The book is concerned mainly with three species of Homo apart from H.sapiens: H. erectus, H. heidelbergensis, and the Neanderthals. When Stringer began his career in the 1970s it was generally thought that the Neanderthals and other types of 'archaic H. sapiens' were variant forms of our own species, and the path by which modern humans developed was hardly considered a subject for research. Most palaeontologists favoured the idea that humans had arisen at more or less the same time in different parts of the world (multiregional hypothesis).
In the 1980s Stringer was one of a small group of 'rebels' who advocated what is often known as the 'Out of Africa' alternative. This propoosed that modern humans arose in Africa comparatively recently and spread to other parts of the world, replacing the other species such as the Neanderthals, which had followed a different evolutionary path. Stringer prefers to call this the Recent Africa Origin theory (RAO) because we now know that there were earlier dispersals from Africa. RAO has largely though not completely won the field and become the prevailing orthodoxy.
Our understanding of human evolution has been hugely impacted by developments in genetics, which have even allowed the sequencing of complete genomes of ancient species such as the Neanderthals and Denisovans. This has lent support to RAO although some modifications to the original theory have been necessary. For example, we now know that some interbreeding with the Neanderthals and other ancient species occurred after our ancestors left Africa, but it seems possible that some of the 'primitive' features in our genome may have been acquired via hybridisation with other kinds of hominin even before we left Africa.
Stringer covers a lot of ground. He starts by looking at evidence from fossils and how dating has been made more accurate by new methods based on physics. But not everything has been high-tech: one anthropologist, Grover Krantz, tried to investigate the possible benefit of the large brow ridge possessed by H. erectus. He strapped a fake ridge on his own head and wore it for several days. He found it kept his hair out of his eyes when he was running 'and also scared people out of their wits on dark nights'. (Stringer thinks this may, in fact, have been the reason for its existence; it may have enhanced the aggressive appearance of early males.)
In the early years of his career Stringer was mainly concerned with fossils, but evolutionary anthropology has been dramatically transformed by modern genetics, and this is something that gets a lot of attention here; it's a research area which String has himself been involved with in collaboration with others. Much of the work has been concerned with the genomes extracted from fossils, but what I found particularly intriguing was the unexpected light that has come from work on our parasites such as lice.
Using mtDNA molecular clocks of lice evolution in humans and apes, [researchers] have estimated the origin of body lice to between 80,000 and 170,000 years ago. This in turn suggests that bedding and clothing would already have been in use by modern humans in Africa, and they then took this valuable innovation with them when they left their ancestral homeland—together with the lice.Another possibility is that the lice were transferred by women's use of a sling to carry their babies.
As well as body lice, humans may harbour head lice and pubic lice, and their genetics are different. Head lice are most closely related to chimpanzee lice, which makes sense because chimps are our closest living relatives. Our pubic lice, in contrast, are most similar genetically to gorilla lice, with an apparent divergence of only about 3 million years. As we are less closely related to gorillas than to chimps, this must mean that our African ancestors had some contact with gorillas— sexual, sociable, involving conflict or predation? It also implies that we had lost much of our intervening body hair by 3 million years ago.
This by no means exhausts the intriguing conclusions about our evolution that studies of louse mtDNA have yielded, and Stringer discusses the question in some detail.
Important though all this genetic research is for our understanding of our evolution, that does not mean that study of fossils has no further interest. On the contrary, new techniques such as computed tomography have yielded fascinating results. For example, differences in the inner ear structure (concerned with balance) in H. erectus, the Neanderthals, and ourselves may tell us something about behaviour and posture. And it has been possible to infer quite a lot about Neanderthal obstetrics from studies of their pelvic anatomy. We will never know everything about how these ancient species lived but we are learning more than seemed possible even a few years ago.
This is a fascinating and important book that should not be missed by anyone with more than a passing interest in human evolution. There are extensive chapter notes with suggestions for further reading.