Constant Change
Don’t look up, look down! To understand life on other planets you need to look down at the rocks as well as up at the stars. Reading rocks to learn about alien life may seem about as sensible as reading tea leaves, but the history of our planet is written into its crust and that history tells us much about the conditions necessary for life. This history also tells us that there are many ways of courting disaster but that – extraordinarily – the Earth has avoided them all. In particular the Earth has followed a narrow path that has avoided climatic catastrophe. Unlike our immediate neighbours in space, the Earth has not become too hot, like Venus, or too cold, like Mars.
Is 4 billion years of good weather on Earth really so surprising? Well, yes it is. The Earth is incredibly old and so there has been ample time for even the slowest of changes to produce catastrophe. A warming trend as small as 1°C every 100 million years would have been enough to make our world uninhabitable by now, and it would not have been surprising had such a trend occurred. The oceans, atmosphere and land surface of our planet have changed continuously and dramatically throughout Earth’s history, as has the amount of heat emitted by our Sun. The details of exactly how much change has occurred, and when, are the subject of intense scientific debate but no one doubts that there have been profound alterations to the Earth’s environment. Any of these changes could have caused devastation but, instead, they usually managed to roughly cancel each other out.
Before I describe these changes in a bit more detail, I want to say a little more about the immense lengths of time involved. Unfortunately, it is practically impossible for the human mind to really grasp the scale of deep time. Nevertheless, many writers of geological and astronomical books have tried. A frequent approach is to use distance to represent time. For example, if every year is replaced by a millimetre, the age of the Earth is equivalent to the distance from London to New York. Personally, I don’t find this at all helpful because I have never walked from London to New York and have no real feeling for how far it is. Another approach is to shrink time by, for example, imagining that one second represents one year. On this scale, I was born 53 seconds ago, the pyramids were built about an hour before that and the dinosaurs died out two years earlier. Using this reduced timescale, the Earth formed about 150 years ago. Shrinking each year down to a second works reasonably well for me but I still find it very abstract. Sadly, doing better is an almost impossible task.
The greatest length of time for which anyone can really claim a concrete grasp is probably a human lifetime. The oldest man I ever met was my great-grandfather who was 85 years older than me. If, when he was a small boy, my great-grandfather had also known someone 85 years older, that person would have been born in 1790. I am therefore separated from the French Revolution by only two human lifetimes, which makes that particular piece of ancient history seem quite recent. Using an 85-year human lifetime as my yardstick, I am separated from early feudalism by ten lifetimes and the beginnings of civilisation by a hundred lifetimes. A few thousand lifetimes link me to the earliest modern humans. At this point, I feel that the human-lifetime yardstick has started to become useless. To go back a truly geological length of time, say to the age of dinosaurs, we need a million lifetimes. I find it impossible to relate to such an enormous stretch of time but the extinction of the dinosaurs is, in turn, a relatively recent event. More than 98 per cent of Earth’s history occurred before they disappeared.
So, I’m afraid I’m going to have to give up after all, but perhaps that’s the point. The lengths of time we are looking at in this book are so far beyond human imagination and experience that almost anything could have happened. The time periods were long enough for hundreds of mountain ranges to grow and be ground back to nothing; and it was long enough for the descendants of bacteria to change so much that they could build rockets that travel to Mars. To do justice to all the major events in the life of our planet would require an entire library; I’ll outline just a few of the more important ones. The key point to bear in mind throughout this chapter is that once life appeared, and despite the enormous changes the Earth went through, the average surface temperature did not vary by more than a few tens of degrees centigrade.
Let’s start around 4.6 billion years ago when there was no Earth, no Sun, and no solar system. Instead, there was just a slowly spinning cloud of interstellar gas and dust, spread over dozens of light years, a nebula large enough to build hundreds of stars and their accompanying planetary systems. Slowly, over millions of years, thicker parts of the cloud became even denser as their slightly higher gravity pulled gas and dust in from their surroundings. The cloud had started to collapse. As material converged on each of the more compact regions, it began to spin rapidly around in much the same way that water spins as it converges on the plug-hole of an emptying bath. As in a bath-tub whirlpool, this rapid spin pushed material away from the centre of convergence and collapse all but ceased. What happened next is still poorly understood, although we see the process going on today in places like the Orion nebula a few hundred light years from the Earth. Spin was somehow transferred out of the central clumps and into orbiting discs of gas and dust, looking like the rings of Saturn but a million times larger. The reduced rotation in the inner regions then allowed these cores to shrink further and soon they became small, dense and hot. Within a few million years, central temperatures exceeded 7 million degrees centigrade, allowing nuclear reactions to begin. One by one the members of a brand-new star cluster ignited.
