Wondering About The Past

Imagine yourself inside a completely empty void. When I say empty, not only do I mean that there are no objects, no mass or energy, but also that there is neither time nor space. There is only compete and utter nothingness, everywhere and everywhen. Bear with me, because I am really quite serious about this: close your eyes, screen out as much of the world around you as you can, and really try to place yourself in just this situation.

Difficult, isn’t it? In fact, I’ll bet that you find it utterly impossible. There is a good reason for this. It is impossible. It is impossible because the very act of imagining a void places you inside the void, whereupon it is a void no longer. The moment you are there you are mass / energy taking up volume, and so there are these phenomenom, and well as space. Events are happening (even if just the beating of your heart), so time exists as well. Mass, energy, space, and time are inexorably linked together. You cannot have one without the other three. As you are mass / energy, taking up space / time, the very act of trying to imagine an empty void ends its existence.

This is, admittedly, more of a philosophical argument than anything derived from physics, and I suspect that any physicists reading it are doing so with a jaundiced eye. It certainly isn’t a rigorous argument, but I think it is a good place to start wondering about the past because, if a lot of modern physics is correct (there are dissenting views), our entire universe has a beginning, before which there really was nothing, mass, energy, space, or time. Perhaps there were no physical laws either, as difficult as it is to see how the very concept can make any sense.

That beginning has a name, a name that ironically has found its way into common usage because its author, Fred Hoyle, intended it to be more of an insult to discredit it than anything else. It is, admittedly, a rather silly name for a scientific theory, compared to, say, General Relativity or Quantum Mechanics or Evolution by Natural Selection. It also has the flaw of not being a particularly accurate way of describing the beginning of the universe; the phrase “Big Bang” implies an explosion of matter and energy within a pre-existing space and time, rather than the initial expansion of space-time as well the matter and energy which occupies it.

In calling it “the initial expansion” I am not being much more accurate, however. There is no point in the evolution of the universe where can point to and say, “The Big Bang ends here; everything after this point is just ordinary expansion.” The truth is, the Big Bang is not a theory of how the universe got started; it is a theory of how it has evolved and still is evolving and will evolve that reaches right back to its very beginnings, back to time zero, assuming that there was a time zero and whether that term has any meaning. It is also not a theory of how or why that expansion began; there is a lot of scientific speculation about what might have been happening before time zero which lead to the Big Bang, but (almost) none of that speculation describes or modifies the Bang itself. Lastly, an important perspective to maintain is that we are not mere observers of but are living amidst the Big Bang, yes, right now even as we go about our busy little lives it is happening right here and everywhere else in the universe, an event which probably has many, many trillions of years to go or more, assuming that it will not continue forever, which at this point in our understanding of it is not at all clear.

And having said all that, I must now confess that I’m still not describing it with sufficient precision. I speak as though it is iron-clad scientific fact that the universe has a beginning. We do not understand the laws of physics well enough, however, to make so powerful a claim. All we can do is: ( a) observe now that the universe is expanding; (b) imagine reversing time so that the universe is contracting; and (c) extrapolate the time-reversed contraction until we inevitably encounter a situation in which all matter, energy, space, and time in the universe are compressed into a single point, a so-called “singularity”, at which moment our extrapolation comes to an end by logical necessity – by mathematical definition, nothing can by smaller than a single point. This is where we run into trouble, however. We cannot follow (c) to its logical conclusion with certainty because nobody knows what the laws of physics are beyond a certain degree of compression and temperature; as the universe gets smaller, hotter, and denser the set of rules of how matter-energy-space-time behave under such conditions are less and less well understood. When you get to the point where you don’t know what the rules are anymore, you simply can’t extrapolate any further. Thus, (c) is really just a conjecture; we cannot say whether there was a beginning or not. It is an important point to keep in mind because you will routinely encounter statements such as “the universe is 13.73 ± 0.12 billion years old” which assume (c) is true. Actually, such statements are really conveniences, because they allow us to apply time markers to various stages of the universe’s evolution.

The result of all this is that we are face to face with a dilemma. Curiosity and wonder cause us to yearn for answers that our current state of knowledge can’t gratify. Worse, we may never have gratification, because we are bumping up against the limits of what technology and even the laws of physics themselves can help us answer. The most powerful telescopes in the world can’t see all the way back to the beginning, if there is one, of the universe (remember that in looking at objects further and further away in space, the time it takes for the light to reach us means we are looking further and further into the past). The most powerful particle accelerators we have built can’t come close to reproducing the earliest fraction of a second of the universe’s existence. Even the theoretical / mathematical approach, through it has paid its share of dividends in nuclear physics (it is how Murray Gell-Mann predicted the existence of quarks for example), might not get us there. It is as though we keep creeping closer and closer to our goal, yes, but we can never actually get there, however patient and persistent we are.

