Without life, the Earth would not be a place suitable for life.
Sounds like a Catch-22? You bet. But it is true. In this blog post, we will explore how life makes the climate on Earth stable enough to host life.
Earth today has an average temperature of around 15 C on the surface, an atmosphere with 21% oxygen and around 0,04% CO2 (creating the right amount the greenhouse effect for us), blue water in oceans and rivers, and an ozone layer for protection against dangerous rays from the sun.
It could have been very different. Many geological events had to take place to make Earth a place for humans to live.
The young Earth, 4,6 billion years ago, was a place with at least around 25 times more CO2 than today in the atmosphere, may be a hundred times more.
Oxygen? None. Not a whiff. Since ozone is a molecule made of three oxygen atoms, O3, there was no protection against radiation either.
The young atmosphere probably also contained some methane, CH4. Methane is at least a 25 times more powerful greenhouse gas than CO2.
Which was good. The earth needed a thick, insulating down jacket, because the sun probably radiated out around only three-quarters of the energy it does today.
And the oceans were greenish, colored by dissolved iron ions.
No wonder it took the Earth four billion years to prepare for higher life, and half a billion years more to make humans.
The first thing was to get Earth a thermostat to keep temperature stable. Plate tectonics is this thermostat, and it works by coupling several chemical reactions, “feedbacks” in science-ish:
This CO2 reacts with silicate minerals in rocks, following the basic reaction:
2CO2 + 3H2O + CaSiO3 -> Ca2+ + 2HCO3– + H4SiO4
These ions, calcium, carbonic acid and silica acid, flows into the sea by rivers and there, the calcium ions and the carbonic acid combines to precipitate limestone, water – and CO2. But, only half the amount of the CO2 that came in, goes out in the other end. The rest is stored safely as limestone on the sea floor. Most of this limestone is really the shells of microorganisms, but it can also be deposited by pure chemistry:
Ca2++ + 2HCO3– -> CaCO3 + CO2 + H2O
Less CO2 in the atmosphere leads to less greenhouse effect, and the Earth cools. Lower temperature slows down the chemical reaction between silicate rocks and CO2, and thus also the CO2 extraction from the atmosphere. The volcanoes, steadily puffing along, refill the atmosphere with CO2, and the temperature goes up again…
This cycle has kept Earth’s surface temperature more or less stable through time and makes it possible for liquid water to exist on the surface – a need to have for life as we know it. It is driven by plate tectonics, which continuously spews out CO2 from volcanoes along the mid-ocean ridges and above subduction zones. CO2 is returned to the atmosphere along the subduction zones, by plunging the limestone into the Earth, and returning it by volcanoes. Plate tectonics also crash continents together, making mountains, which expose fresh rocks, which can react with the CO2.
Plate tectonics clearly seems to be necessary to keep the temperature, which makes liquid water possible.
But plate tectonics also, probably, needs liquid water to work.
Water flows down into the subduction zones through faults, and it comes as part of sediments riding on the slab going down. Water lowers the minerals’ melting temperature, and the soft minerals lubricate the slab gliding down in subduction zones. Without water, there might be no lubrication, and subduction might come to a grinding halt. Plate tectonics seems a planetary Catch-22: Without plate tectonics, no water, but without water, no plate tectonics.
Compare the Earth to Venus, our sister planet, the Roman goddess of beauty. Venus is just slightly smaller than Earth, but it has no water, the surface temperature is 450 C and the atmosphere is 98% CO2. Why?
The obvious answer is that Venus gets more heat from the sun. Venus is closer to the sun than the inner border of the theoretical “habitable zone”, the zone around the sun where liquid water can exist, while Earth is just on the right side of the inner border, and Mars is close to the outer border.
When the sun was young and colder, Venus may have had water, even oceans. But, unfortunately for Venus, as stars grow past their adolesence, the fusion in their core accelerates, and they radiate more energy. Venus got it hot. Any water vaporized and became steam in the atmosphere. Water vapor is a powerful greenhouse gas, which heated Venus even more… Venus became a “runaway greenhouse”. Then, the solar wind gradually blew this vapor away, leaving Venus entirely dry as a government document, and with an atmosphere of CO2 from its many volcanoes.
The Earth, during this time of a colder sun, kept warm thanks to the high CO2, and probably methane. Somewhere on the alien surface of Earth, the first life emerged. It would be life itself, which made our oceans blue, made oxygen and prepared Earth for intelligent life.
We do not know how life came to be, when or where. There are many theories, homing towards possible answers. We are on the way to understand the origin of life, but only time will show if we arrive on an answer.
The origin of life is such an enormous, complex issue that it would require a Lord-of-The-Rings-thick book itself, to give it something close to fair treatment. Therefore – and to save the author from embarrassing myself too much – we will just touch-and-go on topic of the origin of life. This may sound like a cop-out – but the fact is that life did develop, somehow, probably in some geothermal spring with the right mixture of organic compounds.
