The same DNA technology that allows the analysis of a hair or tiny blood spot at a crime scene also enables sequencing the DNA of a bone fragment from a Neanderthal skeleton. Comparing the DNA sequences of different samples and comparing the frequency of specific genetic variants in different populations is a critical part of both forensic and evolutionary investigations. These two fields share a common goal: solving historical mysteries. The book discusses the intrinsic human curiosity about our origins and the desire to solve crimes and seek justice and how the recent emergence of DNA analysis has transformed our ability to address these universal human aspirations.

The unifying theme of the book is the recently developed capacity to use DNA sequence information to make inferences about historical events. Part One is a discussion of how DNA analysis can reconstruct the recent past, in particular, the events that transpired at the scene of a crime. Part Two is a discussion of the application of DNA analysis to reconstructing the ancient past, using DNA sequences from human samples as well as from fossil remains to study the evolution of the human species and the historical relationships among contemporary and extinct human populations. Erlich discuss how he, along with his colleagues at Cetus Corp in the mid-1980s developed the PCR (polymerase chain reaction) technology of specific DNA amplification, a method of synthesizing millions of copies of a specific targeted DNA sequence and applied it to address forensic and evolutionary questions. It is this capacity, that has transformed both forensic science and evolutionary biology and has led to both the identification-and exoneration-of criminal suspects and a deeper understanding of human evolution. These techniques, now widely used, were applied in the first DNA criminal case, the first exoneration case, and the first identification of a missing person. Discussions on the history and the many remaining contentious issues in forensic DNA analysis in Part One are organized around several specific criminal cases, while the book tries to convey the spirit of "doing science."