Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body (4 page)

This is where we work: southern Ellesmere Island, in Nunavut Territory, Canada, 1,000 miles from the North Pole.

Failure again: all the fish we were finding were well-known species that had been collected in sites of a similar age in Eastern Europe. To top it off, these fish weren’t very closely related to land-living animals. In 2004, we decided to give it one more try. This was a do-or-die situation. The Arctic expeditions were prohibitively expensive and, short of a remarkable discovery, we would have to call it quits.

Everything changed over a period of four days in early July 2004. I was flipping rock at the bottom of the quarry, cracking ice more often than rock. I cracked the ice and saw something that I will never forget: a patch of scales unlike anything else we had yet seen in the quarry. This patch led to another blob covered by ice. It looked like a set of jaws. They were, however, unlike the jaws of any fish I had ever seen. They looked as if they might have connected to a flat head.

One day later, my colleague Steve Gatesy was flipping rocks at the top of the quarry. Steve removed a fist-size rock to reveal the snout of an animal looking right out at him. Like my ice-covered fish at the bottom of the pit, it had a flat head. It was new and important. But unlike my fish, Steve’s had real potential. We were looking at the front end, and with luck the rest of the skeleton might be safely sitting in the cliff. Steve spent the rest of the summer removing rock from it bit by bit so that we could bring the entire skeleton back to the lab and clean it up. Steve’s masterful work with this specimen led to the recovery of one of the finest fossils discovered to date at the water–land transition.

The specimens we brought back to the lab at home were little more than boulders with fossils inside. Over the course of two months, the rock was removed piece by piece, often manually with dental tools or small picks by the preparators in the lab. Every day a new piece of the fossil creature’s anatomy was revealed. Almost every time a large section was exposed, we learned something new about the origin of land-living animals.

What we saw gradually emerge from these rocks during the fall of 2004 was a beautiful intermediate between fish and land-living animals. Fish and land-living animals differ in many respects. Fish have conical heads, whereas the earliest land-living animals have almost crocodile-like heads—flat, with the eyes on top. Fish do not have necks: their shoulders are attached to their heads by a series of bony plates. Early land-living animals, like all their descendants, do have necks, meaning their heads can bend independently of their shoulders.

There are other big differences. Fish have scales all over their bodies; land-living animals do not. Also, importantly, fish have fins, whereas land-living animals have limbs with fingers, toes, wrists, and ankles. We can continue these comparisons and make a very long list of the ways that fish differ from land-living animals.

The process of finding fossils begins with a mass in a rock that is gradually removed over time. Here I show a fossil as it travels from the field to the lab and is carefully prepared as a specimen: the skeleton of the new animal. Photograph in upper left by author; other photographs courtesy of Ted Daeschler, Academy of Natural Sciences of Philadelphia.

But our new creature broke down the distinction between these two different kinds of animal. Like a fish, it has scales on its back and fins with fin webbing. But, like early land-living animals, it has a flat head and a neck. And, when we look inside the fin, we see bones that correspond to the upper arm, the forearm, even parts of the wrist. The joints are there, too: this is a fish with shoulder, elbow, and wrist joints. All inside a fin with webbing.

Virtually all of the features that this creature shares with land-living creatures look very primitive. For example, the shape and various ridges on the fish’s upper “arm” bone, the humerus, look part fish and part amphibian. The same is true of the shape of the skull and the shoulder.

It took us six years to find it, but this fossil confirmed a prediction of paleontology: not only was the new fish an intermediate between two different kinds of animal, but we had found it also
in the right time period in earth’s history
and
in the right ancient environment.
The answer came from 375-million-year-old rocks, formed in ancient streams.

This figure says it all.
Tiktaalik
is intermediate between fish and primitive land-living animal.

As the discoverers of the creature, Ted, Farish, and I had the privilege of giving it a formal scientific name. We wanted the name to reflect the fish’s provenance in the Nunavut Territory of the Arctic and the debt we owed to the Inuit people for permission to work there. We engaged the Nunavut Council of Elders, formally known as the Inuit Qaujimajatuqangit Katimajiit, to come up with a name in the Inuktitut language. My obvious concern was that a committee named Inuit Qaujimajatuqangit Katimajiit might not propose a scientific name we could pronounce. I sent them a picture of the fossil, and the elders came up with two suggestions,
Siksagiaq
and
Tiktaalik.
We went with
Tiktaalik
for its relative ease of pronunciation for the non-Inuktitut-speaking tongue and because of its meaning in Inuktitut: “large freshwater fish.”

Tiktaalik
was the lead story in a number of newspapers the day after the find was announced in April 2006, including above-the-fold headlines in such places as
The New York Times.
This attention ushered in a week unlike any other in my normally quiet life. Though for me the greatest moment of the whole media blitz was not seeing the political cartoons or reading the editorial coverage and the heated discussions on the blogs. It took place at my son’s preschool.

In the midst of the press hubbub, my son’s preschool teacher asked me to bring in the fossil and describe it. I dutifully brought a cast of
Tiktaalik
into Nathaniel’s class, bracing myself for the chaos that would ensue. The twenty four-and five-year-olds were surprisingly well behaved as I described how we had worked in the Arctic to find the fossil and showed them the animal’s sharp teeth. Then I asked what they thought it was. Hands shot up. The first child said it was a crocodile or an alligator. When queried why, he said that like a crocodile or lizard it has a flat head with eyes on top. Big teeth, too. Other children started to voice their dissent. Choosing the raised hand of one of these kids, I heard: No, no, it isn’t a crocodile, it is a fish, because it has scales and fins. Yet another child shouted, “Maybe it is both.”
Tiktaalik
’s message is so straightforward even preschoolers can see it.

For our purposes, there is an even more profound take on
Tiktaalik.
This fish doesn’t just tell us about fish; it also contains a piece of us. The search for this connection is what led me to the Arctic in the first place.

How can I be so sure that this fossil says something about my own body? Consider the neck of
Tiktaalik.
All fish prior to
Tiktaalik
have a set of bones that attach the skull to the shoulder, so that every time the animal bent its body, it also bent its head.
Tiktaalik
is different. The head is completely free of the shoulder. This whole arrangement is shared with amphibians, reptiles, birds, and mammals, including us. The entire shift can be traced to the loss of a few small bones in a fish like
Tiktaalik.

Tracing arm bones from fish to humans.

I can do a similar analysis for the wrists, ribs, ears, and other parts of our skeleton—all these features can be traced back to a fish like this. This fossil is just as much a part of our history as the African hominids, such as
Australopithecus afarensis,
the famous “Lucy.” Seeing Lucy, we can understand our history as highly advanced primates. Seeing
Tiktaalik
is seeing our history as fish.

So what have we learned? Our world is so highly ordered that we can use a walk through a zoo to predict the kinds of fossils that lie in the different layers of rocks around the world. Those predictions can bring about fossil discoveries that tell us about ancient events in the history of life. The record of those events remains inside us, as part of our anatomical organization.

What I haven’t mentioned is that we can also trace our history inside our genes, through DNA. This record of our past doesn’t lie in the rocks of the world; it lies in every cell inside us. We’ll use both fossils and genes to tell our story, the story of the making of our bodies.

CHAPTER TWO