The Ancestor's Tale Part 2

This is, in effect, what the evolutionary biologist Alan Templeton did, and he came up with his engagingly t.i.tled theory 'Out of Africa Again and Again'. Templeton used a type of coalescence theory, similar to that in our haemophilia discussion, but he did it for lots of separate genes instead of just one. This enabled him to reconstruct the history and geography of genes over the whole world and over hundreds of thousands of years. At the moment, I favour Templeton's 'Out of Africa Again and Again' theory, because he seems to me to use all the available information in a way that maximises its power to generate inferences; and because he bent over backwards, at every step of his work, to guard against overreaching the evidence.

Here is what Templeton did. He looked through the genetic literature, using strict criteria to skim the cream: he wanted only large studies of human genetics, where samples had been taken from different parts of the world, including Europe, Asia and Africa. The genes examined belonged to long-lived 'haplotypes'. A haplotype, as we have seen, is a chunk of genome which is either impervious to being broken up by s.e.xual recombination (as with Y-chromosomal and mitochondrial DNA), or (as with certain smaller parts of the genome) can be recognised intact through enough generations to cover the timescale of interest. A haplotype is a long-lived, recognisable chunk of genome. You don't go too far wrong if you think of it as a large 'gene'.

Templeton zeroed in on 13 haplotypes. For each of them, he calculated their 'gene tree', and dated the various coalescence points using the molecular clock which is ultimately calibrated with fossils. From these dates, and from the geographical distribution of the samples, he was able to pull out inferences about the genetic history of our species over the past couple of million years. He summarised his conclusions in a helpful diagram, reproduced on page 61 page 61.

Templeton's main conclusion is that there were not two major migrations out of Africa but three. In addition to the OOOA (h.o.m.o erectus (h.o.m.o erectus) exodus around 1.7 million years ago (which everyone accepts and for which the evidence is mostly from fossils) and the recent migration as promoted by the YOOA theory, there was another Great Trek from Africa to Asia between 840,000 and 420,000 years ago. This middle emigration shall we call it MOOA? is supported by extant 'signals' from three of the 13 haplotypes. The YOOA emigration is supported by mitochondrial and Y-chromosomal evidence. Other genetic 'signals' betray a major back-migration from Asia to Africa about 50,000 years ago. A little later, mitochondrial DNA and various smaller genes disclose other migrations: from southern to northern Europe, from southern Asia to northern Asia, across the Pacific and to Australia. Finally, as shown by mitochondrial DNA and archaeological evidence, North America was colonised across what was then the Bering land bridge from north-east Asia, around 14,000 years ago. Colonisation of South America through the Isthmus of Panama rapidly followed. The suggestion, by the way, that either Christopher Columbus or Leif Ericsson 'discovered' America is nothing short of racist. Equally distasteful, in my view, is relativist 'respect' for Native American oral histories which ignorantly deny that their ancestors ever lived outside America.

[image]

Out of Africa again and again. Templeton's summary of major human migrations, based on the study of 13 haplotypes. Vertical lines represent genetic descent; diagonal lines represent gene flow. The major human migrations indicated by genetic data are shown by the thick arrows. Adapted from Templeton [ Templeton's summary of major human migrations, based on the study of 13 haplotypes. Vertical lines represent genetic descent; diagonal lines represent gene flow. The major human migrations indicated by genetic data are shown by the thick arrows. Adapted from Templeton [284] (square brackets refer to sources in the Bibliography).

Between Templeton's three major migrations out of Africa, other genetic signals reveal continual eddies of gene flow back and forth between Africa, southern Europe and southern Asia. His evidence suggests that major and minor immigrations have usually been followed by some interbreeding with indigenous populations, rather than as might just as well have happened complete extermination of one side or the other. Clearly this has large implications for our evolutionary ancestry.

This tale, and Templeton's study, focused on humans and their genes. But of course all species have family trees. All species inherit genetic material. All species with two s.e.xes have an Adam and an Eve. Genes and gene trees are a ubiquitous feature of life on Earth. The techniques that we apply to recent human history can also be applied to the rest of life. Cheetah DNA reveals a 12,000-year-old population bottleneck important to feline conservationists. Maize DNA has stamped upon it the unmistakable signature of its 9,000-year Mexican domestication. The coalescence patterns of HIV strains can be used by epidemiologists and medical doctors to understand and contain the virus. Genes and gene trees reveal the history of the flora and fauna of Europe: the vast migrations driven by ice ages whose waxing pushed temperate species into southern-European refuges, and whose waning stranded Arctic species on isolated mountain ranges. All these events and more can be traced in the distribution of DNA around the globe, a historical reference book which we are only just learning to read.

We have seen how different genes have different stories to tell, which can be pieced together to reveal something of our history, both modern and ancient. How ancient? Amazingly, our oldest MRCA genes can even date back before we were human at all. This is especially so when natural selection favours variety in the population for its own sake. Here's how it works.

Suppose there are two blood types called A and B, which confer immunity to different diseases. Each blood type is susceptible to the disease against which the other type has immunity. Diseases flourish when the blood type that they can attack is abundant, because an epidemic can get going. So if B people, say, happen to be common in the population, the disease that hurts them will enjoy an epidemic. Consequently, B people will die until they cease to be common, and the A people increase and vice versa. Whenever we have two types, the rarer of which is favoured because it is rare, it is a recipe for polymorphism: polymorphism: the positive maintenance of variety for variety's sake. The ABO blood group system is a famous polymorphism which has probably been maintained for this kind of reason. the positive maintenance of variety for variety's sake. The ABO blood group system is a famous polymorphism which has probably been maintained for this kind of reason.

Some polymorphisms can be quite stable so stable that they span the change from an ancestral to a descendant species. Astonis.h.i.+ngly, our ABO polymorphism is present in chimpanzees. It could be that we and chimps have independently 'invented' the polymorphism, and for the same reason. But it is more plausible that we have both inherited it from our shared ancestor, and independently kept it going during our six million years of separate descent, because the relevant diseases have been continuously at large throughout that time. This is called trans-specific polymorphism, and it may apply to far more distant cousins than chimpanzees are to us.

A stunning conclusion is that, for particular genes, you are more closely related to some chimpanzees than to some humans. And I am closer to some chimpanzees than to you (or to 'your' chimpanzees). Humans as a species, as well as humans as individuals, are temporary vessels containing a mix of genes from different sources. Individuals are temporary meeting points on the crisscrossing routes that genes take through history. This is a tree-based way to express the central message of The Selfish Gene The Selfish Gene, my first book. As I put it there, 'When we have served our purpose we are cast aside. But genes are denizens of geological time: genes are forever.' At the concluding banquet to a conference in America, I recited the same message in verse: An itinerant selfish geneSaid 'Bodies a-plenty I've seen.You think you're so cleverBut I'll live for ever.You're just a survival machine.'

