The Boring Billion: Earth's Quiet Middle Age in the Proterozoic

The Boring Billion is the informal name for a long stretch of Earth’s history, from about 1.8 to 0.8 billion years ago, when the planet’s climate, ocean chemistry and atmosphere appear to have stayed unusually stable. It falls in the middle of the Proterozoic Eon, between the first rise of oxygen and the extreme ice ages of the Neoproterozoic. Its apparent calm is a puzzle, because complex cells already existed but took hundreds of millions of years to become diverse and abundant.

Background: after the first rise of oxygen

Around 2.4 billion years ago, the Great Oxidation Event brought free oxygen into Earth’s atmosphere for the first time, thanks to photosynthesising cyanobacteria. That transition was accompanied by glaciations and large swings in the chemistry of the oceans.

By about 2 billion years ago, however, the turbulence had faded. What followed was a long interval with few major glaciations, no huge swings in the carbon cycle and only modest evolutionary change. Geologists usually date it from about 1.8 billion years ago, though some start it at 1.85 or 1.7 billion and end it between 0.85 and 0.75 billion years ago.

Why it looks boring

The name was inspired by the rock record. In the 1990s Roger Buick and colleagues, studying carbonate rocks in Western Australia, described the period as perhaps the dullest in Earth’s history. In 1998 Martin Brasier and John Lindsay published a study of northern Australian rocks titled “A billion years of environmental stability”, and Brasier’s phrase “the boring billion” stuck.

Several features stand out. Carbon isotope ratios in limestone, a sensitive record of the global carbon cycle, stay remarkably flat. There is little clear evidence of glaciation between about 2.1 and 0.72 billion years ago. Banded iron formations, common earlier, become rare.

Life also seems to have changed slowly. Stromatolites, layered mounds built by microbial mats, remained common in shallow seas. Eukaryotes, the group of complex cells with a nucleus, are present as fossils, but chemical traces called biomarkers suggest they remained ecologically minor in the oceans until after about 800 million years ago.

Oceans and oxygen

Most geochemists think atmospheric oxygen during the Boring Billion stayed well below today’s level, perhaps only a small percentage of it, though estimates vary widely. The surface ocean was oxygenated, but deeper waters were largely oxygen-free.

In 1998 the geochemist Donald Canfield proposed that much of the deeper ocean was not only oxygen-poor but sulphidic, full of hydrogen sulphide, a scenario often called the “Canfield ocean”. Later work suggests that sulphidic water was mostly restricted to zones along continental margins, while much of the deep ocean was iron-rich instead.

In 2002 Ariel Anbar and Andrew Knoll suggested a link between ocean chemistry and evolution. Sulphidic waters could have removed trace metals such as molybdenum, which organisms need to process nitrogen. Scarce nutrients might in turn have held back eukaryotic algae.

Plates and supercontinents

The Boring Billion was not quiet beneath the surface. The supercontinent Columbia, also called Nuna, assembled around 1.8 billion years ago and began to break up from about 1.5 to 1.3 billion years ago. Later, the supercontinent Rodinia assembled between about 1.3 billion and 900 million years ago.

Some geologists, including Nick Roberts in a 2013 paper, have argued that the long-lived supercontinent arrangement and slow tectonic change helped to keep the environment stable. This remains one hypothesis among several.

Not so boring after all?

Many researchers now question the label. Shales from the Xiamaling Formation in North China, about 1.4 billion years old, have been interpreted as evidence of a temporary rise in oxygen. Other studies point to short-lived oxygenation pulses at different times.

Biologically, the period saw important firsts. Eukaryotes diversified into several lineages, multicellular forms appeared and the red alga Bangiomorpha, about 1.05 billion years old, is the oldest widely accepted fossil assignable to a living group of eukaryotes. Sexual reproduction and photosynthetic eukaryotes were therefore established well before the period ended.

The debate is partly about scale. Compared with the turbulent eras before and after, the mid-Proterozoic was stable; looked at closely, it contains plenty of change.

Timeline

• c. 2.4-2.1 billion years ago: The Great Oxidation Event and associated glaciations transform Earth’s surface.
• c. 1.8 billion years ago: The Boring Billion is usually said to begin, around the time Columbia (Nuna) assembles.
• c. 1.6 billion years ago: The Mesoproterozoic Era begins.
• c. 1.4 billion years ago: Rocks in North China record a possible temporary rise in oxygen.
• c. 1.1-0.9 billion years ago: Rodinia assembles through the Grenville orogeny.
• c. 1.05 billion years ago: Bangiomorpha lives on tidal flats in what is now Arctic Canada.
• c. 0.8 billion years ago: Algae begin to expand as Rodinia breaks apart, and the stable interval ends.
• c. 717 million years ago: The Sturtian glaciation, the first Snowball Earth episode, begins.

Why it matters

The Boring Billion asks why complex life took so long to take off once complex cells existed. Its answer, still debated, probably lies in the interplay of oxygen, nutrients and plate tectonics, and it shows that long periods of stability can be just as important as sudden revolutions in shaping the history of life.