Ancient marine life left clues to early evolution

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A unique ancient marine creature, preserved in Australian and African rocks, has been discovered by University of Melbourne geologists. It suggests a previously unknown advance in the evolution of complex life. The discovery adds to the scientific record of early Earth.

The creatures, called the Oodnaminta biota, lived around 600–700 million years ago, in a relatively balmy period between two ice ages. This was at the end of the Precambrian period, when multicellular organisms first started to appear.

A research team led by Associate Professor Malcolm Wallace and Dr Ashleigh Hood from the School of Earth Sciences analysed sediment in desert rocks in the Flinders Ranges in South Australia and in the Otavi Mountainland and Kaokoveld Desert in northern Namibia. These areas were once marine reefs and would have been biodiversity hotspots – just like modern-day reefs. There are  a handful of these sites around the world.

The researchers analysed the rocks in place and then took small samples back to their laboratory in Melbourne. They used conventional microscope observation and tomography (a type of 3D X-ray imaging) to scan the samples, which revealed the Oodnaminta biota inside the rocks.

The team initially thought that the Oodnaminta biota were a form of sponge, because they looked similar to other fossils of ancient sponges. But the researchers have since found the fossils lacked some key features of sponges and now suggest that they might be a complex microbe or a primitive precursor to multicellular animals.

The Oodnaminta biota are notably more complex than older known fossils. This means they represent a significant evolutionary advance.

Almost all large complex organisms require oxygen or light to live but the Oodnaminta biota lived in a dark, low-oxygen environment. The fossils were found in the framework of ancient marine reefs, which showed that they lived in very deep water. Research by the same team found that levels of atmospheric and ocean oxygen were very low when the Oodnaminta biota were alive.

Next steps

The research team will compare the chemical composition of ancient reefs in Canada and Australia to learn more about these ancient marine environments.