Your Inner Fish


A Brief Review of Neil Shubin’s Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body

Less lyrically than Loren Eiseley, and with less intensity than Stephen Jay Gould, Neil Shubin of the University of Chicago and the Field Museum has written an excellent, substantive, and lucid work for the popular reader which successfully puts you in touch with Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body.

We’ve already previewed a few selections from Shubin’s book with an earlier excerpt on The Hard Parts – Conodonts & Ostracoderms, another on Why History Makes Us Sick and finally in a brief overview of the extraordinary Tiktaalik.

As may already be evident from these selections, Shubin pursues a thoroughly integrative approach to the study of biology, seeking to understand evolutionary development by utilizing the full spectrum of evidence at hand, from the paleontological to the anatomical, through the embryological to the genetic realm. This multidimensional perspective is not only illuminating, it is captivating, and quite readily accessible for any intelligent and interested reader.

As Shubin writes, “Carl Sagan once famously said that looking at the stars is like looking back in time. The stars’ light began the journey to our eyes eons ago, long before our world was formed. I like to think that looking at humans is much like peering at the stars. If you know how to look, our body becomes a time capsule that, when opened, tells of critical moments in the history of our planet and of a distant past in ancient oceans, streams and forests. Changes in the ancient atmosphere are reflected in the molecules that allow our cells to cooperate to make bodies. The environment of ancient streams shaped the basic anatomy of our limbs. Our color vision and sense of smell has been molded by life in ancient forests and plains. And the list goes on. This history is our inheritance, one that affects our lives today and will do so in the future.”

I heartily recommend Your Inner Fish.

A few more very brief samples from the work:

Discovering ‘the Organizer’

“In the 1920s Hilde Mangold, a graduate student in Spemann’s laboratory, started to work with small embryos. The fine control she had of her fingers made her able to do some incredibly demanding experiments. At the stage of development with which Mangold worked, the salamander embryo is a sphere about a sixteenth of an inch in diameter. She lopped off a tiny piece of tissue, smaller than a pinhead, from one part of the embryo and grafted it onto the embryo of another species. What Mangold transplanted wasn’t just any patch, but an area where cells that were to form much of the three germ layers were moving and folding. Mangold was so skilled that the grafted embryos actually continued to develop, giving her a pleasant surprise. The grafted patch led to the formation of a whole new body, including a spinal cord, back, belly, even a head.

“Why is all this important? Mangold had discovered a small patch of tissue that was able to direct other cells to form an entire body plan. The tiny, incredibly important patch of tissue containing all this information was to be known as the Organizer.

“Mangold’s dissertation was ultimately to win the Nobel Prize, but not for her. Hilde Mangold died tragically (the gasoline stove in her kitchen caught fire) before her thesis could even be published. Spemann won the Nobel Prize in Medicine in 1935, and the award cites ‘his discovery of the Organizer and its effect in embryonic development.’

“Today, many scientists consider Mangold’s work to be the single most important experiment in the history of embryology.

“At roughly the same time that Mangold was doing experiments in Spemann’s lab, W. Vogt (also in Germany) was designing clever techniques to label cells, or batches of them, and thus allow the experimenter to watch what happens as the egg develops. Vogt was able to produce a map of the embryo that shows where every organ originates in the egg. We see the antecedents of the body plan in the cell fates of the early embryo.

“From the early embryologists, people like von Baer, Pander, Mangold, and Spemann, we have learned that all the parts of our adult bodies can be mapped to individual batches of cells in the simple three-layered Frisbee, and the general structure of the body is initiated by the Organizer region discovered by Mangold and Spemann.

“Cut, slice, and dice, and you’ll find that all mammals, birds, amphibians, and fish have Organizers. You can even sometimes swap one species’ Organizer for another. Take the Organizer region from a chicken and graft it to a salamander embryo: you get a twinned salamander.

“But just what is an Organizer? What inside it tells cells how to build bodies? DNA, of course. And it is in this DNA that we will find the inner recipe that we share with the rest of animal life.”

Extracting DNA in Your Kitchen

“As we’ve seen, DNA is an extraordinarily powerful window into life’s history and the formation of bodies and organs. Its role is particularly important where the fossil record is silent. Large parts of bodies – soft tissues, for example – simply do not fossilize readily. In these cases, the DNA record is virtually all we have.