Still, the ability to trace back the evolution of the cosmos back 13.73 ± 0.12 billion years is a remarkable feat, one of the most remarkable in the history of science, even if we struggle to squeeze the previous trillionth of a second or so out of our cosmic clock. We are talking of a time long before our solar system existed, indeed long before there were any stars or planets or galaxies or kinds of structures we see in the universe today. What we can say is, however, is that the primordial seeds of all these structures, including ourselves, must have somehow existed in the super-hot, super-dense soup of quarks, gluons, electrons, neutrinos, photons, and what else have you which constituted the universe – or at least our universe – at that time. How those seeds of so long ago became the reality we experience around and within us today is a fantastic tale, only a fraction of which has been worked out to any detail.

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So much for ultimate beginnings. Let us jump forward to more mundane affairs. I suspect that most people, if asked what the early Earth was like, would probably think of dinosaurs roaming its landscapes, and their reptilian cousins filling the air and the oceans. Some would even think of “cave men” (not all early humans lived in caves, of course) hunting wooly mammoths and other prehistoric creatures. I hope these statements don’t sound condescending, because they aren’t meant to be. Rather, they are meant as cautions regarding our sense of time, particularly our sense of so-called “deep” time, that time scale in which biological, geological, and cosmic evolution reveal their full and wondrous workings. Time of this magnitude overwhelms our imaginations. We simply cannot, however hard we try, conceive of a million years. Even a thousand years – a thousandth of that million, and the approximate distance between Middle Age Europe and today – defies our intuitive sense of time. Actually, as Richard Dawkins points out in The Blind Watchmaker this is not only unsurprising but exactly what we should expect: our intuitive sense of time is the result of the way our brains are wired, and the genes that have wired them have been selected by evolution to give us good, gut-level grasps only of events that take from significant fractions of a second to decades to happen. Any event much shorter than a second or longer than a hundred years or so fall outside of the range of our intuitive time sense, and we shall have a hard time conceiving them. This is why a thousand, a million, a billion, and a trillion years all feel pretty much the same to us, although there is a thousand fold difference between each pair, thousand to a million and so on. Again, as Dawkins has noted, this is probably one of the main reasons for most of the people who reject evolution; they simply cannot imagine the time scales involved, and therefore seriously underestimate what is and isn’t probable during them.

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Typically, this is where most authors of books dealing with deep time try to come up with some kind of scale which makes these times more intuitively manageable. The scale often relates time to distance; so that for example the age of the universe may be represented by a line stretching from Los Angeles to New York City, or something like that. The trick in using this analogy is in avoiding scales which already defy readers’ imaginations at one or both ends; I, for one, am not sure I can intuitively grasp the three thousand mile LA-NYC distance effectively. This of course is because our brains have also been selected to have good, gut-level grasps of distances as well as times, and three thousand miles almost certainly falls well outside of that range as well.

On the other hand, too small a scale, while allowing us to grasp its full length, can be too short to let us sense the fractions of that scale we wish to deal with. For an example of this, a universe timeline only a mile long would put our human lifespans in a fraction of a thousandth of an inch range; most of us, I suspect, find that just as hard to fathom as the LA-NYC distance.

What we want is a scale that is neither too large in its full range, nor too short in the fractions of that range we wish to work with. The scale I’ve chosen to work with seems pretty workable to me. What I propose to do is simply scale all times down by a factor of a billion. I am also leaving it as a time scale, rather than converting it to distance as many people do. In this scale, therefore, the universe is about 13.7 years old. As human beings in the real universe generally live between two and three billion seconds (there are 31,556,926 seconds in a year, or a billion seconds in 31.688765 years), our average lifetimes are reduced to between two and three seconds using this scale. Events thus appear pretty manageable at both ends of the continuum, or at least they do so for me.

If we take this moment, now, December 18, 2008 as I write these words, as the end point of our scale, then the universe “began” (recall my cautionary statements earlier about the beginning of the universe) in early April of 1995. Relating that to my own life, I was thirty-eight years old then compared to fifty-two today, my 21½ year old daughter was nearing her ninth birthday, and my 14½ year old son was only a baby, not yet taking his first steps yet. President Bill Clinton was in his first term, it was about four years after the fall of the Soviet Union, and the terrorist attacks of 9/11/2001 were still over six years in the future. I can relate a lot of other events of that time in my life, and no doubt so can the reader (unless you are a teenager or younger).

There are numerous ways we can proceed here. For example, the expanding universe became transparent – that is, it cooled off enough for the first atoms to form, allowing electromagnetic radiation to decouple from matter and stream freely through space in what is now called the cosmic background radiation – about 3½ hours later on that April day, and the first galaxies that we can detect with our most powerful telescopes would have formed about a year later, in early 1996. We don’t know with certainty when our own Milky Way galaxy condensed from intergalactic gas and dust, but it must have been well before the creation of our solar system 4½ years ago, in June, 2004. This is because our sun is a so-called “first generation star”, one heavily composed from the ejecta of earlier, massive stars which exploded in supernovae and seeded interstellar space with all the elements up to uranium (thereby allowing the sun to have terrestrial planets like Earth).

If we restrict our timeline to events that happened on Earth, then the first fossil traces of simple “prokaryotic” (bacteria or archea) cells appear as either rare microfossils or biochemical traces between 4 and 3½ years ago, although recent hypotheses about early Earth could mean life began sooner. Eukaryotic cells, which are larger, more complex cells which comprise all animals, plants, and fungi today, evolved from prokaryotic cells at around somewhat more that half that distance from the present, or around two years ago, and the first, unequivocal albeit very simple multicellular organisms appear, along with sex, approximately a little over a year ago, although this is uncertain because such organisms contained no hard parts (shells, skeletons, etc.) and would have left few fossils.

