In my opinion, these two chapters were the most interesting yet, though they are “denser” in terms of the amount of information they present. Moalem delves even deeper into evolutionary sciences using killer worms, mind controlling parasites, superbugs, mutating corn, and HIV.
Chapter 5 describes how we interact with organisms that live inside us, for better or for worse. It begins by introducing the Guinea worm, a terrifying parasite capable of burning its way through the skin of an infected person. The burning sensation caused by the worm’s acid leads the host to use cold water for relief. However, water allows the worm to release its larvae, leading to even more infections. Other, more extreme examples of “host manipulation” that Moalem gives include a parasitic wasp that, as a larva, manipulates the nervous system of a host spider, causing it to spin a cocoon for the larvae to develop into a fully grown wasp. Lancet silver flukes thrive in the livers of sheep. They make their way from one sheep liver to another through a complicated process: they leave the sheep’s body through its droppings, which are then consumed by snails, which excrete the worms along with their slime. The slime is in turn consumed by ants. While in the body of ants, it manipulates them in a way that causes them to climb blades of grass, which are eaten by sheep. Smaller organisms have similar mechanisms to get from host to host, such as T. gondii, a protozoan which causes mice to be attracted to the smell of cats (T. gondii’s preferred host). Many things we think of as symptoms of diseases are actually viruses’ way of manipulating our bodies to help themselves move from host to host. For example, sneezing is often the result of a cold virus artificially triggering the sneeze reflex to spread through the air. In contrast to these parasitic relationships, Moalem points to the bacteria in our gut that help us break down food as an example of mutually beneficial relationships we have with organisms that live inside of us.
Chapter 6 deals with special genes called “jumping genes”. These genes help accelerate evolution when populations are under stress. Only three percent of DNA is actually involved in building cells. While the other 97 percent was at first thought to be useless, even dubbed “junk DNA” by scientists, new research has shown that this DNA has an important role in evolution. It has been discovered that a large amount of this “junk DNA” (now called “noncoding DNA”) was integrated into our genome by viruses. Retroviruses, such as HIV, have the ability to use the enzyme reverse transcriptase to transcribe themselves from RNA into DNA, allowing them to integrate their genes with ours. In the 1950s, Barbara McClintock’s research into corn genetics suggested the existence of “jumping genes”, parts of the genome that relocate when an organism is under stress. McClintock also observed that jumping genes seemed to relocate to parts of the genome where they would be most useful. This suggests that evolution is driven not only by random mutations that may or may not be useful, but also by parts of the genome shifting for an organism’s benefit during times of stress. Later research provided evidence for another theory: times of stress lead to increased rates of mutation (called “hypermutation” in orgamisms). Scientists are still unsure of the exact mechanics of jumping genes or hypermutation.
These chapters left me with more questions than the others. How do some parasites manage to make their hosts follow such specific behavior patterns? How can we learn from Jimmy Carter’s efforts to spread understanding about how Guinea worms reproduce and apply that knowledge to slow the spread of other diseases? How exactly to genes from retroviruses pass the Weismann barrier? Which theory is more supported by present-day evolutionary scientists: McClintock’s theory that genes tend to mutate for the benefit of the organism, or the hypermutation theory?