The Magic of Reality

THERE ARE LOTS of myths that attempt to explain why particular kinds of animals are the way that they are – myths that ‘explain’ things like why leopards have spots, and why rabbits have white tails. But there don’t seem to be many myths about the sheer range and variety of different kinds of animals. I can find nothing akin to the Jewish myth of the Tower of Babel, which accounts for the great variety of languages. Once upon a time, according to this myth, all the people in the world spoke the same language. They could therefore work harmoniously together to build a great tower, which they hoped would reach the sky. God noticed this and took a very dim view of everybody being able to understand everybody else. Whatever might they get up to next, if they could talk to each other and work together? So he decided to ‘confound their language’ so that ‘they may not understand one another’s speech’. This, the myth tells us, is why there are so many different languages, and why, when people try to talk to people from another tribe or country, their speech often sounds like meaningless babble. Oddly enough, there is no connection between the word ‘babble’ and the Tower of Babel.

I was hoping to find a similar myth about the great diversity of animals, because there is a resemblance between language evolution and animal evolution, as we shall see. But there doesn’t seem to be any myth that specifically tackles the sheer number of different kinds of animals. This is surprising, because there is indirect evidence that tribal peoples can be well aware of the fact there are many different kinds of animals. In the 1920s a now famous German scientist called Ernst Mayr did a pioneering study of the birds of the New Guinea highlands. He compiled a list of 137 species, then discovered, to his amazement, that the local Papuan tribesmen had separate names for 136 of them.

Back to the myths. The Hopi tribe of North America had a goddess called Spider Woman. In their creation myth she teamed up with Tawa the sun god, and they sang the First Magic Song as a duet. This song brought the Earth, and life, into being. Spider Woman then took the threads of Tawa’s thoughts and wove them into solid form, creating fish, birds, and all other animals.

Other North American tribes, the Pueblo and Navajo peoples, have a myth of life that is a tiny bit like the idea of evolution: life emerges from the Earth like a sprouting plant growing up through a sequence of stages. The insects climbed from their world, the First or Red World, up into the Second World, the Blue World, where the birds lived. The Second World then became too crowded, so the birds and insects flew up into the Third or Yellow World, where the people and other mammals lived. The Yellow World in turn became crowded and food became scarce, so they all, insects, birds and everybody, went up to the Fourth World, the Black and White World of day and night. Here the gods had already created cleverer people who knew how to farm the Fourth World and who taught the newcomers how to do it too.

The Jewish creation myth comes closer to doing justice to diversity, but it doesn’t really attempt to explain it. Actually, the Jewish holy book has two different creation myths, as we saw in the previous chapter. In the first one, the Jewish god created everything in six days. On the fifth day he created fish, whales and all sea creatures, and the birds of the air. On the sixth day he made the rest of the land animals, including man. The language of the myth pays some attention to the number and variety of living creatures – for example, ‘God created great whales, and every living creature that moveth, which the waters brought forth abundantly after their kind, and every winged fowl after his kind,’ and made every ‘beast of the earth’ and ‘every thing that creepeth upon the earth after his kind’. But why was there such variety? We are not told.

In the second myth we get some hint that the god might have thought his first man needed a variety of companions. Adam, the first man, is created alone and placed in the beautiful oasis garden. But then the god realized that ‘It is not good that the man should be alone’ and he therefore ‘formed every beast of the field and every fowl of the air; and brought them unto Adam to see what he would call them’.

Why are there really so many different kinds of animals?

Adam’s task of naming all the animals was a tough one – tougher than the ancient Hebrews could possibly have realized. It’s been estimated that about 2 million species have so far been given scientific names, and even these are just a small fraction of the number of species yet to be named.

How do we even decide whether two animals belong in the same species or in two different species? Where animals reproduce sexually, we can come up with a sort of definition. Animals belong to different species if they don’t breed together. There are borderline cases like horses and donkeys, which can breed together but produce offspring (called mules or hinnies) that are infertile – that is, that cannot have offspring themselves. We therefore place a horse and a donkey in different species. More obviously, horses and dogs belong to different species because they don’t even try to interbreed, and couldn’t produce offspring if they did, even infertile ones. But spaniels and poodles belong to the same species because they happily interbreed, and the puppies that they produce are fertile.

