Chapter 2: The Cook's Body

The Trade Off: Little Jaws, Big Brains?

In Chapter Two: The Cook’s Body, Wrangham discusses the morphological differences between human and ape digestive systems. As predicted, he contributes most of these to cooked food. However, other factors should be considered, such as random gene mutations. I want to summarize these changes quickly before exploring the article I want to bring up. In brief some of the changes mentioned are:

1.       MINISCULE MOUTHS and LIPS:

Humans, as compared to other primates, have relatively small mouth compared to body size; our mouths fit the same amount of food as that of a chimpanzee, although humans weigh on average twice as much as chimps do. (Wrangham, 2009).  Chimpanzees also have much stronger lips, used to compress food against their teeth. Human lips are small, and together with the mouth can only accommodate a relatively small amount of food at a time (Wrangham, 2009)

2.       WEAKER JAWS and TINY TEETH

Human masticatory muscles (the temporalis and the masseter) reach only to about the tops of our ears, where as many apes have larger sagittal crests than anchor chewing muscles from their strong, robust jaws to the tops of their craniums. You would have to trace back along our evolutionary timeline to the robust Australopiths to find such masticatory equipment.  We also have smaller, weaker muscle fibers in our jaws (Wrangham, 2009). This is important, and we’ll return to this in the journal article. Also, after millions of years of soft food items, Wrangham, (2009) believes our early Homo chefs developed more diminutive teeth, because cooked food requires less grinding and shredding.

3.       SMALLISH STOMACHS

The surface area of the human is only 1/3 the expected size for a primate of our body weight- that’s smaller than 97% of other primates (Wrangham, 2009). According to the same author this may be because, eating foods of low quality, gorillas and other apes have to ingest much more food to break even on their caloric intake.

4.       ITTY-BITTY INTESTINES

Finally, while the human small intestine is only a little smaller than the predicted size for a primate of our body weight, our large intestine is 60% less than the expected mass for that organ (Wrangham, 2009). According to Wrangham (2009), this is likely because the colon is where tough fibrous plants ferment in other primates, and through cooking we have largely decreased the amount of difficult-to-digest roughage we consume.

                Returning to the diminutive jaw muscles. Could our little jaws be due only to changes in diet? Could our loss of sagittal crests and large masticatory muscles have played a role in the development of our bigger brains? Stedman et al. (2004) conducted research linking the myosin heavy chain (MYH) Wrangham mentions with the evolutionary trend of encephalization.  This is attributed to a frameshift mutation and loss of the protein that produced larger, powerful masticatory muscles.  Stedman et al. (2004) believes this mutation occurred at approximately 2.4 million years ago, and that without the constraints of sagittal cresting and larger chewing muscles, are craniums could expand to house larger brains. But the advent of cooking, according to Wrangham, occurred ~1.9-1.8 million years ago. Therefore, I think Wrangham could consider other factors in our morphological changes besides those instigated by softer, more nutritious diets. Cooking may have led to morphological change, but considering the research done by Stedman et al. (2004), cooking may have just further spurred on changes long in the making, and independent of our dietary preferences.

References:

Steman H, Kozyak B, Neilson A, Thesler D, Si L, Low D, Bridges C, Sheager J, Minugh-Purvis, N and Mitchell M. 
  2004. Myosin gene mutation correlates with anatomical changes in the human lineage. Nature.   426: 415-419.

Wrangham, R. 2009. Catching fire: how cooking made us human. New York: Basics Books.