Oldest Complex Life Fossils Found in Australian Mud

Decades ago, mineral exploration companies drilled cylindrical cores of rock in northern Australia, looking for valuable ore deposits. What they actually found, without realizing it, were fossils that could help answer one of biology’s oldest questions.

Forgotten Rock Cores From a Darwin Warehouse

The rock cores sit today in an open-air warehouse maintained by the Northern Territory Geological Survey in Darwin, Australia. Many are mudstone, sedimentary rock formed from ancient seafloor mud, drilled from an inland sea that covered much of northern Australia more than 1.5 billion years ago. A new study, published in Nature by researchers Maxwell Lechte, Leigh Anne Riedman, Susannah Porter, Galen Halverson, and Margaret Whelan, examined these forgotten cores in detail.

More Than 12,000 Fossils

The team crushed and dissolved samples of the mudstone, then examined the organic residue left behind under a microscope. In total, they identified more than 12,000 fossils of single-celled organisms with a level of cellular complexity typical of eukaryotes, the broad category of life that includes all animals, plants, fungi, and algae, distinguished from simpler bacteria and archaea by features like a cell nucleus and specialized internal structures. Some of these fossils date back as far as 1.75 billion years, making them the oldest confirmed eukaryote fossils identified anywhere on Earth.

Oxygen: Friend, Foe, or Both?

Nearly every eukaryote alive today depends on oxygen, using it to break down food and generate the large amounts of energy complex life requires. But that dependence has looked less certain in recent years, since researchers have identified some living eukaryotes capable of thriving without oxygen. There’s also solid geological evidence that oxygen was considerably scarcer when eukaryotes first evolved than it is now, raising real doubt about whether early eukaryotes actually needed oxygen from the very beginning.

A Clean Result

To test this directly, the researchers analyzed the chemistry of the mudstone samples themselves to determine whether the ancient seawater that deposited each layer contained oxygen or not. Eukaryote fossils turned up in a wide range of environments, from coastal mudflats to open ocean settings, but only in samples that had formed in oxygenated water. Mudstone from oxygen-free settings contained only simple, prokaryotic organisms, no eukaryotes at all. That clean pattern supports a long-standing hypothesis that oxygen played a genuinely essential role in the evolution of early eukaryotes, at least during this particular period between roughly 1.7 and 1.4 billion years ago.

Sources

This report draws on an account written by the study’s own authors, Maxwell Lechte and Leigh Anne Riedman, published via The Conversation and ScienceDaily, based on their peer-reviewed study in Nature.