At this point, let me backtrack a little. Earlier I mentioned how most people probably think of dinosaurs, or even early humans aka cave men, as living on the early Earth. We begin to see now just how potent a testament to our poor intuitive grasp of time that is. So far I’ve covered about three-quarters of the Earth’s history, and over ninety percent of the universe’s and nowhere have dinosaurs or early humans been mentioned. In fact, we still have some considerable chronology to cover before we get there.

Scientists, as they so often do whatever their field of specialty, have divided Earth’s history into a number of time slices called eons (technically, an eon is a billion years, so it is used only as a rough approximation here). These are, from oldest to most recent, the Hadean eon, covering the time between Earth’s formation and 3.8 billion years ago; the Archeon eon, when life unequivocally first began, lasting from 3.8 billion years ago to 2.5 billion years ago; the Proterozoic eon, from 2.5 billion to 542 million years ago, covering the evolution of eukaryotic cells and the first multi-cellular organisms; and the Phanerozoic eon, which takes us from the end of the Proterozoic to the present. Each of these eons begins and ends with some important developments in the evolution of life on our planet: the first appearance of life, the first eukaryotes, and so on (the burying of the first multicellular organisms and sex in the middle of the Proterozoic seems to be the major exception to this schema).

On our 10⁹ : 1 scaled down history of the universe, the Proterozoic Eon lasts from two and a half years to six and a half months ago. The last three and a half months (starting from ten months or 850 million years BP) of this epoch are of immense importance to biologists, geologists, and paleontologists because it includes a series of events which may have given rise to the world we know today, teaming with highly complex, diversified multicellular organisms – such as ourselves – which dominate almost everywhere we look.

The first such series are a sequence of serious ice ages, lasting from 850-800 million (ten months) to around 630 million (seven and a half months) ago. There is considerable controversy over just how severe these ice ages were; the phrase “Snowball Earth” has been coined by those scientists who believe virtually the entire planet was entirely frozen over, right down to the tropics, while critics of this admittedly dramatic picture believe the planet came nowhere close to such extremes – the phrase “Slushball Earth” has been used by a number of said critics. Other scientists dispute the timing as well as the extents of the ice expanse. It appears also that these ice ages were interspersed with periods of unusual warmth. This is because greenhouse gasses such as carbon dioxide (CO₂), methane (CH₄), and sulfur dioxide (SO₂) belched from active volcanoes would have built up in the atmosphere during the ice ages, yet the extremely cold, dry atmosphere would have been unable to precipitate them out; when rising temperatures caused by this CO₂ / CH₄ / SO₂ driven greenhouse effect finally did melt the ice there might have been temporary “Hot House Earths” until the excess gas was finally removed.

Whatever the exact scenario, these large climactic swings must have had devastating effects on Earth’s biota of the time. We may be quite fortunate that all life was not completely and permanently extinguished. Certainly, many species would not have been able to cope with the prevailing conditions and did become extinct, while those that did survive probably stumbled upon various adaptations which natural selection would have favored. In short, the Snowball / Hot House Earth period probably was a crucible in which evolutionary pressures would have been much greater than at almost any other period in Earth’s history – a biological ice house / smelter from which those who emerged would have many improvements over those who went into it. This is a somewhat speculative statement, but it is a reasonable one. The fact that the first truly complex multicellular organisms appear in the fossil record only a few million years after the end of the Proterozoic ice ages (about two weeks in our timeline) – the so-called Ediacaran fauna of soft-bodies animals – lends support to it.

Let’s stop, catch our breaths, and take stock of where we are and how far we’ve come. In mentioning the first appearance of the Ediacaran fauna we are standing somewhere between 600 and 580 million years BP. That is about seven months BP in our scaled-down timeline. Given that Earth first condenses from the solar nebula 4½ years, or 54 months, ago on this timeline, we have already covered 1  47/54 = 87% of our planet’s history (or, that the universe begins 13.73 years or 165 months ago, 1  157/165 = 96% of its history). By this reckoning, even Ediacaran animals are the new neighbors on the block, although they don’t hang around for long: by six and a half months ago or 86.5% of Earth’s history they have vanished forever.

Dinosaurs? Early humans? Not even close yet.

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As a child, probably my second most favorite activity, after star gazing, was fossil hunting. There was a small creek which ran very close to the house I grew up in, and from time to time I would spend a few hours at one of the sandy banks and rock outcroppings along the beach, turning over stone after stone in hopes of find one with the outlines of some ancient life form on it (those that didn’t I would practice stone-skipping with if I could find stones flat enough). Mind you, I didn’t have a fossil collection, not in any kind of formal sense; I just kept whatever I found of interest in the various places I squirreled stuff, taking them out occasionally to look at and wonder about. I wouldn’t even hazard a guess as to where any of them are now; most of them are probably back in the ground I liberated them from, hopefully to be re-unearthed by future generations.

