Gunflint Chert: Microfossils That Proved Precambrian Life

The Gunflint Chert is a rock layer about 1.88 billion years old on the north shore of Lake Superior in Ontario, Canada, famous for its exquisitely preserved microscopic fossils. In 1954 the geologist Stanley Tyler and the palaeobotanist Elso Barghoorn reported that it contained the remains of ancient microbes, the first Precambrian fossils to win broad scientific acceptance. Their discovery launched the modern search for the earliest life on Earth.

Darwin’s puzzle

In On the Origin of Species (1859), Charles Darwin acknowledged a difficulty for his theory: complex animals appear suddenly in rocks of the Cambrian Period, with almost nothing known from older strata. If evolution was gradual, a long history of earlier life should have existed.

For nearly a century afterwards, claims of Precambrian fossils were repeatedly made and rejected. Some were mineral structures mistaken for organisms. By the mid-twentieth century many geologists doubted that convincing Precambrian life would ever be found.

The discovery

The Gunflint Iron Formation belongs to the Animikie Group, a sequence of sedimentary rocks around the western end of Lake Superior. It contains layers of black and red chert, a hard rock made of very fine-grained silica, along with iron-rich sediments.

In 1953 Stanley Tyler, a geologist at the University of Wisconsin who was studying the iron formations, examined thin slices of black chert collected near Schreiber, Ontario. Under the microscope he saw tiny filaments and spheres that looked organic. He contacted Elso Barghoorn, a Harvard expert on fossil plants, and in 1954 they reported the discovery in Science.

The short 1954 paper described the structures as preserved plants, reflecting the classification of the time, when cyanobacteria were called blue-green algae. A full description followed only in 1965, after Tyler’s death in 1963, when Barghoorn and Tyler formally named a range of fossil microorganisms. The geologist Preston Cloud published his own analysis of their significance in the same year.

What the fossils show

The Gunflint microfossils include thin filaments, small spheres and distinctive star-shaped and umbrella-like forms. They received names such as Gunflintia, Animikiea, Huroniospora, Eosphaera, Eoastrion and Kakabekia, the last named after Kakabeka Falls near Thunder Bay.

Many resemble modern bacteria, including cyanobacteria, which produce oxygen by photosynthesis, and microbes that obtain energy from iron. They lived in shallow-water microbial communities, some forming stromatolites, at a time when the oceans were changing after the Great Oxidation Event. The rocks are part of a wider episode of iron deposition that also produced banded iron formations.

Their preservation is remarkable because the cells were entombed in silica soon after death. The silica hardened before the delicate structures could decay, a process known as permineralisation.

How scientists know they are fossils

Because earlier claims of Precambrian fossils had failed, the Gunflint structures were examined with great care. Several features persuaded most scientists that they were biological. They occur in large numbers, in a limited set of recurring shapes and sizes, much like a population of living microbes. Many are made of organic carbon rather than minerals alone, and some filaments show internal divisions resembling cell walls.

Their setting also made sense. The fossils are found in rocks formed in shallow water, where microbial mats grow today, and they are associated with stromatolites. These criteria, applied together, became a model for assessing later claims of ancient life, including some that have proved far more controversial.

Dating and later research

Volcanic ash beds within the formation have been dated to about 1.878 billion years ago. The top of the Gunflint sequence also records an extraordinary event: a layer of debris thrown out by the Sudbury impact about 1.85 billion years ago, identified in 2005 more than 600 kilometres from the crater.

The Gunflint discovery inspired a generation of researchers, including J. William Schopf, a student of Barghoorn, who went on to study much older microfossils. It established the techniques, especially the study of chert thin sections, that are still used to investigate early life, including much older finds such as the Pilbara stromatolites of Australia.

Timeline

• c. 1.88 billion years ago: Microbial communities are preserved in silica in shallow seas along the edge of the Superior craton.
• c. 1.85 billion years ago: Debris from the Sudbury impact is deposited over the Gunflint rocks.
• 1859: Darwin discusses the absence of fossils below the Cambrian in On the Origin of Species.
• 1953: Stanley Tyler observes microscopic structures in chert from near Schreiber, Ontario.
• 1954: Tyler and Barghoorn report the Gunflint microfossils in Science.
• 1963: Stanley Tyler dies before the full description is published.
• 1965: Barghoorn and Tyler formally describe the microfossils; Preston Cloud publishes his analysis.

Why it matters

The Gunflint Chert transformed the study of the history of life. It proved that fossils could survive from deep in the Precambrian and showed that microbes had been shaping Earth for well over a billion years before animals appeared. Almost every later discovery of ancient microfossils builds on the methods and confidence that came from these rocks.