Tiny 1.7-billion-year-old fossils may clarify how complex life began

Paleontologists are examining some of Earth's oldest rocks for microscopic fossils that could illuminate how simple microbial life gave rise to complex organisms. The transition from a planet dominated by bacteria to one populated by plants, animals, and fungi occurred over billions of years, but the fossil record for this period remains sparse.

Eukaryotes, cells with a nucleus and organelles such as mitochondria, represent the first complex life on Earth. Every animal, plant, and fungus today descends from these organisms, which appeared by at least 1.7 billion years ago. Their emergence set the stage for later multicellularity and the diversification of visible life.

Finding fossils of these early eukaryotes is difficult because they lacked hard shells or skeletons and have endured billions of years of geological alteration. Researchers must rely on rare environments, such as ancient clay deposits in shallow marine settings, that could preserve fragile cellular material.

Ross Anderson of the University of Oxford focuses on a roughly 100-square-kilometer region near Svalbard, Norway, where rocks from a former shallow sea are exposed. Similar work in Australia recently yielded eukaryotic microfossils dated to about 1.75 billion years ago, among the oldest known.

Ancient coastal environments are considered promising because they offered abundant nutrients and organic matter that may have supported greater diversity and the development of multicellularity. Desert and Arctic landscapes today provide accessible outcrops where vegetation does not obscure the ancient rocks.

Even in favorable locations, the search remains challenging. The organisms were microscopic, lacked protective tissues, and the fossil record from this era is poorly sampled. Scientists are improving their ability to identify rock types most likely to preserve early fossils, gradually building a clearer picture.

Understanding how complex life arose on Earth also informs the search for life elsewhere. By learning which environments preserve ancient organisms here, researchers can better recognize potential signs of life on Mars or icy moons such as Europa and Enceladus.

Anderson notes that much of his work on clay deposits was originally motivated by astrobiology. A clearer reconstruction of Earth's early evolutionary history helps scientists estimate how likely life is to arise and become complex on other worlds.