I don’t remember if the connection between the two pastimes, star gazing and fossil hunting, ever occurred to me back then, but it is quite obvious now. Both are attempts to peer into the past, to learn about what was, not just a few years or even centuries but thousands and even millions of years ago. There is something about the vastness of deep time, like deep space, that is paradoxically both very intimate and infinitely far away at the same time; it is, I sometimes suspect, rather like being with someone we have known and loved for a long time, but still realize we don’t know very well.

I didn’t become either a paleontologist or an astronomer because, as I have already confessed to, I’m too much of a scientific dilettante, and too lazy I must also add, to put in the time and effort or endure the tedium that being a professional in these fields requires. So I don’t hunt for fossils anymore; I would no doubt chuck them anywhere convenient, something that I now know but my child self didn’t is akin to blasphemy. Let someone who will treat them with appropriate respect discover them.

The very idea of a fossil is a fantastic one, one that makes us shake our heads in near disbelieving wonderment when we think about it. Think of all the many quadrillions of living things that must have lived and died on this planet. Despite such numbers, that any of them should have had their hard parts (and sometimes soft parts) mineralized into or left impressions in rock, and then be exposed to human eyes millions of years later by weathering or geological processes is, I think, amazing enough. But to in addition be discovered by scientists who study them and use them to piece together so much of the story of life on Earth as they have, is one of the most remarkable accomplishments of science. For me it is certainly well up there with the discovery of the laws of physics or the elucidation of the details of biological heredity.

I have paused in our cosmological timeline to talk about fossils because for most intents and purposes this is where life really begins. True, we do have fossils and fossil traces and other biochemical markers in rocks older than 540-550 million years ago (six and a half months on our timeline) but they are much rarer and difficult to find because before this time few if any living things possessed hard parts to leave behind when they died. This is the start of the Cambrian period of our current era of life, the Phanarozoic eon; this is the beginning of many animals possessing shells or exo or endoskeletons which can mineralize after the animal’s death or leave clear impressions in rock.

One of the most fascinating – and scientifically challenging – things about the Cambrian is how quickly this evolution and radiation of creatures with hard parts occurs. Quickly on the geologic scale that is; within a few tens of millions of years at most, or about a week on our timeline, most of the major phyla (large groups of life, such as vertebrates, arthropods, mollusks, etc.) have made their first appearance in the fossil record. This period is even often termed the Cambrian Explosion in recognition of its extreme rapidity, although of course by human time standards it is still an immense stretch of time, well, well beyond anything we can intuitively grasp. It is as though the speed of evolution had been stepped up by an order of magnitude or more for a good several tens of millions of years or more. Scientists are still debating the cause of the Cambrian Explosion, with some arguing that it really wasn’t really an explosion at all but more of an artifact of the fossil record. Time, if the pun can be pardoned, will tell. Either way, by some 500 million years or six timeline months or so ago almost all the major animal body plans had been laid down and are ready for evolution to elaborate and sculpt them into all the animal species found on Earth today. An Earth that is about ninety percent of its present age, or ninety-seven percent of the universe’s. And yes, we still have a way to go before the first dinosaurs appear.

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I hope that by now you have started to get at least some feel for the overwhelming immenseness of geologic, or more, of cosmic time. During the period we have been covering so far, somewhere around a couple of billion stars have been born, lived, and perished in our Milky Way galaxy alone, and probably over a billion trillion in the universe as a whole, a universe that has expanded from something much smaller than an atom into an empire of galaxies, galactic clusters, and galactic superclusters spreading over tens of billions of light-years of spacetime. It all begs the natural question: have any other civilizations preceding our own come into existence, and perhaps went extinct, in all this enormity? Have any other minds even puzzled over the questions we puzzle over now? It seems so odd that the answer to this question is that we simply don’t know, and may never know, however hard we search for it.

Based on what we do know, our own existence was not even remotely close to guaranteed at the end of the Cambrian, that half billion years / six months ago. The fact is, even today, almost one hundred and fifty years after the publication of the Origin of Species, we still don’t know in any detail why some species lines survive while others go extinct. We comprehend in broad brushstrokes about natural selection and adaptive complexity and the competition for survival and reproduction, yes; but we can’t point to any fossil in a geologic strata and say for sure why it does or doesn’t exist in the strata directly above or below it. There are general trends in the fossil record that can be followed and so offer hope for better understanding: an increasing number and diversification of species and genera (punctuated by mass extinction events which we will come to shortly); long term increases and decreases in the sizes of the largest species; a general overall though not perfectly steady trend towards increasing complexity and intelligence, assuming that cranium versus overall body size is a good indicator of intelligence. We can, if we squint our eyes tight enough and apply a liberal enough dose of imagination and wishful thinking, perceive humans or something like humans eventually rising from the fray. But we don’t really know, whatever we privately, or publically, believe.

