Twelve thousand windows into deep time
Researchers have recovered more than 12,000 microscopic fossils from 1.7-billion-year-old rocks in northern Australia. The cells are early eukaryotes — the group that includes every plant, animal and fungus alive today — and they lived more than a billion years before the first animals appeared.
Most of what scientists know about life that old comes from a handful of specimens. A collection this large allows researchers to compare cell shapes and see which forms were common and which were rare.
Oxygen set the limit
Chemistry of the surrounding rock points to oxygen as the gatekeeper. The eukaryotes turned up only in shallow, oxygenated coastal water. In deeper water, where oxygen had not reached, the fossil assemblages contained only simple single-celled bacteria.
That pattern fits the biology. Eukaryotic cells carry a nucleus and internal compartments and need far more energy than bacteria to run. Aerobic respiration delivers that energy, but only where oxygen is available. As oxygen built up in shallow seas, complex cells could spread — and where it did not, they could not.
What the shapes show
The fossils are not uniform. Some are simple spheres. Others are far more elaborate, with jutting appendages and plates, or surfaces creased like the whorl of a fingerprint. That variety suggests early eukaryotes were already experimenting with different body plans and ways of living.
Because these cells are among the oldest eukaryotes known, they sit close to the base of the branch that eventually produced animals, plants and fungi. Understanding where they could live, and why, helps explain the long delay between the origin of complex cells and the explosion of complex life.
The work was published in an open research write-up and reported by science outlets in late September.
The samples come from the Northern Territory, an area that has produced several of the most important early-life finds of the past decade. Better dating and chemical analysis have turned those rocks into a record of how oxygen moved through shallow seas.