The Greatest Show on Earth Page 27
Islands don’t come much more isolated than St Helena, a single volcano in the South Atlantic some 1,200 miles from the coast of Africa. It has about 100 endemic plants (the young Darwin would have called them ‘aboriginal creations’ and the older Darwin would have said they evolved there). Among these are (or were, for some of them are now extinct) forest trees belonging to the daisy family.
These trees resemble in habit trees on the African mainland to which they are not closely related. The mainland plants to which they are related are herbs or small shrubs. What must have happened is that a few seeds of small herbs or shrubs chanced across the thousandmile gap from Africa, settled on St Helena and, because the niche of forest trees was unfilled, evolved larger and more woody trunks until they became proper trees. Similar tree-like daisies have evolved independently on the Galapagos archipelago. It is the same pattern on islands the world over.
Forest trees on St Helena
Each of the great African lakes has its own unique fish fauna, dominated by the group called cichlids. The cichlid faunas of Lake Victoria, Lake Tanganyika and Lake Malawi, each several hundred species strong, are completely distinct from each other. They have evidently evolved separately in the three lakes, which makes it all the more fascinating that they have converged on the same range of ‘trades’ in all three. In each lake, it looks as though one or two founder species somehow made their way in, perhaps from rivers, in the first place. And in each lake these founders then speciated and speciated again, to populate the lake with the hundreds of species that we see today. How, within the confines of a lake, did the budding species achieve the initial geographical isolation that enabled them to split apart?
When introducing islands, I explained that, from a fish’s point of view, a lake surrounded by land is an island. Slightly less obviously, even an island in the conventional sense of land surrounded by water can be an ‘island’ for a fish, especially a fish that lives only in shallow water. In the sea, think of a coral-reef fish, which never ventures into deep water. From its point of view, the shallow fringe of a coral island is an ‘island’, and the Great Barrier Reef is an archipelago. Something similar can happen even in a lake. Within a lake, especially a large one, a rocky outcrop can be an ‘island’ for a fish whose habits confine it to shallow water. This is almost certainly how at least some of the cichlids in the African great lakes achieved their initial isolation. Most individuals were confined to shallow water around islands, or in bays and inlets. This achieved partial isolation from other such pockets of shallow water, linked by occasional traversings of the deeper water between them to form the watery equivalent of a Galapagos-like ‘archipelago’.
There’s good evidence (for example from sediment core samples) that the level of Lake Malawi (it was called Lake Nyasa when I spent my first bucket-and-spade holidays on its sandy beaches) rises and falls dramatically over the centuries, and reached a low point in the eighteenth century, more than 100 metres lower than the present level. Many of its islands were not islands at all during that time, but hills on the land around the then smaller lake. When the lake level rose, in the nineteenth and twentieth centuries, the hills became islands, ranges of hills became archipelagoes, and the process of speciation took off among the cichlids that live in shallow water, known locally as Mbuna. ‘Almost every rocky outcrop and island has a unique Mbuna fauna, with endless colour forms and species. As many of these islands and outcrops were dry land within the last 200–300 years, the establishment of the faunas has taken place within that time.’
Such rapid speciation is something the cichlid fishes are extremely good at. Lake Malawi and Lake Tanganyika are old, but Lake Victoria is extremely young. The lake basin was formed only about 400,000 years ago, and it has dried up several times since then, most recently about 17,000 years ago. This seems to mean that its endemic fauna of 450 or so species of cichlid fishes have all evolved over a timescale of centuries, not the millions of years that we usually associate with evolutionary divergence on this grand scale. The cichlids of Africa’s lakes impress us mightily with what evolution can do in a short space of time. They almost qualified for inclusion in the ‘before our very eyes’ chapter.