And, as the body's immediate reply to the gene, I parodied the very same Harp Song of the Dane Women Harp Song of the Dane Women quoted previously: quoted previously: What is a body that first you take her,Grow her up, and then forsake her,To go with the old blind watchmaker?

We estimated the date of Rendezvous 0 Rendezvous 0 as probably tens of thousands of years ago, and at most hundreds of thousands. We have not travelled far on our backward pilgrimage. The next rendezvous, our meeting with the chimpanzee pilgrims at as probably tens of thousands of years ago, and at most hundreds of thousands. We have not travelled far on our backward pilgrimage. The next rendezvous, our meeting with the chimpanzee pilgrims at Rendezvous 1 Rendezvous 1, is millions of years away, and most of our rendezvous are hundreds of millions beyond that. To stand a chance of completing our pilgrimage, we shall need to speed up, and begin the move into 'deep time'. We must accelerate past the rest of the 30 or so ice ages that punctuate the last three million years, past such drastic events as the drying and refilling of the Mediterranean that occurred between 4.5 and 6 million years ago. To ease this initial acceleration, I shall take the otherwise unusual liberty of stopping at a few intermediate milestones en route en route, and allowing dead fossils to tell tales. The fossilised 'shadow' pilgrims we shall meet, and the tales they tell, will help satisfy our natural preoccupation with our direct ancestors.

1 When his team went on to decipher the dog genome, it was no surprise to discover that the individual honoured was Dr Venter's own poodle, Shadow. When his team went on to decipher the dog genome, it was no surprise to discover that the individual honoured was Dr Venter's own poodle, Shadow.

2 With characteristic prescience, the great statistician and evolutionary geneticist Sir Ronald Fisher (18901962) wrote the following, in a letter dated 15 January 1929 to Major Leonard Darwin (18501943, Charles's second youngest son): 'King Solomon lived 100 generations ago, and his line may be extinct; if not, I wager he is in the ancestry of all of us, and in nearly equal proportions, however unequally his wisdom may be distributed.' In J. H. Bennett, (Ed. 1983) With characteristic prescience, the great statistician and evolutionary geneticist Sir Ronald Fisher (18901962) wrote the following, in a letter dated 15 January 1929 to Major Leonard Darwin (18501943, Charles's second youngest son): 'King Solomon lived 100 generations ago, and his line may be extinct; if not, I wager he is in the ancestry of all of us, and in nearly equal proportions, however unequally his wisdom may be distributed.' In J. H. Bennett, (Ed. 1983) Natural Selection, Heredity and Eugenics Natural Selection, Heredity and Eugenics. Oxford: Clarendon Press, p. 95.

3 Just after the first print run of this book, a paper by Rohde, Olsen and Chang was published in Just after the first print run of this book, a paper by Rohde, Olsen and Chang was published in Nature Nature magazine (Vol. 431, p. 562) suggesting that magazine (Vol. 431, p. 562) suggesting that Rendezvous 0 Rendezvous 0 occurred a mere 3,500 years ago, even more recently than I dared hope. They too come to the conclusion that Concestor 0 was probably Asian. occurred a mere 3,500 years ago, even more recently than I dared hope. They too come to the conclusion that Concestor 0 was probably Asian.

4 For explanation of the inverted commas around 'black', see the Gra.s.shopper's Tale. For explanation of the inverted commas around 'black', see the Gra.s.shopper's Tale.

ARCHAIC h.o.m.o SAPIENS h.o.m.o SAPIENS.

Our first milestone on the way back to Rendezvous 1 Rendezvous 1 is in the depths of the ice age before last, about 160,000 years ago. I have chosen this way station to look at fossil finds from Herto in the Afar depression of Ethiopia. is in the depths of the ice age before last, about 160,000 years ago. I have chosen this way station to look at fossil finds from Herto in the Afar depression of Ethiopia.1 The Herto humans are intriguing because, in the words of their discoverers, Tim White and his colleagues, they are from a 'population that is on the verge of anatomical modernity but not yet fully modern'. The distinguished palaeoanthropologist Christopher Stringer regards 'the Herto material as the oldest definite record of what we currently think of as modern The Herto humans are intriguing because, in the words of their discoverers, Tim White and his colleagues, they are from a 'population that is on the verge of anatomical modernity but not yet fully modern'. The distinguished palaeoanthropologist Christopher Stringer regards 'the Herto material as the oldest definite record of what we currently think of as modern H. sapiens H. sapiens', a record previously held by younger Middle Eastern fossils dating from about 100,000 years ago. Regardless of hair-splitting distinctions between 'modern' and 'nearly modern', it is clear that the Herto people are on the cusp between modern humans and those predecessors that we know by the catch-all name of 'Archaic h.o.m.o sapiens h.o.m.o sapiens'. Certain authorities use this name back to about 900,000 years ago where it grades into an earlier species, h.o.m.o erectus h.o.m.o erectus. As we shall see, others prefer to give various Latin names to the bridging archaic forms. I shall sidestep the disputes by using anglicisms in the style of my colleague Jonathan Kingdon: 'Moderns', 'Archaics', 'Erects', and others that I'll mention as we come to them. We should not expect to draw a neat line between early Archaics and the Erects from whom they evolved, or between Archaics and the earliest Moderns who evolved from them. Don't be confused, incidentally, by the fact that the Erects were even more archaic (with a small a) than the Archaics (with a large A), and that all three types were erect with a small e!

Archaic forms persisted alongside Modern forms until at least 100,000 years ago (longer still if we include the Neanderthals, of whom more in a moment). Archaic fossils are found all around the world, dating from various times during the last few hundred thousand years: examples are the German 'Heidelberg man', 'Rhodesian man' from Zambia (which used to be called Northern Rhodesia), and the Chinese 'Dali man'. Archaics had big brains like us, averaging 1,200 to 1,300 cubic centimetres. This is a little smaller than our average of 1,400 cubic centimetres but the range comfortably overlaps with ours. Their bodies were more robust than ours, their skulls were thicker, and they had more p.r.o.nounced brow ridges and less p.r.o.nounced chins. They looked more like Erects than we do, and hindsight justly sees them as intermediate. Some taxonomists recognise them as a subspecies of h.o.m.o sapiens h.o.m.o sapiens called called h.o.m.o sapiens heidelbergensis h.o.m.o sapiens heidelbergensis (where we would be (where we would be h.o.m.o sapiens sapiens h.o.m.o sapiens sapiens). Others do not recognise the Archaics as h.o.m.o sapiens h.o.m.o sapiens at all, but call them at all, but call them h.o.m.o heidelbergensis h.o.m.o heidelbergensis. Yet others divide the Archaics into more than one species, for instance h.o.m.o heidelbergensis, h.o.m.o rhodesiensis h.o.m.o heidelbergensis, h.o.m.o rhodesiensis, and h.o.m.o antecessor h.o.m.o antecessor. If you think about it, we should be worried if there was not not disagreement over the divisions. On the evolutionary view of life, a continuous range of intermediates is to be expected. disagreement over the divisions. On the evolutionary view of life, a continuous range of intermediates is to be expected.

