Undeniable (15 page)

Regardless of where they strike, large asteroids would boil seas, fill the air with dust and acidic compounds, and perhaps induce carbon dioxide to cook off out of the rocks and into the air, triggering a strong greenhouse effect, all of which in turn would change the world's climate faster than living things could adjust to. Giant impactors could create enormous waves in the ocean and in the atmosphere that could upset weather patterns around the world for extended periods. Perhaps asteroids have also helped unleash Earth's internal heat and caused subsequent volcanism. There is a joke in the space science community—not a joke, really—that the dinosaurs died because they didn't have a space program, so they had no way to save themselves from that asteroid.

Interestingly, many other large asteroid strikes did not seem to cause mass extinctions. Something about the Chicxulub impact was particularly bad. It was an unusually large impact, for one thing, and it may have struck in a geologically sensitive area. But those volcanoes in India are probably also part of the story. When the asteroid hit, the planet's ecosystems were already under stress. There is a great lesson for us here. Ecosystems can change only so fast, and the more insults you throw at them the harder it is for them to keep up. Human activity is causing another big ecological imbalance right now. How much of life on Earth will be able to keep up?

There is a great lesson to be learned from our neighboring planet Venus. Venus is very much like Earth in size and composition, but its surface temperature is about 460° C (860 F), hotter than your oven when it's set to “broil.” The difference between the temperatures of Earth and Venus is not because Venus is slightly closer to the Sun. No, Venus is hot primarily because its atmosphere is full of carbon dioxide, a greenhouse gas that keeps the Sun's heat trapped in the planet's atmosphere. Venus is the extreme case of climate change: There is no way life, as we know it, could survive at those beyond-broiling temperatures. It would take a big change in Earth's geology and chemistry for it to become exactly like Venus. But humans are pouring carbon dioxide into Earth's atmosphere right now at an alarming rate, shoving our climate in that high-carbon direction, which is a terrifying prospect. We do not want to become even a little like Venus.

We know that a little cooling can cause a mass extinction as well; that's apparently what happened in the Ordovician-Silurian extinction. The key idea is simply that sharp climate swings evidently contributed to the Permian-Triassic and K-Pg mass extinctions, and probably to others along the way.

Industrial emissions are one way humans are changing this planet, but not the only way. We are also directly killing countless species at a rate that dwarfs the rates estimated in the previous Big Five extinctions. We are killing them mostly by destroying their habitats. We are forcing countless species to move, driving them from their ecological niches. The extra carbon in the air is holding in the Sun's heat; it's also soaking into the ocean, forming carbonic acid (like in soft drinks), which is compounding our looming troubles. The problem is not just that the ecosystems are changing; as many people note, conditions on Earth have been changing for as long as the planet has existed. The problem is the rate at which we are causing the changes. It's the speed that has us headed for the sixth mass extinction.

We can evaluate the current situation dispassionately, in evolutionary terms. If we destroy ecosystems, new organisms will take the place of those we kill. But as you might quickly realize, if we destroy the ecosystems we depend on, we will kill ourselves. Humans are smart and resilient. You might figure that a few of us will make it through no matter what happens. But how many of us won't? What will be the human cost, and the economic cost, along the way? How many of our genes, including your genes, will disappear forever?

The sooner we start acting to address climate change, the better. Don't take my word for it. Look at the eons, the eras, the periods, and the epochs. Will some future intelligent creature, digging through the ancient strata, try to figure out what happened to
Homo sapiens
back during the Great Holocene extinction? Earth will be here no matter what we do. Let's work together to save the world—for us …

 

15

ANCIENT DINOSAURS AND THE ASTEROID TEST

When I was in second grade, Mrs. McGonagle, our teacher, read from a large, authoritative-looking book and explained to us why the ancient dinosaurs went extinct. At the time, apparently, the best idea anyone had for their demise was mammals. Our ancestors somehow absconded with all of the dinosaur's food, or ate all of their eggs. Even as a very young person, I could tell that Mrs. McGonagle's heart wasn't in it. I could easily imagine an Ankylosaurus accidentally crushing a family of proto-rabbits with one stride while it was ambling around looking for a fruit snack. The intended message seemed to be that we humans were immune to extinction because we were on the winning team. But Mrs. McGonagle clearly could tell that the idea just wasn't reasonable, and so could I.

