Science

A Brief History of Earth: Dancing With Complex Life and Continents in the Devonian

The Pāhoehoe lava flow, Hawaii. Credit: Brocken Inaglory/Wikimedia Commons, CC BY-SA 3.0

The Pāhoehoe lava flow, Hawaii. Credit: Brocken Inaglory/Wikimedia Commons, CC BY-SA 3.0

The relatively calm region of space we occupy in the Solar System today belies a fiery, violent past, and a spine-chilling future. This series explores the geological and natural history of Earth, beginning with the formation of our Solar System, moving on through asteroid impacts and mass extinctions, and ending with the human impact on the environment today. To really grasp the magnitude of the changes our planet has undergone, we need to speed through immense timescales, pausing at important milestones.

Until now: We saw that life on earth had finally moved from the oceans on to land. There had been one great mass extinction that killed most of it, which coincided with a terrible ice age. Supercontinents formed and broke apart in cycles.

A supercontinent is hard to define. In history, the first few continents that were formed – Vaalbara, Ur, Kenorland – were all called supercontinents merely because they were the only landmasses on the planet. These early continents that formed almost 4.4 billion years ago (bya) were relatively small – smaller than today’s countries.

Continents and supercontinents are formed due to the movement of Earth’s mantle. Below Earth’s surface – the crust that extends to 60 km – exists a layer of viscous molten rock called the mantle. It is nearly 2,500 km deep. And further below the mantle is Earth’s core, divided into an inner and an outer part. The outer core is what we perceive as the ‘core’ of the earth – a swirling liquid of iron, nickel, and other materials. The inner core is just a solid sphere of iron and nickel, a heavy metallic ball that spins right at the centre of Earth, glowing white hot at 5,400º C. The outer core is at about 4,000º C while the mantle is anywhere between 500º and 1000º C.

As the inner and outer cores rotate, they heat up the mantle. The heat spreads through 2,500 kilometres of what is essentially rock. The process the heat transfer follows is convection: hot material rises up while cool material gravitates down. The contents of the mantle are always swirling, never constant. Of course, the change in mantle composition and its movement is visible only when measured over geological time. To us today, mantle behaves just like rock.

This process of heat transfer underneath our crust, imaginatively called mantle convection, drives both plate tectonics and volcanism. The mantle is not a homogenous mixture of rock. Sometimes, when heat rises up from the core, certain parts of the mantle might get more heated than others. When one part of the mantle gets very hot, the heat travels upward towards the underside of the crust, heating it up too. As the region of mantle in contact with the crust expands and becomes less dense due to heat, the crust sinks into it. This process is called subduction.

The mantle can contain only a fixed volume of rock. Sometimes, when large amounts of crust, like the sea floor or a large landmass, subduct into the mantle, the displacement causes some of the mantle to rise and occasionally burst out of the crust in another location, preserving the amount of material held in it. This can happen in either one or two directions. In one scenario, a piece of rock subducts and the mantle material is pushed out from another part of the earth in the form of a volcano. In another scenario, if a very large piece of land subducts, its weight warps the mantle such that volcanism and/or earthquakes occur at the two ends of the warp. Sometimes, the mantle doesn’t release material at all but builds up pressure within in the form of a mound.

Such subduction, volcanism and bulges cause a change in the shape of Earth, the geoid. Since volcanism pushes material upward, the mantle swells below the volcano, raising its height. Large landmasses tend to move from higher elevations to lower, most likely in the direction of a volcanically active area to a subducted area. This slow sliding of continents is what makes or breaks a supercontinent.

The movement of landmasses sometimes brings these pieces of land in positions that block warm oceanic currents from flowing to the poles. Oceanic currents play an important role in regulating Earth’s temperature. Warm currents offer relief to life on the poles and help conduct nutrients. A continental configuration where warm currents are blocked is a catalyst to the occurrence of a glaciation event. Apart from a self-induced negative feedback process caused due to the greenhouse effect, a glaciation can end due to volcanism caused as a result of the ice itself. As ice builds up over land, the land sinks a little into the mantle due to the weight of the ice above it. This causes either subduction from below the land or a warping of land, a bend, that releases mantle material from opposite ends of the concentration of ice, which in turn causes land to be heavier than without ice.

Such processes are the main reason ice ages usually end due to the shifting of landmasses, and both ice ages and continental drift accompany major mass extinctions.

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The last supercontinent we saw, Rodinia, broke apart 650 mya and was promptly followed by the assimilation and immediate breakage of the temporary Pannotia some 500 mya. After Pannotia broke up, there were three large continents on Earth: Laurentia, Gondwana and Baltica. Laurentia contained a large portion of North America. Baltica held Ukraine and parts of Russia. Gondwana, on the other hand, could have been a supercontinent in itself . It contained the cratons – chunk of land masses that are the building blocks for continents – of India, Australia, Africa and South America. It existed entirely in the southern hemisphere.

The next 200 million years were eventful for these continents. A bit of Gondwana containing the Newfoundland and New England cratons broke off and slowly started dancing its way up north. Meanwhile, Baltica drifted toward Laurentia. Both these pieces of land ultimately collided and formed what came to be called Laurasia, a portmanteau of the names Laurentia and Eurasia.

