The Origin of Our Species by Chris Stringer

This week, a pause for some real rather than fictional science. Chris Stringer has been researching human evolution throughout his professional life and currently works at the Department of Palaeontology at the National History Museum, London. He is regarded as the UK's foremost authority on the subject and his latest book, The Origin of Our Species, sets out to explain to interested observers the current state of knowledge in a field which has seen some rapid developments in recent years.

Not only have two additional hominims who lived at the same time as Homo sapiens been discovered - the Denisovans and Homo floresiensis (the "Hobbit"), both in Asia - but gene sequencing has hit the news with the revelation that the genome of modern humans contains some elements from both Neanderthals and Denisovans, indicating that they all interbred at some point. Genetic analysis and advanced dating methods have also provided far more information about the way in which the various species of the genus Homo are related to each other, plus how they spread and interacted.

I read with great interest what Stringer has to say about all this. His approach is thematic and discursive rather than chronological; it focuses on how we know what we do about human origins rather than on simply telling the evolutionary story. This makes for an interesting read but an awkward reference source since material on Neanderthals, for example, is scattered throughout the book, requiring much flipping between text and index to track down.

I liked the fact that Stringer is not didactic. He acknowledges where the data is shaky and where it is firm, and points to alternative interpretations in order to highlight the areas where there is disagreement between the researchers in this field. However, it is also clear from his narrative that most such disagreements tend to be temporary, caused by lack of adequate data, and that they usually go away as the data builds up sufficiently to make one interpretation clearly a better fit with the data than the others (although scientists are human too, so can be reluctant to give up a theory that they've adopted).

In the initial chapter the author outlines the history of the study of human evolution starting with Darwin's The Descent of Man. This is followed up by chapters on: the development of dating techniques (the long-established radiocarbon dating having been joined by several others with different strengths and weaknesses); new high-tech ways of analysing skulls and other bones; recent finds and their implications; the examination of the evidence for the development of thought and behaviour (tools, art, crafts, burials); genes and DNA; and finally two chapters on "The Making of a Modern Human" and "The Past and Future Evolution of Our Species". There is a huge amount of fascinating material in this book and in a review like this I can only pick out a few points which caught my attention.

An early problem, which still exists today, is how to categorise the various fossils which have been discovered to date. One view (particularly associated with the Multiregional theory described below) is that the genus "Homo" and species "sapiens" covers a wide range of hominims, leading to the use of sub-species terms such as Homo sapiens neanderthalensis and Homo sapiens sapiens (i.e., us). At the opposite extreme, another view has a different species name attached to almost every fossil. Stringer sits somewhere in the middle; he doesn't use sub-species terms but just the main species ones.

One of the hottest debates over the past few decades has been between the "Multiregional" and "Out of Africa" models of evolution. The former postulates an early spread of hominims from Africa across Eurasia, after which each group evolved in parallel into the variety of modern humans. The latter (which Stringer prefers to call RAO, for "Recent African Origin", since there is no dispute that all of humanity originated in Africa, as Darwin speculated) argues that there were several stages of dispersion from Africa, with modern humans primarily originating from the most recent one approximately 70,000 years ago (with the addition of a soupçon from older species by interbreeding before they died out). At one time the Multiregional theory was dominant but modern genetic analysis has swung the argument strongly in favour of RAO after the usual academic debate (polite term for a vicious cat-fight!), with some still refusing to be convinced.

The course and timeline of human evolution within Africa is another issue explored in the first chapter. Trying to sort out how the various fragmentary fossils relate to each other - specifically, which were in the ancestral chain leading directly to modern humans and which were dead-ends - is still very much a source of debate. Here, the development of more sophisticated technical dating systems has proved helpful. What has become clear is that the evolutionary history is very far from the tidy progression from an ape-like hominim to modern man as shown in the now notorious "ascent of man" illustration. Different types of hominim coexisted for a very long time in Africa - and probably interbred. The earliest skull fragment of modern human form has been dated to 250,000 years ago (although the first fully modern humans seem to be only half as old), but a primitive skull found in west Africa is only 20,000 years old.

In the light of all of this, any human family tree is tentative and subject to revision as more data are discovered. Keeping that caveat in mind, the author indicates a probable structure as follows: Homo erectus, for which there is fossil evidence in both Africa and Asia, emerged about 1.5 million years ago and survived in Africa into the Homo sapiens era. At some point, perhaps 1.2 million years ago, an offshoot of erectus appeared, designated Homo heidelbergensis. This hominim family subdivided around 400,000 years ago; one branch produced both the Neanderthals and Denisovans, the other became Homo sapiens; modern humans.

