Preface. Neil Shubin describes how he taught a human anatomy course at the University of Chicago, though his degree was in paleontology and his specialty was fish. Yet Shubin’s knowledge of other animals gave him the opportunity to explain complex human anatomy in terms of simpler animal anatomy. The second year he taught the course, Shubin found a fossil fish from the period of transition between water animals and land animals, and this discovery reframed how he thought about the human body.
From the start, Shubin highlights how knowledge of animals can help anyone who wants to study the human body, as humans are so similar to other animals underneath the surface. As a paleontologist and a developmental biologist, Shubin takes a long view of the history of the human body and looks for the historical ancestors to humans, starting from the first animals that even lived on land.
Shubin has now spent many summers in the Arctic looking for fossil fish. Fossils are the only way to see what life was like on Earth in the distant past, and therefore are a key part of understanding how life developed into human life. These fish give fundamental clues to understanding the human body.
Shubin draws connections from the ancient fossils to contemporary humans, though other scientists might think that fossils are a dead area of science. From Shubin’s perspective, understanding the origin of humans is important to understanding modern anatomy.
Digging Fossils – Seeing Ourselves. On Ellesmere Island, with a latitude of 80 degrees north, Shubin finds a fossil fish with a flat head. He is in the Arctic because the Arctic region is one of the best places to reliably find fossils. With new technology that allows paleontologists to scan potential field sites before digging, fossils are in some ways easier to find than they once were. But fossils are so fragile that digging them out by hand is still laborious, time-consuming work – often in harsh terrain and weather conditions.
A flat head is important because it suggests that the fish probably lived in shallow water, an important move from living in deep water to living on land. The details that Shubin provides about his location on Ellesmere Island make the search for this fossil more engaging than a simple list of the facts. Shubin emphasizes the time and care that goes into finding fossils in these Arctic conditions, as well as the significance of finding the fossil itself.
Fossil sites depend on three things: rocks of the right age, rock types that can preserve fossils, and rocks that are exposed on the surface. To look for fossils from the transition between water animals to land animals, Shubin has to find rocks that are older than 365 million years old. Luckily, the arrangement of rock layers on Earth leaves a relatively stable timeline with the oldest rocks on the bottom and more recent rocks on top. Earthquakes and fault shifts can disturb this pattern, but there is usually enough evidence to put the timeline back together.
The order of the fossil layers helps paleontologists like Shubin predict the location of certain fossils if they know approximately which age the fossils they are looking for were formed. Shubin does not explain how paleontologists know the age of the rocks, but methods of radiometric dating first started in 1907 have proven reliable in setting base layer ages for volcanic rock, from which paleontologists can extrapolate the ages of sedimentary rocks found above and below these radioactive dated volcanic layers.
Fossils inside the rock layers also follow the progression of oldest on bottom to youngest on top, starting with jellyfish-type creatures, moving through various animals with skeletons, all the way to humans. Looking at a zoo from today can actually help paleontologists predict what type of animal will be in each age of rock layer. They do this by focusing on the traits that animals share.
Since the rocks are layered with oldest on bottom to youngest on top, it makes sense that the fossils formed within that rock would also follow that pattern. Shubin also makes the assumption that the animals with the “simplest” body plan are the oldest, as these simple traits had to develop before more complicated animals could arise.
Everything in the zoo has a head and two eyes. A subset adds limbs. The next subset adds another feature. The more unique a subset is, the younger it is. Thus Shubin expects to find fossils with a head and two eyes in rock layers below fossils with a head, two eyes, and limbs. By analyzing thousands of animal characteristics and species, paleontologists have formed a catalogue of what age rock holds which type of fossils.
The assumption that “simple” animals are older follows the ideas of descent with modification that Shubin will pick up again in Chapter 11. The idea is that as time goes on, life on Earth grows more complicated by adding more features to animals. While all of this history might not be expressed in the animal physically, the evidence of these types of group progressions are seen in the genetic information of animals.