Every scientific name of an animal or plant consists of two Latin words, usually printed in italics. The first word refers to the ‘genus’ or group of species and the second to the individual species within the genus. Homo sapiens (‘wise man’) and Elephas maximus (‘very big elephant’) are examples. Every species is a member of a genus. Homo is a genus. So is Elephas. The lion is Panthera leo and the genus Panthera also includes Panthera tigris (tiger), Panthera pardus (leopard or ‘panther’) and Panthera onca (jaguar). Homo sapiens is the only surviving species of our genus, but fossils have been given names like Homo erectus and Homo habilis. Other human-like fossils are sufficiently different from Homo to be placed in a different genus, for example Australopithecus africanus and Australopithecus afarensis (nothing to do with Australia, by the way: australo- just means ‘southern’, which is where Australia’s name also comes from).

Each genus belongs to a family, usually printed in ordinary ‘roman’ type with a capital initial. Cats (including lions, leopards, cheetahs, lynxes and lots of smaller cats) make up the family Felidae. Every family belongs to an order. Cats, dogs, bears, weasels and hyenas belong to different families within the order Carnivora. Monkeys, apes (including us) and lemurs all belong to different families within the order Primates. And every order belongs to a class. All mammals are in the class Mammalia.

Can you see the shape of a tree developing in your mind as you read this description of the sequence of groupings? It is a family tree: a tree with many branches, each branch having sub-branches, and each sub-branch having sub-sub-branches. The tips of the twigs are species. The other groupings – class, order, family, genus – are the branches and sub-branches. The whole tree is all of life on Earth.

Think about why trees have so many twigs. Branches branch. When we have enough branches of branches of branches, the total number of twigs can be very large. That’s what happens in evolution. Charles Darwin himself drew a branching tree as the only picture in his most famous book, On the Origin of Species. He sketched an early version in one of his notebooks some years earlier. At the top of the page he wrote a mysterious little message to himself: ‘I think’. What do you think he meant? Maybe he started to write a sentence and one of his children interrupted him so he never finished it. Maybe he found it easier to represent quickly what he was thinking in this diagram than in words. Perhaps we shall never know. There is other handwriting on the page, but it is hard to decipher. It is tantalizing to read the actual notes of a great scientist, written on a particular day and never meant for publication.

The following isn’t exactly how the tree of animals branched, but it gives you an idea of the principle. Imagine an ancestral species splitting into two species. If each of those then splits into two, that makes four. If each of them splits into two, that makes eight, and so on through 16, 32, 64, 128, 256, 512… You can see that, if you carry on doubling up, it doesn’t take long to get up into the millions of species. That probably makes sense to you, but you may be wondering why a species should split. Well, it’s for pretty much the same reason as human languages split, so let’s pause to think about that for a moment.

Pulling apart: how languages, and species, divide

Although the legend of the Tower of Babel is, of course, not really true, it does raise the interesting question of why there are so many different languages.

Just as some species are more similar than others and are placed in the same family, so there are also families of languages. Spanish, Italian, Portuguese, French and many European languages and dialects such as Romansch, Galician, Occitan and Catalan are all pretty similar to each other; together they’re called ‘Romance’ languages. The name actually comes from their common origin in Latin, the language of Rome, not from any association with romance, but let’s use an expression of love as our example. Depending on which country you are in, you might declare your feelings in one of the following ways: ‘Ti amo’, ‘Amote’, ‘T’aimi’ or ‘Je t’aime’. In Latin it would be ‘Te amo’ – exactly like modern Spanish.

To swear your love to someone in Kenya, Tanzania or Uganda you could say, in Swahili, ‘Nakupenda’. A bit further south, in Mozambique, Zambia, or Malawi where I was brought up, you might say, in the Chinyanja language, ‘Ndimakukonda’. In other so-called Bantu languages in southern Africa you might say ‘Ndinokuda’, ‘Ndiyakuthanda’ or, to a Zulu, ‘Ngiyakuthanda’. This Bantu family of languages is quite distinct from the Romance family of languages, and both are distinct from the Germanic family which includes Dutch, German and the Scandinavian languages. See how we use the word ‘family’ for languages, just as we do for species (the cat family, the dog family) and also, of course, for our own families (the Jones family, the Robinson family, the Dawkins family).