Modern h.o.m.o sapiens sapiens h.o.m.o sapiens sapiens are not the only offshoot of the Archaics. Another species of advanced humans, the so-called Neanderthals, were our contemporaries for much of our prehistory. They resembled the Archaics more than we do in some respects, and they seem to have emerged from an Archaic root between about one and two hundred thousand years ago in this case not in Africa but in Europe and the Middle East. Fossils from these regions show a gradual transition from Archaics to Neanderthals with the first unequivocal Neanderthal fossils found just before the beginning of the last Ice Age, about 130,000 years ago. They then persisted in Europe for most of this cold period, vanis.h.i.+ng about 28,000 years ago. In other words, for their entire existence Neanderthals were contemporaries of European Modern emigres from Africa. Some people believe that Moderns were responsible for their extinction, either by killing them directly or by competing with them. are not the only offshoot of the Archaics. Another species of advanced humans, the so-called Neanderthals, were our contemporaries for much of our prehistory. They resembled the Archaics more than we do in some respects, and they seem to have emerged from an Archaic root between about one and two hundred thousand years ago in this case not in Africa but in Europe and the Middle East. Fossils from these regions show a gradual transition from Archaics to Neanderthals with the first unequivocal Neanderthal fossils found just before the beginning of the last Ice Age, about 130,000 years ago. They then persisted in Europe for most of this cold period, vanis.h.i.+ng about 28,000 years ago. In other words, for their entire existence Neanderthals were contemporaries of European Modern emigres from Africa. Some people believe that Moderns were responsible for their extinction, either by killing them directly or by competing with them.

Neanderthal2 anatomy was sufficiently different from ours that some people prefer to give them a separate species name, anatomy was sufficiently different from ours that some people prefer to give them a separate species name, h.o.m.o neanderthalensis h.o.m.o neanderthalensis. They retained some features of Archaics such as large brow ridges which Moderns did not (which is why some authorities cla.s.sify them as just another type of Archaic). Adaptations to their cold environment include stockiness, short limbs and enormous noses, and they surely must have been warmly clothed, presumably in animal furs. Their brains were as big as ours or even bigger. Much is made of slight indications that they ceremonially buried their dead. n.o.body knows whether they could speak, and opinions differ on this important question. Archaeology hints that technological ideas may have pa.s.sed both ways between Neanderthals and Moderns, but this could have been by imitation rather than by language.

The rules for the pilgrimage stated that only modern animals setting off from the present were ent.i.tled to tell tales. We are making an exception for the dodo and the elephant bird, because they lived in recent historical times. And the fossils h.o.m.o erectus h.o.m.o erectus and and h.o.m.o habilis h.o.m.o habilis qualify as 'shadow pilgrims' because a plausible case could be made that they are our direct ancestors. Do the Neanderthals, too, qualify under this rubric? Are we descended from them? Well, as it happens, that very question is the topic of the tale that the Neanderthals want to tell. Think of the Neanderthal's Tale as a plea to be allowed to tell it. qualify as 'shadow pilgrims' because a plausible case could be made that they are our direct ancestors. Do the Neanderthals, too, qualify under this rubric? Are we descended from them? Well, as it happens, that very question is the topic of the tale that the Neanderthals want to tell. Think of the Neanderthal's Tale as a plea to be allowed to tell it.

THE NEANDERTHAL'S TALE.

Written with Yan Wong.

Are we descended from Neanderthals? If so, they would have to have interbred with h.o.m.o sapiens sapiens h.o.m.o sapiens sapiens. But did they? They overlapped for a long time in Europe, and there was surely contact between them. But did it go beyond contact? Do modern Europeans inherit any Neanderthal genes? This is a hotly debated issue, recently reignited by a remarkable extraction of DNA from late Neanderthal bones. So far, we have extracted only the maternally inherited mitochondrial DNA, but this is enough for a tentative verdict. Neanderthal mitochondria are quite distinct from those of all surviving humans, suggesting that Neanderthals are no closer to Europeans than to any other modern peoples. In other words, the female-line common ancestor of Neanderthals and all surviving humans long pre-dates Mitochondrial Eve: about 500,000 years as opposed to 140,000. This genetic evidence suggests that successful interbreeding between Neanderthals and Moderns was rare. And so it is often said that they died out without leaving any descendants.

But don't let's forget that '80 per cent' argument which so surprised us in the Tasmanian's Tale. A single immigrant who managed to break into the Tasmanian breeding population had an 80 per cent chance of joining the set of universal ancestors: the set of individuals who could call themselves ancestors of all surviving Tasmanians in the distant future. By the same token, if only one Neanderthal male, say, bred into a sapiens sapiens population, that gave him a reasonable chance of being a common ancestor to all Europeans alive today. This can be true even if Europeans contain no Neanderthal genes at all. A striking thought. population, that gave him a reasonable chance of being a common ancestor to all Europeans alive today. This can be true even if Europeans contain no Neanderthal genes at all. A striking thought.

So although few, if any, of our genes come from Neanderthals, it is possible that some people have many Neanderthal ancestors. This was the distinction we met in Eve's Tale between gene trees and people trees. Evolution is governed by the flow of genes, and the moral of the Neanderthal's Tale, if we allow him to tell it, is that we cannot, should not, look at evolution in terms of pedigrees of individuals. Of course individuals are important in all sorts of other ways, but if we are talking pedigrees it is gene trees that count. The words 'evolutionary descent' refer to gene ancestors, not genealogical ancestors.

Fossil changes too are a reflection of gene pedigrees, not (or only incidentally) genealogical pedigrees. Fossils indicate that Modern anatomy pa.s.sed to the rest of the world via young out-of-Africa migrations. But Alan Templeton's work (described in Eve's Tale) suggests that we are also partly 'descended from' non-African Archaics, possibly even non-African h.o.m.o erectus h.o.m.o erectus. The description is both simpler and more powerful if we switch from people talk to gene talk. The genes that determine our Modern anatomy were carried out of Africa by the YOOA migrants, leaving fossils in their wake. At the same time, Templeton's evidence suggests that other genes we now possess were flowing around the world by different routes, but left little anatomical evidence to show for it. Most of our genes probably took the young out-of-Africa route, while just a few came to us through other routes. What could be a more powerful way to express it?

So, have the Neanderthals established their right to tell a tale? Maybe a tale of genealogy if not a tale of genes.