For the first fossils with limbs, the rock layer comes from the critical time period from 380 million years ago to 365 million years ago. 360-million-years-old rocks already show diverse life forms that look like modern day amphibians (frogs and salamanders). Shubin decided to focus on 375-million-years-old rocks to maximize his chances of finding fossils of the first creatures with limbs.
Since only fossils of fish are found at rock layers 385 million years old, and fossils of land animals with limbs are found in rocks 365 million years old, the natural conclusion is that the transition between water and land animals happened in the time period in between. The fact that these calculations are made in terms of millions of years highlights the long, drawn-out process of change in life on Earth.
The best type of rock for finding fossils is sedimentary rock, as volcanic and metamorphic rocks form in conditions too violent to allow fragile fossils to stay intact. Sedimentary rocks all over the world show that the geography and climate of Earth has changed significantly over time, with oceans or tropical rainforests where there are now mountains and deserts.
Sedimentary rock forms by pressing small rocks, pebbles, and sand together with enough force that the rocks fuse into a solid layer. Any bones caught between these small rocks will also be pressed down and the spaces in between the organism’s cells are filled in with mineral-rich water that hardens into a rock-like structure. Sedimentary rock typically forms in stream beds or areas with water, showing that water on Earth was once distributed much differently than it is now.
The last step for choosing a fossil site is finding a layer of sedimentary rocks of the right age that is not covered by human settlements. These three factors are almost always easier to find in deserts. However, it is very expensive to mount a full fossil-finding expedition to a desert like the Gobi or the Arctic.
The desert winds help wear away rock, exposing possible fossils, and paleontologists have an easier time digging in deserts because cities are not usually built in areas with so little water. Yet these factors also mean that the expedition itself takes more work and planning.
Shubin starts his fossil-finding expedition researching the origin of limbs in his hometown of Philadelphia. The Catskill Formation of Pennsylvania actually holds rocks from the Late Devonian Period that contain valuable fish specimens. Shubin and one of his students, Ted Daeschler, check sites of exposed rock recently blasted by the Pennsylvania Department of Transportation to make new roads. They are rewarded with a shoulder bone from a hynerpeton, a small amphibian.
The Devonian Period was the geologic age from 420 million to 358 million years ago. This time period is also called Age of Fishes, due to the many deep water predators that “ruled” the oceans and the apparent lack of many significant land creatures until the late Devonian. Shubin and Daeschler display the ingenuity of scientists on a budget, letting the PDOT do the heavy blasting work for them and then looking for fossils in the newly uncovered rock. A hynerpeton has a very primitive limb, showing that Shubin and Daeschler are close to the origin of limbs.
With one new fossil found, Shubin and Daeschler are ready for more. Looking at a geology textbook, they notice that rocks from the Devonian Period are also in the Alaskan Yukon (which has already been well-studied), the coast of Greenland (where Jenny Clack found an early creature with limbs), and the Canadian Arctic—which has rocks almost identical to the rocks in Pennsylvania. Shubin and Daeschler decide to go to Canada because it has not yet been explored by vertebrate paleontologists.
The rocks from the Devonian period show that Arctic areas such as Greenland and Alaska were once temperate forests with streams fit for small amphibians like hynerpeton. Shubin and Daeschler decide to maximize the cost of mounting a fossil expedition by going to an area that has not yet been covered, in the hopes that they will find new fossils. The success in other Devonian areas is a huge clue that Shubin will not be wasting his time or money in the Canadian Arctic.
An expedition to the Arctic comes with many dangers, including the local wildlife, unpredictable weather, and the limited ability to carry supplies when the team is airlifted to dig sites. Furthermore, Shubin and his team can only go to the Arctic during the summer. Shubin brings in Dr. Farish Jenkins, his graduate advisor from Harvard, who has years of experience leading expeditions in similar conditions in Greenland.
Shubin again focuses on the work of fossil finding, showing that this science is not an easy task even when all the theoretical and academic factors fall into place. Shubin builds on the knowledge of an older and more experienced paleontologist to help ensure that his expedition will be successful.
