The history of life in the oceans on early Earth is well preserved in thick sequences of marine sediments exposed in Australia and South Africa. These rocks reveal the progression of life over the course of billions of years, from simple bacteria to more complex eukaryotic cells. However, the history of life on land is more difficult to ascertain. Microfossils are rare, and the presence of microbes in terrestrial environments is most often inferred by variations in the isotopes of elements such as carbon and iron, and from the presence of structures reminiscent of microbial mats. Indeed, the earliest putative marine bacteria are 3.5 billion years old, whereas the most commonly cited terrestrial microfossils date back only 1.2 billion years.

Credit: © NPG 2011

These terrestrial fossils — known as the Torridonian microfossils — come from ancient lake and river sediments in northwest Scotland. Paul Strother of Boston College, Massachusetts, and colleagues returned to the Torridonian site and found a wealth of well-preserved microfossils that allowed them to describe the three-dimensional structure of the individual fossils (Nature doi:10.1038/nature09943; 2011). The reconstructions revealed an array of body shapes and organic structures, and even some cell clusters that verged on tissue-grade organization.

The fossils, which range from tens of micrometres to almost a millimetre in diameter, exhibit a wide variety of morphologies. A number of fossils were broadly spherical, whereas some had spines and others arm-like projections. Many had complex cell walls, with different thicknesses and compositions. Some walls had ornamentation, as well. Intriguingly, the fossils also showed a number of features — such as sutures or highly ordered arrangements of numerous cells — consistent with eukaryotic organisms.

The cells of eukaryotes comprise complex structures enclosed by membranes. These structures are lacking in the bacteria and archaea that made up the earliest life on Earth. Eukaryotes appeared in the marine record at least 1.5 billion years ago (probably much earlier), but reports of their terrestrial counterparts before 540 million years ago are sparse.

The finding of a wide range of eukaryotic fossils in the 1.2-billion-year-old Torridonian rocks suggests that freshwater habitats were colonized by eukaryotes earlier than previously thought. However, although the organisms were able to survive during periods of desiccation, it is unclear whether they were able to thrive for long periods outside of aquatic conditions. The search for early life on dry land is therefore far from over.