1 The same 'Afar' after which the much older The same 'Afar' after which the much older Australopithecus afarensis Australopithecus afarensis, or Lucy, is named.

2 Pedants' Corner: Pedants' Corner: Thal Thal or, in modern German, or, in modern German, Tal Tal, means valley. The Neander Valley is where the first fossil of this type was discovered. When German spelling was reformed at the end of the nineteenth century, the valley changed from a Thal Thal to a to a Tal Tal, but the Latin name, h.o.m.o neanderthalensis h.o.m.o neanderthalensis, was left high and dry, trapped by the laws of zoological terminology. To conform with tradition and with the Latin, I prefer to leave the English spelling in its original form and stick with the h.

ERGASTS.

Moving deeper in time, we touch down again at one million years in search of ancestors. The only likely candidates of this age are of the type usually called h.o.m.o erectus h.o.m.o erectus, although some would call the African ones h.o.m.o ergaster h.o.m.o ergaster and I shall follow them. In seeking an anglicised form for these creatures, I shall call them Ergasts rather than Erects, partly because I believe the majority of our genes trace back to the African form, and partly because, as I've already remarked, they were no more erect than their predecessors ( and I shall follow them. In seeking an anglicised form for these creatures, I shall call them Ergasts rather than Erects, partly because I believe the majority of our genes trace back to the African form, and partly because, as I've already remarked, they were no more erect than their predecessors (h.o.m.o habilis) or their successors (us). Whatever name we prefer, the Ergast type persisted from about 1.8 million until about a quarter of a million years ago. They are widely accepted as the immediate predecessors, and partial contemporaries, of the Archaics who are in turn the predecessors of us Moderns.

The Ergasts were noticeably different from modern h.o.m.o sapiens h.o.m.o sapiens, and, unlike the Archaic sapiens sapiens people, they differed from us in some respects that show no overlap. Fossil finds show they lived in the Middle East and Far East including Java, and represent an ancient migration out of Africa. You may have heard them referred to by their old names of Java Man and Peking Man. In Latin, before they were admitted into the people, they differed from us in some respects that show no overlap. Fossil finds show they lived in the Middle East and Far East including Java, and represent an ancient migration out of Africa. You may have heard them referred to by their old names of Java Man and Peking Man. In Latin, before they were admitted into the h.o.m.o h.o.m.o fold, they had the generic names fold, they had the generic names Pithecanthropus Pithecanthropus and and Sinanthropus Sinanthropus. They walked on two legs like us, but had smaller brains (900 cc in early specimens to 1,100 cc in late ones), housed in lower, less domed, more 'swept-back' skulls than ours, and they had receding chins. Their jutting brow ridges made a p.r.o.nounced horizontal ledge above the eyes, set in wide faces, with a pinching in of the skull behind the eyes.

Hair doesn't fossilise, so there is no natural place in our history to discuss the obvious fact that at some point in our evolution we lost most of our body hair, with the luxuriant exception of the tops of our heads. Very likely the Ergasts were hairier than us, but we can't rule out the possibility that Ergasts had already lost their body hair by a million years ago. They could have been as hairless as we are. Equally, n.o.body should complain of an imaginative reconstruction as hairy as a chimpanzee, or any intermediate level of s.h.a.gginess. Modern people, males at least, remain quite variable in how hairy they are. Hairiness is one of those characteristics that can increase or decrease in evolution again and again. Vestigial hairs, with their a.s.sociated cellular support structures, lurk in even the barest-seeming skin, ready to evolve into a full coat of thick hair at short notice (or shrink again) should natural selection at any time call them out of retirement. Look at the woolly mammoths and woolly rhinoceroses that rapidly evolved in response to the recent ice ages in Eurasia. We shall return to the evolutionary loss of human hair in strangely enough the Peac.o.c.k's Tale.

Subtle evidence of repeatedly used hearths suggests that at least some groups of Ergasts discovered the use of fire with hindsight a momentous event in our history. The evidence is less conclusive than we might hope. Blackening from soot and charcoal does not survive immense timespans, but fires leave other traces that last longer. Modern experimenters have systematically constructed fires of various kinds and then examined them afterwards for their trace effects. It emerges that deliberately built campfires magnetise the soil in a way that distinguishes them from bushfires and from burnt-out tree stumps I don't know why. But such signs provide evidence that Ergasts, both in Africa and Asia, had campfires nearly one and a half million years ago. This doesn't have to mean that they knew how to light fires. They could have begun by capturing and tending naturally occurring fire, feeding it and keeping it alive as one might look after a Tamagochi pet. Maybe, before they began to cook food, they used fires to scare away dangerous animals and provide light, heat, and a social focus.

The Ergasts also shaped and used stone tools, and presumably wooden and bone ones too. n.o.body knows whether they could speak, and evidence is hard to come by. You might think that 'hard to come by' is an understatement, but we have now reached a point in our backward journey when fossil evidence starts to tell. Just as campfires leave traces in the soil, so the needs of speech call forth tiny changes in the skeleton: nothing so dramatic as the hollow bony box in the throat with which the howler monkeys of the South American forests amplify their stentorian voices, but still telltale signs such as one might hope to detect in a few fossils. Unfortunately, the signs that have been unearthed are not telltale enough to settle the matter, and it remains controversial.

There are two parts of the modern human brain which seem to go with speech. When in our history did these parts Broca's Area and Wernicke's Area enlarge? The nearest approach we have to fossil brains is endocasts, to be described in the Ergast's Tale. Unfortunately the lines dividing different regions of the brain do not fossilise very clearly, but some experts think they can say that the speech areas of the brain were already enlarged before two million years ago. Those who want to believe that Ergasts possessed the power of speech are encouraged by this evidence.

They are discouraged, however, when they move down the skeleton. The most complete h.o.m.o ergaster h.o.m.o ergaster we know is the Turkana Boy, who died near Lake Turkana, in Kenya, about 1.5 million years ago. His ribs, and the small size of the portholes in the vertebrae through which the nerves pa.s.s, suggest that he lacked the fine control over breathing that seems to be a.s.sociated with speech. Other scientists, studying the base of the skull, have concluded that even Neanderthals, as recently as 60,000 years ago, were speechless. The evidence is that their throat shape would not have allowed the full range of vowels that we deploy. On the other hand, as the linguist and evolutionary psychologist Steven Pinker has remarked, 'e lengeege weth e smell nember ef vewels cen remeen quete expresseve'. If written Hebrew can be intelligible without vowels, I don't see why spoken Neander or even Ergaster couldn't too. The veteran South African anthropologist Phillip Tobias suspects that language may pre-date even we know is the Turkana Boy, who died near Lake Turkana, in Kenya, about 1.5 million years ago. His ribs, and the small size of the portholes in the vertebrae through which the nerves pa.s.s, suggest that he lacked the fine control over breathing that seems to be a.s.sociated with speech. Other scientists, studying the base of the skull, have concluded that even Neanderthals, as recently as 60,000 years ago, were speechless. The evidence is that their throat shape would not have allowed the full range of vowels that we deploy. On the other hand, as the linguist and evolutionary psychologist Steven Pinker has remarked, 'e lengeege weth e smell nember ef vewels cen remeen quete expresseve'. If written Hebrew can be intelligible without vowels, I don't see why spoken Neander or even Ergaster couldn't too. The veteran South African anthropologist Phillip Tobias suspects that language may pre-date even h.o.m.o ergaster h.o.m.o ergaster, and he may just possibly be right. As we have seen, there are a few who go to the opposite extreme and date the origin of language to the Great Leap Forward, just a few tens of thousands of years ago.

