## Metadata - Author: [[Neil Shubin]] - Full Title: Your Inner Fish - Category: #books ## Highlights - It turns out that being a paleontologist is a huge advantage in teaching human anatomy. Why? The best road maps to human bodies lie in the bodies of other animals. The simplest way to teach students the nerves in the human head is to show them the state of affairs in sharks. The easiest road map to their limbs lies in fish. Reptiles are a real help with the structure of the brain. The reason is that the bodies of these creatures are often simpler versions of ours. ([Location 30](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=30)) - 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. ([Location 342](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=342)) - What is it about a hand that seems quintessentially human? The answer must, at some level, be that the hand is a visible connection between us; it is a signature for who we are and what we can attain. Our ability to grasp, to build, and to make our thoughts real lies inside this complex of bones, nerves, and vessels. ([Location 365](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=365)) - The important, and often surprising, fact is that most of the major bones humans use to walk, throw, or grasp first appear in animals tens to hundreds of millions of years before. The first bits of our upper arm and leg are in 380-million-year-old fish like Eusthenopteron. Tiktaalik reveals the early stages in the evolution of our wrist, palm, and finger area. The first true fingers and toes are seen in 365-million-year-old amphibians like Acanthostega. Finally, the full complement of wrist and ankle bones found in a human hand or foot is seen in reptiles more than 250 million years old. The basic skeleton of our hands and feet emerged over hundreds of millions of years, first in fish and later in amphibians and reptiles. ([Location 526](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=526)) - As we’ve seen, when we discover creatures that reveal different and often simpler versions of our bodies inside their own, a wonderfully direct window opens into the distant past. But there is a big limitation to working with fossils. We cannot do experiments on long-dead animals. Experiments are great because we can actually manipulate something to see the results. For this reason, my laboratory is split directly in two: half is devoted to fossils, the other half to embryos and DNA. Life in my lab can be schizophrenic. The locked cabinet that holds Tiktaalik specimens is adjacent to the freezer containing our precious DNA samples. ([Location 572](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=572)) - A skin cell is different from a neuron because different genes are active in each cell. When a gene is turned on, it makes a protein that can affect what the cell looks like and how it behaves. Therefore, to understand what makes a cell in the eye different from a cell in the bones of the hand, we need to know about the genetic switches that control the activity of genes in each cell and tissue. ([Location 585](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=585)) - The first experimental embryologists interested in limbs in the 1930s and 1940s faced several problems. They needed an organism in which the limbs were accessible for observation and experiment. The embryo had to be relatively large, so that they could perform surgical procedures on it. Importantly, the embryo had to grow in a protected place, in a container that sheltered it from jostling and other environmental disturbances. Also, and critically, the embryos had to be abundant and available year-round. The obvious solution to this scientific need is at your local grocery store: chicken eggs. ([Location 620](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=620)) - In the 1950s and 1960s a number of biologists, including Edgar Zwilling and John Saunders, did extraordinarily creative experiments on chicken eggs to understand how the pattern of the skeleton forms. ([Location 624](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=624)) - A strip of tissue at the extreme end of the limb bud is essential for all limb development. Remove it, and development stops. Remove it early, and we are left with only an upper arm, or a piece of an arm. Remove it slightly later, and we end up with an upper arm and a forearm. Remove it even later, and the arm is almost complete, except that the digits are short and deformed. ([Location 629](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=629)) - Take a little patch of tissue from what will become the pinky side of a limb bud, early in development, and transplant it on the opposite side, just under where the first finger will form. Let the chick develop and form a wing. The result surprised nearly everybody. The wing developed normally except that it also had a full duplicate set of digits. Even more remarkable was the pattern of the digits: the new fingers were mirror images of the normal set. ([Location 633](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=633)) - This patch of tissue was named the zone of polarizing activity (ZPA). Essentially, the ZPA is a patch of tissue that causes the pinky side to be different from the thumb side. ([Location 640](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=640)) - Sharks and their relatives are the earliest creatures that have fins with a skeleton inside. ([Location 710](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=710)) - We would never have scales, feathers, or breasts if we didn’t have teeth in the first place. The developmental tools that make teeth have been repurposed to make other important skin structures. ([Location 1044](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1044)) - Bones that support the upper and lower jaws in sharks are used in us to swallow and hear. ([Location 1204](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1204)) - There is a pattern common to every skull on earth, whether it belongs to a shark, a bony fish, a salamander, or a human. The discovery of this pattern was a major accomplishment of nineteenth-century anatomy, a time when anatomists were putting