The biggest monkey wrenches thrown into our hopeful perception of progress from the Cambrian onward are the mass extinction events that pock-mark the fossil record like shotgun blasts from a drunkard. There are five recognized major ones during Phanerozoic eon (the criterion for being major is that over fifty percent of existing animal species were killed off), along with a myriad of smaller events. The most important effect of these events is how they often drove many hitherto successful species, genera, families, and even entire orders into extinction, essentially “clearing the slate” so that new kinds of animals could rise to prominence. Life on Earth today would be much different had they not occurred. We, for one, would certainly not be here to discuss them. With almost equal certainty neither would the dinosaurs, whose rise and fall appear to be the result of three of the five major events.

Let’s return to our timeline, where we left off six months, or 500 million years ago, bearing in mind again that we are almost at ninety percent of Earth’s present age. If we fast-forward to half that distance in the past, three months / 250 million years ago, we find that our Cambrian beginnings have covered quite a bit of ground towards the planet’s current biota. The oceans, lakes, and rivers are filled with fish, many of which are indistinguishable from fish of today, although the proportions of the different classes are different; there are more lobe-finned fishes (the ancestors of land animals) than today. Many of the land and water arthropods are also fairly modern-looking, one of the obvious exceptions being the trilobite, an ancient arthropod reaching back to the Cambrian but whose days are severely numbered by this time. Another difference is that some of them grow to amazing sizes; there are, for example, dragonflies with wing spans close to a meter across. The land is already populated by a wide variety of amphibians and reptiles, although especially for the latter, they don’t resemble those of today very much at all; some of the latter are the so-called mammal-like reptiles, named for exactly the reason you imagine, and indeed all modern mammals are descended from a lineage of them. Many ferns, mosses, and conifer trees also fill the landscape along with other plants, though none of them sport flowers or are pollinated by insects or birds – for that matter, there are no birds or other flying vertebrates to pollinate anything. As for the state of the world, much of its climate at this time is hot and arid, and all of the major land masses are joined together in a single, super-continent named Pangaea, the center of which is possibly the largest desert ever to have existed in Earth’s history.

It was on this world that what has been called the Mother of Mass Extinctions then happened 250 million / three months, the Permian-Triassic, or P-T, extinction event. Over the next several million years (about one or two days on our timeline) the great majority of species in both land and marine environments vanish from the fossil record, leaving no descendants. Estimates of the total carnage are as high as 90-95% of marine species, and 70% of land. Of most important for this discussion were the extinction of most of the mammal-like reptiles (excepting the one that gave rise to mammals, of course), leaving room for the rise of the group of reptiles called archosaurs, which appeared either somewhat before or after the extinction, depending on their exact definition. Whatever definition is accepted, the archosaurs are the ancestors not only of modern crocodiles and birds, but also the dinosaurs and pterosaurs (the flying reptiles of the dinosaur age), and the marine reptiles which would come later.

The first true dinosaurs appear in the fossil record some 225-230 million years ago, in the middle of the Triassic, the first period of the Mesozoic Age, or age of reptiles. And thus finally we are here, a mere two and a half months ago on our timeline, with our planet being 95% of its current age. So the dinosaurs aren’t ancient at all, but are more like our next door neighbors in time. The next major extinction event, at the end of the Triassic 205 million years ago, cleared out most of the rest of the dinosaurs’ competition and led to their being the main land animals on Earth for the next 140 million years, until they themselves met their end in the most famous (but not largest) extinction event of all, the Cretaceous-Tertiary or K-T event, 65 million years / 24 days / 1½% of Earth’s age ago. Indeed, looked at this way it seems like just yesterday that Tyrannosaurus Rex and its cousins spread their terror across the land, making the world of Jurassic Park almost sound plausible.

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The causes of mass extinction events are still strongly disputed in the scientific community. One of reasons for this is that there are many possible contenders for causes, geologic, climate-related, or astronomic; so many, in fact, that we probably should count ourselves absurdly fortunate that life was never completely wiped out by one or a combination of them by now – assuming that that there isn’t something other than fortune going on here. Over the last 20-30 years, however, two particular contenders have come to acquire particular interest: asteroid impacts and massive volcanism known as flood basalt events. Interestingly, although both scenarios are very different in the events that begin them, the consequences to Earth’s biota are due largely to changes they cause in the planet’s atmosphere. The impact scenario, which was first hypothesized for the K-T (dinosaur) extinction by Alvarez et al in 1980, proposes that a large (5-10 miles across) asteroid struck Earth 65 million years ago, injecting so much dust, water vapor, soot, and ash into the atmosphere (the latter two from massive forest fires caused by burning debris from the impact) that the amount of sunlight reaching ground level would have been seriously reduced for as much as several years. The results would have been severe cooling and reduction in photosynthesis, killing all those plants and animals that could not adapt to these extreme changes.

The Alvarez hypothesis is supported not only by geologic data such as high iridium concentrations and shocked quartz in the K-T boundary stratum, but also by the discovery of an impact crater of appropriate the right size in 1979 just off the Yucatán peninsula. Despite all this, the hypothesis is not without its problems, one of the main ones being an alternative which might be better (another problem is that there are known large impacts in the geologic record that are not associated with mass extinctions). This alternative is the massive volcanism / flood basalt event hypothesis.

