Strother of his samples: Each gram of rock contained “thousands of microfossils.” Photograph: Lee Pellegrini

The study of life in the Precambrian era—from the earth’s beginnings some 4.6 billion years ago to about 600 million year ago, when complex organisms are said to have developed—has long focused almost exclusively on the marine, or oceanic, environment. The terrestrial regions, scientists have believed, were, for this 90 percent of the planet’s existence, either devoid of life or, at most, home to very simple bacteria known as cyanobacteria. “So much for speculation,” wrote Paul Strother, an earth and environmental sciences research professor at Boston College’s Weston Observatory, in a 2010 conference abstract of work that he undertook with colleagues from the University of Sheffield and Oxford University. “We have spent two field seasons collecting samples . . . and [for] the first time we have had a good look at what actually lived in Precambrian terrestrial habitats”—environments that included lakes, streams, stream banks, and soils. What they discovered, the abstract continued, was a “surprising array of far more complicated evidence of life” than anyone, including the researchers themselves, expected.

The findings, which were published this year in Nature, derive from rock samples of a seaside stone called Torridonian gathered by Strother and Charles Wellman, a reader in palaeobiology at the University of Sheffield, from outcroppings along the northwest coast of Scotland. The area has long been known as a source of ancient microfossils (less than a millimeter in size, requiring a hand lens or a microscope for viewing), but has mostly been passed over by scientists.

Strother works in the field of paleontology, specializing, as he puts it, in “the fossil record of how plants originated.” About a decade ago, he visited Wellman at Sheffield, where the two examined long-neglected slides of Torridonian microfossils collected in the 1960s. They noticed “all kinds of weird things,” Strother says, including “multicellular balls of tiny cells, membranous sheets of organic matter, and various irregular clusters of cells stuck together.” They decided to build a more extensive collection of Torridonian microfossils and attempt to decipher their content and age, and they joined forces with Leila Battison and Martin Brasier of Oxford’s department of earth sciences. In 2007, the team received funding for this project from the National Aeronautics and Space Administration’s exobiology program, which according to its website “supports research on the origins and early evolution of life, the potential of life to adapt to different environments, and the implications for life elsewhere.”

Strother and Wellman spent a total of three weeks in 2008 and 2009 collecting samples of the gray Torridonian shale in 17 locations along a 125-mile stretch of coastline from the Isle of Skye to the Stoer peninsula. Because previous investigators had visited a limited number of sites, and “didn’t know how to process very well,” says Strother, “they just got one or two slides out of it, and they looked and said there’s not much here.” According to Strother, he and Wellman collected close to 100 samples, “maybe 60 of which are extremely well preserved,” with each gram of rock containing “thousands of microfossils.”

The rock, which is of aggregate composition, was processed at a laboratory in Sheffield using a technique known as maceration: 10- to 20-gram slivers of an individual sample are mixed with hydrofluoric acid, which dissolves the minerals that make up the rock, releasing the 1 or 2 percent of organic matter—spores, algal cysts, pollen—that is, in Strother’s words, “trapped between mineral grains.” After further separation and very fine screening, the carbon remains are placed on slide mounts and studied with either a conventional microscope or by transmission electron microscopy.

When they examined the fossilized bits, Strother says, “We expected to find evidence of cyanobacterias.” And they did. But the researchers also found a great number of unclassified spherical microfossils possessing a true nucleus that are thought to be the cysts (resting stages) of freshwater algae. Many of these cells are surprisingly large—three to four times the size of modern phytoplankton—and one specimen, measuring almost a millimeter in width, is the largest non-marine fossil known to be Precambrian. Such enormous girth, says Strother, indicates that, contrary to prior thinking, “large cell size is not restricted in time and not confined to the marine realm.”

In addition to single-celled organisms, Strother and Wellman discovered larger, more complex forms that are almost certainly multicellular and were likely exposed to the atmosphere periodically, due to the wet/dry cycles that would have occurred in at least some terrestrial water bodies. (The researchers also found fossilized impressions of raindrops.) Some of the cells were clustered in geometric arrangements that suggest they had undergone cell division by meiosis. This is an indication that by one billion years ago “a sexual phase had evolved in the land-based eukaryotic life cycle,” some 500 million years earlier than previously thought, says Strother.

To date, Strother and his fellow researchers have noted roughly 50 species among the samples they have examined, “five times greater than the diversity we knew about [previously],” he says. In the Nature article, the researchers write that this heterogeneity most likely reflects adaptations to a terrestrial environment that was both more varied and more severe than the ocean, with greater extremes in temperature and salinity. The challenging conditions of the land-based environments, Strother suggests, were a spur to speciation.

Strother and his colleagues have “tons of work” to do to classify the Scottish samples into species. Strother is also investigating a rock deposit in Michigan that is similar to the Torridonian outcroppings in age and composition. He says the rock is “loaded” with microfossils, including “some species overlap” with the Scottish finds. The Michigan rock also needs taxonomic work. When that is completed, Strother says, “we can begin to reconstruct what the biological component of life was like one billion years ago.”

Read more by Thomas Cooper