As humans heat up the Earth with greenhouse gases, we're recreating an ancient climate unlike anything our species has ever seen. This is drawing more attention to the history of Earth's climates, especially warm periods like the Pliocene Epoch, which many scientists consider a model for where we're headed.
At the same time, though, researchers are also shedding new light on other, very different periods in Earth's past. These too can reveal key details about our planet, and even ourselves, despite bearing little resemblance to the world we know today.
One such period is the the Cryogenian, which lasted from about 720 million to 635 million years ago. That's when Earth experienced the most extreme ice age in its history, including a global freeze known as "Snowball Earth."
Somehow, though, it was also when the first signs of complex animals appeared in the fossil record, left by creatures who set the stage for a golden age of animal life that continues today. In a new study, researchers examined the chemistry of Cryogenian rocks to learn more about this unfamiliar world — including why it was able to not only support animal life, but also seemingly launch it to new heights.
Let it snow
The planet's surface became fully or almost fully frozen during the Cryogenian, with enormous ice sheets stretching down to the tropics. (There is still some debate about the extent of this freeze, though.) Most landmasses were united in the supercontinent Rodinia, but thanks to the global glacier, Earth's entire surface may have been effectively solid. The average surface temperature probably didn't go far above freezing, and some research suggests temperatures were much colder, possibly falling below minus 50 degrees Celsius (minus 58 Fahrenheit).
There were actually two big freezes during the Cryogenian, known as the Sturtian and Marinoan glaciations, separated by a brief intermission of heat, melting ice and erupting volcanoes. This was a wild time for our planet, which was see-sawing between extremes of ice and fire, but also an important one. That's because, despite seeming like a terrible time to be alive, the Cryogenian Period apparently helped spark the dawn of complex animals — including our own ancestors.
If you're wondering how animals survived on Snowball Earth, you're not alone. It would've been incredibly difficult for animals to survive on the ice sheets, but also in the seawater below, since a global coating of ice would severely hinder the oceans' ability to absorb oxygen. Scientists have long puzzled over this apparent paradox, but the new study, published this week in the Proceedings of the National Academy of Sciences, is the latest in a growing body of research that's finally offering answers.
Explosion of animal life
Life on Earth began long before the Cryogenian, but it was mostly single-celled microbes. Even when multicellular animals did arise, they were simple, often stationary creatures, calmly filtering seawater or grazing on mats of microbes. These early animals did not yet have innovations like eyes, legs, jaws or claws, and in a world without predators, they didn't really need them.
That would soon change, however, thanks to the Cambrian Explosion, a world-altering diversification of life that gave rise to the age of animals. This may have unfolded in as few as 20 million years, which is incredibly fast for such big evolutionary changes, and it has been described as the "big bang" of animal evolution, although some research suggests it might have been more like a series of smaller bangs. Either way, the Cambrian Explosion was a giant leap in the evolution of life on Earth, giving rise to the major animal groups we know today, including the ancestors of humans and all other vertebrate animals.
Yet before this explosion began, the fossil record suggests the ascent of complex animals was already in the works. It may not have been the elaborate new creatures that came later, but complex life apparently did exist prior to the Cambrian Explosion, and seems to have begun early enough in the Cryogenian that it had to endure a Snowball Earth. These pioneers included eukaryotes, a broad term for organisms with advanced cell structures, and possibly primitive animals like sponges.
Oxygen-rich waters would've been vital for many of these early complex organisms, especially the animals, but due to limited oxygen in ice-covered oceans, scientists have long believed that kind of environment was unavailable at the time. Still, we know these early creatures survived the snowball, since we are their descendants. Faced with that contradiction, some scientists have suggested other ways eukaryotes might have made it through the Cryogenian, such as living in meltwater pools atop the ice sheets instead of in the oceans below.
According to the new study, though, even a frozen ocean might not have been as inhospitable to these ancient organisms as we tend to think.
A 'glacial oxygen pump'
The edge of the Venable Ice Shelf juts into the ocean between two peninsulas from Antarctica. (Photo: Operation IceBridge/NASA Goddard Space Flight Center [CC BY 2.0]/Flickr)
The study's authors looked at iron-rich rocks known as ironstones from Australia, Namibia and California, all of which date back to the Sturtian glaciation. These rocks were deposited in a range of glacial environments, the researchers found, providing a well-rounded picture of what marine conditions were like at the time.
Their findings suggest seawater farther from shore did have extremely low oxygen levels and high levels of dissolved iron, which would have rendered those environments uninhabitable for oxygen-dependent life such as animals. Closer to the ice-covered shorelines, however, the Sturtian seawater was surprisingly rich with oxygen. This is the first direct evidence for oxygen-rich marine environments during Snowball Earth, the researchers say, and it might explain how Cryogenian creatures managed to survive the snowball and later evolve during the Cambrian Explosion.
"The evidence suggests that although much of the oceans during the deep freeze would have been uninhabitable due to a lack of oxygen, in areas where the grounded ice sheet begins to float there was a critical supply of oxygenated meltwater," says lead author Maxwell Lechte, a postdoctoral researcher at McGill University, in a press release about the study. "This trend can be explained by what we call a 'glacial oxygen pump'; air bubbles trapped in the glacial ice are released into the water as it melts, enriching it with oxygen."
Glaciers are created by snow, which slowly becomes compressed into glacier ice as it accumulates. The snow holds air bubbles, including oxygen, that become trapped in the ice. Those bubbles move down through the ice over time, eventually escaping with meltwater from the underside of the glacier. In certain places, that might have provided just enough oxygen to help early marine animals survive Snowball Earth.
By studying 'snowball' and 'slushball' phases of Earth's history, scientists hope to learn more about other worlds that seem to have frozen oceans, like the exoplanet Kepler-62f. (Illustration: NASA Ames/JPL-Caltech/Tim Pyle)
In fact, Snowball Earth might have been more than just a hardship for those creatures to overcome. There are hints that specific conditions of the Cryogenian might have helped pave the way for the Cambrian Explosion. "The fact that the global freeze occurred before the evolution of complex animals suggests a link between Snowball Earth and animal evolution," Lechte says. "These harsh conditions could have stimulated their diversification into more complex forms."
That was also the conclusion of another recent study, which linked the rise of animals to a global boom of algae during the Cryogenian. That algae boom, in turn, had been triggered by melting ice after the Sturtian glaciation. During the hot interval between the Sturtian and Marinoan freezes, huge amounts of meltwater surged into Earth's oceans — along with a few key ingredients, courtesy of Snowball Earth.
"The Earth was frozen over for 50 million years. Huge glaciers ground entire mountain ranges to powder that released nutrients, and when the snow melted during an extreme global heating event, rivers washed torrents of nutrients into the ocean," lead author and Australian National University professor Jochen Brocks explained in a statement.
As the hot interval gave way to another snowball phase, the combo of dense nutrients and cooling seawater created ideal conditions for an explosion of marine algae around the world. Oceans previously ruled by bacteria were now dominated by larger, more complex organisms, whose abundance provided the fuel for even bigger, more elaborate species to evolve. These were the forebears of the Cambrian Explosion, but if not for Snowball Earth, they — and therefore we — might never have had the opportunity to evolve.
"These large and nutritious organisms at the base of the food web provided the burst of energy required for the evolution of complex ecosystems," Brocks said. And it was only in these complex environments, he added, "where increasingly large and complex animals, including humans, could thrive on Earth."