Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body (10 page)

Human mouths reveal that we are all-purpose eaters, for we have several kinds of teeth. Our front teeth, the incisors, are flat blades specialized for cutting. The rearmost teeth, the molars, are flatter, with a distinctive pattern that can macerate plant or animal tissue. The premolars, in between, are intermediate in function between incisors and molars.

The most remarkable thing about our mouths is the precision with which we chew. Open and close your mouth: your teeth always come together in the same position, with upper and lower teeth fitting together precisely. Because the upper and lower cusps, basins, and ridges match closely, we are able to break up food with maximal efficiency. In fact, a mismatch between upper and lower teeth can shatter our teeth, and enrich our dentists.

Paleontologists find teeth wonderfully informative. Teeth are the hardest parts of our bodies, because the enamel includes a high proportion of the mineral hydroxyapatite—higher even than is found in bones. Thanks to their hardness, teeth are often the best-preserved animal part we find in the fossil record for many time periods. This is lucky; since teeth are such a great clue to an animal’s diet, the fossil record can give us a good window on how different ways of feeding came about. This is particularly true of mammal history: whereas many reptiles have similar teeth, those of mammals are distinctive. The mammal section of a typical paleontology course feels almost like Dentistry 101.

Living reptiles—crocodiles, lizards, snakes—lack much of what makes mammalian mouths unique. A crocodile’s teeth, for example, all have a similar blade-like shape; the only difference between them is that some are big and others small. Reptiles also lack the precise occlusion—the fit between upper and lower teeth—that humans and other mammals have. Also, whereas we mammals replace our teeth only once, reptiles typically receive visits from the tooth fairy for their entire lives, replacing their teeth continually as they wear and break down.

A very basic piece of us—our mammalian way of precise chewing—emerges in the fossil record from around the world that ranges from 225 million to 195 million years ago. At the base, in the older rocks, we find a number of reptiles that look superficially dog-like. Walking on four legs, they have big skulls, and many of them have sharp teeth. There the resemblance stops. Unlike dogs, these reptiles have a jaw made up of many bones, and their teeth don’t really fit well together. Also, their teeth are replaced in a decidedly reptilian way: new teeth pop in and out throughout the animals’ lives.

Go higher in the rocks and we see something utterly different: the appearance of mammalness. The bones of the jaw get smaller and move to the ear. We can see the first evidence of upper and lower teeth coming together in precise ways. The jaw’s shape changes, too: what was a simple rod in reptiles looks more like a boomerang in mammals. At this time, too, teeth are replaced only once per lifetime, as in us. We can trace all these changes in the fossil record, especially from certain sites in Europe, South Africa, and China.

The rocks of about 200 million years ago contain rodent-like creatures, such as
Morganucodon
and
Eozostrodon,
that have begun to look like mammals. These animals, no bigger than a mouse, hold important pieces of us inside. Pictures cannot convey just how wonderful these early mammals are. For me, it was a real thrill to see creatures like them for the first time.

When I entered graduate school, I wanted to study early mammals. I chose Harvard because Farish A. Jenkins, Jr., whom we met in the first chapter, was leading expeditions to the American West that systematically scoured the rocks for signs of how mammals developed their distinct abilities to chew. The work was real exploration; Farish and his team were looking for new localities and sites, not returning to places other people had discovered. Farish had assembled a talented group of fossil finders comprising staff from Harvard’s Museum of Comparative Zoology and a few free-lance mercenaries. Chief among them were Bill Amaral, Chuck Schaff, and the late Will Downs. These people were my introduction to the world of paleontology.

Farish and the team had studied geological maps and aerial photos to choose promising areas where they might find early mammals. Then, each summer, they got in their trucks and headed off into the deserts of Wyoming, Arizona, and Utah. By the time I joined them, in 1983, they had already found a number of important new mammals and fossil sites. I was struck by the power of predictions: simply by reading scientific articles and books, Farish’s team could identify likely and unlikely places to find early mammals.

