Chapter 7: Adventures in Bodybuilding

1. Refer to the timeline on p.121 in Your Inner Fish – what is most surprising to you about the timescale?  Explain your choice.

The most surprising thing about the timescale is how recently modern humans came into existence. This is shocking to me, because the events we learn in history seem so numerous and ancient. It is difficult to imagine an earth without human life.

2. What is the most common protein found in the human body?  Name it and describe it.

The most common protein found in the human body is collagen. When magnified 10,000 times, it resembles a rope, consisting of bundles of tiny molecular fibers. Like rope, it is strong when held taut, but weak when relaxed.

3. Explain how cells “stick” to one another; give at least one example.

Cells are held together by a biological ‘glue’ that also allows them to communicate. It contains a variety of different molecules and gives our tissues and organs their distinct appearance and function. Some tissues have cells in organized strips or columns, while others have cells that are randomly scattered or loosely attached to each other. For example, the molecules in between bone cells determine the strength of the bone and more loosely attached proteins in eyes make them more squishy and yielding.

4. How do cells (generally) communicate with one another?
Cells generally communicate with one another by sending molecules back and forth. Basic cell-to-cell communication involves one cell emitting a signal (some molecule) that attaches to the membrane of the cell receiving the signal. Once attached to the outer membrane, the molecule sets off a chain reaction of molecular events (i.e. signal transduction pathway) that travels from the outer membrane all the way, in many cases, to the nucleus of the cell. Consequently, this molecular signal can cause genes to be turned on or off, with the end result being that the cell receiving the information now changes its behavior. The combination of molecular tools allowing cell communication, structural molecules like collagens and proteoglycans, and molecular rivets to hold cells together makes bodies possible.
**Click HERE for an animated tutorial of signal transduction pathways.

5. What are choanoflagellates and why have they been studied by biologists?
Choanoflagellates are single-celled microbes, known to be the closest microbe relatives of animals with bodies, placozoans, and sponges. Because of the discovery of choanoflagellates, the genetic distinction between ‘single-celled microbe’ and ‘animal with body’ completely broke down because most of the genes active in choanoflagellates are also active in animals, with many of those genes serving as part of the machinery that builds bodies. Functions of cell adhesion and cell communication, even parts of the molecules that form the matrix between cells and cell signaling cascades, are present in choanoflagellates, in addition to collagens and molecular rivets. These organisms gave biologists a road map for comparing the bodybuilding apparatus to microbes.
**Click HERE for Chapter 3 discussion of how genes relate to the development of animal bodies.
**Click HERE for an animated tutorial of the three-domain system of classifying life, relevant to what distinguishes ‘microbes’ from ‘animals with bodies’.
6. What are some of the reasons that “bodies” might have developed in the first place? Include any environmental conditions that might have favored their evolution.
After 3.5 billion years of nothing but microbes, why did bodies come about? Theories explaining the appearance of bodies include a very simple one: bodies arose when microbes developed new ways to eat each other or avoid being eaten. Having a body with many cells allows for creatures to get bigger—a good defense against being engulfed by predators. An additional explanation is that the molecules that allow microbes to catch prey and hold on to them are likely candidates for the molecules that form the rivet attachments between cells in bodies. Some microbes can communicate with each other by making compounds that influence the behavior of other microbes, and predator-prey interactions often involve molecular cues. These may be the precursors to the kinds of signals that our own cells use to exchange information. An experiment by scientists with single-celled algae proved that if the pressure of predation is there, a multicellular form can arise.
Thus, part of the explanation for development of bodies was obviously evolutionary function, but bodies would not have developed without permissive conditions. A body allows an animal to eat or avoid each other and also move long distances, both of which consume a great amount of energy, especially if the body incorporates oxygen-expensive collagen. Levels of oxygen were not high enough on ancient earth until roughly a billion years ago, when the amount of oxygen increased dramatically and made high energy-consuming bodies possible. In conclusion, microbes developed ways of interacting with one another for billions of years, and in doing so, hit on a number of molecular parts and tools to build bodies. A cause for the origin of bodies was also in place: by a billion years ago, microbes had learned to eat each other and so there was a reason to build bodies. Finally, when enough oxygen was present to support them, bodies popped up.
**Click HERE  for Chapter 6 discussion of the actual components of body plans.

Leave a comment


  1. Yes, it’s hard to imagine Earth without humans. However the world would go on, if we went extinct! It would be different, no doubt.
    Isn’t collagen amazing?!
    Good review/reinforcement for us all re: cell to cell communication and cell signaling; significance of choanoflagellates and explanations for the appearance of multicellular, heterotrophic (animals) bodies on earth.
    Are you seeing the connections between textbook content and real world science, as Neil Shubin presents it?

  2. I do not understand how a single-celled organism can express genes similar to those of multicellular eukaryotes yet be so primitive at the same time. I guess this just shows that humans have been around for a very insignificant amount of time.

    • Remember, it’s not enough just to have the genes. You have to have the right transcription factors too, and they have to be turned on/off in tandem/sequence with other genes in order to get a functional protein and eventually a pathway. ML

      • 2012wangv

         /  May 2, 2012

        These transcription factors allow for much more diversity between species than just having the genes present would allow. It’s not just the genes, but which genes are expressed and when that play a big role in the development of physical traits.

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