However, this definition of "modern human" concerns only a skeleton and skull like ours. Were the earliest sapiens like us in every other way? To determine this, we have to look beyond biological evidence and try to assess their behaviour from clues they left behind. It seems reasonably clear that up to about 100,000 years ago, sapiens stone tools were much like those of the Neanderthals. However, at some point human behaviour began to change: tools became more varied, specialised and sophisticated; cave art and stones engraved with geometric designs began to appear along with necklaces and musical instruments; there is evidence for more permanent occupation of caves; and also for a wider range of food sources including marine fish as well as shellfish (which implies special tools to catch them). These changes didn't all happen at the same time - the earliest burial evidence dates from around 100,000 years ago with the changes becoming comprehensive by about 40,000 years ago - but the evidence leads some scientists to believe that genetic changes rewired the human brain during this period. Certainly the Cro-Magnons of 35,000 years ago, the first modern humans in Europe, exhibited the full range of such behaviours.

For me, some of the most fascinating questions concern the Neanderthals; how different were they from contemporary Cro-Magnons and why did they die out less than 30,000 years ago, after living in Europe for hundreds of thousands of years? Physically, they were stockier and more massively built than humans; their children developed more rapidly and the adults only lived to about 35-40 at best, with the middle-aged and old comprising a much smaller proportion of the population than with humans. There is evidence that they lived hard and injury-prone lives - the women as much as the men. Their tool technology was simple but they showed some indications of culture in their burials and in decorative items such as beads. Whether or not they wore clothes is unknown, but there is no evidence for the sewing and weaving which the Cro-Magnons possessed. However, it seems likely that they did use fur wraps and ponchos in the cold conditions in Europe (they were physically better adapted to cold than humans, but not that much better). Their foot bones do not indicate that they wore shoes - unlike those of Cro-Magnons. Their diet seems to have been more restricted - they were far more carnivorous than the ominvore humans.

Could Neanderthals speak? The shape of the throat indicates that speech would have been physically possible, albeit at about the same level as a human two-year old. They also have the same variant of the FOX2P gene as humans - for whom it is necessary for speech. So while we can never be certain, there seems to be no reason why they could not have had some kind of speech, albeit without the range and sophistication of humans.

So why did Neanderthals die out? Were they killed off by the more advanced newcomers, the Cro-Magnons? As the author points out, there was probably no single reason for their extinction. They died out at a time of great climate stress, with rapid fluctuations in temperatures. Their more restricted diet would have counted against them. The high death rate - possibly the result of having to close with their animal prey rather than using throwing weapons like the humans - would have taken its toll. Poorer communications due to more restricted language skills could have handicapped them. More subtly, the relative lack of older people would have made it more difficult to pass on acquired knowledge or to help with child care while the younger adults were out hunting. The evidence of long-term decline indicates that the Neanderthals may have been on the way out anyway, although competition for resources from the Cro-Magnons might well have played a part in finishing them off.

This raises another interesting question discussed in the book - the importance of population density. Evidence suggests that at various times during human development in Africa, relatively advanced technologies were employed, only to be lost. They kept having to reinvent the wheel (metaphorically speaking). The reason for this was probably that the small groups who developed the new technologies may have died out without passing on their knowledge, or been put under such survival stress that they stopped having the time to use them - resulting in their being forgotten. Only with a sufficiently high population density, plus frequent communication between groups, could new ideas be disseminated, preserved and built upon. The Neanderthals never enjoyed such advantages.

Perhaps the most striking detail in the book concerns "the Hobbit"; the tiny hominim from the Indonesian island of Flores, providing the name Homo floresiensis. Although the heat and humidity has effectively destroyed the DNA, the morphology of the remains indicates that the Hobbit was unrelated not only to modern humans, Neanderthals and Denisovans, but even to their precursor, Homo erectus. While there is still disagreement over its origins, Stringer suggests that it could be a dwarfed descendent of Homo habilis or even the australopithecines which dispersed from Africa over two million years ago (stone tools found on Flores are at least 800,000 years old). Yet the most recent remains of the Hobbit have been dated to about 18,000 years ago, only a few thousand years before modern humans colonised the island. In evolutionary terms, that's a blink of an eye. It sends a chill down my spine to think that we came that close to coexisting with such an early hominim.

Finally (in case you were wondering) yes, there is evidence that humanity is continuing to evolve - and at an accelerating rate. One surprise is that average human brain size has shrunk by about 10% over the last 20,000 years; whether this will continue is interesting to speculate, but brains are very energy-intensive to maintain and will shrink if they are used less. Perhaps our modern information and other technologies, which require us to do less memorising and even thinking, will accelerate this reduction? The profound changes in lifestyle over the last 10,000 years, with the spread of agriculture and urban population centres, have led to other changes, particularly the development of disease resistance and of adult lactose tolerance (among Africans and Europeans). Analysis of the human genome, and in particular the rate of mutations in DNA, have indicated that some 20% of our genes have come under selection pressure over this period. And this of course is without considering the potential of genetic engineering to alter humanity in the future.

As you may have gathered, I am highly impressed with this book and warmly recommend it to anyone interested in the subject. It does require a degree of concentration - although aiming for a popular audience the author thankfully hasn't dumbed down to the lowest common denominator - but it's well written and easy enough to follow.