This may be one of those disagreements that can never be resolved. All considerations of the origin of language begin by citing the Linguistic Society of Paris which, in 1866, banned discussion of the question because it was deemed unanswerable and futile. It may be difficult to answer, but it is not in principle unanswerable like some philosophical questions. Where scientific ingenuity is concerned, I am an optimist. Just as continental drift is now sewn up beyond all doubt, with multiple threads of convincing evidence, and just as DNA fingerprinting can establish the exact source of a bloodstain with a confidence that forensic experts could once only dream of, I guardedly expect that scientists will one day discover some ingenious new method of establis.h.i.+ng when our ancestors started to speak.

Even I, however, have no hope that we shall ever know what they said to each other, or the language in which they said it. Did it begin with pure words and no grammar: the equivalent of an infant babbling nounspeak? Or did grammar come early and which is not impossible and not even silly suddenly? Perhaps the capacity for grammar was already deep in the brain, being used for something else like mental planning. Is it even possible that grammar, as applied to communication at least, was the sudden invention of a genius? I doubt it, but in this field I wouldn't rule anything out with confidence.

As a small step towards finding out the date at which language arose, some promising genetic evidence has appeared. A family codenamed KE suffers from a strange hereditary defect. Out of approximately 30 family members spread over three generations, about half are normal, but fifteen show a curious linguistic disorder, which seems to affect both speech and understanding. It has been called verbal dyspraxia, and it first shows itself as an inability to articulate clearly in childhood. Other authorities think the trouble stems from 'feature blindness', meaning an inability to grasp certain grammatical features such as gender, tense and number. What is clear is that the abnormality is genetic. Individuals either have it or they don't, and it is a.s.sociated with a mutation of an important gene called FOXP2 FOXP2, which the rest of us have in unmutated form. Like most of our genes, a version of FOXP2 FOXP2 is present in mice and other species, and it probably does various things in the brain and elsewhere. is present in mice and other species, and it probably does various things in the brain and elsewhere.1 The evidence of the KE family suggests that in humans The evidence of the KE family suggests that in humans FOXP2 FOXP2 is important for the development of some part of the brain that is involved in language. is important for the development of some part of the brain that is involved in language.

So, we naturally want to compare human FOXP2 FOXP2 with the same gene in animals that lack language. You can compare genes either by looking at the DNA sequences themselves, or by looking at the amino acid sequences in the proteins that they encode. There are times when it makes a difference, and this is one of them. with the same gene in animals that lack language. You can compare genes either by looking at the DNA sequences themselves, or by looking at the amino acid sequences in the proteins that they encode. There are times when it makes a difference, and this is one of them. FOXP2 FOXP2 codes for a protein chain 715 amino acids long. The mouse and chimpanzee versions of the gene differ in only one amino acid. The human version differs from both these animals in an additional two amino acids. You see what this might mean? Although humans and chimpanzees share the great majority of their evolution and their genes, the codes for a protein chain 715 amino acids long. The mouse and chimpanzee versions of the gene differ in only one amino acid. The human version differs from both these animals in an additional two amino acids. You see what this might mean? Although humans and chimpanzees share the great majority of their evolution and their genes, the FOXP2 FOXP2 gene is one place where humans seem to have evolved rapidly in the short time since we split from them. And one of the most important respects in which we differ from chimpanzees is that we have language and they don't. A gene that changed somewhere along the line towards us, but after the separation from chimpanzees, is exactly the sort of gene we should be looking for if we are trying to understand the evolution of language. And it is the very same gene that has mutated in the unfortunate KE family (and also, in a different way, in a wholly unrelated individual with the same kind of language defect). Perhaps it was changes in gene is one place where humans seem to have evolved rapidly in the short time since we split from them. And one of the most important respects in which we differ from chimpanzees is that we have language and they don't. A gene that changed somewhere along the line towards us, but after the separation from chimpanzees, is exactly the sort of gene we should be looking for if we are trying to understand the evolution of language. And it is the very same gene that has mutated in the unfortunate KE family (and also, in a different way, in a wholly unrelated individual with the same kind of language defect). Perhaps it was changes in FOXP2 FOXP2 that made humans, as opposed to chimpanzees, capable of language. Did the Ergasts have the mutated that made humans, as opposed to chimpanzees, capable of language. Did the Ergasts have the mutated FOXP2 FOXP2 gene? gene?

Wouldn't it be wonderful if we could use this genetic hypothesis to date the origin of language in our ancestors? While we can't do it with certainty, we can do something quite suggestive, along these lines. The obvious approach would be to triangulate backwards from variants among modern humans, and try to calculate the antiquity of the FOXP2 FOXP2 gene. But with the exception of rare unfortunates like the members of the KE family, there is no variation among humans in any of the gene. But with the exception of rare unfortunates like the members of the KE family, there is no variation among humans in any of the FOXP2 FOXP2 amino acids. So there isn't enough variation there to triangulate from. Luckily, however, there are other parts of the gene which are never translated into protein and which are therefore free to mutate without natural selection 'noticing': they are 'silent' code letters, in those parts of the gene that are never transcribed and are called introns (as opposed to 'exons' which are 'expressed' and therefore 'seen' by natural selection). The silent letters, unlike the expressed ones, are quite variable among individual humans, and between humans and chimpanzees. We can get some understanding of the evolution of the gene if we look at the patterns of variation in the silent areas. Even though the silent letters are not subject to natural selection themselves, they can be swept along by selection of neighbouring exons. Even better, the mathematically a.n.a.lysed pattern of variation in the silent introns gives a good indication of amino acids. So there isn't enough variation there to triangulate from. Luckily, however, there are other parts of the gene which are never translated into protein and which are therefore free to mutate without natural selection 'noticing': they are 'silent' code letters, in those parts of the gene that are never transcribed and are called introns (as opposed to 'exons' which are 'expressed' and therefore 'seen' by natural selection). The silent letters, unlike the expressed ones, are quite variable among individual humans, and between humans and chimpanzees. We can get some understanding of the evolution of the gene if we look at the patterns of variation in the silent areas. Even though the silent letters are not subject to natural selection themselves, they can be swept along by selection of neighbouring exons. Even better, the mathematically a.n.a.lysed pattern of variation in the silent introns gives a good indication of when when the sweeps of natural selection occurred. And the answer for the sweeps of natural selection occurred. And the answer for FOXP2 FOXP2 is less than 200,000 years ago. A naturally selected change to the human version of is less than 200,000 years ago. A naturally selected change to the human version of FOXP2 FOXP2 seems roughly to coincide with the change from archaic seems roughly to coincide with the change from archaic h.o.m.o sapiens h.o.m.o sapiens to anatomically modern to anatomically modern h.o.m.o sapiens h.o.m.o sapiens. Could this be when language was born? The margin of error in this sort of calculation is wide, but this ingenious genetic evidence counts as a vote against the theory that h.o.m.o ergaster h.o.m.o ergaster could talk. More importantly for me, the unexpected new method boosts my optimism that one day science will find a way to confound the pessimists of the Linguistic Society of Paris. could talk. More importantly for me, the unexpected new method boosts my optimism that one day science will find a way to confound the pessimists of the Linguistic Society of Paris.