After all, we are here.
Fossil cells are usually round blobs, thin threads or some other shape, which are difficult to discern from round blobs or threads of another origin, especially if they are four billion years old. Instead, we look for indirect evidence, for organic material with isotope compositions, which suggests it comes from life. The oldest convincing geochemical evidence for life is 3,2 billion years old, from North Pole Hills in northwest Australia – quite an ironic name, because they are now some of the hottest and driest places on Earth. Earth seems to have spent around the first quarter of its time to invent life.
For around one billion years, life was simple, prokaryote bacteria. The bacteria evolved to fit in all kinds of climates, from the top of mountains to the bottom of the sea. And then they invented photosynthesis.
All the oxygen in our atmosphere is just a by-product of cells inventing this method to build more cells, and extract energy:
CO2 + H2O + sun energy -> C6H12O6 (sugar) + O2
In the beginning, this oxygen reacted with the iron ions in the sea, and created iron oxide, which settled on the sea bottom. This is our main iron ore – the Banded Iron Formations, mainly the mineral hematite:
4 Fe2+ + 3 O2 -> 2 Fe2O3
The story is somewhat complicated, but the TL;DR version is that it rained hematite to the sea floor for several hundred million years. Finally, around 2,45 billion years ago, most of the iron was drained and settled as sediments, and the sea had become the familiar blue. Only then did oxygen start filling the atmosphere, a nearly two billion long work to prepare it for the much later animals that would breathe it, and to make the protecting ozone layer.
2,45 billion years ago is also the time of the first of several Snowball Earths – called so, because the Earth probably froze over, tip to toe. From the poles to the equator, the Earth and its oceans became covered in ice.
Snowball Earth, covered by ice from tip to toe. This is how the Earth would look if it became a snowball today; you can see the Andes and Rocky Mountains as the only land standing out above the ice. The Precambrian snowballs had different mountain chains and continent positions, but otherwise would have looked quite like this. Image by Neethis [Copyrighted free use], via Wikimedia Commons
We know, because we have found glacier rocks, which were deposited near the equator back then. They are fossil moraines, composed of a characteristic hodgepodge of large and small rocks, sand and mud. Fossil moraines are called tillites, and the rocks underneath tillites often have scouring marks from the glaciers. They are tell-tale signs of past cold climate, and we have found them in rocks, which, according to magnetic measurements, must have been at low latitudes back then.
Tillites from Moelv in south Norway. These tillites are younger than the Snowball Earths, around 620 million years old, but show well the hodgepodge of rocks of all sizes, typical of a moraine.
How could Earth become one big Antarctica? There is no single cause, a smoking gun. Many unlucky stars aligned, some known, some surely still in the dark, a string of proverbial climate snowballs rolling towards the snowball Earth.
Some of the stars aligned round the Equator. Those stars were the continents, the cold bed for glaciers. Today, the largest continental glaciers are the Antarctic and Greenland ice sheets, but there are glaciers also in the tropics: There is (still) snow on Kilimanjaro, and glaciers in the Andes and the Himalayas.
White glaciers reflect light and return the energy from the sun back into space. Glaciers therefore cool the Earth, and the cooling creates more glaciers, which in turn reflect even more light, which cools the Earth… and if there are continents along the Equator, glaciers that form there will have an even greater effect on the climate. At Equator, rays from the sun hits the Earth at a vertical angle, and therefore carries more energy than rays hitting at low angle near the poles. Reflections from glaciers at low latitude will thus cool the Earth more than reflections from polar glaciers.
We have an obvious parallel today: Our currently, historically cool, climate may partly be due to the two very long mountain chains; the Rockies and Andes from north to south, and the Himalayas and the Alps from east to west.
Mountain chains at low latitude would also push the global tectonic thermostat towards “cold”, because large amounts of erosion products would be ready for the thermostat to react with CO2 in the air.
Hence, one likely driver for the first snowball Earth was that continents clashed and made a mega-mountain chain along the Equator.
Then, there was the oxygen. Oxygen may – may! – have helped create the snowball by increasing the amount of CO2 in the atmosphere. Remember the methane in the young atmosphere? Methane is a greenhouse gas with at least 25 times the power of CO2, and may have kept the Earth warm in its infancy. But methane is unstable in the presence of oxygen. It reacts to form CO2 and water – basically a low-intensity combustion. When oxygen filled the atmosphere, the methane was replaced by CO2, and the Earth actually cooled.
With the sun getting hotter, it was a good thing for Earth to throw some of its down jacket. It just threw away too much, and froze over.
Luckily – or, “luckily” – the snowball also shut down the cooling part of the tectonic thermostat. No more weathering. Below the ice, volcanoes kept churning out CO2, filling and warming the atmosphere, until it finally tipped above zero. When ice melting started, it went fast, pushing the albedo effect in reverse. In a few thousand years, earth was again blue and…grey, with some green slime.