It isn’t hard to work out how families of related languages arise over the centuries. Listen to the way you and your friends speak to each other, and compare it to the way your grandparents speak. Their speech is only slightly different and you can easily understand them, but they are only two generations away. Now imagine talking, not to your grandparents but to your 25-greats-grandparents. If you happen to be English, that might take you back to the late fourteenth century – the lifetime of the poet Geoffrey Chaucer, who wrote descriptions like this:

Well, it is recognizably English, isn’t it? But I bet you’d have a hard time understanding it if you heard it spoken. And if it was any more different you’d probably consider it a separate language, as different as Spanish is from Italian.

So, the language in any one place changes century by century. We could say it ‘drifts’ into something different. Now add the fact that people speaking the same language in different places don’t often have the opportunity to hear each other (or at least they didn’t before telephones and radios were invented); and the fact that language drifts in different directions in different places. This applies to the way it is spoken as well as to the words themselves: think how different English sounds in a Scottish, Welsh, Geordie, Cornish, Australian or American accent. And Scottish people can easily distinguish an Edinburgh accent from a Glasgow accent or a Hebridean accent. Over time, both the way the language is spoken and the words used become characteristic of a region; when two ways of speaking a language have drifted sufficiently far apart, we call them different ‘dialects’.

After enough centuries of drift, different regional dialects eventually become so different that people in one region can no longer understand people in another. At this point we call them separate languages. That is what happened when German and Dutch drifted, in separate directions, from a now extinct ancestral language. It is what happened when French, Italian, Spanish and Portuguese independently drifted away from Latin in separate parts of Europe. You can draw a family tree of languages, with ‘cousins’ like French, Portuguese and Italian on neighbouring ‘branches’ and ancestors like Latin further down the tree – just as Darwin did with species.

Like languages, species change over time and over distance. Before we look at why this happens, we need to see how they do it. For species, the equivalent of words is DNA – the genetic information every living thing carries inside it that determines how it is made, as we saw in Chapter 2. When individuals reproduce sexually, they mix their DNA. And when members of one local population migrate into another local population and introduce their genes into it by mating with individuals of the population they have just joined, we call this ‘gene flow’.

The equivalent of, say, Italian and French drifting apart is that the DNA of two separated populations of a species becomes less and less alike over time. Their DNA becomes less and less able to work together to make babies. Horses and donkeys can mate with each other, but horse DNA has drifted so far from donkey DNA that the two can no longer understand each other. Or rather, they can mix well enough – the two ‘DNA dialects’ can understand each other well enough – to make a living creature, a mule, but not well enough to make one that can reproduce itself: mules, as we saw earlier, are sterile.

An important difference between species and languages is that languages can pick up ‘loan words’ from other languages. Long after it developed as a separate language from Romance, Germanic and Celtic sources, for example, English picked up ‘shampoo’ from Hindi, ‘iceberg’ from Norwegian, ‘bungalow’ from Bengali and ‘anorak’ from Inuit. Animal species, by contrast, never (or almost never) exchange DNA ever again, once they have drifted far enough apart to have stopped breeding together. Bacteria are another story: they do exchange genes, but there isn’t enough space in this book to go into that. In the rest of this chapter, assume that we are talking about animals.

Islands and isolation: the power of separation

So the DNA of species, like the words of languages, drifts apart when separated. Why might this happen? What might start the separation? An obvious possibility is the sea. Populations on separate islands don’t meet each other – not often, anyway – so their two sets of genes have the opportunity to drift away from one another. This makes islands extremely important in the origins of new species. But we can think of an island as more than just a piece of land surrounded by water. To a frog, an oasis is an ‘island’ where it can live, surrounded by a desert where it can’t. To a fish, a lake is an island. Islands matter, both for species and for languages, because the population of an island is cut off from contact with other populations (preventing gene flow in the case of species, just as it prevents language drift) and so is free to begin to evolve in its own direction.

