Pilbara Stromatolites: Evidence of Life 3.5 Billion Years Ago

The Pilbara stromatolites are fossil mounds built by microbes in what is now Western Australia between about 3.48 and 3.43 billion years ago. Found in the Dresser and Strelley Pool formations of the Pilbara Craton, they are the oldest evidence of life on Earth that most scientists accept. They show that microbial communities were already widespread and varied less than a billion years after the planet formed.

What stromatolites are

A stromatolite is a layered structure made of sediment and minerals, built by communities of microorganisms. Microbes such as cyanobacteria grow as thin, sticky mats on the sea floor or in shallow water. Sediment settles on the mat, the microbes grow up through it, and minerals precipitate around them. Over many years this builds up domes, cones or columns with fine internal layering.

Stromatolites are therefore not fossils of individual organisms but records of their activity. Living examples still form today in a few places with extreme conditions, notably Hamelin Pool in Shark Bay, Western Australia, where very salty water keeps away the snails and other animals that would otherwise graze the mats.

The Pilbara Craton

The Pilbara Craton is one of the best-preserved pieces of ancient crust on Earth. It covers a large area of north-western Australia around the towns of Marble Bar and Port Hedland. Its rocks, between about 3.6 and 2.8 billion years old, have escaped the intense heating and deformation that destroyed or altered most rocks of similar age elsewhere.

The craton consists of dome-shaped bodies of granite surrounded by greenstone belts, sequences of volcanic and sedimentary rock. Within these greenstone belts, thin layers of chert and carbonate preserve features formed in shallow water, including the stromatolites. Geologists have mapped the region for decades, and the Geological Survey of Western Australia has played a key role in this work.

The fossil sites

The Dresser Formation

The Dresser Formation lies in an area called the North Pole Dome, named in jest because it is one of the hottest places in Australia. Its rocks have been dated to about 3.48 billion years ago. In 1980 the geologists Malcolm Walter, Roger Buick and John Dunlop described stromatolite-like structures from these rocks in the journal Nature, one of the first reports of such ancient microbial mounds.

The Dresser rocks formed in a volcanic setting, with hot fluids and minerals such as barite. Later work, notably by Martin Van Kranendonk and his colleagues, documented well-preserved stromatolites there. In 2017 a team led by Tara Djokic reported geyserite, a mineral deposit typical of hot springs on land, alongside stromatolites in the Dresser Formation. They argued that microbial life lived in freshwater hot-spring pools about 3.48 billion years ago, an idea that bears on whether life began in the sea or on land.

The Strelley Pool Formation

The Strelley Pool Formation is slightly younger, generally dated to about 3.43 to 3.35 billion years ago. In 2006 Abigail Allwood and colleagues published a study in Nature describing a fossil stromatolite reef exposed across roughly 10 kilometres of outcrop. They identified seven distinct forms of stromatolite, including cones, domes and egg-carton shapes, whose distribution matched different environments along an ancient coastline.

This variety mattered. Purely chemical processes might form a few simple layered structures, but a whole reef with different shapes adapted to different settings is much easier to explain as the work of living communities. In 2011 David Wacey and colleagues reported microscopic cell-like structures from sandstone at the base of the Strelley Pool Formation, which they interpreted as cells that used sulphur compounds for energy.

Debates and doubts

Claims of very ancient life are always tested hard, and the Pilbara has its own controversy. In 1993 J. William Schopf described what he interpreted as microfossils of filamentous bacteria in the Apex Chert, about 3.46 billion years old. In 2002 Martin Brasier and colleagues argued that the structures were mineral artefacts formed in hot fluids. Later studies have supported both sides, and the Apex Chert remains contested.

The stromatolites themselves are more widely accepted. Their forms, layering, chemistry and geological setting together make a strong case for a biological origin, even though individual cells are rarely preserved.

How scientists recognise ancient life

Since microbes rarely leave bodies behind, researchers judge ancient stromatolites by several tests at once. They look at shape: biological mounds often grow upward in cones or columns that steepen and branch, and their layers can thicken over crests in ways that settling sediment alone would not produce. They examine fine internal laminae under the microscope for wrinkled textures typical of microbial mats.

Chemistry adds another line of evidence. Organic carbon trapped in the layers, its isotopes, and the distribution of trace elements and minerals can point to biological activity. Finally, geologists study the setting: a structure that varies sensibly with water depth or flow along an ancient shoreline is more convincing than an isolated oddity. The Pilbara stromatolites score well on several of these tests, which is why they are so widely cited.

Myths and facts

A common claim is that stromatolites were built by cyanobacteria producing oxygen. Modern stromatolites are often dominated by cyanobacteria, but scientists do not know which microbes built the Pilbara mounds. They may have relied on other forms of photosynthesis that do not release oxygen. The atmosphere 3.48 billion years ago contained almost no free oxygen, which only accumulated during the Great Oxidation Event more than a billion years later.

Another misconception is that these are the oldest rocks on Earth. They are not; older rocks exist in Canada and Greenland, and older mineral grains in Australia. They are the oldest widely accepted fossils. Older claims from the Isua Greenstone Belt in Greenland and the Nuvvuagittuq Belt in Quebec remain disputed.

Pilbara and the search for life on Mars

Because the Pilbara preserves early life so well, it has become a training ground for planetary scientists. In 2019 members of NASA’s Mars 2020 science team, together with European researchers, visited the region to see how ancient biosignatures appear in the field. The Perseverance rover, which landed in Jezero Crater on Mars in 2021, is searching rocks of similar age for comparable traces.

By the numbers

Timeline

• c. 3.48 billion years ago: Microbial mats build stromatolites in the Dresser Formation.
• c. 3.43-3.35 billion years ago: A stromatolite reef develops along the Strelley Pool coastline.
• 1980: Walter, Buick and Dunlop describe stromatolites from the North Pole Dome.
• 1993: J. William Schopf reports possible microfossils in the Apex Chert.
• 2002: Martin Brasier’s team challenges the Apex Chert microfossils.
• 2006: Abigail Allwood’s team describes the Strelley Pool stromatolite reef.
• 2017: Tara Djokic’s team reports hot-spring deposits with stromatolites in the Dresser Formation.
• 2019: Mars 2020 scientists visit the Pilbara for field training.

Why it matters

The Pilbara stromatolites are the firmest evidence that life was established on Earth by about 3.5 billion years ago. They push the origin of life back into the early Archean and give scientists a benchmark for judging any claim of ancient life, on Earth or beyond.