h.o.m.o ergaster is the first fossil ancestor we have met on our pilgrimage who is unequivocally of a different species from ourselves. We are about to embark on a portion of the pilgrimage in which fossils provide the most important evidence, and they will continue to bulk large though they will never overwhelm molecular evidence until we reach extremely ancient times and relevant fossils start to peter out. It is a good moment to look in more detail at fossils, and how they are formed. The Ergast will tell the tale. is the first fossil ancestor we have met on our pilgrimage who is unequivocally of a different species from ourselves. We are about to embark on a portion of the pilgrimage in which fossils provide the most important evidence, and they will continue to bulk large though they will never overwhelm molecular evidence until we reach extremely ancient times and relevant fossils start to peter out. It is a good moment to look in more detail at fossils, and how they are formed. The Ergast will tell the tale.

THE ERGAST'S TALE.

Richard Leakey movingly describes the discovery, by his colleague Kimoya Kimeu on 22 August 1984, of the Turkana Boy (h.o.m.o ergaster), at 1.5 million years the oldest near-complete hominid skeleton ever found. Equally moving is Donald Johanson's description of the older, and unsurprisingly less complete, australopithecine familiarly known as Lucy. The discovery of 'Little Foot', yet to be fully described, is just as remarkable (see page 92). Whatever freak conditions blessed Lucy, 'Little Foot', and the Turkana Boy with their version of immortality, would we not wish it for ourselves when our time comes? What hurdles must we cross to achieve this ambition? How does any fossil come to be formed? This is the subject of the Ergast's Tale. To begin, we need a small digression into geology.

Rocks are built of crystals, though these are often too small for the unaided eye to see. A crystal is a single giant molecule, its atoms arranged in an orderly lattice with a regular s.p.a.cing pattern repeated billions of times until, eventually, the edge of the crystal is reached. Crystals grow when atoms come out of the liquid state and build up on the expanding edge of an existing crystal. The liquid is usually water. On other occasions, it is not a solvent at all but the molten mineral itself. The shape of the crystal, and the angles at which its plane facets meet, is a direct rendition, in the large, of the atomic lattice. The lattice shape is sometimes projected very large indeed, as in a diamond or amethyst whose facets betray to the naked eye the three-dimensional geometry of the self-a.s.sembled atomic arrays. Usually, however, the crystalline units of which rocks are made are too small for the eye to detect them, which is one reason why most rocks are not transparent. Among important and common rock crystals are quartz (silicon dioxide), feldspars (mostly silicon dioxide again, but some of the silicon atoms are replaced by aluminium atoms), and calcite (calcium carbonate). Granite is a densely packed mixture of quartz, feldspar and mica, crystallised out of molten magma. Limestone is mostly calcite, sandstone mostly quartz, in both cases ground small and then compacted from sediments of sand or mud.

Igneous rocks begin as cooled lava (which in turn is molten rock). Often, as with granite, they are crystalline. Sometimes their shape may be visibly that of a gla.s.s-like solidified liquid and, with great good fortune, molten lava may sometimes be cast in a natural mould, such as a dinosaur's footprint or an empty skull. But the main usefulness of igneous rock to historians of life is in dating. As we shall see in the Redwood's Tale, the best dating methods are available for igneous rocks alone. Fossils usually cannot be precisely dated themselves, but we can look for igneous rocks in the vicinity. We then either a.s.sume that the fossil is contemporaneous, or we seek two datable igneous samples that sandwich our fossil and fix upper and lower bounds to its date. This sandwich dating is open to the slight risk that a corpse has been carried by floodwater, or by hyenas or their dinosaur equivalents, to an anachronistic site. With luck this will usually be obvious; otherwise we have to fall back on consistency with a general statistical pattern. rocks begin as cooled lava (which in turn is molten rock). Often, as with granite, they are crystalline. Sometimes their shape may be visibly that of a gla.s.s-like solidified liquid and, with great good fortune, molten lava may sometimes be cast in a natural mould, such as a dinosaur's footprint or an empty skull. But the main usefulness of igneous rock to historians of life is in dating. As we shall see in the Redwood's Tale, the best dating methods are available for igneous rocks alone. Fossils usually cannot be precisely dated themselves, but we can look for igneous rocks in the vicinity. We then either a.s.sume that the fossil is contemporaneous, or we seek two datable igneous samples that sandwich our fossil and fix upper and lower bounds to its date. This sandwich dating is open to the slight risk that a corpse has been carried by floodwater, or by hyenas or their dinosaur equivalents, to an anachronistic site. With luck this will usually be obvious; otherwise we have to fall back on consistency with a general statistical pattern.

Sedimentary rocks such as sandstone and limestone are formed from tiny fragments, ground by wind or water from earlier rocks or other hard materials such as sh.e.l.ls. They are carried in suspension, as sand, silt or dust, and deposited somewhere else, where they settle and compact themselves over time into new layers of rock. Most fossils lie in sedimentary beds. rocks such as sandstone and limestone are formed from tiny fragments, ground by wind or water from earlier rocks or other hard materials such as sh.e.l.ls. They are carried in suspension, as sand, silt or dust, and deposited somewhere else, where they settle and compact themselves over time into new layers of rock. Most fossils lie in sedimentary beds.

It is in the nature of sedimentary rock that its materials are continually being recycled. Old mountains such as the Scottish Highlands have been slowly ground down by wind and water, yielding materials which later settle into sediments and may ultimately push up again somewhere else as new mountains like the Alps, and the cycle resumes. In a world of such recycling, we have to curb our importunate demands for a continuous fossil record to bridge every gap in evolution. It isn't just bad luck that fossils are often missing, but an inherent consequence of the way sedimentary rocks are made. It would be positively worrying if there were no gaps in the fossil record. Old rocks, with their fossils, are actively being destroyed by the very process that goes to make new ones.

