Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body (11 page)

I consulted with Farish on my emerging plans, and he not only offered money but suggested that I take the fossil-finding experts, Bill and Chuck. Money, Bill, Chuck, Paul Olsen, excellent rocks, and decent exposures—what more could you want? The following summer, I led my very first fossil expedition.

Off I went in a rented station wagon to the beaches of Nova Scotia with my field crew, Bill and Chuck. The joke, of course, was on me. With Bill and Chuck along, who between them had more years of field experience than I had birthdays, I was the leader in name only. They called the fossil-finding shots, while I paid the dinner bills.

The rocks in Nova Scotia were exposed in absolutely gorgeous orange sandstone cliffs along the Bay of Fundy. The tides would go in and out about half a mile each day, exposing enormous flats of orange bedrock. It wasn’t long before we started to find bones in many different areas. Small white flecks of bone were coming out along the cliffs. Paul was finding footprints everywhere, even in the flats opened by the moving tides each day.

Paul Olsen finding footprints in the tidal flats of Nova Scotia. At high tide, the water would come all the way to the cliffs at left. The arrowhead points to a spot where, if we timed our trip wrong, we would be stuck on the cliffs for hours at a time. Photograph by the author.

Chuck, Bill, Paul, and I spent two weeks digging in Nova Scotia, finding bits, flakes, and fragments of bones sticking out of the rocks. Bill, being the fossil preparator of the group, continually warned me not to expose much of the bones in the field but rather to wrap them up still covered in sandstone so that he could trace the bones in the laboratory under a microscope in more controlled conditions. We did this, but I’ll admit to being disappointed with what we brought home: just a few shoeboxes of rocks, with small chips and flakes of bones showing. As we drove home, I recall thinking that even though we hadn’t found much, it had been a great experience. Then I took a week’s vacation; Chuck and Bill returned to the lab.

When I returned to Boston, Chuck and Bill were out to lunch. Some colleagues were visiting the museum and, having caught sight of me, came up to shake my hand, offer congratulations, and slap me on the back. I was being treated like a conquering hero, but I had no idea why; it seemed like a bizarre joke, as if they were setting me up for some big con. They told me to go to Bill’s lab to see my trophy. Not knowing what to think, I ran.

Under Bill’s microscope was a tiny jaw, not more than half an inch long. In it were a few minute teeth. The jaw’s owner was clearly a reptile, because the teeth had only a single root at the base, whereas mammal teeth have many. But on the teeth were tiny bumps and ridges that I could see even with the naked eye. Looking at the teeth under the microscope gave me the biggest surprise: the cusps had little patches of wear. This was a reptile with tooth-to-tooth occlusion. My fossil was part mammal, part reptile.

Unbeknownst to me, Bill had unwrapped one of our blocks of rock, seen a fleck of bone, and prepared it with a needle under the microscope. None of us had known it in the field, but our expedition was a huge success. All because of Bill.

What did I learn that summer? First, I learned to listen to Chuck and Bill. Second, I learned that many of the biggest discoveries happen in the hands of fossil preparators, not in the field. As it turned out, my biggest lessons about fieldwork were yet to come.

The reptile Bill had found was a tritheledont, a creature known from South Africa as well as now from Nova Scotia. These were very rare, so we wanted to return to Nova Scotia the next summer to find more. I spent the whole winter tense with anticipation. If I could have chipped through the winter ice to find fossils, I would have done it.

In the summer of 1985, we returned to the site where we had found the tritheledont. The fossil bed was just at beach level, where a little piece of the cliff had fallen off several years before. We had to time our daily visit just so: the site was inaccessible at high tide because the water came up too high around a point we had to navigate. I’ll never forget that first day of excitement when we rounded the point to find our little patch of bright orange rock. The experience was memorable for what was missing: most of the area we had worked the year before. It had weathered away the previous winter. Our lovely fossil site, containing beautiful tritheledonts, was gone with the tides.

The good news, if you could call it that, was that there was a little more orange sandstone to scan along the beach. Most of the beach, in particular the point we had to go around each morning, was made up of basalt from a 200-million-year-old lava flow. We were positive no fossils could be found there, for it is virtually axiomatic that these rocks, which were once super hot, would never preserve fossil bone. We spent five or more days timing our visits to the sites by the tides, pawing away at the orange sandstones beyond it, and finding absolutely nothing.

Our breakthrough came when the president of the local Lions Club came by our cabin one night looking for judges for the local beauty contest, to crown Parrsboro’s Miss Old Home Week. The town always relied on visitors for this onerous task, because internecine passions typically run high during the event. The usual judges, an elderly couple from Quebec, were not visiting this year, and the crew and I were invited to substitute.