The woods and forests of Australia are dominated by trees of a single genus, Eucalyptus, and there are more than 700 species of them, filling a huge range of niches. Once again, Darwin’s dictum about finches can be coopted: one could almost imagine that one species of eucalypt had been ‘taken and modified for different ends’. And, along parallel lines, an even more famous example is the Australian mammal fauna. In Australia there are, or were until recent extinctions possibly caused by the arrival of aboriginal people, the ecological equivalents of wolves, cats, rabbits, moles, shrews, lions, flying squirrels and many others. Yet they are marsupials, quite different from the wolves, cats, rabbits, moles, shrews, lions and flying squirrels with which we are familiar in the rest of the world, the so-called placental mammals. The Australian equivalents are all descended from just a few, or even one, ancestral marsupial species, ‘taken and modified for different ends’. This beautiful marsupial fauna has also produced creatures for which it is harder to find a counterpart outside Australia. The many species of kangaroo mostly fill antelope-like niches (or monkey or lemur-like niches in the case of the tree kangaroos) but get about by hopping rather than galloping. They range from the large red kangaroo (and some even larger extinct ones, including a fearsome, bounding carnivore) to the small wallabies and tree kangaroos. There were giant, rhinoceros-sized marsupials, Diprotodonts, related to modern wombats but 3 yards long, 6 feet tall at the shoulder, and weighing 2 tons. I shall return to the marsupials of Australia in the next chapter.
It is almost too ridiculous to mention it, but I’m afraid I have to because of the more than 40 per cent of the American population who, as I lamented in Chapter 1, accept the Bible literally: think what the geographical distribution of animals should look like if they’d all dispersed from Noah’s Ark. Shouldn’t there be some sort of law of decreasing species diversity as we move away from an epicentre – perhaps Mount Ararat? I don’t need to tell you that that is not what we see.
Why would all those marsupials – ranging from tiny pouched mice through koalas and bilbys to giant kangaroos and Diprotodonts – why would all those marsupials, but no placentals at all, have migrated en masse from Mount Ararat to Australia? Which route did they take? And why did not a single member of their straggling caravan pause on the way, and settle – in India, perhaps, or China, or some haven along the Great Silk Road? Why did the entire order Edentata (all twenty species of armadillo, including the extinct giant armadillo, all six species of sloth, including extinct giant sloths, and all four species of anteater) troop off unerringly for South America, leaving not a rack behind, leaving no hide nor hair nor armour plate of settlers somewhere along the way? Why were they joined by the entire infraorder of caviomorph rodents, including guinea pigs, agoutis, pacas, maras, capybaras, chinchillas and lots of others, a large group of characteristically South American rodents, found nowhere else? Why did an entire sub-order of monkeys, the platyrrhine monkeys, end up in South America and nowhere else? Shouldn’t at least a few of them have joined the rest of the monkeys, the catarrhines, in Asia or Africa? And shouldn’t at least one species of catarrhine have found itself in the New World, along with the platyrrhines? Why did all the penguins undertake the long waddle south to the Antarctic, not a single one to the equally hospitable Arctic?
An ancestral lemur, again very possibly just a single species, found itself in Madagascar. Now there are thirty-seven species of lemur (plus some extinct ones). They range in size from the pygmy mouse lemur, smaller than a hamster, to a giant lemur, larger than a gorilla and resembling a bear, which went extinct quite recently. And they are all, every last one of them, in Madagascar. There are no lemurs anywhere else in the world, and there are no monkeys in Madagascar. How on Earth do the 40 per cent history-deniers think this state of affairs came about? Did
all thirty-seven and more species of lemur troop in a body down Noah’s gangplank and hightail it (literally in the case of the ringtail) for Madagascar, leaving not a single straggler by the wayside, anywhere throughout the length and breadth of Africa?
Once again, I am sorry to take a sledgehammer to so small and fragile a nut, but I have to do so because more than 40 per cent of the American people believe literally in the story of Noah’s Ark. We should be able to ignore them and get on with our science, but we can’t afford to because they control school boards, they home-school their children to deprive them of access to proper science teachers, and they include many members of the United States Congress, some state governors and even presidential and vice-presidential candidates. They have the money and the power to build institutions, universities, even a museum where children ride life-size mechanical models of dinosaurs, which, they are solemnly told, coexisted with humans. And, as recent polls have shown, Britain is not far behind (or should that read ‘ahead’?), along with parts of Europe and most of the Islamic world.