These collisions were among the latest on our planet, and therefore present us with a lot of evidence of their happenings. They caused the formation (and sometimes eventual breakage) of multiple mountain ranges. Some of the mountains that exist today, and which were formed as a result of collisions that created Eurasia, are the Scandinavian mountains that run across Sweden, Norway and Finland, and the Appalachian mountain ranges in North America. Other mountain chains formed too, but were eventually destroyed. As mountain building and continental collisions continued, lifeforms flourished after the first major mass extinction.

A mass extinction isn’t necessarily bad for the planet and life as a whole. Just like forest fires that raze a forest to the ground and provide organic nutrients for new life to grow, mass extinctions exterminate lifeforms that have occupied niche-dominating territories, giving a chance for new suppressed and starved types of life to grow and diversify. It’s only because the dinosaurs were wiped out that humans came to be the dominant species on the planet.

About 400 mya, life had reached a very important milestone. The first vascular plants had started growing on land. This was a big deal. It meant that at least one kind of life had not only learned how to grow against gravity and but was also able to survive without being immersed in the nutrient-conductive medium of water. Plants still needed water, as do we all today. So the root system that acted as support lodging a plant in place also tapped into underground water and nutrients. A vascular plant is one that contains vascular tissues: tissues that are capable of distributing water, nutrients and energy. These plants have a xylem and a phloem to conduct water/nutrients from the soil and move the products derived from photosynthesis to the plant.

The first true plants were ferns, still burgeoning on the planet today. They reproduce with spores and have neither flowers nor seeds. They started spreading through land in the Devonian period, developing fibrous roots. These roots reached further into the sand than anything has done before. They first grew on the coastal regions. As water receded in the summers, the minerals got deposited in the soil, providing nutrients for plants. Once roots could effectively tap into groundwater, plants developed rapidly moving further inland. And as they covered large parts of land, they grew bigger. Primitive forests appeared, with abundant photosynthesis now happening over land for the first time.

Animals moved to land, too. The first among them were the arthropods, those with segmented bodies and exoskeletons. Some 450 mya, these were millipedes evolving to burrow in and out of the soil more effectively. The worms took a while to combat gravity and grow sturdy legs. A few million years later, small scorpions and mites started to appear. Today, arthropods include spiders, crabs, scorpions, shrimp, cicadas and even butterflies. A 100 million years after millipedes appeared, the first insects started to appear. These were without wings, still developing their ability to walk on land and grow in size.

The first seed-bearing plant appeared at about the same time, roughly 400 mya. Plants had shot up in height from 25 cm to nearly 25 m in these 100 million years. Toward the end of the Devonian, 375 mya, plants had their roots firmly planted underground and arthropods had miles and miles to roam with their sturdy, well-developed legs. The spread of plant and animal life finally established a soil system, but not without loosening the top layers of rock and causing changes in erosion patterns first.

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Meanwhile, the ocean had quickly become a rainbow of radial diversity, with each species sprouting into multiple variants. Fish came to dominate the seas in numbers with their variance. Trilobites that reigned upon the seas before the first big mass extinction continued to be widespread as fish developed jaws and got bigger. Sea scorpions, now extinct, were the most remarkable species of this time.

These creatures were massive 375 mya even by today’s standards. They grew up to nearly two meters in length, and were the largest arthropods known to humans. Their fossils have been found in oceans all over the world, which indicated that they were spread globally. They evolved in shallow waters around landmasses and killed for prey. Given their size and their shell, it’s been believed guessed that they would hunt in the waters. They have been often described as the “first real predator” on the planet.

A model of a sea scorpion at the Smithsonian Museum of Natural History. Credit: ideonexus/Flickr, CC BY-SA 2.0

A model of a sea scorpion at the Smithsonian Museum of Natural History. Credit: ideonexus/Flickr, CC BY-SA 2.0

In the Devonian period, there was a lot of reef and coral-building activity in the seas. This was characterised by hollow, solid growth on continental shelves by colonial sea animals. A coral is typically built as a colony, each consisting of a number of millimetre-length creatures called polyps that excrete what is essentially their exoskeletons. Over the years, this builds up to form a colony of coral. A lot of coral fossils have been found from this time, along with trilobites.

Armoured marine life, like the sea scorpions and other arthropods, grew quick and grew strong in the initial million years of the Silurian and Devonian periods. However, their populations gradually declined because they had to compete for prey with the briskly evolving fish. Fishes grew from small creatures to bony fish, armoured fish, shark-like smaller fish, flat fish and eventually to actual sharks and rays. They varied in size and shape, and each variant sporting a plethora of colours. As they evolved, they also grew bigger in size, confidently conquering the seas. The largest known fossil from this era shows fish that were nearly seven full metres long.

The Devonian period also saw some fish evolving to breathe air. Some of them could slide their way over land, push with their fins, and stay outside of water for hours. These strong fins were called lobe fins and they would eventually evolve to become the legs of primitive amphibians. Specifically, tiktaalik is an extinct fish that is thought to be the transitional link between life in the seas and the first amphibians.

