Man is never watchful enough against dangers that threaten him every hour.
Horace, Carmina, II.13
Destruction may come not just from the distressingly long list of celestial hazards. Some threats are much closer to home. We have already mentioned the most obvious worry: meteorite impact. Tiny meteorites fall to Earth every day; medium-sized objects land every few years; large objects — say, 20 km wide — hit Earth every few hundred million years. Although large meteorites only hit Earth infrequently, when they do hit they cause total devastation. If a 20-km-wide asteroid hit Earth today, it would almost certainly kill every human being. Multiply the small chance of an event occurring by the number of people it would kill, and you arrive at the probability of death per person for the event. It turns out that, averaged over a human lifetime, the chance of being killed by meteorite impact is about the same as dying in an aircraft crash. Paradoxically, we spend vast amounts of money on air safety, yet essentially nothing on detecting the near-Earth objects that could destroy our civilization.
Presumably, ETCs also have to contend with the threat posed by meteorite impact, as these objects are probably common in planetary systems. But there are many other hazards, and below I discuss a few more.
The threats need not even come from space. Recent evidence — particularly the discovery of glacial debris near sea level in the tropics — suggests that, over geological history, Earth has repeatedly been covered in a layer of ice. One event may have happened 2.5 billion years ago, and there may have been four of these Snowball Earth events in the last 800 million years, with each episode lasting for 10 million years or more. Do not mistake these events with the textbook images of the last Ice Age; compared to a
Snowball Earth, the last Ice Age was positively tropical. During a Snowball Earth, a kilometer-thick layer of ice covers the oceans, and ice even covers equatorial oceans (though perhaps not to the same depth). Average temperatures drop to —50° C. Most organisms are unable to cope with such conditions, and life can hang on only by the thinnest of threads — perhaps around volcanoes, or under clear thin ice at the equators.203
figure 55 Melting icefloes in open water. On a Snowball Earth conditions at the equator would, at best, be like this. All else would be covered in thick
How our planet can descend into a Snowball Earth is well understood: The ice cover can increase for a variety of reasons, and when it increases the ice reflects an increasing amount of sunlight straight back out into space. This decrease in solar heating of the surface causes the temperature to drop and more ice to form. Once a critical amount of ice cover is reached, a "runaway icehouse" effect takes place and the planet descends into a Snowball Earth event. What is difficult to understand, and what caused scientists to dismiss the idea of a Snowball Earth for many years, is how the planet can escape from the ice cover. Once Earth is encased in ice, most of the sunlight falling on the planet is reflected into space before it can warm the surface. The solution came with the realization that volcanic activity does not stop during a Snowball Earth event. Volcanoes pump out vast amounts of carbon dioxide — a greenhouse gas. Of course, volcanoes are still belching out carbon dioxide, but under normal conditions this CO2 is absorbed by falling rainwater, which eventually carries it to the ocean where it becomes locked up in solid carbonate deposits on the ocean floor. On a Snowball Earth there is no liquid water to evaporate, and therefore no clouds, and therefore no rain: for 10 million years, maybe more, the CO2 from volcanoes would build up in the atmosphere. Eventually, there would be about a thousand times more atmospheric CO2 than in today's atmosphere. The temperatures would rise and quickly melt the ice: from icehouse to greenhouse in a geological instant.
The implications of the Snowball Earth hypothesis are profound, and we shall examine some of them later.
Although volcanoes proved to be life's savior during the Snowball Earth events of the Neoproterozoic era, more recently they almost proved disastrous for intelligent life on Earth: they have almost wiped out Homo sapiens.
Recent research indicates that, genetically, humans are all remarkably similar. To explain this lack of genetic diversity, some biologists have suggested that Homo sapiens must have emerged from a "genetic bottleneck" about 75,000 years ago. A bottleneck occurs when the size of a population reduces dramatically. In the case of our species, the total number of humans alive on Earth may have dropped as low as a few thousand. We almost became extinct.
If this bottleneck really did occur, then we do not have to look far for a smoking gun that may have caused it. The Toba volcano in Sumatra erupted 74,000 years ago; so great was the eruption, it earns the title of a "super-volcano." The eruption was much more violent than recent vol canoes like Mount Pinatubo and Mount St. Helens. Climatologists have suggested that a super-volcanic eruption can cause a volcanic winter — similar in effect to a nuclear winter, but without the radiation. It is not implausible that the years of drought and famine following such an explosion could drive a pre-technological human species to the brink of extinction.