Volcanic eruptions occur in different scales and varieties. Some, like the volcanoes of Hawaii, involve a more-or-less steady flow of lava from an underground magma reservoir over a long period of time. Others are explosive, releasing large amounts of hot ash and gasses, along with pyroclastic flows and lava and rock “bombs” over a relatively short time period. Mount Vesuvius, which destroyed the Roman cities of Pompeii and Herculaneum in a 79 AD eruption, is one example of this latter type of volcano. Another is Mount Tambora in Indonesia, whose eruption in 1815 was probably the largest in history: not only did it outright kill over 71,000 people, it injected so much gas and dust into the atmosphere that there was significant global cooling over the next several years, killing many more from famine due to crop failures.

I mention Tambora not because it was so horrific a catastrophe, but because compared to many eruptions in our planet’s geological history, even its recent history, Tambora was a firecracker which possibly didn’t result in a single species’ extinction. When it comes to serious volcanic eruptions once again we should breathe a sigh of relief and count our lucky stars. And worry about what could happen in the future.

If you want an example of what not only could be but one day will be, though nobody knows when, take a trip to Yellowstone National Park. While you are admiring the many geysers, hot springs, fumaroles, and steam-belching mud pots, and standing in awe of the massive yellowish volcanic tuffs whose color gives the park its name, imagine what lies under your feet that is causing all these natural splendors. What you are standing on is an enormous volcanic caldera, one approximately 1500 square miles in extent (the Tambora caldera is about 20 square miles), which itself overlays a magma chamber of comparable size. The caldera is the result of a colossal volcanic eruption, or “supervolcano”, some 640 thousand years ago, which was orders of magnitude larger than the 1815 Tambora event. If such an eruption were to happen today, which it very well could, the human death toll would be easily in the millions if not tens of millions, from both direct and indirect (climatic, etc.) causes. And there have been eruptions of comparable magnitude that occurred more recently. The Lake Toba caldera on the Indonesian island of Sumatra, comparable in size to Yellowstone, exploded around 70,000 years ago in an eruption so large that it may have almost caused all human species existing at that time to go extinct.

Yet Yellowstone and Toba are nowhere near the upper limit when it comes to volcanic events on this planet. Flood basalt events involve single or multiple volcanic eruptions (of any type) so enormous that they cover up to a million square miles and more of Earth’s surface with lava, to a depth of a mile or more. Such eruptions can dwarf supervolcanoes as much if not more than the latter dwarf any historic volcanoes. Actually, a good example of flood basalts is the maria, or “seas”, on the moon. These are the dark, relatively crater-free, regions which cover much of our satellite world (I should say the half of that world that always faces us due to tidal locking; the “far” side of the moon is almost devoid of maria). The two largest of these events on Earth, within recent geologic history, are the Deccan flood basalts of India and the Siberian flood basalts. I don’t want to even speculate what would happened if either of these events occurred today, because what is so interesting about them is their timing: the Deccan basalts were laid down approximately 65 million years ago, and the Siberian basalts around 250 million years. Both dates coincide with what are probably the two most important mass extinction events: the Deccan with the K-T event which finished the dinosaurs, and the Siberian with the P-T event, the largest extinction event in the geologic record. If you find it difficult to believe that such timing is just coincidence, you have a great deal of company, including me.

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For myself, the on-going debates about the causes of mass extinctions is wonderful and exciting because it shows science, and the people who devote their lives to it, at its best. The combination of simple human inquisitiveness, wide-eyed imagination, the struggle to avoid dogmatism, and hard-nosed skepticism based on details has no equal in any other form of human endeavor. We see an enormous mystery, we open our minds as wide as lotus blooms to come up with possible solutions, but the solutions we come up with must run the gauntlet of data (often incomplete, unclear, and contradictory), experimentation, calculations, and competition from alternatives that other minds as bright as our own have dreamed up while we were busy proving our own ideas. And then, just when we think we’re on the verge of having it all figured out, someone or something else – usually new data – comes along to re-ignite the controversy.

Until 1980 the whole field of explaining mass extinctions was, from what I can tell, rather moribund. It wasn’t that there was a dearth of ideas on what could wipe out such a large proportion of species over a short period of (geologic) time. The two main problems, again from my own reading, is first, a lack of detail in the fossil record, in terms of exactly which species went extinct and exactly when they did so – for example, had an extinction occurred abruptly or over several millions years? – and second, the difficulties in determining what kind of evidence a certain kind of event would leave – an example in this case would be, if a nearby supernova (exploding giant star) had caused the extinction event, what markers in the geologic record would reveal it? The result was that the extinction events looked more like the random acts of a god or gods instead of anything that could be explained scientifically. Many scientists didn’t even want to think about them, even.

The main effect of the Alvarez impact hypothesis, more than anything else if I am reading history right, has been to bring the spotlight, not only of publicity but of serious scientific thinking on the whole subject of explaining mass extinctions. What makes this achievement all the more amazing was, it wasn’t what the Alvarez’s were trying to do at all! Many people by now have heard the story of how they stumbled upon anomalously high levels of iridium in samples of K-T boundary clays, inferred a large asteroid impact from that single fact, then suggested that said impact might be the reason for the dinosaurs’ mysterious vanishing act 65 million years ago. What most people probably don’t know is how much resistance the impact hypothesis met among paleontologists and biologists at the time, many of whom argued that the fossil record showed a gradual decline of dinosaur numbers and diversity for millions of years leading up to their final extermination – data that the impact scenario decidedly does not predict – and that in any case it was hard to reconcile the pattern of extinctions with the consequences of a large impact. What is more interesting, to me at any rate, is how these objections didn’t go away even as further evidence – shocked quartz, soot from mass fires in the boundary clays, and the discovery (or re-discovery) of the 65 million year old “Chicxilub” crater in the Yucatan peninsula – appeared the cinch the case for an impact-caused extinction.