The next important point is that the population of an island need not be totally isolated for ever: genes can occasionally cross the barrier surrounding it, whether this be water or uninhabitable land.

On 4 October 1995 a mat of logs and uprooted trees was blown onto a beach on the Caribbean island of Anguilla. On the mat were 15 green iguanas, alive after what must have been a perilous journey from another island, probably Guadeloupe, 160 miles away. Two hurricanes, called Luis and Marilyn, had roared through the Caribbean during the previous month, uprooting trees and flinging them into the sea. It seems that one of these hurricanes must have torn down the trees in which the iguanas were climbing (they love sitting up in trees, as I have seen in Panama) and blown them out to sea. Eventually reaching Anguilla, the iguanas crawled off their unorthodox means of transport onto the beach and began a new life, feeding and reproducing and passing on their DNA, on a brand new island home.

We know this happened because the iguanas were seen arriving on Anguilla by local fishermen. Centuries earlier, although nobody was there to witness it, something similar is almost certainly what brought the iguanas’ ancestors to Guadeloupe in the first place. And something like the same story almost certainly accounts for the presence of iguanas on the Galapagos islands, which is where we turn for the next step in our story.

The Galapagos islands are historically important because they probably inspired Charles Darwin’s first thoughts on evolution when, as a member of the expedition on HMS Beagle, he visited them in 1835. They are a collection of volcanic islands in the Pacific Ocean near the equator, about 600 miles west of South America. They are all young (just a few million years old), formed by volcanoes punching up from the bottom of the sea. This means that all the species of animals and plants on the islands must have arrived from elsewhere – presumably the mainland of South America – and recently, by evolutionary standards. Once arrived, species could make the shorter crossings from island to island, sufficiently often to reach all the islands (maybe once or twice every century or so) but sufficiently seldom that they were able to evolve separately – ‘drift apart’ as we have been saying in this chapter – during the intervals between the rare crossings.

Nobody knows when the first iguanas arrived in the Galapagos. They probably rafted across from the mainland just like the ones that arrived in Anguilla in 1995. Nowadays the nearest island to the mainland is San Cristobal (Darwin knew it by the English name of Chatham), but millions of years ago there were other islands too, which have now sunk beneath the sea. The iguanas could have arrived first on one of the now sunken islands, and then crossed to other islands, including those still above water today.

Once there, they had the opportunity to flourish in a new place, just like the ones that arrived in Anguilla in 1995. The first iguanas on Galapagos would have evolved to become different from their cousins on the mainland, partly by just ‘drifting’ (like languages) and partly because natural selection would have favoured new survival skills: a relatively barren volcanic island is a very different place from the South American mainland.

The distances between the different islands are much smaller than the distance from any of them to the mainland. So accidental sea crossings between islands would be relatively common: perhaps once per century rather than once per millennium. And iguanas would have started turning up on most or all of the islands eventually. Island-hoppings would have been rare enough to allow some evolutionary drifting apart on the different islands, between ‘contaminations’ of the genes by subsequent island-hoppings: rare enough to allow the different groups of iguanas to evolve so much that when they eventually met again they could no longer breed together. The result is that there are now three distinct species of land iguana on Galapagos, which are no longer capable of cross-breeding. Conolophus pallidus is found only on the island of Santa Fe. Conolophus subcristatus lives on several islands including Fernandina, Isabela and Santa Cruz (each island population possibly on its way to becoming a separate species). Conolophus marthae is confined to the northernmost of the chain of five volcanoes on the big island of Isabela.

That raises another interesting point, by the way. You remember we said that a lake or an oasis could count as an island, even though neither consists of land surrounded by water? Well, the same goes for each of the five volcanoes on Isabela. Each volcano in the chain is surrounded by a zone of rich vegetation, which is a kind of oasis, separated from the next volcano by a desert. Most of the Galapagos islands have only a single large volcano, but Isabela has five. If the sea level rises (perhaps because of global warming) Isabela could become five islands separated by sea. As it is, you can think of each volcano as a kind of island within an island. That’s how it would seem to an animal like a land iguana (or a giant tortoise), which needs to feed on the vegetation found only on the slopes around the volcanoes.

