In the final section of the book, Zimmer wraps up his narrative but leaves us with many of our own questions. The development of the genetic technology CRISPR and the implications of the technology are revisited. One of the first applied uses of CRISPR was to enhance and alter crops quickly. From there, the use of CRISPR expanded to multiple cell types and species, raising many ethical questions along the way.
Part IV of SHHML takes us away from traditional methods of heritability into the weird world of DNA-less information transmission. "You, My Friend Are a Wonderland" starts off with a fascinating tale of a glowing fish that doesn’t actually glow. It turns out this clever fish has formed a mutually beneficial relationship with a special glowing bacteria that has infected each newly hatched fish for thousands (at least) of generations. Zimmer points out this phenomenon can be found throughout the animal kingdom from fish, clams, and all the way to our own cells.
In Ex Ovo Onmia Zimmer talks about the development of ecto meso endoderm and how when a cell divides it decides which of its characteristics it wants to pass on to its offspring (cell differentiation). He discusses Conklin and how he traced the trajectory of cells from zygotes to differentiated cell types to tissues and organs. Once a cell differentiates it loses some of its ability to become certain types of cells; germ cells are the only cells that maintain their ability to become anything and have a complete genetic profile.