In this book Thomas takes up the subject of humanity and, as a medical doctor, spends most of the book opining about the problems of public health. Since it was written a full two decades ago it is rather outdated (even though many of the public issues he writes about are still issues he addresses them from a very different perspective) and I think that has a lot to do with why it falls a bit flat.
However, nearly all the way to the end Thomas describes one of the most interesting lines of research I have ever read about. In the late 1960s some researchers at the University of Buffalo, including one Dr. Jeon, were doing some work on amoebas. They had found a way to transplant the nucleus of one amoeba strain into another. Amoebas are unicellular eukaryotes, the very simplest creatures in the lineage that eventually reached us. This transplanting technique offered up a good deal of insight into genetics.
Tangentially, it would seem, some of Jeon's cultures became infected with bacteria. Rather than just destroy the cultures and start fresh, Jeon fought to keep his precious experimental organisms alive and healthy. He succeeded in getting them back to apparent health; the amoebas returned to reproducing at their normal rate, indicating they were healthy. However each still contained a load of about 50,000 bacteria.
Jeon began some experiments to get rid of the bacteria entirely. He found whenever he was able to destroy the bacteria, such as by exposing them to a heat they could not tolerate but the amoebas could, the amoebas promptly died. Quickly Jeon discovered that they amoebas and the bacteria had become symbiotic. Perhaps more accurately, the bacteria had become organelles; they were now vital to the amoebae.
By thinking long enough about it I felt I had resolved how something like our relationship with mitochondria could have evolved but this experiment shows that it can happen in a remarkably short amount of time. By continuing this line of work the researchers found that it could happen in as little as six weeks. Six weeks! To potentially develop an entirely new kind of living thing. Potentially an entirely new kingdom could develop out of this kind of event. This is, in fact, probably how eukaryotes first arose from prokaryotic ancestors. One bacteria infected another bacteria and then they simply became organelles.
Weird but true science. I feel like a description of this experiment should find itself somewhere into any introductory biology course whether at the high school or college level. This is just remarkably cool stuff.