Researchers have found what could be the oldest known fossils for two single-celled microbes, the tube-shelled foraminifera and the hairy ciliates.

The newly discovered fossils, which dates back more than 100 million years earlier than the oldest foraminifera and ciliates previously known, was found in ancient rock deposited on the ocean floor between 635 and 715 million years ago during a time known as "Snowball Earth."

According to Tanja Bosak, a study researcher and assistant professor of geobiology at the Massachusetts Institute for Technology, these types of single-celled creatures may have been around for considerably more than 1 billion years.

The fossils have evaded researchers because the deposits do not contain a type of rock that typically preserves fossils, particularly something this small and fragile, Bosak said.

Both types of fossil first appear in layers of rock called cap carbonates, laid down as the world was leaving the earlier snowball state, which occurred 716 million years ago. Fossils belonging to foraminifera were found in rocks from Namibia, while ciliates were found in rocks from Mongolia.

The microbe foraminifera has protective shells made by picking up tiny grains of mineral which they stick to their exterior using a sugary compound. The ciliates, on the other hand, are covered with tiny hairs called cilia and closely resemble the planktonic organisms called tintinnids.

Bosak said the discovery of these organisms reveals a possible mechanism by which the oxygen levels in the atmosphere increased, allowing life to become more complex.

The research was published in two journals: Geology, and Earth and Planetary Science Letters.