Gondwana, Snowball Earth and the Cambrian explosion of life

In times when marine life exploded into greater biodiversity than ever before, all southern continents had merged in one enormous landmass, a supercontinent called Gondwana. It didn't sprout from mythology like Atlantis and other mysterious vanished lands. But its once great mountains, the fierce climatic changes that scourged it, the land ice, swamp forests and deserts that enveloped it, and the life to which it provided a home, do make Gondwana a place of mythical proportions.

Text: Kathelijne Bonne

Gondwana lay on the South Pole, a geological Terra Australis. It encompassed Africa, South America, Australia, Antarctica, India, Madagascar, and many fragments of smaller continents, such as Adria, Turkey, Iberia, Arabia, Iran, pieces of China and Indochina, and more mythical-sounding Avalonia, Armorica, the Hun terrains, Cimeria, and others. 

Gondwana formed about 650 million years (650 Ma) ago, at the end of the Neoproterozoic when there was only unicellular life in the sea, and it began to disintegrate during the Jurassic (180 Ma), time of dinosaurs.

Gondwana formed when older continents, "cratons," collided during several great phases of mountain building, collectively called the Pan-African Orogeny. The geological forces that forged Gondwana are still recognizable in rock outcrops and in the way the rocks are deformed and fractured. The broad fold belts that crisscross the southern continents are the eroded vestiges of the once massive Trans-Gondwana Mountains. The Mozambique Belt that stretches all along East Africa, must have appeared like the Himalayas, or higher, but has long been worn down since. But the old Pan-African structures are not entirely dead or passive, as nothing ever is in geology. The East African Rift, which runs across Africa like a zipper, is making its way through the Earth's crust via the ancient structures that once sutured Gondwana; or said more simply, new cracks open up where the crust has been pre-broken. 

Half a billion years may have passed, but the planet reuses everything.

Greatest cycle of all

Gondwana is not the only supercontinent that ever existed. Its jigsaw pieces were once assembled in a more ancient supercontinent, Rodinia, which in turn was preceded by Kenorland, Nuna, Ur and Vaalbara in a deeper, more hazy past. There is indeed a cycle of supercontinents, which seem to follow the pulse of an extremely slow beating, immensely ancient heart. It may well be the slowest and most majestic of all cycles that make our planet a dynamic one.

Snowball Earth

Gondwana emerged after a massive ice age. During the Cryogenic (720-635 Ma), our planet must have looked like a featureless white ball, a Snowball Earth. From space, Gondwana's contours were invisible, hidden below ice and snow. It was the coldest and longest ice age the planet has ever endured. Near the equator, there may have been ice-free open sea (which is why scientists abandon the hard Snowball Earth concept and call it a Slushball), which allowed the hydrological cycle to persist and microscopic life to have better chances of survival - although it is also possible to survive in deep dark oceans, in the absence of sunlight. After a few more gelid convulsions, the ice finally retreated, probably due to the build-up of greenhouse gases emitted by volcanoes, inert as they are to the messy and complex happenings on earth's surface. A warmer world set in and the Cryogenic gave way to the more pleasant Ediacara.

Ediacara Fauna

During the Ediacara (635-541 Ma), the development of multicellular life was in full swing. It gave rise to the most extraordinary creatures ever found: the Ediacara biota. They are fairy-like, soft-bodied organisms, many of which still cannot be allocated into the tree of life. They show no obvious resemblance or kinship to later, more familiar life forms. While later life probably sprang from Ediacara-like organisms, the Ediacara biota in the strict sense may well have been an amazing but doomed experiment of life. We are fortunate that their soft bodies have been preserved at all. Otherwise, we would have remained oblivious to this wonderful world. 

The end of Ediacaran times also closed the long Precambrian, which occupied 80% of Earth's age: from its birth 4.6 billion years ago, over the formation of the Moon, the origins of life (possibly aided by lightning), until the start of the Cambrian 541 million years ago.

Cambrian explosion

Much, if not all, changed at the dawn of the Cambrian, the first period of the Phanerozoic Eon. Life exploded in many shapes and forms and changed the face of the earth forever. Remarkable animals had emerged in the fossil record, armored with exo-skeletons and hard parts, which fossilize well, as part of a great diversity of species. Amongst the most charismatic animals were trilobites, masterfully brought to live by palaeontologist Richard Fortey in his delightful book Trilobite: Eyewitness to Evolution (his other book 'Life' sparked my interest in geology). 

Life was suddenly "visible" in the rocks, hence the name Phanerozoic, which is Greek for visible life. Before the 20th Century, it looked as if life had suddenly appeared in the Cambrian, as if by a divine hand, because older life (tiny unicellular or soft-bodied) had not yet been found. Darwin, among others, was deeply vexed with this "sudden appearance". Surely there must have been something before the Cambrian – in the mysterious and unfathomable pre-Cambrian? 

This remarkable biological event in which much diversity developed in a geologically short period of time has been described as the Cambrian Explosion. It is one of the most perplexing transformations in the evolution of life, as there was a  uniqueness in the disparity of new forms and the appearance of extremely diverse body plans of animals, especially according to Stephen J Gould as defended in his book Wonderful Life (1989). However, the 'explosiveness' or 'suddenness' of the Cambrian explosion – both in time and in disparity – has now been somewhat softened based on more recent interpretations. 

