When there are more significant researchers than significant specimens, and the question is what has made us human, the scholarly knives are sharper than knapped flints.

Palaeoanthropology is a highly strung discipline in which strength of conviction makes up for a shortage of raw material and cherished hypotheses are constantly vulnerable to unexpected new discoveries. It makes an exciting spectator sport, notwithstanding the frustrating tendency of factions to draw opposite conclusions from the same weathered fragments of ancient bone.

Chris Stringer, the leader of human origins research at the Natural History Museum, occupies a fascinating position in the volatile study of our own natural history. Over forty years he has become a world authority, the go-to expert without a quote from whom no media report on new discoveries is complete, and a leading partisan in the controversies over how modern humans emerged. Stringer has leaned heavily towards the view that we are all descended from a single small African population which replaced older populations rather than mingling with them. This model was boosted immensely by early findings from DNA analysis, which has turned living humans into evolutionary specimens. Stringer’s theory came to prevail over its rival, which saw modern human evolution as a networking process that shared genes across populations in different regions.

More recently, however, Stringer’s comments have come to sound increasingly ecumenical. This is an effect not merely of seniority, which may sometimes encourage a statesmanlike tone, but of new evidence impinging on an open mind. Stringer’s imagination is piqued by surprises, which is just as well, for the fossil record seems to have developed a sense of mischief in recent years. Ninety-nine years ago, the Piltdown hoax – a modern skull and an ape jaw planted in a Sussex gravel pit – offered its victims evidence that supported their favoured story of human evolution. The ‘Hobbit’ found on the Indonesian island of Flores in 2003–4 did just the opposite, presenting palaeoanthropologists with a genuine find that could not have confounded the standard story – it was about 5,000 miles and more than two million years out of place – more impudently if it had been planted there. Even the boring explanation, that the skeleton is the remains of a very small modern human with a pathologically small brain, is deeply improbable. Stringer is inclined to accept the find as a genuine, utterly unexpected addition to the variety of hominins (upright apes, proto-humans, archaic humans and ourselves).

He has a soft spot for imaginative hypotheses, such as the suggestion that the massive brow ridges that dominated the faces of Neanderthal males may have evolved to intensify the impact of intimidatory glares. They weren’t robust enough to protect skulls in combat, but they may have been impressive enough to prevent hostilities from breaking out. Nowadays brow ridges are doing their bit to reduce conflicts among palaeoanthropologists. They are the characteristic features of archaic faces, and the persistence of such features into times when anatomically modern humans were on the scene is one of the factors that has induced Stringer to qualify his view of human evolution. He now suggests that a network of gene exchange may have operated in Africa, making the story there one of give-and-take rather than straightforward replacement.

Outside Africa, the boundaries between modern and archaic humans are looking blurred too. The fossil record is still as fragmentary as the bones themselves, but the game has changed now that fragments can yield DNA. A previously unsuspected human stock was recently identified by extracting DNA threads from a finger bone and a tooth found in Siberia. The Neanderthals have a genome project dedicated to them: it suggests that they have left their mark in 1 to 4 per cent of the DNA of living people whose ancestry lies in Europe or Asia. Stringer’s grand ‘Out of Africa’ narrative still stands, but the plot details have got more complicated and intriguing.

This makes The Origin of Our Species the right book by the right author at the right time. It highlights just how many tantalising discoveries and analytical advances have enriched the field in recent years, and folds them into an appropriately comprehensive, generous and nuanced reflection. In doing so, however, it smoothes over one or two jagged edges that onlookers would do well to mark.

One is that becoming human in the fullest sense of the term may not be easy or inevitable. As Stringer notes, early signs of modern humanity – including jewellery and a marked preference for red pigment – flicker intermittently like candles and then disappear for thousands of years. Large brains aren’t enough: if they were, Neanderthals now would be recovering DNA from fossils of small-browed, round-headed humans like us. Stringer has a sympathetic word for the ideas of the radical anthropologist Chris Knight, who sees the red ochre as the remains of rituals that inaugurated symbolic culture. But Stringer doesn’t engage with Knight’s contention that without the trust that ritual engendered, language could not have evolved. Becoming human must have been a tricky business. It may or may not be significant that Neanderthals preferred black.

The other issue is that of diversity. However vehemently the word ‘race’ is rejected, the question of cognitive differences between human groups haunts all accounts of modern human evolution. In On the Origin of Species, Darwin gingerly tucked his one brief, explosive reference to humans into his closing remarks. Stringer adopts the same ploy, observing edgily on his way out that ‘it is likely that selection has favoured different behaviours and cognitive abilities as modern humans have diversified in different environments and social complexities’. The tone varies, according to whether the matter unsettles authors or affirms their prejudices, but that subtext never really gets written out.