“Extracting DNA from bodies is incredibly easy, so easy you can do it in your kitchen. Take a handful of tissue from some plant or animal – peas, or steak, or chicken liver. Add some salt and water and pop everything in a blender to mush up the tissue. Then add some dish soap. Soap breaks up the membranes that surround all the cells in the tissue that were too small for the blender to handle. After that, add some meat tenderizer. The meat tenderizer breaks up some of the proteins that attach to DNA. Now you have a soapy, meat-tenderized soup, with DNA inside. Finally, add some rubbing alcohol to the mix. You’ll have two layers of liquid: soapy mush on the bottom, clear alcohol on top. DNA has a real attraction to alcohol and will move into it. If a goopy white ball appears in the alcohol, you’ve done everything right. That goop is the DNA.

“You are now in a position to use that white glop to understand many of the basic connections we have with the rest of life . . . .”

Ears to Hear

“In 1837, the German anatomist Karl Reichert was looking at embryos of mammals and reptiles to understand how the skull forms. He followed the gill arches of different species to understand where they ended up in the various skulls. As he did this again and again, he found something that appeared not to make any sense: two of the ear bones in the mammals corresponded to pieces of the jaw in the reptiles. Reichert could not believe his eyes, and his monograph reveals his excitement. As he describes the ear-jaw comparison, his prose departs from the normally staid description of nineteenth-century anatomy to express shock, even wonderment, at this discovery. The conclusion was inescapable: the same gill arch that formed part of the jaw of a reptile formed ear bones in mammals. Reichert proposed a notion that even he could barely believe – that parts of the ears of mammals are the same thing as parts of the jaws of reptiles. Things get more difficult when we realize that Reichert proposed this several decades before Darwin propounded his notion of a family tree for life. What does it mean to call structures in two different species ‘the same’ without a notion of evolution?

“Much later, in 1910 and 1912, the German anatomist Ernst Gaupp picked up on Reichert’s work and published an exhaustive study on the embryology of mammalian ears. Gaupp provided more detail and, given the times, interpreted Reichert’s work in an evolutionary framework. Gaupp’s story went like this: the three middle ear bones reveal the tie between reptiles and mammals. The single bone in the reptilian middle ear is the same as the stapes of mammals; both are second-arch derivatives. The explosive bit of information, though, was that the two other middle ear bones of mammals – the malleus and incus – evolved from bones set in the back of the reptilian jaw. If this was indeed the case, then the fossil record should show bones shifting from the jaw to the ear during the origin of mammals. The problem was that Gaupp worked only on living creatures and didn’t fully appreciate the role that fossils could play in his theory.

“Beginning in the 1840s a number of new kinds of fossil creatures were becoming known from discoveries in South Africa and Russia. Often abundantly preserved, whole skeletons of dog-size animals were crated and shipped to Richard Owen in London for identification and analysis. Owen was struck that these creatures had a mélange of features. Parts of their skeletons looked reptile-like. Other parts, notably their teeth, looked like mammals. And these were not isolated finds. It turns out that these ‘mammal-like reptiles’ were the most common skeletons being uncovered at many fossil sites. Not only were they very common, there were many kinds. In the years after Owen, these mammal-like reptiles became known from other parts of the world and from different time periods in earth history. They formed a beautiful transitional series in the fossil record between reptile and mammal.

“Until 1913, embryologists and paleontologists were working in isolation from one another. At this time, the American paleontologist W.K. Gregory, of the American Museum of Natural History, saw an important link between Gaupp’s embryos and the African fossils. The most reptilian of the mammal-like reptiles had only a single bone in its middle ear; like other reptiles, it had a jaw composed of many bones. Something remarkable was revealed as Gregory looked at the successively more mammal-like reptiles, something that would have floored Reichert had he been alive: a continuum of forms showing beyond doubt that over time the bones at the back of the reptilian jaw got smaller and smaller, until they ultimately lay in the middle ear of mammals. The malleus and incus did indeed evolve from jawbones. What Reichert and Gaupp observed in the embryos was buried in the fossil record all along, just waiting to be discovered.”

Published in: on March 25, 2009 at 11:10 am  Leave a Comment  

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