Our Sun was one star in that cluster. It would have been inconspicuous and unspectacular compared to the most flamboyant of its siblings but it was, in fact, significantly larger and brighter than average. The very brightest members of the cluster burned themselves out within a few million years. In contrast, our more frugal star was destined to shine for 10 billion years. In fact, the young Sun was even more frugal then than it is today. As the Sun has aged, the hydrogen it uses for nuclear-fusion fuel has been consumed and an inactive core of helium has slowly grown at its centre as this ‘nuclear ash’ has sunk through the lighter hydrogen. Present-day hydrogen fusion is therefore restricted to a zone surrounding an inactive core and, rather counter-intuitively, this has had the effect of increasing the total energy produced as our star has grown older. As a result, the young Sun radiated only about 70 per cent of the heat that it does now. Despite this, the Sun’s visible surface has not actually become much hotter. Instead, like a slowly inflating balloon, it has expanded to become a bigger radiator able to pump out more heat. If you could have looked at our Sun 4.5 billion years ago you might have noticed that it was a little redder but you would certainly have noticed that it looked smaller.
Looking at our juvenile star you would also have been struck by the beautiful disc of gas and dust that still surrounded it, but this beauty was short-lived. Clumping began again, on a planetary rather than stellar scale, as rocky and icy bodies in the disc collided and stuck together. Gradually, the larger bodies consumed the smaller ones and, within a few million years, collisions involved bodies the size of moons and planets. A particularly violent collision nearly demolished one planet as it smashed into another, slightly smaller, one. Most of the debris from this impact was flung into distant space, but some remained to form a ring of rubble orbiting 10,000 kilometres above the surface of a world that had only barely escaped complete destruction. That traumatised planet was the young Earth and the rubble-ring condensed in less than a thousand years to become our Moon. The Earth–Moon system was therefore born in great violence and, as we shall see in detail in a later chapter, the subsequent evolution of this system has had profound effects on the history of our planet.
The collision that formed the Moon was not the last impact our planet suffered, although it was quite possibly the largest. Intense bombardment of the Earth continued at a gradually declining rate for another 500 million years. Even today the Earth suffers collisions with bodies several kilometres across every few million years or so. However, in those early days impacts of that size occurred many times a day and truly enormous collisions must have happened every few years. These collisions generated a great deal of heat and, for 200 million years, the Earth’s surface was too hot for liquid water and the atmosphere mostly consisted of steam. Then the rate of collisions dropped, the surface temperature cooled and atmospheric water started to condense. A 100 million-year rainstorm began and oceans flooded our planet for the first time.
Life appeared within a few hundred million years of liquid water. We do not know exactly when, where or how life emerged on our world but we know that it happened early on because we have convincing fossils from 3.5 billion years ago and fairly convincing chemical signatures of life in rocks approaching 4 billion years old. As for how it happened, one speculation is that increasingly complex chemical reactions in the tiny spaces between hot, rocky grains buried beneath the volcanically active sea floor led to the emergence of a molecule that could make copies of itself from the chemicals in its surroundings. The first self-replicator had therefore appeared. This very first replicator made a copy of itself and so there were now two. But these two also copied themselves to give four and these reproduced to give eight and so on until, in a very short time, the number of replicators was enormous. A complex series of chemical reactions that had taken millions of years to happen once, by chance, quickly became widespread. This was not yet life as we know it, but it was a start.
There are many variations to this story. Charles Darwin speculated that life may have emerged in some warm, muddy pool. Others have suggested that it happened in shallow seas or even in the clay of river beds. Perhaps most speculative are proposals that life didn’t form on Earth at all but was imported from elsewhere, Mars for example. Whatever the details, the key step was the appearance of entities that make copies of themselves. Self-replicators were the beginning.
However they emerged, the first primitive self-replicators were not very good copiers and many mistakes were made. Most of the badly copied replicators were not viable and their lines died out immediately but some of the mistakes were not fatal and so, after a short while, many varieties of replicator existed. Some of these different species of replicator just happened to be better at scavenging their chemical constituents from the dilute sources available and they thrived while their less efficient cousins died out. In other words, natural selection had begun. As a result the replicators rapidly became very efficient and accurate makers of replicators. Mistakes were still made sometimes, and occasionally these mutants proved to be even better than their parents so that a new variety took over the world, for a while. At some point, replicators emerged that were able to steal resources from what had become the widest available source, their fellow replicators. These first predators thrived until all the easy prey were gone and only the fortuitously less-digestible replicators were left.