Any kind of isolation by a geographical barrier which can be crossed sometimes but not too often leads to evolutionary branching. (Actually, it doesn’t have to be a geographical barrier. There are other possibilities, especially in insects, but for simplicity’s sake I won’t go into them here.) And once the divided populations have drifted far enough apart that they can no longer breed together, the geographical barrier is no longer necessary. The two species can go their separate evolutionary ways without contaminating each other’s DNA ever again. It is mainly separations of this kind that were originally responsible for all the new species that have ever arisen on this planet: even, as we shall see, the original separation of the ancestors of, say, snails from the ancestors of all vertebrates including us.

At some point in the history of iguanas on Galapagos, a branching occurred which was to lead to a very peculiar new species. On one of the islands – we don’t know which – a local population of land iguanas completely changed their way of life. Instead of eating land plants on the slopes of volcanoes, they went to the shore and took to feeding on seaweed. Natural selection then favoured those individuals that became skilled swimmers, until nowadays their descendants habitually dive to graze on underwater seaweeds. They are called marine iguanas and, unlike land iguanas, they are found nowhere but Galapagos.

They have lots of strange features that equip them for life in the sea and this makes them really rather different from the land iguanas of Galapagos and everywhere else in the world. They have certainly evolved from land iguanas, but they are not especially close cousins of today’s land iguanas of Galapagos, so it is possible that they evolved from an earlier, now extinct genus, which colonized the islands from the mainland long before the present Conolophus. There are different races of marine iguanas, but not different species, on the different islands. One day these different island races will probably be found to have drifted apart far enough to be called different species of the marine iguana genus.

It’s a similar story for giant tortoises, for lava lizards, for the strange flightless cormorants, for mockingbirds, for finches, and for many other animals and plants of Galapagos. And the same kind of thing happens all over the world. Galapagos is just a particularly clear example. Islands (including lakes, oases and mountains) manufacture new species. A river can do the same thing. If it is difficult for an animal to cross a river, the genes in populations on either side of the river can drift apart, just as one language can drift to become two dialects, which can later drift to become two languages. Mountain ranges can play the same role of separation. So can just plain distance. Mice in Spain may be connected by a chain of interbreeding mice all across the Asian continent to China. But it takes so long for a gene to travel from mouse to mouse across that vast distance that they might as well be on separate islands. And mouse evolution in Spain and China might drift in different directions.

The three species of Galapagos land iguana have had only a few thousand years to drift apart in their evolution. After enough hundreds of millions of years have passed, the descendants of a single ancestral species can be as different as, say, a cockroach is from a crocodile. In fact it is literally true that once upon a time there was a great-great-great- (lots of greats) grandparent of cockroaches (and lots of other animals including snails and crabs) which was also the grand ancestor (let’s use the word ‘grancestor’) of crocodiles (not to mention all the other vertebrates). But you’d have to go back a very very long way, maybe more than a billion years, before you found a grancestor as grand and ancient as that. That is much too long ago for us even to begin to guess what the original barrier was that separated them in the first place. Whatever it was, it must have been in the sea, because in those far-off days no animals lived on land. Maybe the grancestor species could only live on coral reefs, and two populations found themselves on a pair of coral reefs separated by inhospitable deep water.

As we saw in the previous chapter, you’d only have to go back six million years to find the most recent shared grancestor of all humans and chimpanzees. That’s recent enough for us to guess at a possible geographical barrier that might have occasioned the original split. It’s been suggested that it was the Great Rift Valley in Africa, with humans evolving on the east side and chimpanzees on the west. Later, the chimp ancestral line split into common chimpanzees and pygmy chimpanzees or bonobos: it’s been suggested that the barrier in that case was the Congo river. As we saw in the previous chapter, the shared grancestor of all surviving mammals lived about 185 million years ago. Since then, its descendants have branched and branched and branched again, producing all the thousands of species of mammals we see today, including 231 species of carnivores (dogs, cats, weasels, bears etc.), 2,000 species of rodents, 88 species of whales and dolphins, 196 species of cloven-hoofed animals (cows, antelopes, pigs, deer, sheep), 16 species in the horse family (horses, zebras, tapirs and rhinos), 87 rabbits and hares, 977 species of bats, 68 species of kangaroos, 18 species of apes (including humans), and lots and lots of species that have gone extinct along the way (including quite a few extinct humans, known only from fossils).