embryos of all kinds of species under the microscope. In 1872, the Oxford anatomist Francis Maitland Balfour first saw the basic plan of heads when he looked at sharks and saw the bulges, the gill arches, and the structures inside. ([Location 1212](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1212)) - In the 1800s, some natural philosophers looked to embryos to try to find the common plan for life on earth. Paramount among these observers was Karl Ernst von Baer. ([Location 1290](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1290)) - As they looked at embryos, they found something fundamental: all organs in the chicken can be traced to one of three layers of tissue in the developing embryo. These three layers became known as the germ layers. They achieved almost legendary status, which they retain even to this day. ([Location 1295](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1295)) - Von Baer compared the three layers of Pander’s chicken embryos with everything else he could get his hands on: fish, reptiles, and mammals. Yes, every animal organ originated in one of these three layers. Significantly, the three layers formed the same structures in every species. Every heart of every species formed from the same layer. Another layer gave rise to every brain of every animal. And so on. No matter how different the species look as adults, as tiny embryos they all go through the same stages of development. ([Location 1299](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1299)) - We may be more complicated than we were at twenty-one days after conception, but we are still a tube within a tube, and all of our organs derive from one of the three layers of tissue that appeared in our second week after conception. ([Location 1328](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1328)) - Whether the body belongs to a salmon, a chicken, a frog, or a mouse, all of its organs are formed by endoderm, ectoderm, and mesoderm. ([Location 1333](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1333)) - Von Baer’s approach is very different from the “ontogeny recapitulates phylogeny” idea you might have learned in school. Von Baer simply compared embryos and noted that the embryos of different species looked more similar to each other than do the adults of those species. The “ontogeny recapitulates phylogeny” approach championed decades later by Ernst Haeckel made the claim that each species tracked its evolutionary history as it proceeded through development. Accordingly, the embryo of a human went through a fish, a reptile, and a mammal stage. Haeckel would compare a human embryo to an adult fish or a lizard. The differences between the ideas of von Baer and Haeckel might seem subtle, but they are not. In the past one hundred years, time and new evidence have treated von Baer much more kindly. ([Location 1341](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1341)) - The embryos of different species are not completely identical, but their similarities are profound. All have gill arches, notochords, and look like a tube within a tube at some stage of their development. And, importantly, embryos as distinct as fish and people have Pander and von Baer’s three germ layers. ([Location 1349](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1349)) - Different kinds of creatures have different numbers of Hox genes. Flies and other insects have eight, mice and other mammals thirty-nine. The thirty-nine Hox genes in mice are all versions of the ones that are found in flies. This similarity has led to the idea that the large number of mammalian Hox genes arose from a duplication of the smaller complement of genes in the fly. ([Location 1434](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1434)) - Genes interact with other genes at all stages of development. One gene may inhibit the activity of another or promote it. Sometimes many genes interact to turn another gene on or off. Fortunately, new tools allow us to study the activity of thousands of genes in a cell at once. Couple this technology with new computer-based ways of interpreting gene function and we have enormous potential to understand how genes build cells, tissues, and bodies. ([Location 1458](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1458)) - Noggin alone does not instruct any cell in the embryo about its position on the top–bottom axis; rather, it acts in concert with several other genes to do this. Another gene, BMP-4, is a bottom gene; it is turned on in cells that will make the bottom, or belly side, of an embryo. There is an important interaction between BMP-4 and Noggin. Wherever Noggin is active, BMP-4 cannot do its job. The upshot is that Noggin does not tell cells to develop as “cells on the top of the body” instead, it turns off the signal that would make them bottom cells. These off-on interactions underlie virtually all developmental processes. ([Location 1463](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1463)) - Draw a line from the mouth to the base of the animal. Biologists have given that line a name: the oral–aboral axis. But naming it doesn’t make it more than an arbitrary line. If it is real, then its development should resemble the development of one of our own body dimensions. ([Location 1482](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1482)) - Martindale and his colleagues discovered that primitive versions of some of our major body plan genes—those that determine our head-to-anus axis—are indeed present in the sea anemone. And, more important, these genes are active along the oral–aboral axis. This in turn means that the oral–aboral axis of these primitive creatures is genetically equivalent to our head-to-anus axis. ([Location 1484](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1484)) - Like a cake recipe passed down from generation to generation—with enhancements to the cake in each—the recipe that builds our bodies has been passed down, and modified, for eons. We may not look much like sea anemones and jellyfish, but the recipe that builds us is a more intricate version of the one that builds them. ([Location 1498](https://readwise.io/to_kindle?action=open&asin=B0010SKTRA&location=1498))