The Cryogenian Glaciations and the Birth of Animals

720 million years ago, our world froze over during the Cryogenian glaciations, but that doesn’t seem to have prevented the rise of the very first animals.

When we think of the ice age, the first image that usually comes to mind is a frigid tundra dominated by magnificent prehistoric beasts like mammoths and woolly rhinos. However, the last glacial period, which ended 11,700 years ago, was nothing compared to the ice ages of the Cryogenian Period.

Throughout most of its history, our planet has been ice-free, even at the poles. In fact, there was even a time when palm trees grew in Antarctica. It was during times such as these that, unsurprisingly, global ecosystems were most abundant with life, and biodiversity was at its most splendid.

With that in mind, it might seem counterintuitive that the very first animals probably appeared at least 650 million years ago, right in the midst of a series of glaciations that saw the Earth turn into a gigantic ball of ice and snow.

What Did the Cryogenian World Look Like?

Pixabay

The Cryogenian Period was a time when icy deserts dominated the Earth.

The Cryogenian Period lasted between 720 and 635 million years ago. It was the second-last geological period of the Proterozoic Aeon, which ended 541 million years ago with the Cambrian Explosion. For decades, natural historians assumed that animals arose in this famous event, an explosion of biodiversity that saw early animals proliferate in oceans all over the world. More recent studies, however, have pushed back the birth of animals by up to 130 million years to a time when the world was going through its harshest ice age ever.

Although debate still rages on as to whether ice covered the entire planet or left a small band of open sea around the equator, ample evidence exists that tell us a story of a world far colder than it is today. Some simulations have demonstrated that air temperatures, even at the equator, averaged between -43 to -23°C during the height of the glaciations. That’s some 74 to 54°C colder than the equatorial regions today, making it more like the present surface of Mars.

The Cryogenian Period saw two major glaciations, the Sturtian and Marinoan, the end of the latter heralding the beginning of the much warmer Ediacaran Period. Throughout the period, the world was a barren place. The landmasses, which were largely assembled together in the form the supercontinent of Rodinia, were utterly void of life, and would remain so for hundreds of millions of years. Owing to the bitter cold of the period, we can probably safely assume that the land was a lonely, snowy desert rather like modern-day inland Antarctica.

Aside from the freezing cold, most modern land-dwelling animals, including humans, would have serious difficulty surviving in the Cryogenian world. Atmospheric oxygen content was around half of what it is today, but it rose steadily throughout the period thanks to the proliferation of photosynthetic microorganisms. It was likely also these organisms that were responsible for the higher atmospheric CO2 levels of the time which, eventually led to the much warmer climate of the Ediacaran.

How to Evolve in a Hostile World

NOAA

Hydrothermal vent ecosystems rely purely on volcanic activity for power.

The two competing hypotheses on the Cryogenian glaciations exhibit two major differences when it comes to explaining the evolution of the very first mammals.

One of these hypotheses suggests that the Earth was completely covered in ice, right down to the equator. In other words, the world’s oceans were locked deep beneath a near-impenetrable layer of permafrost and thus far beyond the reach of any sunlight. One could perhaps compare it to Jupiter’s moon Europa which, beneath its several-mile-thick surface of ice as hard as granite, is home to a vast subterranean ocean. Such an environment has some very interesting implications for the evolution of animals.

NASA

The Cryogenian Earth likely had moon in common with present-day Europa, a moon of Jupiter with a subterranean ocean.

If indeed the world were a frigid snowball during the Cryogenian, with not even a band of ice-free ocean in existence, then the first animals must have evolved around hydrothermal vents – home to the only known ecosystems on Earth that have no dependence, indirect or direct, on the Sun. Meanwhile photosynthetic organisms, which had already been around for well over two-billion years, must have still thrived on the icy surface. However, they certainly didn’t give birth to the first animals, since the first terrestrial animals did not appear until the Silurian Period, 428 million years ago.

If the Snowball Earth hypothesis is true, then we can probably trace our own origins and, indeed, the origins of all animals, to the darkest depths of the oceans. However, the Slushball Earth hypothesis presents a more ‘earthly’ origin for our species. If indeed there did remain a band of ice-free ocean around the equator, then the first animals may instead have arisen from photosynthetic ecosystems on or close to the ocean surface.

Of course, the origins of animal life may have resulted from a combination of the two. For example, even in the case of the Snowball Earth theory, there could have still been meltwater volcanic lakes or areas where the ice was thin enough to let in some light.

Deciphering Our First Common Ancestor

NOAA

Sea sponges are the oldest known animal on Earth

Chances are, we’ll never know exactly what the last common ancestor of all animals was, but that doesn’t stop us from having a name for it. We call this elusive creature an urmetazoan which, in this sense, means ‘original animal’. All we know about this animal is that it undoubtedly lived in the sea but, beyond that, we know nothing else other than that it evolved in either a hydrothermal vent or photosynthetic ecosystem.

All animals are eukaryotes (that means ‘true nuts’), organisms that consist of cells with nuclei. Eukaryotes first emerged in the form of primitive microorganisms, likely during the Rhyacian Period around 2100 million years ago. The earliest true animals, the existence of which there is definitive evidence, emerged during the Ediacaran Period, at least 580 million years ago. Most modern animal phyla appeared during this time.

However, more recent evidence suggests that the first animals appeared in Earth’s oceans much earlier, as far back as 650 million years ago in the form of sponges. Although ample intact fossil evidence of sponges, members of the animal phylum porifera, exist from 580 million years ago, they likely emerged much earlier. This makes sense, since sponges are known to be among the world’s longest-living and most resilient animals. With life spans of up to 10,000 years and the ability to live in a vast range of diverse maritime environments, there’s a lot of evidence to suggest that, if any animal could have appeared at all during the inhospitable Cryogenian Period, it was a sponge.

Whether the rise of the first animals coincides with a relatively brief interglacial period during the Cryogenian or, perhaps did indeed evolve at a time when Earth was at one of its least hospitable stages of the Proterozoic Aeon remains uncertain. Furthermore, the frigid climate of the time is likely what prevented any such animals from proliferating to the same extent they did in the Late Ediacaran and, to an even greater degree, during the Cambrian Explosion of 541 million years ago.

One thing seems certain, however, and that’s that the oldest true animal in the world is indeed the humble sea sponge, from which emerged jellyfish and a multitude of other reef-building organisms. It would still be hundreds of millions of years before these sessile, sea-dwelling creatures would eventually move beyond the sea bed and, eventually, start to colonize the land in the form of amphibious fishapods and air-breathing millipedes.

*

It’s incredible to think that animal life began in such an inhospitable world yet, born from this frigid gloom was a major change in direction of the story of life on our planet. Perhaps, indeed, we all owe our origins to the deepest, darkest reaches of the abyssal ocean. If you’d like to find out more, check my recommended reading list below.

Further Reading