Undeniable (13 page)

 

13

FOSSIL RECORDS AND EXPLOSIONS

When people talk about ancient life, they frequently talk about the “fossil record.” But let's be clear: The fossil record isn't a tidy clean recording. No one went to a studio and methodically laid down some tracks, the way a rock and roll band records an album. The evidence embedded in Earth's rocks is more like the work of a band that recorded with a faulty microphone and then accidentally recorded over most of the tracks. On top of that, when they were finished, they lost almost all of the final versions. Most living things never get fossilized, and most fossils end up in places where they are impossible to discover. It is impressive that paleontologists have been able to reconstruct the lives of individual plants and animals. It is downright amazing that they have found enough fossils to deduce the broad sweep of evolution's great busts and booms—including one wild episode of innovation that led to all of the large animals living today.

Let's step back for a moment and consider what it takes to become a fossil. First of all, you have to get buried. I don't know about you, but I'd prefer to be dead first. However, from a fossil hunter's point of view, the less time the deceased has been left out in the sun, or washed around in the sea, or scavenged upon by chewy-beaked birds, the better. Creepy as it sounds, getting your specimens buried alive is the best thing for a paleontologist. The resting place generally has to be wet in order for the organism to get buried effectively. Then that wet sand or soil has to dry out completely so that microorganisms don't rot the remains. Then it has to sit there for years and years and years, generally millions of years, while minerals slowly trickle through and turn once-living structures to stone.

This extraordinary string of events produces about the most incomplete kind of record keeping you can imagine. Almost every animal and plant that has ever lived has disappeared without a trace. Suppose a fossil were perfectly preserved, but then got subducted (pulled downward) under one of Earth's continent-sized tectonic plates. It would become part of the magmatic mantle. It would melt into molten rock. The pattern would disappear like a corporate banquet ice sculpture on the next day's afternoon.

It is only through very unusual happenstance that we are able to find any trace of the past. That's why, when exciting new (or are they old?) fossils are found, it is—thrilling. Notice also, it's probably easier to find big fossils. A tyrannosaur femur is easier to come across than a pigeon-sized animal's toe, even if they're both preserved in the same fossil bed. This selection effect may have influenced our understanding of ancient dinosaur ecosystems. We might presume the big creatures dominated, when perhaps actually there were a great many smaller animals that just don't show up as often at our dinosaur digs.

The deeper we dig, the older the animals and plants we find. We can trace the development of certain species for millions of years. Trilobite fossils are so plentiful that scientists are able to classify them into orders, suborders, families, subfamilies, and so on, all the way down to genus and species, just like biologists do for living creatures. We can trace the trilobite lineage back more than 250 million years. Trilobites remind me of crabs and lobsters. They had hard shells that held their shape as they were buried and slowly turned to stone. When I went on walks in central New York State, I would find trilobite fossils all over the place. On the other hand, there are relatively few fossils on Earth of animals with soft body parts. When we find fossils of ancient rhinoceroses for example, we almost never find fossils of their ears. Soft things tend to decay instead of fossilizing.

There are a few important examples of fossils that include the soft and hard body parts of long extinct creatures. They figure importantly in the history of fossil finding and the history of our understanding of evolution. I refer to some fossils found in shale, the sedimentary rock that old-style classroom blackboards were made of. The shale found in the Burgess Shale Formation in Canada is the most well-known, because the fossils are just so perfect and because it covers a truly extraordinary period in Earth's history.

In 2005, when I was doing a television show about great scientific discoveries, I had the wonderful opportunity to hold a few Burgess fossils in my hands. These fossils are astonishing, with silvery traces and lines preserved in layers of very smooth almost black rock. This solid rock can be broken apart pretty easily in perfect, flat, distinct, obvious layers. Skilled rock hounds can gently tap open pieces of the shale with their ever-present rock hammers, so that the layers split apart like pages in a book. The Burgess area is in the Walcott Quarry in British Columbia. It's all part of the Canadian Rockies now, but it was once a great big wall of mud and part of an ancient ocean reef, about 500 million years ago, during the Cambrian Period, which is named for an area in Wales, in the United Kingdom. (By long tradition, geologic time spans get their names from the place in which they were first discovered or cataloged. It's coincidental that both Wales and the Burgess area were once part of the British Empire.)