The snowball had a key role in making us humans. It probably forced bacteria to make the first eukaryote cells.
Eukaryotes are the complex cells that make up all higher life, including us. Eukaryotes have a core, nucleus, which is covered by its own membrane and holds the cell’s DNA. They also have other organelles – “mini-organs”, including the mitochondrium, which is the cell’s digestion centre, and converts sugars to energy, and uses oxygen in the process.
Mitochondria are extra special, because they have their own DNA, which is a key to understand how eukaryotes developed: It probably happened by huddling several prokaryotes together. Triggered by the cold of the snowball, one big prokaryote gobbled up smaller ones, which specialized and became the different parts of the eukaryote. The piece that became the mitochondrium kept its old DNA.
Our bodies consist of eukaryote cells, all specialized to do the various tasks of the skin, muscles, intestines, hearing, sight, smell… Without the eukaryote cells and their specialization, higher life could never have developed. If the snowball hadn’t nearly pushed life to the brink, may be eukaryotes never would have developed either, and thus no higher life.
Now, let’s jump forward in time. We jump across “the boring billion”, a time when not very much happened to life.
We jump to two other snowball Earths, which followed each other from 720 million years ago. Being much younger, these snowballs are well documented by tillites from around the world. The first ideas of large Precambrian glaciations actually came in the late 19th century, with finds of fossil moraines in Scotland, Australia and India. But, long before the advent of plate tectonics, their significance was not really understood.
Ironically, the discovery of plate tectonics and moving continents in the 1960s first served to dismiss global glaciations: The continents surely were at high latitude when they had glaciers, so nothing special about the tillites. Even more ironically, it was tillites from Spitsbergen, near the North Pole, which broke the spell: In 1964, magnetic data from the rocks showed that they had been at tropical latitude when they deposited.
These snowballs started around 720 million years ago and ended around 630 million years ago, and together they comprise what is appropriately called the Cryogenian period- from Greek “kryos”, which means “ice”.
Why were there two successive snowballs? The explanation may, paradoxically, be that these snowballs happened not because of high mountains,
but the opposite: The break-up of a supercontinent. Towards the end of the “boring billion”, the land mass on Earth was assebled in one supercontinent called Rodinia. Rodinia was quite stable – why is a question in itself – which may be why the billion was boring.
Around 750 million years ago Rodinia started to break up in its seams. These rifts exposed much fresh rock to the atmosphere at the same time, feeding the tectonic thermostat, and pushing it towards cold.
When volcanic CO2 melted the first Cryogenian snowball, there was still enough rifting going on, to feed the tectonic thermostat. After the excessive CO2 was consumed, the growing glacier story repeated itself. When that second snowball thawed, the continents had spread, at least enough to prevent another global ice age. At least, this is a possible explanation for why two snowballs followed in a row.
These snowballs had severe impact on life, yet again bringing it to the brink. But then, life made another big leap. The first multicellular organisms appeared. How is still uncertain, but some> connection to the snowballs is likely.
Hence, we now jump forward to the time of the literal hard evidence for evolution: The Cambrian explosion, the dawn of animals as we think of them, which started 542 million years ago.
The Cambrian explosion is called so because life in a geological instant seemed to change from blobs and worms, to make the first real animals. The most notable fossils were exoskeletons; the shells of insects, crabs, mussels, lobsters and most of animal life. In between, there were the tiny, modest Pikaia, a few cm long distant relative of jaw-less lampreys. Pikaia had a string along its inside. This string would later develop into the spine of fish and ultimately of humans. Pikaia is our oldest known direct ancestor.
Meet your great-great-etc grandfather: The small jaw-less fish Pikaia was the first known animal with a back-string, the precursor to the backbones in all vertebrates. Image by Nobu Tamura email:firstname.lastname@example.org http://spinops.blogspot.com/ http://paleoexhibit.blogspot.com/ [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
The Cambrian explosion happened because cells became able to excrete calcium carbonate, and thus to make hard stuff. Hard stuff can become shells for protection, for teeth to gnaw at the shells, and serve as attachment for muscles, which enable animals to move to hunt other animals, and to escape. (Compare this to a blob trying to slowly hunt another blob…) Shells and spines gave evolution tools to work with, which triggered an evolutionary arms race, and this race has continued until today.
Trilobites – the favourite invertebrates for children of all ages! Trilobites illustrate well how the invention of calcite shells made possible the leap from blobs to animals, with specialised head, tail, legs and muscles. Image by Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
The race crowded the sea and rivers, but on land there was ample free real estate. Finally, around 375 million years ago – 92% of Earth’s history until now – our ancestors finally escaped the water and got onto land.
And for that story, please go back to the previous episode in this series :)