Stirring, selection and survival

I want to round off the chapter by telling the story again in slightly different language. I’ve already briefly mentioned gene flow; scientists also talk of something called the gene pool, and I now want to spell out more fully what that means. Of course there can’t literally be a pool of genes. The word ‘pool’ suggests a liquid, in which genes might be stirred around. But genes are found only in the cells of living bodies. So what does it mean to talk of a gene pool?

In every generation, sexual reproduction sees to it that genes are shuffled. You were born with the shuffled genes of your father and your mother, which means the shuffled genes of your four grandparents. The same applies to every individual in the population over the long, long reach of evolutionary time: thousands of years, tens of thousands, hundreds of thousands of years. During that time, this process of sexual shuffling sees to it that the genes within the whole population are so thoroughly shuffled, indeed stirred, that it makes sense to talk of a great, swirling pool of genes: the ‘gene pool’.

You remember our definition of a species as a group of animals or plants that can breed with each other? Now you can see why this definition matters. If two animals are members of the same species in the same population, that means their genes are being stirred about in the same gene pool. If two animals are members of different species they cannot be members of the same gene pool because their DNA cannot mix in sexual reproduction, even if they live in the same country and meet each other frequently. If populations of the same species are geographically separated, their gene pools have the opportunity to drift apart – so far apart, eventually, that if they happen to meet again they can no longer breed together. Now that their gene pools have moved beyond mixing they have become different species and can go on moving further apart for millions of years to the point where they might become as different from one another as humans are from cockroaches.

Evolution means change in a gene pool. Change in a gene pool means that some genes become more numerous, others less. Genes that used to be common become rare, or disappear altogether. Genes that used to be rare become common. And the result is that the shape, or size, or colour, or behaviour of typical members of the species changes: it evolves, because of changes in the numbers of genes in the gene pool. That is what evolution is.

Why should the numbers of different genes change as the generations go by? Well, you might say it would be surprising if they didn’t, given such immensities of time. Think of the way language changes over the centuries. Words like ‘thee’ and ‘thou’, ‘zounds’ and ‘avast’, phrases like ‘stap me vitals’, have now more or less dropped out of English. On the other hand, the phrase ‘I was like’ (meaning ‘I said’), which would have been incomprehensible as recently as 20 years ago, is now commonplace. So is ‘cool’ as a term of approval.

So far in this chapter, I haven’t needed to go much further than the idea that gene pools in separate populations can drift apart, like languages. But actually, in the case of species, there is much more to it than drifting. This ‘much more’ is natural selection, the supremely important process that was Charles Darwin’s greatest discovery. Even without natural selection, we’d expect gene pools that happen to be separated to drift apart. But they’d drift in a rather aimless fashion. Natural selection nudges evolution in a purposeful direction: namely, the direction of survival. The genes that survive in a gene pool are the genes that are good at surviving. And what makes a gene good at surviving? It helps other genes to build bodies that are good at surviving and reproducing: bodies that survive long enough to pass on the genes that helped them to survive.

Exactly how they do it varies from species to species. Genes survive in bird or bat bodies by helping to build wings. Genes survive in mole bodies by helping to build stout, spade-like hands. Genes survive in lion bodies by helping to build fast-running legs, and sharp claws and teeth. Genes survive in antelope bodies by helping to build fast-running legs, and sharp hearing and eyesight. Genes survive in leaf-insect bodies by making the insects all but indistinguishable from leaves. However different the details, in all species the name of the game is gene survival in gene pools. Next time you see an animal – any animal – or any plant, look at it and say to yourself: what I am looking at is an elaborate machine for passing on the genes that made it. I’m looking at a survival machine for genes.

Next time you look in the mirror, just think: that is what you are too.