By looking things over carefully, paleontologists have surmised that an ancient huge wall of mud gave way all at once. It slumped or slid downhill extremely quickly and buried countless sea creatures in a slurry of fine silt and seawater in just a few moments. There is no evidence in the Burgess Shale Formation of any of the creatures struggling to get free or burrow their way out. They must have been immobilized and suffocated in a snap. If you think too hard about those moments, it can be troubling. But for us learning about our place in the process of evolution and the progress of life here on Earth, it is an extraordinary find. In no other excavation site ever explored have the fossils been of as high a quality as those found at the Burgess Shale Formation, apparently because the ancient silt was so fine-grained.

The Burgess Shale Formation is full of beautifully preserved shells and flesh from dozens of animals that were absolutely unknown to science before the shale's discovery by paleontologist Charles Wolcott in 1909. Even then, the age and significance of those fossils was not appreciated until 1966, when a number of researchers reexamined the site and recognized the true age and diversity of the creatures preserved there. By carefully photographing, carefully tracing, and then carefully abrading the rocky slate prisons of the images, fine layer after fine layer, investigators were able to reconstruct the sizes and shapes of ancient creatures. They realized that they were looking at animals that swam in weird ways, walked in weird ways, and captured prey in ways that many never thought possible. What animal do you know that has five eyes, for example—two pair and one in the middle—as Opabinia once did? His and her brains would have to process images in some way that we even-count-eyed folk can't quite imagine. How about an animal so confusing to the world's experts that it was given the name
Hallucigenia
, because they felt like they were seeing things.

Some evolutionary biologists have argued that the abundance of fossils preserved in shale from this part of the Cambrian Period, about 500 million years ago, indicate that life on Earth was once much more diverse than it is today, but most disagree. The quite reasonable presumption is that these crazy-looking creatures arose by natural selection, but their body plans proved to be unworkable in the long run. They got outcompeted by the more recent animals and their ancestors that we observe today. These crazy critters then disappeared from subsequent layers in the fossil record.

As I consider these animals, they don't look—to me—that much different from what we see today, if one accepts the idea that each appendage and each organ served those animals, and in modified forms they serve the invertebrate sea creatures of today. Then again, you can look at these creatures and focus on the specific ways in which they are drastically different from anything swimming around now. They are from an ancient, completely unfamiliar underwater world.

The Burgess fossils are unique in their preservation of life in the Cambrian, but to me it is not so extraordinary that this site is so extraordinary. It is difficult to produce fossils in nature in the first place, and extremely difficult to find fossils of soft body tissue anywhere for any geologic period, period. It seems reasonable to me that if we just knew where to look, we could find other extraordinary animals and plants preserved in the fossil record. After all, more than two thirds of our world is under water. I imagine that there are countless rich fossil beds that we'll never be aware of because they're in deep water and buried in modern or much more recent detritus; that's the fine sediment that falls from the surface as plankton live and die above. The Burgess Shale Formation was discovered a full fifty years after Darwin published his seminal work. I have to think that there are plenty more places to look.

Along with the stunning fossils themselves, the Burgess Shale Formation is notable for the era of life that it records—a period of breakneck evolutionary innovation called the “Cambrian Explosion.” Over the course of 20 million years, the number of novel species in the fossils found around the world increased twentyfold. This burst of diversification is often presented as a big evolutionary mystery. Creationists, in particular, often talk about the Cambrian Explosion as if it happened in an instant. This is, for me, another example of extraordinary ignorance or very limited critical thinking.

