Stored in an open-air warehouse in tropical Darwin, Australia, are dozens of trays containing cylindrical cores of rock. They are from drill holes bored hundreds of metres below the surface by mineral exploration companies decades ago.

Some of these cores at the Northern Territory Geological Survey are mudstone – a type of sedimentary rock formed from hardened seafloor mud. The companies that drilled these cores were largely unaware that within these mudstones were fossils of microscopic organisms buried on the seafloor of an ancient inland sea that covered much of northern Australia over 1.5 billion years ago.

As our new study, published today in Nature, shows, these fossils are crucial for addressing a longstanding puzzle about the major evolutionary leap that led to all complex life on Earth: the origin of eukaryotes.

All life on Earth can be placed into one of two types which are fundamentally different at the cellular level.

Prokaryotes (bacteria and archaea) have simple cellular organisation and are mostly single celled. Eukaryotes – including all animals, plants, algae and fungi – are very different. They have much more complicated cells featuring a nucleus and other........

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