They found the presence of ciliate micro-fossils with a temporal range of 635 to 715 million years old. Hundreds were uncovered, each with a beautiful, goblet-like shape created by constricted necks and flaring collars covered in a bulbous structure. Tanja Bosak from the Cecil and Ida Green Career Developmental Assistant Professor at MIT's Department of Earth, Atmospheric and Planetary scientists, found that they were almost exactly the same as a modern day group of ciliates called the tintinnids.

Tintinnids are a fascinating group of protozoans because of their tough, bubble-like shell, which is flexible yet resilient. This means they fossilize more readily than many other micro-organisms. These new fossils as well as representing the oldest known protozoans on Earth are also the oldest tintinnids. Nicholas Butterfield, a lecturer of palaeobiology at the University of Cambridge, suggests that protozoans may be even older.

The resistant shell means that tintinnids play a vital role in the oceans' carbon chemistry. 'You have this resistant material that sinks to anaerobic oceans, where it takes longer to degrade,' Bosak says. 'As a result, you could sequester more carbon … that in turn releases more oxygen. More oxygen in the atmosphere would foster complex, oxygen-breathing life. According to Bosak, the geologic timing is consistent with this theory.


The ciliate fossils date to the period between the two ice ages. Soon after the second ice age, fossils of the first animal embryos were identified. 'It’s conceivable that they only evolved, or became ecologically important, at this time,' says Butterfield. 'Ciliates probably do play an important role in how the oceans work, but there’s no reason to believe that that role wasn't defined much earlier.' The team stated that they planned to conduct a closer analysis of the fossils to try and extract as much data as possible.