But in judging the beauty contest and arguing over its conclusion, we stayed up way too late, forgot about the next morning’s tides, and ended up trapped around a bend in the basalt cliffs. For about two hours, we were stuck on a little promontory about fifty feet wide. The rock was volcanic and not the type one would ever choose to search for fossils. We skipped stones until we got bored, then we looked at the rocks: maybe we’d find interesting crystals or minerals. Bill disappeared around a corner, and I looked at some of the basalt behind us. After about fifteen minutes I heard my name. I’ll never forget Bill’s understated tone: “Uh, Neil, you might want to come over here.” As I rounded the corner, I saw the excitement in Bill’s eyes. Then I saw the rocks at his feet. Sticking out of the rocks were small white fragments. Fossil bones, thousands of them.

This was exactly what we were looking for, a site with small bones. It turned out that the volcanic rocks were not entirely volcanic: slivers of sandstone cut through the cliff. The rocks had been produced by an ancient mudflow associated with a volcanic eruption. The fossils were stuck in the ancient muds.

We brought tons of these rocks home. Inside were more tritheledonts, some primitive crocodiles, and other lizard-like reptiles. The tritheledonts were the gems, of course, because they showed that some kinds of reptiles already displayed our mammalian kind of chewing.

Early mammals, such as those Farish’s team uncovered in Arizona, had very precise patterns of biting. Scrapes on the cusps of an upper tooth fit against mirror images of these scrapes on a lower tooth. These patterns of wear are so fine that different species of early mammals can be distinguished by their patterns of tooth wear and occlusion. Farish’s Arizona mammals have a different pattern of cusps and chewing than those of the same age from South America, Europe, or China. If all we had to compare these fossils to were living reptiles, then the origin of mammalian feeding would appear to be a big mystery. As I’ve mentioned, crocodiles and lizards do not have any kind of matching pattern of occlusion. Here is where creatures like tritheledonts come in. When we go back in time, to rocks about 10 million years older, such as those in Nova Scotia, we find tritheledonts with an incipient version of this way of chewing. In tritheledonts, individual cusps do not interlock in a precise way, as they do in mammals; instead, the entire inner surface of the upper tooth shears against the outer surface of the lower tooth, almost like a scissors. Of course, these changes in occlusion did not happen in a vacuum. It should come as no surprise that the earliest creatures to show a mammalian kind of chewing also display mammalian features of the lower jaw, skull, and skeleton.

A tritheledont and a piece of its upper jaw discovered in Nova Scotia. Jaw fragment illustrated by Lazlo Meszoley.

Because teeth preserve so well in the fossil record, we have very detailed information about how major patterns of chewing—and the ability to use new diets—arose over time. Much of the story of mammals is the story of new ways of processing food. Soon after we encounter tritheledonts in the fossil record, we start seeing all sorts of new mammal species with new kinds of teeth, as well as new ways of occluding and using them. By about 150 million years ago, in rocks from around the world, we find small rodent-size mammals with a new kind of tooth row, one that paved the way for our own existence. What made these creatures special was the complexity of their mouths: the jaw had different kinds of teeth set in it. The mouth developed a kind of division of labor. Incisors in the front became specialized to cut food, canines further back to puncture it, and molars in the extreme back to shear or mash it. These little mammals, which resemble mice, have a fundamental piece of our history inside of them. If you doubt this, imagine eating an apple lacking your incisor teeth or, better yet, a large carrot with no molars. Our diverse diet, ranging from fruit to meat to Twinkie, is possible only because our distant mammalian ancestors developed a mouth with different kinds of teeth that can occlude precisely. And yes, initial stages of this are seen in tritheledonts and other ancient relatives: the teeth in the front have a different pattern of blades and cusps than those in the back.

TEETH AND BONES—THE HARD STUFF

It almost goes without saying that what makes teeth special among organs is their hardness. Teeth have to be harder than the bits of food they break down; imagine trying to cut a steak with a sponge. In many ways, teeth are as hard as rocks, and the reason is that they contain a crystal molecule on the inside. That molecule, known as hydroxyapatite, impregnates the molecular and cellular infrastructure of both teeth and bones, making them resistant to bending, compression, and other stresses. Teeth are extra hard because their outer layer, enamel, is far richer in hydroxyapatite than any other structure in the body, including bone. Enamel gives teeth their white sheen. Of course, enamel is only one of the layers that make up our teeth. The inner layers, such as the pulp and dentine, are also filled with hydroxyapatite.

There are lots of creatures with hard tissues—clams and lobsters, for example. But they do not use hydroxyapatite; lobsters and clams use other materials, such as calcium carbonate or chitin. Also, unlike us, these animals have an exoskeleton covering the body. Our hardness lies within.