Meteor impact, global glaciation, super-volcanoes. Even on a placid planet like Earth, life has to contend with a lot. Sometimes, whether the cause is one of the three mechanisms mentioned above, or one of the celestial agents of destruction, life barely hangs on.
Life on Earth has suffered several mass extinctions — a mass-extinction event being defined as a period that sees a significant reduction in Earth's biodiversity. There have been fifteen such events over the past 540 million years. (There may have been many more extinctions earlier in Earth's history, particularly in Snowball Earth events, but only in the past half billion years have creatures with hard skeletons become common; so only relatively recently could creatures become fossils. Indeed, the time since the Cambrian age is known as the Phanerozoic era, from Greek words meaning "visible life." The 4 billion years before the Cambrian age is known as the Cryptozoic era, from Greek words meaning "hidden life." For most of Earth's history, virtually all organisms lived and died without leaving traces.) In six great mass-extinction events, more than half of all species then alive were killed.204 These six events are, in chronological order, the Cambrian, Ordovician, Devonian, Permian, Triassic and Cretaceous.
The Cambrian extinction (actually two extinctions) occurred 540 to 500 million years ago. Their precise cause is uncertain, but in some ways they were the most serious of the mass extinctions. During the Cambrian explosion, a time of immense biological innovation, Nature experimented with many different body plans; perhaps as many as a hundred different animal phyla evolved. All the animal phyla we are familiar with today emerged during the Cambrian explosion, and no new phyla have evolved since. But during the Cambrian extinctions some of these phyla — each containing species that seem bizarre and even nightmarish to our eyes — died out.205
The Ordovician extinction 440 million years ago and the Devonian extinction 370 million years ago both saw more than a fifth of the marine families disappear. The effects on land life are less well known, mainly because the fossil record is so poor for these ages. Nor is the cause of these extinction events known; if impact events caused them, no trace of the resultant craters has been found.
The Permian extinction 250 million years ago was even more severe than the Cambrian extinction. Perhaps more than 90% of marine species became extinct; eight of the 27 orders of insects were lost; the loss was devastating. The cause of this catastrophic event is uncertain; several mechanisms, possibly acting in synergy, have been proposed to explain this global catastrophe.
The Triassic extinction 220 million years ago saw significant reductions in the number of marine and land species. Many scientists believe a meteorite was the cause of this extinction event.
The Cretaceous extinction 65 million years ago is the most celebrated and most well-known of all the mass extinctions. This event saw the end of the age of the dinosaurs (and provided the conditions that led to the rise of the mammals). Almost certainly, the cause of this extinction was the aftereffects of a large meteorite impact. There are several reasons for believing in the impact theory of this extinction event. First, the 200-km-wide Chicx-ulub crater on the Yucatan peninsula in Mexico is of precisely the right age. Second, no matter from where in the world they are drawn, rock samples from the Cretaceous-Tertiary boundary show a high concentration of iridium, which is what one would expect if a large asteroid hit Earth. Third, many of the same sites contain shocked quartz grains — another sign of violent impact. Fourth, geologists often find fine soot particles in clays from the Cretaceous-Tertiary boundary — particles that could have come only from burning vegetation; the implication is that much of Earth's plant matter was on fire.206 The immediate aftermath of the impact would clearly have killed large numbers of organisms. The precise mechanism for eradicating large numbers of species is less clear; it could have been atmospheric change, a nuclear winter, large-scale long-term fires, acid rain, a combination of these effects . . . or something else entirely. The effects were also dependent upon when and where the meteorite struck Earth, and also on the mass and velocity of the meteorite. Had the meteorite struck just a few hours later, the effects might have been less deadly; had the meteorite been just twice as large, the extinction of life might have been total.
It is difficult to say what we can learn from these extinction events. They seem to be different in character, cause and severity. Only in the cases of the Cretaceous and Permian events are there obvious causal mechanisms for the extinctions. The other extinctions may have been caused by something quite different; after all, we have considered many potential threats. Life-forms on other planets presumably face the same hazards, and they may face risks that life on Earth has been spared. For example, some planetary systems may have life-bearing planets in orbits that become chaotic — and a mass extinction would be probable. Or a change in the rotational rate of a planet might trigger a mass extinction. Anything that causes extensive climate change — either a global cooling or warming outside of temperatures that are tolerable for animal life — might induce a mass extinction. Perhaps the lesson is simply that planetary systems are dangerous: over the course of billions of years, mass extinctions are inevitable.
