When Shubin did his first human medical dissection, he was unbothered by the creepiness of working on a person until he had to focus on the hand. The hand is the most quintessentially human feature, and one of the most complex parts of the human body in terms of bones, muscles and tendons. Sir Charles Bell, a Scottish surgeon in the early 1800s, took this complexity as evidence of a divine creator.
Shubin explains that his desire to study animals stems from a desire to understand all life, including the human body. The complexity of the human hand leads some, like Bell, to believe that the human body never could have developed by “accident,” as some detractors of evolution have argued.
Sir Richard Owen, one of the most famous anatomists of the 1800s, also believed in a divine order within bodies. Owen catalogued thousands of animal specimens, and realized that almost all of the animals with limbs had the same bone structure: one bone that connects to two bones, followed by lots of small bone blobs and finally the digits (fingers or toes). The shape and size of these bones changes radically, but the underlying blueprint remains. Charles Darwin took this similarity further to suggest that all animals with limbs shared a common ancestor that gave them this limb structure, going all the way back to fish fins.
Though Shubin acknowledges that men like Bell and Owens had reasons to believe in the possibility of divine creation, he puts far more emphasis on the idea that the basic similarities between many animals make it likely that the different species we have today developed from one common ancestor. In Owen’s case, the similar bone structure between all limbs is a huge similarity between all animals.
Seeing the Fish. Fish fins look nothing like limbs, being mostly made up of webbing with four bones arranged in a line. Yet the lungfish, a fish that has lungs, actually has a single bone that attaches the four fin bones to the fish’s shoulder. Another fish from the Devonian period, Esuthenopteron, goes even farther, with one bone connecting to two bones in the fin.
Fish fins are an excellent example of Shubin’s point that all animals are fundamentally similar, because fins and limbs look nothing alike on the surface. Yet some fins seem to transition into the same bone structure that human limbs have, suggesting that the fish that are most similar to land animals (those that have lungs) are the ones that started to develop land limbs. Shubin acknowledges that it is still a long way from these primitive fin-limbs to true limbs, but rationalizes that there are millions of years in which these fin-limbs could evolve.
Swedish Paleontologist Gunnar Säve-Söderbergh found a “missing link” fossil from the Devonian Period in expeditions between 1929 and 1934. This fish fossil, Ichthyostega soderberghi, has a land animal neck and back with fully developed fingers and toes on its fin-limbs. Another Säve-Söderbergh fossil remained a mystery until 1988, when Jenny Clack analyzed the limb as a flipper. This appendage, with fully-formed wrist and finger bones, suggests that the earliest limbs developed for the purpose of swimming instead of walking.
A “missing link” is the popular term for a fossil or animal that seems to fill in a gap between two distinct sets of animals, such as fish and amphibians. Shubin rejects this term, however, as the link would no longer be missing once it is found, of course, and there also has to be more than one singular link to show the many transitional stages animals go through to arrive at the distinct animal species now living. Clack’s work is another example of how scientists can build on the work of previous generations in order to make greater discoveries.
Finding Fish Fingers and Wrists. In 1995, Daeschler and Shubin find an isolated fin fossil in a Pennsylvania highway construction zone. The fin has the “standard” bone structure of a limb, even though the fin has all the webbing and scales of a normal fish. This fin looks as if it is a good candidate for the origin of limbs, but Daeschler and Shubin need a full skeleton.
On the surface, Shubin and Daeschler’s fossil looks like a fish, but the bones are in some ways similar to a human – another example of how human developmental history is actually tied up in fish. Yet a fin in isolation does little good for Shubin, as one fin cannot fully explain the animal’s lifestyle, and it is easy to misinterpret a fossil fragment. Without a full skeleton, Shubin and Daeschler may be wrong about the bone structure they think this fin has.
Shubin and his team bring back three chunks of Devonian rock from their 2004 expedition to the Canadian Arctic. Fossil preparators Fred Mullison and Bob Masek work on these chunks for the next two months, gradually uncovering intact skeletons of flat-headed fish that have human style wrists in their fins. One of these fish, another Tiktaalik, seems to have a limb that is part fin and part limb. Tiktaalik would have lived during the exact time period of the transition between water and land animals.
The importance of Tiktaalik is not only that it has the limb structure that suggests a movement towards limbs from fins, but also it is the right age. If Tiktaalik were much younger than 375 million, Shubin’s team would have just found another example of a strange amphibian, instead of the origin of limbs. If it were much older than 375 million, Shubin and his team would have to revise the expected predictions about when animals moved out of the sea, or explain why fish needed limbs if they did not live close to land.
Now that they have uncovered Tiktaalik’s wrist, Shubin and his team analyze the most likely function of this limb. Due to the structure of Tiktaalik’s joints, it seems that the limb was designed to allow the fish to do “push-ups” off the shallow stream bed and maneuver around rocks. Tiktaalik probably lived in a shallow rocky environment to avoid larger predators in the deep river water.
Shubin’s explanation for the limb relies on Tiktaalik living in a shallow stream bed, which is an educated guess considering the rocks that Tiktaalik was found in, but is not a proven fact. All fossil analysis includes some level of doubt because many specifics of the environment are unknown. Shubin also guesses at the muscles his fossil most likely had, as these details are not fossilized Here is another area where similarities between animals can help biologists, as Shubin can make assumptions based on what animals with similar bone structure alive today do with their limbs. The things we have observed in animal behavior today can be applied even to ancient fish.
It is a long journey from Tiktaalik’s “push-ups” to the range of complicated motions and movements humans can do with their wrists. But the blueprint for the human skeleton already existed within this fish, and would later become refined through amphibians and reptile species from 250 million years ago. Shubin sees this especially in the ability to rotate the thumb relative to the elbow. Humans can do this because our elbow joint is a ball-and-socket fit that lets the radius bone in our lower arm rotate around the humerus bone in our upper arm. Tiktaalik’s upper arm bone already has a primitive version of this joint, which becomes more defined in amphibians and reptiles.
Shubin does not underestimate the amount of time and miniscule changes in successive generations that are necessary to go from Tiktaalik’s very primitive wrist to the highly specialized human wrist, but he does understate the difference somewhat. Though Tiktaalik may have the beginnings of the ball-and-socket joint that lets humans rotate their thumb, a groove and elongated bump are very different from a fully realized joint. Shubin doesn’t detail the long process and the many survival motivations that benefitted animals that had more developed limbs.
Another key anatomical structure for humans is our kneecap. Our knees and elbows bend in opposite directions, allowing us to walk on two legs. In the womb, human fetus limbs face the same direction, much like a primitive fish like Euthenopteron. The knees and elbows then rotate as the legs project under our body instead of to the side – thanks to the bow-shaped pelvis and deep hip sockets. This feature, unique to humans and bipedal primates, is another reminder that all of the extraordinary things the human body does came from the humble structural anatomy of a fish.
In a rare move for a book that highlights the similarities between all animals, Shubin focuses on something that makes humans different from other animals. By explaining the bone structure that allows only primates and humans to walk upright, Shubin ensures that he is not glossing over factual difference to further his argument. However, Shubin does not back up his explanation of the bones with the reasons behind the changes that allowed humans and primates to walk upright, leaving this analysis somewhat less convincing than other portions of the book that delve into the complicated history of a certain body structure.