Scientific analogies are a bit treacherous, but in some ways this one reminds me of Neils Bohr’s work on explaining the hydrogen atom in 1913 with the then new ideas coming out of quantum theory: it was brilliant and original, predicted properties of atomic hydrogen perfectly such as its spectral line series, was critical in the development of atomic theory, and rightfully earned Bohr his fame and Nobel Prize, but …but that was all it did. It couldn’t predict the properties of any other atom or molecule, nor did it offer any explanation for why electron orbits should be quantized. It was an absolutely necessary and essential step, but that’s all it was, a step.

By the 1990’s impact theory was so in vogue that some scientists were attempting to explain all extinction events with them. It was even postulated that the sun had a stellar companion, suitably dubbed Nemesis, that was too dim to have been detected so far and which had a highly elliptical orbit that brought it close to the inner solar system once every 26 million years or so, disturbing enough comets in the Oort cloud and / or Kuiper Belt to rain destruction down on Earth and other planets. The 26 million year cycle in extinction events didn’t hold up well under statistical analysis, however, but that didn’t stop people from finding iridium anomalies and shocked quartz and other geologic evidence associated with the big die-offs.

Still, a lot of the paleontological evidence just could not be made to fit simple impact hypotheses, leading some scientists to look for other causes. And here I think they were helped by yet another one of those convergences like those that Schmidt mentions in his book. The 1990s and 2000s were times of increasing scientific work on the effect of anthropogenic greenhouse gasses on Earth’s climate, and the possible implications of quick temperature rises of around 5° – 10° C or more, rises that seem plausible given our current state of knowledge. One of these implications involves the release of a large amount of methane, another potent greenhouse gas, from methane “clathrates” or “hydrates” lying on the ocean floors (these are methane deposits held in an essentially frozen state in water ice by the cold and great pressure of the ocean depths; once brought to the surface they disintegrate rapidly, releasing the methane into the atmosphere). A large scale release of this methane, which exists in enormous quantities on the bottoms of northern and southern oceans, could lead to even more drastic, short-term warming of the planet.

This issue led to a reexamination of the possible consequences of large-scale volcanic eruptions on our planet’s climate, in particular the Deccan and Siberian flood basalt eruptions which are by far the largest over the last 500 million years ago – er, six months on our timeline, if anyone is still counting. Eruptions do release large quantities of various gasses, mostly water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), sulfur dioxide (SO₂) and hydrogen sulfide (H₂S); all of these are greenhouse gasses, though some, like SO₂, can lead to cooling also due to the sulfate aerosols they often form; in addition, H₂S can also erode the ozone layer in large enough quantities. We’ve already seen how a large eruption in the historic record, Tambora in 1815, had serious consequences for Earth’s climate for several years. What might enormously large eruptions on the scale of the Deccan and Siberian lead to?

It isn’t an easy question to answer, in part because it is difficult to determine the quantities of volcanic gasses released in these eruptions and the time period(s) they were released over, let alone the global effects of such releases. But by the mid 2000s some scenarios had been worked out which showed how these events could have played a major role in the P-T and K-T extinctions which coincided with them. Also, by then evidence for these scenarios had been found in the geologic and fossil records.

What follows is worrisome to me because of what it might imply about our current problem of anthropogenic greenhouse gas warming. In my chapter on the future, I placed man-made environmental catastrophes aside in considering our own future evolution while stressing that something like Homo sapiens and our technological civilization on this planet is a gigantic experiment which may never have been played out in the universe before (and may never again), so we don’t know how it is really going to turn out. There are simply too many unknowns, and trying to wade through all possibilities is a Sisyphean task a hundred books wouldn’t be enough to cover. But if the connection between mass extinctions and massive flood basalt eruptions proves out to be true, what it suggests about our current greenhouse warming is sobering. We may need to take much more drastic steps than we are taking now in order to prevent any significant warming.