Even if we leave Mount Ararat to one side; even if we refrain from lampooning those who take the Noah’s Ark myth literally, similar problems apply to any theory of the separate creation of species. Why would an all-powerful creator decide to plant his carefully crafted species on islands and continents in exactly the appropriate pattern to suggest, irresistibly, that they had evolved and dispersed from the site of their evolution? Why would he put lemurs in Madagascar and nowhere else? Why put platyrrhine monkeys in South America only, and catarrhine monkeys in Africa and Asia only? Why no mammals in New Zealand, except bats who could fly there? Why do the animals in island chains most closely resemble those on neighbouring islands, and why do they nearly always resemble – less strongly but still unmistakably – those on the nearest continent or large island? Why would the creator put only marsupial mammals in Australia, again except bats who could fly there, and those who could arrive in man-made canoes? The fact is that, if we survey every continent and every island, every lake and every river, every mountaintop and every Alpine valley, every forest and every desert, the only way to make sense of the distribution of animals and plants is, yet again, to follow Darwin’s insight about the Galapagos finches: ‘One might really fancy that from an original paucity . . . one species had been taken and modified for different ends.’
Darwin was fascinated by islands, and he tramped the length and breadth of a good few during the voyage of the Beagle. He even worked out the surprising truth about how islands of one major class, those built by the animals called corals, are formed. Darwin later came to recognize the crucial importance of islands and archipelagoes for his theory, and he did several experiments to settle questions about the theory of geographical isolation as a prelude to speciation (he didn’t use the word). For example, in a number of experiments he kept seeds in sea water for long periods, and demonstrated that some retained the power to germinate even after immersion for long enough to have drifted from continents to neighbouring islands. Frogspawn, on the other hand, he found to be immediately killed by sea water, and he made good use of this to explain a signal fact about the geographical distribution of frogs:
With respect to the absence of whole orders on oceanic islands, Bory St. Vincent long ago remarked that Batrachians (frogs, toads, newts) have never been found on any of the many islands with which the great oceans are studded. I have taken pains to verify this assertion, and I have found it strictly true. I have, however, been assured that a frog exists on the mountains of the great island of New Zealand; but I suspect that this exception (if the information be correct) may be explained through glacial agency. This general absence of frogs, toads, and newts on so many oceanic islands cannot be accounted for by their physical conditions; indeed it seems that islands are peculiarly well fitted for these animals; for frogs have been introduced into Madeira, the Azores, and Mauritius, and have multiplied so as to become a nuisance. But as these animals and their spawn are known to be immediately killed by sea-water, on my view we can see that there would be great difficulty in their transportal across the sea, and therefore why they do not exist on any oceanic island. But why, on the theory of creation, they should not have been created there, it would be very difficult to explain.
Darwin was well aware of the significance of the geographical distribution of species for his theory of evolution. He noted that most of the facts could be accounted for if we assume that animals and plants have evolved. From this, we should expect – and we find – that modern animals tend to live on the same continent as fossils that could plausibly be their ancestors, or close to their ancestors. We should expect, and we find, that animals share the same continent with species that resemble them. Here is Darwin on the subject, paying special attention to the animals of South America that he knew so well:
the naturalist in travelling, for instance, from north to south never fails to be struck by the manner in which successive groups of beings, specifically distinct, yet clearly related, replace each other. He hears from closely allied, yet distinct kinds of birds, notes nearly similar, and sees their nests similarly constructed, but not quite alike, with eggs coloured in nearly the same manner. The plains near the Straits of Magellan are inhabited by one species of Rhea (American ostrich), and northward the plains of La Plata by another species of the same genus; and not by a true ostrich or emeu, like those found in Africa and Australia under the same latitude. On these same plains of La Plata, we see the agouti and bizcacha, animals having nearly the same habits as our hares and rabbits, . . . but they plainly display an American type of structure. We ascend the lofty peaks of the Cordillera and we find an alpine species of bizcacha; we look to the waters, and we do not find the beaver or musk-rat, but the coypu and capybara, rodents of the American type.