My baptism in field paleontology came from walking out in the Arizona desert with Chuck and Bill. At first, the whole enterprise seemed utterly random. I expected something akin to a military campaign, an organized and coordinated reconnaissance of the area. What I saw looked like the extreme opposite. The team would plunk down on a particular patch of rock, and people would scatter in every conceivable direction to look for fragments of bone on the surface. For the first few weeks of the expedition, they left me alone. I’d set off looking for fossils, systematically inspecting every rock I saw for a scrap of bone at the surface. At the end of each day we would come home to show off the goodies we found. Chuck would have several bags of bones. Bill would have his complement, usually with some sort of little skull or other prize. And I had nothing, my empty bag a sad reminder of how much I had to learn.

After a few weeks of this, I decided it would be a good idea to walk with Chuck. He seemed to have the fullest bags each day, so why not take some cues from the expert? Chuck was happy to walk with me and expound on his long career in field paleontology. Chuck is all West Texas with a Brooklyn flourish: cowboy boots and western values with a New York accent. While he regaled me with tales of his past expeditions, I found the whole experience utterly humbling. First, Chuck did not look at every rock, and when he chose one to look at, for the life of me I couldn’t figure out why. Then there was the really embarrassing aspect of all this: Chuck and I would look at the same patch of ground. I saw nothing but rock—barren desert floor. Chuck saw fossil teeth, jaws, and even chunks of skull.

An aerial view would have shown two people walking alone in the middle of a seemingly limitless plain, where the vista of dusty red and green sandstone mesas, buttes, and badlands extended for miles. But Chuck and I were staring only at the ground, at the rubble and talus of the desert floor. The fossils we sought were tiny, no more than a few inches long, and ours was a very small world. This intimate environment stood in extreme contrast to the vastness of the desert panorama that surrounded us. I felt as if my walking partner was the only person on the entire planet, and my whole existence was focused on pieces of rubble.

Chuck was extraordinarily patient with me as I pestered him with questions for the better part of each day’s walk. I wanted him to describe
exactly
how to find bones. Over and over, he told me to look for “something different,” something that had the texture of bone not rock, something that glistened like teeth, something that looked like an arm bone, not a piece of sandstone. It sounded easy, but I couldn’t grasp what he was telling me. Try as I might, I still returned home each day empty-handed. Now it was even more embarrassing, as Chuck, who was looking at the same rocks, came home with bag after bag.

Finally, one day, I saw my first piece of tooth glistening in the desert sun. It was sitting in some sandstone rubble, but there it was, as plain as day. The enamel had a sheen that no other rock had; it was like nothing I had seen before. Well, not exactly—I was looking at things like it every day. The difference was this time I finally saw it, saw the distinction between rock and bone. The tooth glistened, and when I saw it glisten I spotted its cusps. The whole isolated tooth was about the size of a dime, not including the roots that projected from its base. To me, it was as glorious as the biggest dinosaur in the halls of any museum.

All of a sudden, the desert floor exploded with bone; where once I had seen only rock, now I was seeing little bits and pieces of fossil everywhere, as if I were wearing a special new pair of glasses and a spotlight was shining on all the different pieces of bone. Next to the tooth were small fragments of other bones, then more teeth. I was looking at a jaw that had weathered out on the surface and fragmented. I started to return home with my own little bags each night.

Now that I could finally see bones for myself, what once seemed a haphazard group effort started to look decidedly ordered. People weren’t just scattering randomly across the desert; there were real though unspoken rules. Rule number one: go to the most productive-looking rocks, judging by whatever search image or visual cues you’ve gained from previous experience. Rule number two: don’t follow in anybody’s footsteps; cover new ground (Chuck had graciously let me break this one). Rule three: if your plum area already has somebody on it, find a new plum, or search a less promising site. First come, first served.

Over time, I began to learn the visual cues for other kinds of bones: long bones, jawbones, and skull parts. Once you see these things you never lose the ability to find them. Just as a great fisherman can read the water and see the fish within, so a fossil finder uses a catalogue of search images that make fossils seem to jump out from the rocks. I was beginning to gain my own visual impressions of what fossil bones look like in different rocks and in different lighting conditions. Finding fossils in the morning sun is very different from finding them in the afternoon, because of the way the light plays along the ground.