Today, we understand a great deal more about the demise of the ancient dinosaurs. We know they weren't evolutionary mistakes; unsuccessful adaptations get weeded out rapidly, but the dinosaurs stuck around for about 160 million years. (Human beings have barely made it a thousandth of that.) We know that mass extinctions are the result of environmental changes on a global scale. And we now have strong evidence that the ancient dinosaurs got walloped with a particularly extreme kind of change: not the hungry nibbling of some small mammals, but a long string of noxious volcanic eruptions topped off with a giant flaming rock falling from the sky.

Let's take a closer look at that asteroid that struck at the end of the Cretaceous. A 10-kilometer-wide rock might not seem all that bad, considering it was going up against our 13,000-kilometer-wide planet. But the asteroid was probably moving about 20 kilometers per second, or around 45,000 miles per hour. At such speeds, it carried the energy of a thousand billion (a trillion) tons of TNT. The scale of this is hard to imagine. This impactor must have tossed debris right up through the atmosphere 200,000 kilometers high, halfway to the Moon. Our planet was surrounded by a cloud of red-hot rock for days or weeks. Some of this material stayed aloft and blotted out the Sun. Some came crashing back down and set the world on fire. Sea creatures were cooked. And the ancient dinosaurs were either immolated where they stood, or they could not find food for their next meal. Meanwhile, our distant, distant ancestors were holed up in caves and burrows, and here you and I are.

We share a great deal with those ratlike ancestors. We have hair. We breathe air. Our females produce milk to nurture our offspring. We have four limbs, stereovision, and five appendages at the ends of each limb. What's not to love?… And all of this not only because a medium-large asteroid helped clear the way for you and me … but also because we haven't run into another asteroid that put us through the same infernal test.

As we look into the night sky with our stereoscopic vision system, we wonder whether or not we are alone in the cosmos. As I often remark, if you meet someone who insists that he or she has not wondered about our being alone, they're lyin'. Everyone has wondered about this fundamental question. So, how about this: Perhaps the reason we have never heard from someone or something from another world, from another civilization, is that those living things, somewhere out there in the cosmos, failed to pass the asteroid test.

You and I have the good fortune to live on a planet that is equipped with a large moon. In our case, we spell it M-o-o-n, capitalized because that is its name. We also happen to have had two superpowers that came into existence after a series of world-consuming conflicts. Through a terrible act of disenchantment, one world leader was killed, and his policy of racing to the Moon became a nation's policy, which drove members of our species to create space programs around the world. As a result, when an asteroid that could once again destroy the dominant form of life comes along, that dominant species (you and me) is ready to do something about that rock or block of ice.

We have the technology to prevent another mass extinction. We could give the asteroid a nudge, and life here would go on as usual. There are a number of relevant technologies under study right now. We could ram a rocket right into it, strap a rocket to it, or redirect it with a bomb. If we had enough fuel, we could pull it with the gravity of a massive spacecraft. At the Planetary Society we propose harnessing the energy of sunlight to zap it with lasers and nudge it onto a safe path. The ancient dinosaurs couldn't do any of these things—as far as we know. We can.

This is a train of science-fiction-style thought. But, I emphasize that it's not crazy. It's extraordinary, but hardly unreasonable. It is the study of evolution, and doing our best to learn about where we all came from takes us down this giant-impactor-from-space path. It's part of the process of science. It tells us something important about how we evolved, and it tells us something crucial about how to make sure we survive.

Asteroid impacts, by any reasonable reckoning, are the only preventable natural disasters. So my fellow Earthlings, let's get busy and see to it that we never take this kind of hit.