This is mostly common sense, and Darwin was able to account for an enormous range of observations by means of it. But there are certain facts about the geographical distribution of animals and plants, and the distribution of rocks, that need a different kind of explanation: one that is anything but common sense, and which would have staggered and enthralled Darwin, if only he had known about it.
DID THE EARTH MOVE?
Everybody in Darwin’s time thought that the map of the world was pretty much a constant. Some of Darwin’s contemporaries did countenance the possibility of large land bridges, now submerged, to explain, for example, the similarities between the floras of South America and Africa. Darwin himself was not greatly enamoured of the land bridge idea, but he surely would have exulted in the modern evidence that entire continents move over the face of the Earth. This provides by far the best explanation of certain major facts of animal and plant dispersion, especially of fossils. For example, there are similarities between the fossils of South America, Africa, Antarctica, Madagascar, India and Australia, which nowadays we explain by invoking the once great southern continent of Gondwana, uniting all those modern lands. Once again, our late-coming detective is forced to the conclusion that evolution is a fact.
The theory of ‘continental drift’, as it used to be called, was first championed by the German climatologist Alfred Wegener (1880–1930). Wegener was not the first to look at a map of the world and notice that the shape of a continent or island often matches the coastline opposite as if the two land masses were pieces of a jigsaw puzzle, even when the opposite coastline is far away. I’m not talking about little local examples, such as the Isle of Wight’s neat dovetailing into the Hampshire coast, almost as though the Solent wasn’t there. What Wegener and his predecessors noticed was that something along the same lines seemed to be true of the whole facing sides of the giant continents of Africa and America. The Brazilian coast looks tailor-cut to fit under the bulge of West Africa, while the northern part of Africa’s bulge is a nice fit to the North American coast from Florida to Canada. Not only do the shapes roughly match: Wegener also pointed to matching geological formations up and down the east side of South America and
corresponding parts of the west side of Africa. Slightly less clearly, the west coast of Madagascar forms a pretty good fit to the east coast of Africa (not the section of South African coast that is opposite it today but the coast of Tanzania and Kenya further north), while the long, straight line of Madagascar’s east side is comparable to the straight edge of western India. Wegener also pointed out that the ancient fossils to be found in Africa and South America were more similar than you would expect if the map of the world had always been the way it is today. How could this be, given the width of the South Atlantic ocean? Were the two continents once much closer, or even joined? The idea was tantalizing, but ahead of its time. Wegener also noticed matchings between the fossils of Madagascar and India. And there are similarly telling affinities between the fossils of northern North America and of Europe.
Such observations led Wegener to propose his daringly heretical hypothesis of continental drift. All the great continents of the world, he suggested, had once been joined up in a gigantic super-continent, which he called Pangaea. Over an immense span of geological time, he proposed, Pangaea gradually dismembered itself to form the continents we know today, which then slowly drifted to their present positions and have not finished drifting yet.
One can almost hear Wegener’s sceptical contemporaries wondering, to use the street-talk of today, what he had been smoking. Yet we now know that he was right. Or almost right. Far-sighted and imaginative as Wegener was, I must make it clear that his hypothesis of continental drift was significantly different from our modern theory of plate tectonics. Wegener thought that the continents ploughed through the oceans like gigantic ships, not quite floating in water like Dr Dolittle’s hollow island of Popsipetl, but floating atop the semi-liquid mantle of the planet. Reasonably enough, other scientists erected fortresses of scepticism. What titanic forces could propel an object the size of South America or Africa for thousands of miles? I shall explain how the modern theory of plate tectonics differs from Wegener’s theory before coming to its supporting evidence.