Lauren Fuge
Geologists are fond of giving dramatic names to past events, such as “The Great Dying” or “Snowball Earth” or “The Cambrian Explosion”. But perhaps the most inaccurate is the “Boring Billion”, also charmingly known as “The Dullest Time in Earth’s History”.
This drab term refers to nearly a quarter of the planet’s entire history, spanning from 1.8–0.8 billion years ago. It’s thought to have been nowhere near as dynamic and changeable as the planet today, but instead rather slimy and static. Volcanoes lay dormant, the atmosphere had much less oxygen than today, the oceans swilled about stagnant, and ice ages were absent.
Tectonic plates ground to a standstill, too – the supercontinent Rodinia remained surprisingly stable from 1.8 billion to 750 million years ago.
“The climate appears to be very stable, and life evolution was sluggish,” adds geologist Ming Tang from Peking University in Beijing. “For example, eukaryotes appeared on Earth about 1.7 billion years ago but only rose to dominance some 0.8 billion years ago. How Earth got stuck in this one-billion-year stasis is unclear.”
Tang is the lead author of a recent study that suggested no new mountains formed during these years, which could have led to the evolution of life stalling.
“Erosion and weathering of mountain rocks provide life-essential nutrients such as phosphorus and many trace metals,” he explains. “These nutrients are critical to sustaining a productive primary biomass, which produces food and oxygen to be used by more complex life.”
Their research uses hardy, ancient minerals called zircons to estimate past crustal thickness. Just the concept of this is pretty impressive – geologists are only just starting to be able to study the planet back into the deep recesses of time.
Until 20 years ago, according to geologist Alan Collins from the University of Adelaide in Australia, the only time period we could really study was the last 500 million years – because that’s the period that has fossils in it.
Advances in dating techniques, he says, have “really opened up these really deep time periods – the eight-ninths of Earth history that’s before fossils”.
Collins – who was not involved in Tang’s study – contends that these new advances are beginning to tell us that this so-called Boring Billion was not so yawn-inducing after all.
“Things definitely were different back then, but the paleoclimate and the paleobiological community are realising that actually quite a lot of it is quite interesting, not just boring,” he says.
There’s no doubt that the term “Boring Billion” is catchy. It originated in 1995, when geologists noticed that the isotopic proxies for temperature and life were quite stable through this period. This led co-author Robert Buick to paraphrase Winston Churchill and write that “never in the course of Earth’s history did so little happen to so much for so long”.
Paleontologist Martin Brasier took inspiration from this and spun it into the unfortunate phrase “Boring Billion”, which has stuck around in geological circles ever since.
But further research has suggested that these isotopes were more variable than thought. Moreover, as Collins points out, this period is “actually where the most fundamental biological innovation happened – that’s endosymbiosis, which is the formation of eukaryote fossils”.
The first eukaryote cells – which today make up every plant, animal and fungi today – evolved at the very beginning of the Boring Billion. Then, around 1.6 billion years ago, plants diverged from animals and fungi and 1.5 billion years ago animals and fungi split.
What happened during this “dull” stretch of time therefore set the stage for explosions of evolution and diversity later on, providing a pathway to a planet that was able to support complicated ecosystems and complicated life.
A 2018 study from of the University of Tasmania, for example, suggests that the stress of limited nutrients during the Boring Billion may have actually promoted endosymbiosis, where single cells are ingested by another.
“There’s always been a lot of focus on macro evolution and the Cambrian explosion 541 million years ago,” says lead author of the study, Indrani Mukherjee. “Yet, evolution really starts with the transformation of simple cells into complex ones, and it is in the Boring Billion that scientists have previously found the first fossilised evidence of a complex cell.”
Complex life could have even been attempting and failing to get going this whole time, if we pay attention to pulses of oxygen visible in the geochemical record. Since life inserts an appreciable level of oxygen into the atmosphere through photosynthesis, Collins reckons these pulses could have been the result of “blooms of life in different places that don’t survive – they bloom and then they die out again”.
There were probably a lot of false starts until around about 700 million years ago, when we reached a tipping point where the processes of life became self-sustaining.
This point – ending the Boring Billion once and for all – was likely triggered by a combination of plate tectonics and climactic changes: glaciers grinding up rocks and moving elements into oceans, as well as mountains becoming eroded, providing the nutrients for life.
“But we don’t we don’t really understand all those things yet,” Collins says.
He was recently part of a collaborative effort to produce a full tectonic plate reconstruction over the past billion years – something that will help scientists understand how all these Earth systems are connected into the deep past. Taking such reconstructions even further back in time could shed new light on the Boring Billion.
Full-plate tectonic animation of the last billion years. White areas represent oceanic crust, green areas are modelled continental lithosphere, and blue areas are additional present-day continental crust. Spiked lines are subduction zones; blue lines are mid-ocean ridges; black lines are transform boundaries. Ma = Millions of years ago. 1 second = 25 million years. Credit: Merdith et al. 2021 (Earth Science Reviews).
Plus, studying periods like this could even help us understand potential extraterrestrial life.
“It’s not beyond the realms of possibility that we will find microbes on Mars,” Collins says. “The life that could exist on Mars will be very much like the life in the Mesoproterozoic – it will be single cellular archaea and bacteria…very much like that sort of world.”
Paleontologists are beginning to uncover increasingly rich evidence of life from across that period, such as fossilised algae from 1.56 billion years ago, and lichen fungi that grazed on microbial mats around 1.3 billion years ago.
It’s really what has happened since that time that has made our Earth so different to other terrestrial worlds, and so understanding where we have come – that is, the pathway of life through the Boring Billion – is fundamental to understanding our unique place in the Solar System.
Perhaps we should follow the lead of geologists Peter Cawood and Chris Hawkesworth, and call this less explosive time “Earth’s Middle Age” instead.