It is a short step from arguing that mass extinctions are inevitable to arguing that they play a role in resolving the Fermi paradox. In fact, people have used the idea of mass extinctions to suggest two quite antithetical solutions to the paradox. The straightforward suggestion is that mass-extinction events have impeded the development of intelligent life on other planets. The more subtle suggestion is that, in the immortal capitalization of Sellars and Yeatman, mass extinctions are a Good Thing that occur too infrequently on other planets! (At least, the right sort of extinction events happen too infrequently.)
It is easy to understand why mass extinctions might be a Bad Thing. Many people would argue that life — at least life as we know it — has only two defenses against mass extinction. The first defense is simplicity: this is the approach taken by prokaryotes (see page 190), which have survived for billions of years. Bacteria have essentially kept their single-cell body plan over the aeons; indeed, it is probable, though difficult to prove conclusively, that modern bacteria are genetically identical to the earliest living cells of 3.7 billion years ago. Their ability to evolve biochemical responses to new environmental challenges enables them to take most things Nature can throw at them. Only a catastrophe on a massive scale would remove all prokaryote life from Earth. On the other hand, we cannot communicate with bacteria. When considering Fermi's question, we are interested in complex multicellular life-forms. How do they survive the slings and arrows of a billion years of fortune?
The second defense against mass extinction is diversity — an approach taken by animals and plants. If a phylum contains many different species, if it has different ways of earning a living, then there is a chance that one or two of the species will survive the extinction event. Later, the diversity of the phylum can be replenished. So even though animal and plant life is less hardy than bacterial life, and is much more susceptible to extinction, in the long run it can survive. (Perhaps the main reason the Cambrian extinction was so deadly is because, although there were many different phyla, each phylum contained only a few species. Entire phyla became extinct because they contained insufficient diversity. It is something of a theme of this book: do not put all your eggs in one basket.)
We have no idea how evolution has proceeded on other planets; but perhaps Earth is rare in having phyla with many different species. (See page 181 for a reason why this might be the case.) Complex life on other worlds may be less likely to survive the inevitable extinction events. We can imagine worlds that are home to many different, weird-looking, truly alien creatures — creatures possessing a variety of peculiar body plans. There might be a large number of phyla on such worlds — phyla that took aeons to evolve to their present state. But if those phyla are represented by only a few species — well, when the meteorite strikes, or the climate heats up, or the planet's obliquity changes, those phyla may well die out. Maybe Earth has just been lucky (there is that word "lucky" again). This is a gloomy resolution of the Fermi paradox.
We have encountered the more subtle suggestion regarding mass extinctions — namely, that they may be necessary for the development of intelligent life - when we discussed the suggestion of a "pump of evolution." Of course, it would be no fun being around when a 20-km-wide asteroid smashes into Earth or global temperatures plummet. But in the long run — a run measured in tens of millions of years — life might benefit from such catastrophes. After the deluge, new and radically different forms have a chance to evolve; Nature can use the changed environment to create and experiment with different species, and perhaps even different body plans. Certainly, following mass-extinction events, biodiversity has always regained the pre-extinction level and then exceeded it.
One currently controversial suggestion is that two key events in the history of life on Earth — the development of the eukaryotic cell, and the Cambrian explosion (more of which in later sections) — were a direct result of the escape from Snowball Earth events. The chemical changes a Snowball Earth would cause in the oceans, the genetic isolation of species, the great environmental pressure on life, the rise in temperature and the rapid melting of ice — all these factors might combine to produce a time of rapid evolutionary activity. According to some scientists, neither animals nor higher plants would exist today if it were not for past Snowball Earth events.
Perhaps the "right" global glaciation events are uncommon on other planets. A planet has to be in the CHZ, it has to have oceans of water, it has to descend into an icehouse, and it must possess active volcanoes spewing out greenhouse gases to remove the ice. Perhaps the norm for most water-planets is a descent into a Snowball with no means of escape; the mass extinctions would be total.
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