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 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.
Small but complex
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 organization 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 specialized structures such as organelles which perform specific jobs.
The eukaryotic revolution transformed the planet. It led to the rise of animals and, eventually, to us. Based on observations from the genes of living organisms, it is now widely agreed that the last common ancestor of all living eukaryotes resulted from the symbiotic union of (at least) two prokaryotic microbes: an archaeon and a bacterium.
The first evidence for eukaryotic life comes in the form of these fossils of single-celled organisms. They show a level of cellular complexity not seen among prokaryotes, but common in eukaryotes.
Eukaryote fossils can be found around the world in rocks dating back at least 1.5 billion years. The fossils of the Northern Territory, the oldest of which date back to 1.75 billion years ago, are the oldest currently known eukaryote fossils globally.
But the ancient world in which early eukaryotes evolved remains shrouded in mystery. And so many fundamental aspects regarding their nature are unknown.
Oxygen – friend or foe?
Many types of bacteria can live and grow in places without oxygen. But nearly all eukaryotes alive today use oxygen for their survival. That’s because aerobic respiration – breaking down food using oxygen – provides the vast amounts of energy that complex life demands.
But the idea that oxygen has always been beneficial for all eukaryotes has come under fire in recent years. This follows the surprising discoveries of enigmatic eukaryotes that can thrive in conditions without oxygen.
There is also mounting evidence from the geological record that when eukaryotes were first evolving, oxygen was likely much scarcer. This means oxygen-free marine habitats would have been the norm. Collectively, these observations have called into question the assumption eukaryotes have depended on oxygen since their inception.
Genetic studies of living microbes belonging to groups considered closest to the ancestors of the first eukaryote can offer key insights into eukaryote ancestry. But only the fossil record can tell us about long-extinct lineages. And only geology can offer a window into the kind of world these organisms lived in.
More than 12,000 fossils
For our new study, we crushed up samples of the mudstone cores stored in Darwin, then dissolved them. We identified more than 12,000 fossils by analysing the organic residue left behind by this dissolution under a microscope.
We also studied the mudstones the fossils were preserved in to better understand what the environment was like when the sediments were deposited. This offered insight about the habitats in which these eukaryotes lived. And by analysing the chemistry of these mudstones, we could determine whether oxygen was present in the ancient seawater.
Our results show that eukaryote fossils were found in environments ranging from coastal mudflats to the open sea. But they were present only in samples deposited in oxygenated settings. Samples from oxygen-free environments contained only simple, prokaryotic forms.
This suggests that even the oldest known eukaryotes that lived on Earth 1.7 to 1.4 billion years ago were dependent on oxygen. These data lend support to a long-held hypothesis that oxygen played a key role in driving the evolution of early eukaryotes.
Resolving the drivers and context of the major evolutionary leap represented by early eukaryotes is one of the major outstanding questions in the life sciences. Ongoing studies of these enigmatic, ancient microfossils will no doubt tell us more about our own origins – and our place in the cosmos.