Twenty years later, I know that I must go through a similar experience every time I look for fossils someplace new, from the Triassic of Morocco to the Devonian of Ellesmere Island. I’ll struggle for the first few days, almost as I did those days with Chuck in Arizona twenty years ago. The difference is that now I have some confidence that a search image will kick in eventually.

The whole goal of the prospecting I did with Chuck was to find a site with enough bones to mark a fossil-rich layer that we could expose. By the time I joined the crew, Farish’s team had already discovered such a zone, a patch of rock about a hundred feet long that contained skeleton after skeleton of small animals.

Farish’s fossil quarry was in some very fine-grained mudstone. The trick to working on it was to realize that the fossils were coming from one thin layer, no more than a millimeter thick. Once you exposed that surface, you had a very good chance of seeing bones. They were tiny, no more than an inch or two long, and black, so they looked almost like black smudges against the brownish rock. The little animals we found included frogs (some of the earliest), legless amphibians, lizards and other reptiles, and, importantly, some of the earliest mammals.

The key point is that the early mammals were small. Very small. Their teeth were not much more than 2 millimeters long. To spot them, you had to be very careful and, more often, very lucky. If the tooth was covered by a crumb of rock or even by a few grains of sand, you might never see it.

It was the sight of these early mammals that really hooked me. I’d expose the fossil layer, then scan the entire surface through my 10-power hand lens. I’d scrutinize the whole thing on my hands and knees, with my eye and hand lens only about two inches from the surface of the ground. Thus engrossed, I’d often forget where I was and accidentally trespass on my neighbor’s spot only to have a bag of dirt dumped on my head as a sharp reminder to keep to my space. Occasionally, though, I’d hit the jackpot and see a deep connection for the first time. The teeth would look like little blades, with cusps and roots. The cusps on those little teeth revealed something very special. Each tooth had a characteristic pattern of wear at the face where upper and lower teeth fit together. I was seeing some of the first evidence of our pattern of precise chewing, only in a tiny mammal 190 million years old.

The power of those moments was something I’ll never forget. Here, cracking rocks in the dirt, I was discovering objects that could change the way people think. That juxtaposition between the most child-like, even humbling, activities and one of the great human intellectual aspirations has never been lost on me. I try to remind myself of it each time I dig somewhere new.

Returning to school that fall, I developed the expedition bug big-time. I wanted to lead my own expedition but lacked the resources to do anything big, so I set off to explore rocks in Connecticut that were about 200 million years old. Well studied during the nineteenth century, they had been the setting for a number of important fossil discoveries. I figured that if I hit those same rocks with my hand lens and my wonderfully successful early mammal search image, I’d find lots of goodies. I rented a minivan, grabbed a case of collecting bags, and set off.

Yet another lesson learned: I found nothing. Back to the drawing board, or more precisely, the geology library at school.

I needed a place where 200-million-year-old rocks were well exposed: in Connecticut there were only roadcuts. The ideal place would be along the coast, where wave action would provide lots of freshly broken rock surface to look at. Looking at a map made my choice clear: up in Nova Scotia, Triassic and Jurassic rocks (roughly 200 million years old) lay along the surface. To top it off, the tourist literature about the area advertised the world’s highest tides, occasionally over fifty feet. I couldn’t believe my luck.

I called the expert on these rocks, Paul Olsen, who had just started teaching at Columbia University. If I was excited about fossil-finding prospects before I talked to Paul, I was frothing afterward. He described the perfect geology for finding small mammals or reptiles: ancient streams and dunes that had just the right properties to preserve tiny bones. Even better, he had already found some dinosaur bones and footprints along a stretch of beach near the town of Parrsboro, Nova Scotia. Paul and I hatched a plan to visit Parrsboro together and scan the beach for little fossils. This was wonderfully generous on Paul’s part because he had dibs on the area and was under no responsibility to help me out, let alone collaborate.