What makes the flood basalt driven mass extinction hypothesis especially interesting is that it brings together a sequence of different environmental triggers to accomplish its devastating effects. First, large scale emission of greenhouse gasses, such as H₂O, CO₂, CH₄, SO₂, H₂S (the last gives rotten eggs their odor), and others warm not only the atmosphere over a period of thousands of years or more, but also the upper layers of the oceans and other bodies of waters. Note that as mentioned, this warming may have released large amounts of methane (CH₄) from methane hydrates on the ocean floors, further exacerbating this warming. As warming water reduces its ability to dissolve gasses, reduced O₂ in the oceans leads to widespread anoxia, which in combination with increased acidity due to CO₂ absorption, the direct effect of killing off many organisms in these layers followed. The indirect effect of reduced O₂ / increased CO₂ is far more insidious, however. In different areas of the ocean and some seas (the Black Sea of today is a well-known example), dissolved O₂ only reaches a certain depth; the waters below this depth, known as the chemicline, support only anaerobic organisms, some of which are bacteria which produce copious amounts of H₂S. This H₂S normally never makes it to the surface, but the anoxic waters and die-offs of aerobic organisms during the flood basalt events may have resulted in the H₂S producing bacteria proliferating and the chemicline rising to the surface, releasing large amounts of this gas into the atmosphere. The effects of high concentrations of H₂S in the atmosphere would be catastrophic for most land dwelling organisms. Not only is this gas directly toxic, more so even than the hydrogen cyanide (HCN) used to kill concentration camp prisoners in Nazi Germany, it is also light enough to rise into the stratosphere, where it would poison the ozone layer, allowing in higher levels of lethal ultraviolet light from the sun.

If the flood basalt / greenhouse warming / ocean anoxia / H₂S producing bacteria increase scenario is correct, it should leave certain evidence in the geological / fossil record. And indeed, for the P-T event we do find multiple forms of evidence, in the form of biomarkers for these bacteria in oceanic sediments, in lower oxygen levels in the atmosphere, in the patterns and types of extinctions, and in fossils showing the effects of increased ultraviolet radiation. So it would appear that the Siberian flood basalt events hypothesis of P-T extinctions is well on its way to being confirmed. But what about the effects of the Deccan floods basalts on the K-T extinctions 65 million years ago? The scenario is essentially the same, but can they too account for that event, with or without a large impact?

The Deccan flood basalt caused K-T extinction was actually first proposed by Dewey McLean more-or-less concurrently with the Alvarez impact hypothesis. Although initially overshadowed by the more dramatic image of an asteroid striking Earth and the possible consequences of such an event, this may be due more to yet another convergence in scientific thinking; for this was about the same time that the “Nuclear Winter” hypothesis was being popularized by Carl Sagan and other scientists, and the parallels between the two ideas were undeniably striking. Sagan’s Nuclear Winter hypothesis claimed that even a relatively small nuclear war (relatively truly being a relative term in this case) could release into the atmosphere so much dust and ash and soot from massive firestorms that photosynthesis could be blocked for up to several years and the planet thrown into a deep-freeze, very much like the results of a large impact. Naturally, the re-discovery of the Chicxulib crater also lent a great deal of support to the Alvarez impact theory.

In the end, scientific hypotheses rise or totter and fall based on physical data, in this case the data being the precise timing of the K-T extinctions compared to that of the Chicxilub impact and the Deccan flood basalts. And what appears to be happening over the last ten years or so as far as I can see is that the timing is coming to favor the flood basalt hypothesis better and better. The Chicxilub impact may have actually occurred several hundred thousand years before the final extinction K-T extinction pulse. Of course, it is possible that the impact did still play an important rule, in driving some species closer to extinction. But it is becoming difficult, at least from my read of the controversy, to doubt the central importance of volcanism.

* * *

It’s high time to return to our timeline, although all this talk about mass extinctions and their possible causes leaves me hungry for more. We left off at the end of the dinosaurs, the K-T event, some 65 million years / 24 days / 1½% of Earth’s age ago if I may reemphasize just how recent the dinosaurs are in Earth’s history. We have entered the Cenozoic Age, or age of mammals. The first mammals actually appear in the Triassic, along with the dinosaurs, but until this point they have remained relatively small, mostly nocturnal animals, who eked out their existence in ecological niches the dinosaurs, for whatever reasons, never penetrated. The extermination of their dominating cousins in the K-T mass extinction finally allows the mammals to multiply, diversify, and grow into those now empty niches (along with the dinosaurs’ closer cousins, the birds). Remember that on our timeline Earth and the solar system condensed 4½ years ago and the universe began almost fourteen. We have come a long ways indeed.

How far back can we trace our own ancestors, genus Homo and the australopithecines or upright-walking apes they evolved from? No evolutionary line can be said to start anywhere, of course, but we can identify certain points as being of special importance. For humans, this is probably the split between the human and chimpanzee / bonobo evolutionary lineages which, according to a combination of genetic and fossil evidence, began somewhere between six and seven million years ago, somewhere in Africa. This is one tenth the distance back to the K-T extinction, so we are speaking of about two and a half days ago on our timeline or 0.15% of Earth’s total age.

What about modern humans? This depends on your exact definition of modern. Humans that look essentially modern, but from an intellectual point of view probably weren’t there yet appear several hours ago, while those ancestors we would call fully modern probably about two hours before the present. These modern humans first leave Africa to colonize the rest of the world not much more than a half-hour ago, establishing the first proto-civilized settlements within the last four or five minutes. The rise of the modern industrial state takes to some thirty seconds before the present, and you, depending on your age, probably between ½ and 2½ seconds old, have another couple of seconds or so to go before it’s lights out – well, that may very well depend on how some of the predictions I made in the last chapter pan out. Very comforting, no doubt.

And now let us head into the more distant past, the earliest moments of the future, or what most people refer to as the Big Bang.