Survivors: The Animals and Plants that Time has Left Behind. Richard Fortey
additional evidence for the velvet worm’s place on the tree of life has come from the genome of the living species. Ancient fossils do not preserve DNA, which is a large and delicate molecule, readily fracturing into pieces. But by studying the molecules of living survivors from deep branches in the tree of life we are afforded a kind of telescope to see back in time. For the genetic code of DNA records another kind of history, it retains the accumulated narrative of all the changes at the fundamental molecular level that have built up slowly over time. Mutations that have been incorporated in the genome provide a kind of ancient fingerprint. But the code of life is famously huge – which means that the investigator may be obliged to seek out the particular piece of the genome that contains the information he needs. Although, as this is written, more and more organisms are having their entire DNA sequenced, this is still the prerogative of a privileged few – unsurprisingly, those like wheat or influenza that have a particular importance to Homo sapiens. For many organisms, it is more feasible to use a particular chunk of its genetic code to compare with the same chunk from a range of its potential relatives. This might be a particularly suitable gene or series of genes, for example, that do not change too rapidly to be useful through long periods of geological time. Obviously, the chosen gene has to be present in all the organisms under study. Other workers favour sequencing parts of the RNA molecule in the ribosomes that are present in the cells of all living organisms as the centres for protein synthesis. Comparing the similarity of gene sequences is one way assessing how closely (or not) organisms are related to one another. The results can be drawn up as another kind of tree, with branches drawing together the closest related species, and deeper patterns of branching inferred from still more fundamental inherited similarities. This is not as easy as it might sound from this bald description, as various kinds of ‘noise’ can obscure the signal the investigator seeks, and there are always genes that change too fast to retain meaningful signals from deep time. I need hardly add that computer programs have been designed to help out. The technical problems are not part of our story, except in so far as they have produced different ‘trees’ of relationships between organisms since the methods were first developed. Indeed, early attempts sometimes look quaint or improbable. But recent studies seem to have stabilised, and produce trees that appeal to prior knowledge and common sense, mostly by lumping together evidence from many different genes and finding the best fit. These then make a meaningful contribution to the summary trees of evolutionary history like those on our endpapers. The latest molecular analyses to treat the velvet worm and its relatives show interesting results. It places our chosen survivor as the bottom branch of a tree that includes all the arthropods above it – which must therefore have arrived later. Another name appears between the lobopods and the arthropods. This is Tardigrada (water bears), a group of tiny creatures that often live between sand grains and in other cryptic habitats. They are interesting in their own right, but they have but one known fossil, so they will not be described in detail here. Many tiny animals have no fossil record at all, but that does not mean that they did not exist in the past. The important point for us is that the molecular evidence supports the idea that lobopods are a branch even lower on the tree of life than arthropods. Those stumpy legs have walked on and on from a time even before the Cambrian. The very earliest Cambrian strata contain the traces of animals, but not their bodies. This is probably because those early animals lacked readily fossilisable hard parts, and the special conditions required to preserve the slightly younger Chengjiang fossils were not present at this particular time. No matter, for some of the tracks and trails that are preserved as fossils show clearly the traces made by arthropods of normal size digging their way into soft sediments with their numerous paired legs. It is even possible that these could have been tracks left behind by soft-bodied ‘proto’ trilobites since they are similar to tracks made by the same animals higher in the geological column; at the moment we simply do not know. But we now do know that there must have been lobopods on that same sea floor, too, stomping ever onwards. More than that, they must have been present even earlier, before the first arthropods, because both the molecules and the anatomy of the animals tell us that they preceded the jointed-legged organisms. This takes us back into the mysterious world of the Ediacaran, a period whose remains lie above the Precambrian, and below the Cambrian, before the time of abundance and variety of marine life and before the appearance of shells.*
The story of the lobopods now disappears. There are no velvet worms or indeed any kind of lobopods in strata of Ediacaran age. There has been no shortage of attempts to find them. Geologists and palaeontologists have been cracking open likely rocks for decades now. The fact is that there are no trilobites, no early horseshoe crabs, nor any old familiar biological friends to be found in Ediacaran age strata. As in The Hunting of the Snark by Lewis Carroll searchers vowed: ‘To seek it with thimbles, to seek it with care; To pursue it with forks and hope’, but to no avail. Even big hammers did not work. Instead a whole series of fossil animals have been recovered which have proved as enigmatic as they are exciting: not snarks but boojums. They are not small – some of them are bigger than a dinner plate – and neither are they uncommon if the searcher goes to the right place. The Ediacaran Period takes its name from the Ediacara Hills in the Flinders Ranges in South Australia where a diverse selection of these remarkable early fossils was first collected. They appear as impressions on fine sandstones, many looking like strange leaves or fronds. Most of them show evidence of divisions or compartments dividing up the body, but they are not simple segments, because they are usually offset from one side of the animal to the other. Similar fossils are now known from more than thirty localities all over the world: from Arctic Russia, Canada, America, Newfoundland, and Great Britain. Everyone agrees that these fossils lacked skeletons, but otherwise the experts disagree on almost everything else. Most of them would now concur that the Ediacaran animals were not obvious ancestors of the animals we know from the Cambrian onwards; they were genuinely inhabitants of a former world that did not survive. It seems only fitting that in a book about survivors I should also go to visit a world that failed to endure. The journey took me back to Newfoundland, where I had spent a year at Memorial University in St John’s when I was a young scientist. So I was travelling into my own past as well as towards a far, far deeper time.
Newfoundland is an island at the tip of eastern Canada and is itself something of a survivor. Built on the fortunes made from codfish on the Grand Banks, it has survived the great crash in the population of its most important crop. It is the textbook case for the effects of over-fishing. In the thirty years I have known the ‘rock’ (as the natives call it) I have watched with bewilderment as fishermen have laid up their boats, and an apparently endless resource has all but disappeared. The codfish has not become extinct, of course, but the decline of this otherwise unfussy fish does prove that nothing in nature can be assumed to be unassailably fecund. High-tech factory ships from outside the island indiscriminately scooping up huge quantities of fish are mostly to blame. The Newfoundlanders, ever resourceful, have now taken to oil. The name of the Come-by-Chance refinery is somehow appropriate to their persistence in the face of setbacks not of their making. The little fishing villages along the coast are known as ‘outports’, and ever since they have been required to eschew the cod, those young outport men who have not gone to Come-by-Chance have left to find work at Churchill Falls, the huge hydroelectric plant in northern Labrador, or even to become hands on the extraction of the Athabasca ‘tar sands’ on the other side of Canada. They are a breezy bunch, despite their peripatetic life, and have an unusual accent: Irish with added stretched vowels, and wheezy interpolations of interjections like ‘Jeez, my son’. The outports are all freshly painted these days, with wooden houses in cheery colours scattered up the hillsides. For the few who stay behind, there is nothing much to do except repaint the picket fences.
The drive south along the Avalon Peninsula from the capital St John’s passes several sheltered coves tucked away inside a coastline of magnificent cliffs. The geology is laid bare all along the rim of this island: the only problem is reaching it. Inland, the opposite is true; an endless forest of short conifers interspersed with scattered birch and aspen trees is interrupted only by shallow lakes called ‘ponds’ hereabouts, which are a legacy of the last ice age; the bedrock is hard to see among the scrub. As we approach the end of the Peninsula the trees get shorter and shorter, planed off by the fierce winds. Finally they crouch against the ground, as if terrified to poke up a twig. Usually the whole of this exposed area is swathed in fog, so the landscape supplies a passable setting for a vampire movie starring Vincent Price. But the