Often fossils are formed when mineral-charged water penetrates the fabric of a buried creature. In life, bone is porous and spongy, for good engineering and economic reasons. When water seeps through the interstices of a dead bone, minerals are slowly deposited as the ages pa.s.s. I say slowly almost as a ritual, but it isn't always slow. Think how fast a kettle furs up. On an Australian beach I once found a bottletop embedded in stone. But the process usually is slow. Whatever the speed, the stone of a fossil eventually takes on the shape of the original bone, and that shape is revealed to us millions of years later, even if which doesn't always happen every atom of the original bone has disappeared. The petrified forest in the Painted Desert of Arizona consists of trees whose tissues were slowly replaced by silica and other minerals leached out of ground water. Two hundred million years dead, the trees are now stone through and through, but many of their microscopic cellular details can still be clearly seen in petrified form.

I've already mentioned that sometimes the original organism, or a part of it, forms a natural mould or imprint from which it is subsequently removed, or dissolved. I fondly recall two happy days in Texas in 1987 spent wading through the Paluxy River examining, and even putting my feet in, the dinosaur footprints preserved in its smooth limestone bed. A bizarre local legend grew up that some of these are giant manprints contemporary with undoubted dinosaur prints, and in consequence the nearby town of Glen Rose became home to a thriving cottage industry, artlessly faking giant manprints in blocks of cement (for sale to gullible creationists who know, all too well, that 'There were giants in the earth in those days': Genesis 6:4). The story of the real footprints has been carefully worked out, and is fascinating. The obviously dinosaurian ones are three-toed. The ones that look faintly like a human foot have no toes, and were made by dinosaurs walking on the back of the foot rather than running on their toes. Also, the viscous mud would have tended to ooze back in at the sides of the footprint, obscuring the side toes of the dinosaurs.

More poignant for us, at Laetoli in Tanzania are the companionable footprints of three real hominids, probably Australopithecus afarensis Australopithecus afarensis, walking together 3.6 million years ago in what was then fresh volcanic ash (see plate 3) (see plate 3). Who does not wonder what these individuals were to each other, whether they held hands or even talked, and what forgotten errand they shared in a Pliocene dawn?

Sometimes, as I mentioned when discussing lava, the mould may become filled with a different material, which subsequently hardens to form a cast of the original animal or organ. I am writing this on a table in the garden whose top is a six-inch thick, seven-foot square slab of Purbeck sedimentary limestone, of Jura.s.sic age, perhaps 150 million years old.2 Along with lots of fossil mollusc sh.e.l.ls, there is an alleged (by the distinguished and eccentric sculptor who procured it for me) dinosaur footprint on the underside of the table, but it is a footprint in relief, standing out from the surface. The original footprint (if indeed it is genuine, for it looks pretty nondescript to me) must have served as a mould, into which the sediment later settled. The mould then disappeared. Much of what we know about ancient brains comes to us in the form of such casts: 'endocasts' of the insides of skulls, often imprinted with surprisingly full details of the brain surface itself. Along with lots of fossil mollusc sh.e.l.ls, there is an alleged (by the distinguished and eccentric sculptor who procured it for me) dinosaur footprint on the underside of the table, but it is a footprint in relief, standing out from the surface. The original footprint (if indeed it is genuine, for it looks pretty nondescript to me) must have served as a mould, into which the sediment later settled. The mould then disappeared. Much of what we know about ancient brains comes to us in the form of such casts: 'endocasts' of the insides of skulls, often imprinted with surprisingly full details of the brain surface itself.

Less frequently than sh.e.l.ls, bones or teeth, soft parts of animals sometimes fossilise. The most famous sites are the Burgess Shale of the Canadian Rockies, and the slightly older Chengjiang in South China which we shall meet again in the Velvet Worm's Tale. At both these sites, fossils of worms and other soft, boneless and toothless creatures (as well as the usual hard ones) wonderfully record the Cambrian Period, more than half a billion years ago. We are outstandingly lucky to have Chengjiang (see plate 4) (see plate 4) and the Burgess Shale. Indeed, as I have already remarked, we are pretty lucky to have fossils at all, anywhere. It has been estimated that 90 per cent of all species will never be known to us as fossils. If that is the figure for whole species, just think how few individuals can ever hope to achieve the ambition with which the tale began, and end up as fossils. One estimate puts the odds at one in a million among vertebrates. That sounds high to me, and the true figure must be far less among animals with no hard parts. and the Burgess Shale. Indeed, as I have already remarked, we are pretty lucky to have fossils at all, anywhere. It has been estimated that 90 per cent of all species will never be known to us as fossils. If that is the figure for whole species, just think how few individuals can ever hope to achieve the ambition with which the tale began, and end up as fossils. One estimate puts the odds at one in a million among vertebrates. That sounds high to me, and the true figure must be far less among animals with no hard parts.

1 Many genes have more than one effect: a phenomenon known as pleiotropism. Many genes have more than one effect: a phenomenon known as pleiotropism.

2 A journalist interviewed me at this two-tonne megalith for over an hour and then described it in his newspaper as a 'white wrought-iron table': my favourite example of the fallibility of eyewitness evidence. A journalist interviewed me at this two-tonne megalith for over an hour and then described it in his newspaper as a 'white wrought-iron table': my favourite example of the fallibility of eyewitness evidence.

HABILINES.

Back another million years from h.o.m.o ergaster h.o.m.o ergaster, 2 million years ago there is no longer any doubt in which continent our genetic roots lie. Everyone agrees, 'multiregionalists' included, that Africa is the place. The most compelling fossil bones at this age are normally cla.s.sified as h.o.m.o habilis h.o.m.o habilis. Some authorities recognise a second, very similar contemporary type, which they call h.o.m.o rudolfensis h.o.m.o rudolfensis. Others equate it with Kenyapithecus Kenyapithecus, described by the Leakey team in 2001. Yet others cautiously refrain from giving these fossils a species name at all, and just call them all 'Early h.o.m.o h.o.m.o'. As usual I shan't take a stand on names. What matters is the real flesh and bone creatures themselves, and I shall use 'Habilines' as an anglicism for all of them. Habiline fossils, being older, are understandably less plentiful than Ergasts. The best-preserved skull bears the reference number KNMER 1470 and is widely known as Fourteen Seventy. It lived about 1.9 million years ago.

The Habilines were about as different from Ergasts as Ergasts from us, and, as we should expect, there were intermediates which are hard to cla.s.sify. Habiline skulls are less robust than Ergast skulls, and lack the p.r.o.nounced brow ridges. In this respect, Habilines were more like us. This should cause no surprise. Robustness and brow ridges are peculiarities that, possibly like hair, hominids seem able to acquire and lose again at the drop of an evolutionary hat.