The Devonian period – which saw a burst of variation, growth and evolution of fish – is thus also known as the Age of Fish.

An artist's rendition of a tiktaalik. Credit: Nobu Tamura/Wikimedia Commons

An artist’s rendition of a tiktaalik. Credit: Nobu Tamura/Wikimedia Commons

 

The climate in these approximate 100 million years we’ve been talking about started off warm. This was due to the increase in tectonic and volcanic activities. But temperatures steadily dropped as plants colonised land and more forests began to grow. In the same period, there were four short mass extinctions. This was followed by a very large, major extinction event, the second of the Big Five.

Following a familiar pattern, the major extinction event, known as the Late Devonian Extinction, was caused by a global drop in temperatures combined with the loss of oxygen in waters. Like in the first major mass extinction but on a much greater scale, deep roots caused displacement of large quantities of rock and their deposition into water. Since this mud was heavy with nutrients, it dumped a large portion of minerals into the water along with it. The excess input of nutrients into water systems by artificial or non-natural means is called eutrophication. Today, eutrophication happens in water bodies by draining of sewage, fertilisers, detergent and soap into them. The nutrients are quickly absorbed by a spurt of algal bloom, which drain oxygen from the water in the process of breaking these nutrients down for their multiplication. This causes anoxia – a lack of oxygen – in the water, killing marine life.

Plants had colonised so much land with deep roots that the loss of the top layer of rock and plants attached to it barely made a difference to life on land. Oxygen in the atmosphere remained nearly steady. In this major mass extinction, only marine life suffered losses: of over 50% of all species and genera.

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The onset of the glaciation event that coincided with this mass extinction was not a major ice age. And it was not caused just by the mass extinction or vice versa. There are more important factors that cause periodic large scale changes in climate. These factors – three of them, to be precise – together are called the Milankovitch Cycles.

As Earth rotates on its axis while speeding around the Sun, its tilt and orbit vary minutely each passing day. Over millennia, these variations culminate in visibly large changes that cause dramatic variations in Earth’s climate. How? By changing the amount of sunlight that reaches us. The three properties that are responsible for these effects are earth’s axial tilt, orbital eccentricity and precession.

Axial tilt: We know that Earth rotates about an axis that isn’t precisely perpendicular to the plane of the Solar System. Earth’s axis is tilted at an angle and it’s this axial tilt that gives our planet seasons: the hemisphere of Earth that is tilted toward the Sun has summers and the other hemisphere has winters. It is commonly accepted that the angle of tilt is 23.5º. But in reality, the axial tilt shifts between 22º and 24.5º in a cycle that repeats every 41,000 years. Currently, our axial tilt is 23.2º and.

When the tilt increases, the pole that points toward the Sun has warmer summers and the one that points away has colder winters. Conversely, when the tilt decreases, summers become cooler and winters warmer. This condition is conducive for a glaciation event as moderate temperatures cool easier than extreme heat.

Illustration showing Earth's precession. Credit: Wikimedia/Commons

Illustration showing Earth’s precession. Credit: Wikimedia/Commons

Precession: As Earth rotates around its axis, it also wobbles. This wobble is independent of the motion of the axis. This wobble changes the way Earth is oriented against the backdrop of the stars, axis and all. What essentially happens is that the wobble makes Earth’s axis point to a different location in the sky. So as the earth goes around the sun, we can imagine the axial tilt to be changing only on the y-axis of a graph, while the precession varies through the z-axis in a three-dimensional motion (akin to a spinning top). Earth’s axis traces the path of a circle in the skies, with the north pole pointing toward a different group of stars through these motions.

The north star is called Polaris. It is the star that our Earth’s axis directly points toward above the north pole and therefore remains stationary in the sky. It is frequently used as a reference point during navigation. However, the north star hasn’t always been Polaris. Precession changes where the axis points. This is what is thought to have made the Egyptians build the Great Pyramid where it is. When ancient Egyptians finished building it in the year 2,560 BC, the north star was the star Thuban and the pyramid pointed directly at it. Some 12,000 years from now, the north star will be the bright, blue Vega.

A precession Milankovitch Cycle repeats itself every 26,000 years.

Orbital eccentricity: We know that the orbit of Earth around the Sun is an ellipse. However, it doesn’t always remain an ellipse. Earth (and other bodies) are constantly being acted upon by the gravitation of other large bodies, particularly Jupiter and Saturn. This changes the shape of the orbit. Earth’s orbit varies from a nearly perfect circle to an ellipse over a period of 21,000 years.

It is estimated by scientists that the axial tilt Milankovitch Cycle played a leading role in the onset of the ice age that coincided with the Late Devonian great mass extinction. (Do remember that many climate change detractors use Milankovitch cycles as an explanation for global warming. While Earth is always in some phase of a Milankovitch cycle, it is imperative to not ignore the overwhelming evidence of global rise in temperatures caused by human activities.)

The next instalment in this series will talk about the first wooden trees on land, the first vertebrates, the diversification of sharks, the first animals to fly, the highest atmospheric oxygen levels in history, gigantic insects and the formation of coal.

Sandhya Ramesh is a science writer focusing on astronomy and earth science.