In this visibly alive world, life forms were no longer delicate-looking lik the Ediacara fauna but fierce and frantic like the creatures found at locations of exceptional preservation known as 'lagerstätten'. The Burgess Shale (Canada) and the Maotianshan Shales (China) of the Cambrian are among the best known lagerstätten in the world, with extraordinary animals such as Wiwaxia, Anomalocaris and Marella splendens. In the Cambrian as well, the basic foundation for the body plan of all modern animals was laid down, including the very familiar one that has a spinal cord, i.e. that of the Chordata, including all vertebrates and humans.

Meanwhile, the shape of Gondwana itself changed, riding earth's tectonic caterpillar tracks in slow motion, seemingly inert to all the swarming, splashing and evolving of life in the surrounding seas. The mainland of Gondwana at first looked like a vast barren wasteland, very Mars-like, until life would start exploring the shores and eventually leave the water to venture inland. 

In the Silurian, life firmly established itself on land. First only fungi, lichen and microscopic organisms left the water, but mosses, ferns, vascular plants, and animals followed soon, creating a green world abuzz with large arthropods and amphibia and rustling of leaves. One scary dragonfly, i.e. meganeura, must have hovered ominously above other more timid creatures that hid in the undergrowth of dense swamp forests. There must have been much humming, buzzing, creaking and gurgling, but no birdsong yet. There were no flowers either, or mammals. There were fish and coelacanths but no whales, who were tetrapods that returned to the sea eons later.

Pangaea

At the end of the Paleozoic Era, the last and largest supercontinent had formed, as Gondwana had collided with Laurasia (North America and Eurasia). Pangaea was surrounded by the globe-spanning Panthalassa ocean. The Tethys Ocean was an an inlet on Pangaea's eastern side. Pangea reached its greatest extent in Permian times. But it was great in size, not in sweetness. The climate became scorching hot, torrid and dry, and an almost unlivable, hostile global environment led to a collapse of global biodiversity, which sank to its nadir in the Permian-Triassic mass extinction. Even the trilobites, which had thrived for about a quarter of a billion years, did not survive this climate shock and perished during the "Great Dying" together with 90% of all life.

Yet, the life that did survive and made it to the other end of the bottleneck, multiplied and radiated into a whole range of new forms occupying the niches that were left empty, to create ecosystems capable of supporting the most charismatic of all animals: dinosaurs.

Demise of Gondwana

During the Mesozoic Era (252-66 Ma), Gondwana slowly disintegrated. Rift valleys opened, became oceans - like the Red Sea today - and finally Gondwana fragmented into the continents we now see on maps. The breakup of South America and Africa is most symbolic for Gondwana's final demise, their now faraway coasts still matching, their once connected mountain chains truncated by 'new' Atlantic beaches.

Some researchers place the last vestiges of Gondwana even later, in the Cenozoic (66-0 Ma). South America and Antarctica were the last "children of Gondwana" to hold each other's hand until the very end, a narrow land bridge holding them together. This last connection ruptured when the Drake Passage formed, now one of the world's most ruthless seas. Since, Antarctica became isolated on the south pole, it became encircled by the cold and unbidding Antarctic Circumpolar Current, putting it into a deep freeze and slowly pushing the planet towards a cooler world, a trend that would culminate into the Pleistocene ice ages. 

Biased by life

It seems as if life, plate tectonics and climate advance independently of each other and at different rates. But they are all connected, even plate tectonics is probably slightly biased by the actions of life. The emergence of multicellular life during the Ediacara, after the Snowball Earth, may not be a coincidence (nothing ever is). The extreme cold may have forced life to develop new solutions. It is also being investigated whether Gondwana itself, and the massive erosion of its mountains, may have contributed to the Cambrian Explosion through the greatly increased supply of sediment and minerals to the sea. It literally fed life and gave it fins, legs, eyes and wings.

Pangea Proxima

Geologists can already model what the next supercontinent will look like, Pangaea Proxima. Africa will drift further north and the Mediterranean Sea, which has already had convulsions in the form of the Messinian crisis, will disappear. But we shouldn't worry, according to some maps, a new Mediterranean Sea will emerge somewhere within Pangaea Proxima.

What we do not know is whether humanity will still be around to live and thrive, on Pangea Proxima, to see a supercontinent with our own eyes.

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Last note: Discovery of Gondwana. The name Gondwana was first used by Eduard Suess (1831-1914), inspired by the land of the Gonds in India, see the article on plate tectonics and how it was discovered. He had reason to believe that an ancient great continent had existed, but the time was not ripe for the concept of plate tectonics. It was Alfred Wegener (1880-1930) gave shape to this hypothesis, and after his death he was proved right, as Arthur Holmes (1890-1965) demonstrated using geomagnetism. The vague, previously almost impossible idea of Gondwana had crystallized into a world that had really existed, a new paradise for geologists.

Kathelijne: I am intrigued by how earth, life, air, ocean and societies interact on geological and human timescales.

Why I started GondwanaTalks.

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