A great number of chemical and physical innovations then occurred in succession since any advantage, however slight, produced a population explosion for the innovator. Within a very short time all replicators were stored within tiny fatty bubbles that concentrated life-giving chemicals and made the replicators safer from attack by other replicators. Single-celled microbes had appeared. By 3.8 billion years ago sophisticated bacteria infected the crust, land surfaces and oceans across the entire planet. Then the first of several disasters struck the nascent biosphere.
A period of renewed heavy bombardment began as large objects were deflected into the inner solar system by recently formed outer planets. The evidence for this is visible on any clear moonlit night. The large, dark marks that make up the features of ‘the man in the Moon’ are easily visible to the unaided eye. Our ancestors also saw these features and believed them to be seas on the surface of the Moon. In reality, they are enormous impact craters – and analysis of the rocks, brought back by the Apollo astronauts, shows that these craters formed 3.8 billion years ago. It seems obvious that similar-sized impacts must have also happened on Earth and that these would have sterilised the surface so that life only just clung on in the deep, dark places of the world. Much of the atmosphere and ocean would have been blasted off our planet in these cataclysms, to be slowly replaced, over millions of years, by exhaust from volcanoes.
Despite this catastrophe, by 3.5 billion years ago the Earth was a vibrant yet still alien world. The Sun, slightly redder and noticeably smaller than today, shone through a cloudless but hazy sky onto a warm salt-water sea covering most of the planet. In many places the sea floor was lifted by massive flows of hot magma moving deep below the surface of the hot young Earth. The resulting shallow seas were carpeted by countless bacteria in colonies large enough to form reefs. Fierce tides ebbed and flowed across these microbial mats every seven hours because the days were short and the Moon close. Single-celled organisms floated in the planet-wide ocean, grew on the deep sea floor, or lived deep within the Earth’s crust.
These early bacteria obtained their energy and their building materials from the chemicals around them but these were available in only a few places. Life’s conquest of the rest of the planet had to wait until photosynthetic microbes evolved. This was followed by the appearance, around 2.8 billion years ago, of the first organisms to give off pure oxygen gas: cyano-bacteria. It took hundreds of millions of years for the full consequences of this to develop, but over that time, oxygen started to become an important constituent of the atmosphere. The geological evidence for a slow, but eventually substantial, increase in atmospheric oxygen is robust. Sediments deposited more than 2 billion years ago frequently contain banded iron formations (BIFs) but they virtually disappear from the rock record after this time. BIFs cannot form in the presence of oxygen. However, it is not clear exactly how much oxygen there was in the atmosphere at various times in our planet’s history. We are fairly sure that there was hardly any 2.2 billion years ago and that, by half a billion years ago, oxygen levels were close to modern values. But the experts continue to debate among themselves exactly how fast our atmosphere got from one state to the other.
Even if the rise in oxygen was near to the lowest estimates, oxygen-intolerant microbes, which included most life-forms at the time, soon became restricted to special environments where oxygen was absent. These anaerobic bacteria moved to where they still live today – in buried rotting matter, deep in the crust, or in ocean depths undisturbed by oxygen-carrying currents. Newly evolved oxygen-breathing bacteria took over most of the world. This drastically altered atmosphere had a profound effect on global temperatures and, as we’ll see in later chapters, produced the most unstable climate our world has ever seen.
It was also about 2 billion years ago, give or take a few hundred million years, that a dramatically new life-form appeared. These new organisms were so radically different from their bacterial forebears that, by comparison, oak trees and people are close relatives. Criminal behaviour was at the root of this innovation. Many life-forms, including humans, prefer theft to hard work since it’s easier to take food from another organism than it is to make it yourself. Large organisms do this by swallowing small ones and small organisms do it by infecting large ones. However, it’s not always clear which organism is predator and which is prey. It is particularly unclear when, as was the case for all life at this time, the organisms are simple, single-celled ones. Is a small germ inside a larger one being eaten or is it a pathogen?
When this situation first occurred an arms-race ensued. Sometimes the predators had the advantage, sometimes the infectors. Eventually a mutually beneficial truce was arrived at – or, more accurately, some groups of organisms stumbled by chance upon an arrangement that immediately made them very successful and so they thrived and became numerous. Certain small microbes became welcome, permanent residents of larger ones. They then had access to plenty of nutrients and could avoid being swallowed by less friendly predators. In return, the small microbes provided chemical services for their hosts. These aggregated organisms had taken life to a whole new level of organisational complexity.