 

16

PUNCTUATED EQUILIBRIUM

If you've never been, I encourage you to visit and tour the sandstone slot canyons in Zion National Park, where the history of Earth is laid out before you, fine layer upon fine layer, like the pages of a book. If you want to sort out the story of how new species appear, and how other species go away, this is an excellent place to look. It's mesmerizing—just stand there and try to count the layers. The formations run from the Late Permian to Early Cretaceous Periods. That's 200 million years' worth of deep time. When you look closely, the layers are stacked like pieces of paper in a copier tray; only this stack is 1,000 meters—more than half a mile—high.

When I looked at these layers in 1997, while shooting the
Science
Guy
show, I had the distinct feeling that Earth's history was a steady business. Each exquisitely fine layer seems to have been laid down in regular fashion. Wind carried the grains there, forming enormous dunes. From time to time, things got wet. From time to time, things got dry. When things were wet, the minerals calcite, CaCO
3
(carbonate with a calcium atom hooked on), and hematite, Fe
2
O
3
(iron oxide, also known as rust), dissolved in the ancient water and cemented every sand grain into place. The rust makes for beautiful red tones. The scene mesmerized the crew and me with its steady drumbeat of sandstone building.

I am not the only one to form first impressions like this. Charles Lyell, the famous nineteenth-century geologist, had a similar impression.
Natura non-facit saltum
—nature does not make leaps—was the common wisdom at the time, and the belief in the steady unfolding of geologic history was called uniformitarianism. To his credit Lyell realized how much time he was dealing with. Basically, it's a lot more than you probably think. In fact, for most of us, it's more than we can imagine.

Uniformitarianism struck Darwin as being the way of nature, geologic or otherwise. He and his contemporaries figured species came into being at a pretty slow but steady clip, and had been doing so since the beginning of time. But is it true? That turns out to be a very important question if you want to know where new species come from—if you want to find a way to combine the ancient fossil evidence with the modern genetic evidence and take Darwin's ideas into the twenty-first century.

As Darwin and countless subsequent researchers examined all the fossils they could find, they ran into a persistent puzzle. There seemed to be a lot of missing animals and plants. Many of the key fossils that you'd expect—early types of birds, for example, the transitional forms—just weren't around. Darwin called the missing fossils, “… the most obvious and gravest objection which can be urged against my theory…” It was a puzzle a lot of scientists wanted to solve, and remained so for a long time. The subtlety of the eventual answer throws millions of people off even today. Creationists and uninformed people everywhere still harbor doubts about evolution partly because of the concerns about missing fossils that Darwin and other nineteenth-century investigators had early on.

Some of the problem was a lack of information. In Darwin's day, there were far fewer fossils available to study than we have today. The museums and the vast collections we have now didn't yet exist. There was no economical way to share the specimens that did exist, no transmission of electronic images between handheld screens, and so on. Most of the fossils scientists expected to find based on their studies had not yet been unearthed. This is especially true of fossil remains that were to connect us humans to an ancestor common with great apes, bonobos, and chimpanzees. This hypothetical single fossil or set of fossils came to be called the missing link or links. I remember well as a kid the derisive description of someone that you thought was uncouth, illiterate, or culturally inept as being a “missing link.” My parents referred to one of my older sister's boyfriends as the missing link. They felt he was an unsuitable suitor. I am pretty sure he was a human like the rest of us, and not a missing link in the fossil record. My old boss on the other hand …

During the decades after Darwin published, field geologists and paleontologists have gone on to uncover and discover thousands and thousands of fossils. They have uncovered an astonishing number of ancient dinosaurs, an overwhelming assortment of long-gone mammals, and an uncountable number of sea creature fossils. Just two years after Darwin expressed his concern about the missing fossils, the famous fossil of the birdlike
Archaeopteryx
was found in Germany, and that's just one example. Later, fossil hunters found a whole range of human ancestors, including
Sahelanthropus tchadensis
, which may in fact be a shared ancestor with chimps as well. You could say that every one of these is a no-longer-missing “missing link.”