For starters, I have to quibble with the word
explosion
. I've been at rock quarries, when the engineers and technicians are, well … blowing stuff up. A typical shot, or explosion sequence, involves a few thousand charges and runs less than thirty seconds. When a sequence takes millions upon millions of years, should we call it an explosion? In geologic time, 20 million years is not very much, but it's not like the milliseconds between explosions in a rock quarry shot. It's a long time.

Along with that, is it that life on Earth actually increased in diversity by a factor of twenty, or is it that the fossil record doesn't reveal much about the past until living things came up with hard shells that were readily preserved in solid rock as fossils? The Cambrian Explosion is more (or was more) like inflating an enormous air mattress. Once some marine organisms evolved hard shells, it's not surprising that these creatures were successful and diversified. If you were to puff on almost anything for 20 million years, you might expect an increase in volume, no?

For me, the idea that there used to be more diversity than we see today is reasonable, but not likely. There have been mass extinction events along the way (more on that in the next chapter), but they don't seem to be, if I may say,
massive enough
to drive us to this conclusion. In the same vein, the Cambrian Explosion strikes me as more of an artifact of the fossils we can find in rocky layers corresponding to about twenty million years rather than an actual very, very fast production of diversity in species. Instead, there was an increase in the size and robustness of the shells of invertebrate sea creatures that are inherently better preserved as fossils than were the animals and plants that preceded them. But of course, I may be wrong. Please investigate this business for yourself and reach your own conclusion.

In general, the difficulty of making fossils guarantees that the fossil record will be full of gaps (or maybe “skips,” for those of us youthful deejays or old fogies familiar with vinyl records). It could not be any other way. The smaller the organism, the softer its body, the longer ago it lived, and the more likely it is to have fallen into one of those gaps. The amazing thing is not that we do not have more examples like the Burgess Shale Formation; the amazing thing is that we have even one near-perfect specimen.

More secrets are out there, waiting to be brought out into the open. Investigators working with existing collections and rock hounds climbing on cliffs today are continually expanding the fossil record. As we move into the future, we keep learning more and more about the past. Every day we each have less life left to us, and yet every day we have access to a greater and greater stretch of evolutionary time. I frankly hope that one of you (or me) reading this work finds a fossil—or becomes a fossil someday—so that the record for our descendants will be that much more complete.

 

14

MASS EXTINCTIONS AND YOU

For my debate with creationist Ken Ham in Kentucky, I estimated that there are about 16 million species of life on Earth. What if 14 million of them suddenly disappeared? Sounds hard to believe, like some dystopian Hollywood summer blockbuster? But that is exactly what seems to have happened at least five times over the past half billion years. In each instance, a catastrophic event (or combination of events) killed up to 90 percent of the world's sea and land creatures, and it happened in the blink of an eye—geologically speaking, at least. Other than the origin of life itself, mass extinctions are the most dramatic and mysterious events in the history of life on Earth.

Some of the best evidence for these mass extinctions is found in the rocks that make up, or once made up, the ocean floor. Scientists head out to sea with massive but elegant devices to cut cylindrical samples out of the ocean floor, where sediments are laid down with some regularity. And once down there, the sediments don't get weathered very much. There are no winds, or rains, or freeze-thaw cycles. The Earth is 4.54 billion years old. Its crust has repeatedly broken up, moved around, melted, and reformed. Most of our planet's extraordinarily ancient history is lost to us. Yet locked away in ancient sediments, some evidence of those five extinction events still remains.

One of the great challenges in reconstructing a mass extinction is making sense of what happened when. In the same way we have divided living things into a hierarchy of divisions—domain, kingdom, phylum, class, order, family, genus, and species—geologists have broken apart the long history of our planet into eon, era, period, epoch, and age. As biologists understood the relationships between living things more completely, they added other terms, like superorder and subspecies. In similar fashion, geologists sometimes use the term
supereon
to describe all of Earth's history before the Cambrian, from the planet's formation through the Precambrian.