In the earliest of these symbiotic relationships, the small microbes were the oxygen-breathing bacteria I mentioned earlier, and so they gave their hosts the ability to use oxygen gas. The resultant compound creatures were the common ancestors of amoebas, mushrooms, oak trees and people. A little later some of these organisms also took on board cyanobacteria and gained the trick of photosynthesis. The resulting algae were the direct ancestors of all plants. The differences between the simpler, pre-existing life-forms and these newer organisms are so profound that biologists treat this division as the most fundamental one in the tree of life. Simpler organisms are called prokaryotes and the compound creatures are called eukaryotes. Plants, animals and fungi are merely different sorts of multicelled eukaryotes and so, in this scheme, are relatively close cousins.
At roughly the same time as all this was going on, but possibly much earlier, the planet’s interior underwent one of the most important transitions in its history: the appearance of plate tectonics. To understand plate tectonics, we need to look at one of the greatest scientific revolutions of the 20th century. This did not take place in a scientific field like theoretical physics or biology, which are rather prone to bouts of radicalism. It took place in the usually well behaved field of geology. After hundreds of years of scientific study we suddenly realised that a single, simple fact was responsible for virtually all the geological processes we could see on the surface of our planet. Geological activity almost all results from the fact that the continents slowly move across the surface of the Earth.
This book is full of coincidences that aren’t coincidental, and the discovery of plate tectonics starts with one of these. Sixteenth-century European explorers were the first to notice that the eastern coastline of South America fits the western coastline of Africa like two pieces from a giant jigsaw puzzle. This coincidence is reinforced by the much more recent discovery that even the rock types are the same when we compare outcrops in South America to those in the equivalent parts of West Africa. It seems obvious that, at one time, the pieces must have been stuck together. For most of the 400 years since this was first suggested the match between the two coastlines was dismissed as a simple coincidence, although bizarre explanations were occasionally offered. Some crackpots, for example, suggested that the Earth used to be smaller and that Africa and South America had been forced apart as the planet expanded. Other madmen were convinced that entire continents simply drifted across the surface of the Earth. Convincing evidence that the second group of lunatics were completely correct was finally obtained in the 1960s when we first began to explore the deep ocean floor.
The mechanism of continental drift is quite straightforward. New ocean floor is constantly being created at ridges that lie roughly along the centre-lines of each ocean. Actually, to call them ridges doesn’t really do them justice, since they form a 70,000 kilometre-long mountain chain covering nearly a quarter of the Earth’s surface. This mountain chain is, by far, the largest on Earth but we were unaware of it until recently because it nearly all lies a kilometre or more beneath the surface of the sea. Mid-ocean ridges form as molten material wells up from the hot interior of the planet and cools to form brand-new sea floor. This new ocean crust slowly moves away from the ridge centre to be replaced by even younger magma oozing up behind. The ocean floor therefore drifts away from its point of origin, at a speed of about a metre every few decades, for a few hundred million years until it gets to a point where it is so old, cold and heavy that it sinks back into the Earth to be re-absorbed by the molten interior. This sea floor destruction process is called subduction and it’s the tug of the heavy ocean floor, as it sinks back into the Earth’s interior, that largely drives plate tectonics.
Continents ride on the resulting system of moving oceanic plates and are therefore slowly transported around the surface of the Earth. However, the oceanic plates have not always had continents sitting on their backs. Rather, continental crust has been slowly created as a by-product of subduction. One way this happens is that volcanic residues deposited on the surface of oceanic plates pile up at the subduction zones, in a process called accretion. New Zealand, for example, is growing this way today as islands and submerged sea-mounts are scraped off the Pacific plate as it sinks beneath the east coast of Te Ika-a-Maui (North Island). However, the more important continent-forming process, arc volcanism, occurs because adding water to hot rocks makes them melt. Most minerals contain small amounts of water bound tightly into their crystal lattices and the effect of this is to lower the melting point substantially compared to a dehydrated rock with an otherwise similar composition. The rocks in a subducting plate contain more water than material within the Earth’s interior and so they tend to melt very easily as the plate subducts. This produces magma, which rises to the surface to produce intense volcanism and, as a result, brand-new land. As with continent growth by accretion, continent growth by arc volcanism is still going on today. For example, the islands of Indonesia were formed, and continue to grow, by this mechanism. Subduction-related volcanoes are particularly explosive and dangerous, good examples being the volcanoes in the Andes, in the north-west of North America and in Japan.