Habilines mark the place in our history where the brain, that most dramatic of human peculiarities, starts to expand. Or more accurately, starts to expand beyond the normal size of the already large brains of other apes. This distinction, indeed, is the rationale for placing the Habilines in the genus h.o.m.o h.o.m.o at all. For many palaeontologists, the large brain is the distinguis.h.i.+ng feature of our genus. Habilines, with their brains pus.h.i.+ng the 750 cc barrier, have crossed the rubicon and are human. at all. For many palaeontologists, the large brain is the distinguis.h.i.+ng feature of our genus. Habilines, with their brains pus.h.i.+ng the 750 cc barrier, have crossed the rubicon and are human.

As readers may soon become tired of hearing, I am not a lover of rubicons, barriers and gaps. In particular, there is no reason to expect an early Habiline to be separated from its predecessor by a bigger gap than from its successor. It might seem tempting because the predecessor has a different generic name (Australopithecus) whereas the successor (h.o.m.o ergaster) (h.o.m.o ergaster) is 'merely' another is 'merely' another h.o.m.o h.o.m.o. It is true that when we look at living species, we expect members of different genera to be less alike than members of different species within the same genus. But it can't work like that for fossils, if we have a continuous historical lineage in evolution. At the borderline between any fossil species and its immediate predecessor, there must be some individuals about whom it is absurd to argue, since the reductio reductio of such an argument must be that parents of one species gave birth to a child of the other. It is even more absurd to suggest that a baby of the genus of such an argument must be that parents of one species gave birth to a child of the other. It is even more absurd to suggest that a baby of the genus h.o.m.o h.o.m.o was born to parents of a completely different genus, was born to parents of a completely different genus, Australopithecus Australopithecus. These are evolutionary regions into which our zoological naming conventions were never designed to go.1 Setting names to one side frees us for a more constructive discussion about why why the brain suddenly started to enlarge. How would we measure the enlargement of the hominid brain and plot a graph of average brain size against geological time? There is no problem about the units in which we measure time: millions of years. Brain size is harder. Fossil skulls and endocasts allow us to estimate brain size in cubic centimetres, and it is easy enough to convert this to grams. But absolute brain size is not necessarily the measure you want. An elephant has a bigger brain than a person, and it isn't just vanity that makes us think we are brainier than elephants. the brain suddenly started to enlarge. How would we measure the enlargement of the hominid brain and plot a graph of average brain size against geological time? There is no problem about the units in which we measure time: millions of years. Brain size is harder. Fossil skulls and endocasts allow us to estimate brain size in cubic centimetres, and it is easy enough to convert this to grams. But absolute brain size is not necessarily the measure you want. An elephant has a bigger brain than a person, and it isn't just vanity that makes us think we are brainier than elephants. Tyrannosaurus Tyrannosaurus's brain was not much smaller than ours, but all dinosaurs are regarded as small-brained, slow-witted creatures. What makes us cleverer is that we have bigger brains for our size for our size than dinosaurs. But what, more precisely, does 'for our size' mean? than dinosaurs. But what, more precisely, does 'for our size' mean?

There are mathematical methods of correcting for absolute size, and expressing an animal's brain size as a function of how big it 'ought to be' given its body size. This is a topic worthy of a tale in its own right, and h.o.m.o habilis h.o.m.o habilis, handyman, from his uneasy vantage point straddling the brain-size 'rubicon', will tell it.

THE HANDYMAN'S TALE.

We want to know whether the brain of a particular creature such as h.o.m.o habilis h.o.m.o habilis is larger or smaller than it 'ought' to be, given that animal's body size. We accept (slightly unwillingly in my case but I'll let it pa.s.s) that large animals just have to have large brains and small animals small brains. Making allowance for this, we still want to know whether some species are 'brainier' than others. So, how do we make allowance for body size? We need a reasonable basis for calculating the expected brain size of an animal from its body size, so that we can decide whether the actual brain of a particular animal is larger or smaller than expected. is larger or smaller than it 'ought' to be, given that animal's body size. We accept (slightly unwillingly in my case but I'll let it pa.s.s) that large animals just have to have large brains and small animals small brains. Making allowance for this, we still want to know whether some species are 'brainier' than others. So, how do we make allowance for body size? We need a reasonable basis for calculating the expected brain size of an animal from its body size, so that we can decide whether the actual brain of a particular animal is larger or smaller than expected.

In our pilgrimage to the past, we happen to have met the problem in connection with brains, but similar questions can arise with respect to any part of the body. Do some animals have larger (or smaller) hearts, or kidneys, or shoulder-blades than they 'ought' to have for their size? If so, this might suggest that their way of life makes special demands on the heart (kidney or shoulder-blade). How do we know what size any bit of an animal 'ought' to be, given that we know its total body size? Note that 'ought to be' doesn't mean 'needs to have for functional reasons'. It means 'would be expected to have, knowing what comparable animals have'. Since this is the Handyman's Tale, and since the Handyman's most surprising feature is his brain, we'll go on using brains for the sake of discussion. The lessons we learn will be more general.

We begin by making a scatter plot of brain ma.s.s against body ma.s.s for a large number of species. Each symbol in the graph on the opposite page (from my colleague the distinguished anthropologist Robert Martin) represents one species of living mammal 309 of them, ranging from the smallest to the largest. In case you are interested, h.o.m.o sapiens h.o.m.o sapiens is the point with the arrow, and the one immediately next to us is a dolphin. The heavy black line drawn through the middle of the points is the straight line that, according to statistical calculation, gives the best fit to all the points. is the point with the arrow, and the one immediately next to us is a dolphin. The heavy black line drawn through the middle of the points is the straight line that, according to statistical calculation, gives the best fit to all the points.2 A slight complication, which will make sense in a moment, is that things work better if we make the scales of both axes logarithmic, and that is how this graph was made. We plot the logarithm of an animal's brain ma.s.s against the logarithm of its body ma.s.s. Logarithmic means that equal steps along the bottom of the graph (or equal steps up the side) represent multiplications multiplications by some fixed number, say ten, rather than additions of a number, as in an ordinary graph. The reason ten is convenient is that we can then think of a logarithm as a count of the number of noughts. If you have to multiply a mouse's ma.s.s by a million to get an elephant's, this means you have to add six noughts to the mouse's ma.s.s: you have to add six to the logarithm of the one, to get the logarithm of the other. Half way between them on the logarithmic scale three noughts lies an animal that weighs a thousand times as much as a mouse, or a thousandth of an elephant: a person, perhaps. Using round numbers like a thousand and a million is just to make the explanation easy. 'Three and a half noughts' means somewhere between a thousand and ten thousand. Note that 'half way between' when