We still speak of the majority of animals as "invertebrates", although we've long known that those creatures don't have anything in particular in common beyond not being vertebrates - in a sense, it would make as much sense to divide the animal kingdom into spiders and non-spiders.

Still, vertebrates are a remarkable group of animals for many reasons beyond the fact that they count our own species among their number, and their many distinctive traits - which along with the backbone itself include such less obvious things like the adaptive immune system - must somehow have arisen from invertebrate ancestors that lacked them. The task Gee has set himself is to sketch how that may have happened, helped by the substantial advances in molecular biology and the spectacular fossil finds of recent decades.

I found the book rather delightful. It quickly surveys the closest relatives of vertebrates and the most basal vertebrates as well as various relevant fossils, and tries to "cross the bridge" across the chasm between invertebrate and vertebrate by determining the order of acquisition of the essential vertebrate traits. At about 300 pages it necessarily skims over a lot, but plentiful further reading is suggested for the reader who wants to dive deeper into the palaeontology of armoured jawless fish or whatever.