The net result of both accretion and arc volcanism is that light volcanic residues slowly accumulate at the Earth’s surface. The Earth’s continents are entirely made from these residues along with the results of reworking of this material into sedimentary rocks. Continent construction has been going on for billions of years and the area of the continents has therefore slowly increased through time. Four billion years ago there was hardly any dry land at all; whereas now, one third of the Earth’s surface is continental. The resulting continents are too light to sink into the Earth’s interior and so, compared to the oceans, they are almost immortal. This lightness, together with the greater thickness of the continents, also means that the continental surfaces sit much higher than the ocean floors, typically 5 to 10 kilometres higher, and therefore stick out above the sea. To a geologist, the fact that oceans are wet and continents dry is a minor side-effect rather than their defining characteristics.
Sometimes a new ocean-spreading centre forms inside an existing continent. This is happening today along the Great Rift Valley of East Africa, which, at its northern end, joins up with a brand-new ocean called the Red Sea. Something very similar occurred 190 million years ago, in what is now the South Atlantic, resulting in a single giant continent splitting apart to form the two separate continents of South America and Africa. So, those 16th-century explorers were right – South America and Africa do fit together very well and this is not a coincidence at all. The slow movement of Earth’s tectonic plates caused South America and Africa to separate in a process that continues today. From the point of view of this book, plate tectonics is important because it has caused a steady increase in the amount of dry land and, as we will see later, because it recycles chemicals between the Earth’s interior, its surface and its atmosphere. Continental drift and the cycling of materials both have a profound effect upon our climate.
And so, by 2 billion years ago when our planet was about half its present age, Earth had developed a substantial amount of dry land, the Sun had reached 85 per cent of its modern heat output, and the Moon had drifted away from Earth to almost its present distance. The climate at this time was experiencing a period of great instability and our biosphere was undergoing the most profound changes since the appearance of life itself. The stage was set for the final major biological act in the creation of modern Earth, the appearance of complex multi-celled organisms.
The evolution of multi-cellularity happened not once but at least three times, with the independent evolution of multi-celled plants, multi-celled animals and multi-celled fungi. Indeed, some experts claim that multi-celled organisms appeared, independently, as many as ten times. Colonies of single-celled organisms had been common for most of life’s history but a multi-celled organism is much more than just a colony of identical cells. The cells in a truly multi-cellular creature are differentiated: they take on different tasks while remaining genetically identical. The cells in my arm muscles are very different from the cells forming my brain but all these cells have a nucleus containing exactly the same genes. Although the ability to produce these different tissues took a long time, perhaps as much as a billion years, the eventual appearance of multi-celled organisms had a very profound effect on the entire planet. The evolution of large plants had the most significant consequences, because once the resulting organisms conquered dry land, they quickly covered much of the continental area, causing big changes in its heat-absorbing properties and also in the rate at which it was eroded. The appearance of these plants also allowed the eventual colonisation of the land by animals and, half a billion years later still, the emergence of human beings.
By the time land plants appeared, the Earth’s story, so far, was almost 90 per cent complete, but life large enough to be visible had only just emerged. Most books on palaeontology only start to get into their stride just before this point. The emergence of animals large enough, and with body parts hard enough, to leave easily visible fossils resulted in the, geologically speaking, instantaneous appearance of animal life about 540 million years ago. The history of life since that time is familiar, with the emergence of fish (500 million years ago), amphibians (400 million years ago), reptiles (350 million years ago) and mammals (240 million years ago). Note that this story is completely biased towards our own line of descent. How many people know as much about the evolutionary history of trees or spiders? In any case, from the point that animals appeared, the story starts to get bogged down in petty details. Most of the really important evolutionary innovations occurred before even fish evolved. Quite frankly, our climate has not been strongly affected by the peculiarities of vertebrate evolution at all and so I’m not planning to say any more about it.
To conclude, I should in all honesty admit that experts would argue over almost every one of the details in the story I have just given, but all would agree that the Earth has seen dramatic transformations many times in the last 4 billion years. As we shall see in the next few chapters, these changes in land, sea and air could have altered the temperature by hundreds of degrees centigrade but, fortunately for us, this didn’t happen. So, the story of our home planet is a story of constant and massive environmental change and, despite this, also one of reasonably stable temperatures.