From the glory days of archeology and anthropology. If you want to know some of the details about how early man lived, what tools he used, how he was dispersed into numerous cultures across the globe, and how archeologists discovered all of this information.
And as we learn about new discoveries and techniques of interpretation of the evidence by the day, it helps to review Braidwood's fascination with the subject in this work published over 50 years ago.
Over 99 per cent of human history is "prehistory".  Writing is only about 5000 years old. "Physical anthropology" is based on bones, teeth, and body parts. Physical anthropologists who work with prehistorical fossils are sometimes called "human paleontologists". The scientists who study what prehistoric humans did are called archeologists. "Making the interpretation of his finds is the most important part of the archeologists's job." 
"Paleo" is Greek for "old". Paleontologists study fossil animals, paleoclimatologists study plants and climates. Planned archeological digging has only been done in earnest since the early 1900s, perhaps since the discovery of cave men on Mount Carmel in Palestine, and the discoveries by the British dentist, A. T. Marston, in 1935, in a gravel pit at Swanscombe who put together separate fossils of a human skull. Braidwood also mentions early diggers like Thomas Jefferson. The German grocer's clerk, Heinrich Schliemann, who made a fortune in the California gold-rush of 1849, became an American citizen, and then in retirement, began digging in mounds in Turkey and Greece, which contained the remains of Troy and Mycenae.  It was Schliemann who conclusively proved that city mounds can be "stratified"--many towns on top of each other. And the materials of each layer that go together are called an "assemblage". 
"We have never found a group of skeletons so absolutely similar among themselves--so cast from a single mould, so to speak--that could claim to have a 'pure' race."  We come from diverse mixed changing sources.
The potasium-argon age determination may enable us to speak of mankind as at least two million years old.  If people tend to have children after 20 years, then we have 50,000 generations in a million years. Julius Caesar was alive about 100 generations ago. 250 generations take us into prehistory.