Shubin spends the first few weeks at the dig site worrying about polar bears. The Arctic landscape is vast and empty, making the search for fossils less than four feet long even more improbable. In 1999, Shubin and his team find plenty of fish fossils from deep water, but none of the shallow-water fish they need to start looking for the transition to land-living animals.
Even though Shubin has planned his expedition carefully, finding fossils isn’t a guarantee. There is a certain amount of luck involved for even the most meticulous and careful fossil finders. And even when Shubin is successful, his fossils might not reveal anything that is not already well-studied.
In 2000, Shubin and his team move their dig site east to Ellesmere Island. There, a college undergraduate named Jason Downs is late returning to camp one night. Just as the senior members of the team are ready to mount a search party for Downs, Downs returns to camp with his pockets full of fossil fragments. The whole team heads out to the river bed where Downs identified these fossils, and they spend several days identifying the exact rock layer that might hide intact fossil fish skeletons. They eventually find intact skeletons, but the fish are all of species that have already been documented.
Jason Downs represents the many levels of the scientific community that are valuable at a dig site. Though Downs may not have all the experience and knowledge that Shubin or the older professors have, his contributions are still important. Even though the fish fossils they found were already catalogued, finding something is better than nothing in terms of continuing to receive funding for these Arctic missions. Had Downs not found these fish, Shubin and his team might not have been able to justify another trip to Ellesmere Island.
In 2004, Shubin and his team make one last expensive trip to the Arctic. Finally, Shubin finds a fish fossil fragment with a jaw that suggests the fish had a flat head. Then another team member, Steve Gatesy, finds a full fish skeleton with the same flat jaw. Over the next two months, fossil preparators meticulously expose this fossil from the rock, discovering that it is an intermediate between fish and land animals. This fish fossil has scales like a true fish, but a neck, flat head, and small limbs like a land animal.
Though Shubin was the lead paleontologist on this mission, he makes sure that his book recognizes the many scientists, like Steve Gatesy, who contributed to the amazing find. This fish is special because it blends traits that previously appeared only in animals that live solely in water with traits that appear in animals that live in land and water. It is an indirect ancestor of all animals that now live on land, as it paved the way for body systems that facilitate limb development for motion on land.
This fish find is a huge success for the idea that there is a transitional stage between fish and amphibians at the 375-million-year time period. Shubin, Daeschler, and Jenkins decide to thank the Inuit people for allowing them to work in the Nunavut territory by giving the fish fossil a name that reflects the Inuit heritage. The Inuit Committee of head elders suggests Siksagiaq or Tiktaalik. Shubin decides on Tiktaalik, which means large freshwater fish.
Shubin also highlights the contributions of people who are not members of the scientific or academic community by honoring the Inuit people with the name of the fossil. The Inuit also made this discovery possible by allowing Shubin and his team to excavate this land at all. Tiktaalik was the winner partly because it was easier to say, another sign that Shubin wants this discovery to be as accessible to the average person as possible.
Tiktaalik’s discovery is a huge news story in 2006, but Shubin is most affected by a moment in his son’s preschool class. Shubin takes the Tiktaalik fossil to his son Nathaniel’s show-and-tell. When one child asks if the fossil is a fish or a crocodile, another child responds that it can be both. More than bridging the gap between fish and reptiles, Tiktaalik offers insight into all the body structures that land animals share.
It is easy to put animals in strict categories, and often those categories help the average person understand different animals. Yet Tiktaalik is easier to understand when people embrace the similarities it has to both fish and amphibians instead of trying to force it into one box or another. Tiktaalik’s primitive legs also offer a blueprint to understanding the various limbs of all land creatures.
Tiktaalik shows human’s history as fish the same way that the famous “Lucy” (an early human ancestor discovered in Ethiopia) shows human’s history as highly advanced primates. Human anatomy is the result of millennia of small shifts in the bone structure of all animals. These shifts can be seen in the fossil record, as well as in genes and DNA.
Tiktaalik is a much older fossil than Lucy, and seems much further removed from human anatomy than Lucy does. Yet Shubin stresses that Tiktaalik is just as important in the entire story of human development, because Lucy never would have developed if fish like Tiktaalik had not paved the way for animals to live on land.