A team of scientists has discovered the remains of an ancient lost world made up of mysterious organisms believed to have existed 1.6 billion years ago. The discovery is based on the groundbreaking discovery of what scientists call “protosterol biota” in ancient Australian rocks.

The scientists say these microscopic creatures help fill a huge gap in man’s understanding of the early evolution of the nucleated family of cell lifeforms known as eukaryotes. These creatures are believed to have grown in wetlands all over the Earth billions of years before plants and animals appeared. However, they remained hidden from fossils until this discovery.

Scientists are working hard to try to unravel the roots of how humans and eukaryotes were related during what they call Earth’s “middle ages,” believed to have begun about 1.7 billion years ago. This Middle Ages lasted a billion years as the simple organisms of that time began to evolve into more complex life. This newly discovered ancient lost world may finally give us important information about them.

The researchers say the eukaryotes discovered represent “early stages of eukaryotic evolution that do not yet have a complete pathway for sterol biosynthesis.” This makes them “witnesses of an ancient lost world” of organisms, the ancient world of root-group eukaryotes that researchers believe may have been more during Earth’s middle ages.

“It was a real eureka moment for us to realize that 1.64 billion-year-old rocks contain fossil protosteroids.” Benjamin Nettersheim, one of the leaders of the new study, told Vice. It is possible that the protosterol biota that lived during this period was the direct or indirect ancestor of eukaryotes living today. Although they’re most likely cousins, researchers believe.

However, what these organisms look like and even how they move around our planet is difficult to determine, or even predict, because they are known only by the chemical byproducts they form. The study has been published in the journal Nature, and the researchers hope to learn more about this ancient lost world, and even more about the mysteries of evolution, by studying organisms more deeply.