Here’s the basic conceit: start with modern humans, and work backward, genealogically. At some point you will “join up” with the ancestors of another living group (in this case, the chimps and bonobos). Keep going, and your joined forces will have another reunion, with the ancestors of another group of animals that are alive today (the gorillas). Keep doing this, and you’ll have successive “reunions” with (ancestors of) modern representatives, each one of course seeming stranger and more distant from us.
Now, all this is wrapped up in a Chaucerian metaphor about a pilgrimage back to the origin of life, and is written in part in the form of “Tales” told by the pilgrims. But this hardly matters, and you can ignore it if literary metaphors for science are not to your taste. The main thing is that it’s a perfect device for an account of the history of life – and focusing on our genealogical line (while briefly telling tales from other lines as they join ours) is a nice way to balance breadth of coverage while still catering to our own interest in ourselves. (Even though it might be just as scientifically eventful, working backward from modern mushrooms wouldn’t have the same kind of shelf appeal.)
How many reunions are there? 39, give or take some disputed branchings. This doesn’t seem like a lot, until you think about how big a tree of depth 39 could be… I guess that the tree isn’t very balanced, and our line has gone through a lot of speciation.
The most fun aspect of the book is just the depth of detail, the strange creatures, and the weird and unexpected facts. Lots of really strange and funky stuff has happened along the way (like reversion to asexual reproduction! Yes, an entire branch of rotifers decided that sex just wasn’t worth the trouble.) Most surprising to me was the fact that the closest living relatives of the hippopotamus are … the whales. Similarly, elephants and manatees are very close cousins. Who knew?