To balance the information provided on colour in animals, Chapter 7 will introduce the variety of eyes. It will show that all animals have to be adapted to the existence of eyes not only in terms of their colour, but also in their shape and behaviour - all factors affecting an animal’s appearance on a retina. When this retina belongs to a predator, the image formed on it becomes a matter of life and death for the potential prey. But is the danger of visual appearance a recent one? The history of predation will be discussed in Chapter 8. By returning to the fossil record I will show that eyes, predators and probably the link between them go back a long way. But exactly how long? This will become a fundamental question.
By the beginning of the penultimate chapter the reader will have all the clues necessary to decipher the probable cause of the Cambrian explosion. In many ways it seems the most obvious explanation, but to reach it one must take this indirect, winding road. Encountered along the way will be a number of unfamiliar but fascinating examples of the sophisticated and finely balanced ecosystems that exist in nature - but to begin with, it’s back to the bare bones and a modern perspective on the lifeless rocks once kept safely within dusty Victorian display cases. Now we are bringing the past to life.
2
The Virtual Life of Fossils
Nothing ever becomes real until it is experienced
JOHN KEATS
Beginning, as it were, with the very beginning, Chapter 1 summarised a history of life on Earth. In this chapter, the evidence used to create such a story will be examined, making a closer inspection of the rocks. But here time shall be traversed from today, travelling back to the Cambrian via some landmark attractions. And good old-fashioned fossils will provide the attractions.
Although the study of evolution is increasingly becoming consumed by genetic studies, the inferences from genetics are, and always will be, theoretical. The genes of many living species have been exposed, but the animals we see today did not evolve directly from each other. Intermediate stages were involved - species, for instance, that became extinct. So in order to reveal evolution, the genetics of the living and the extinct are required. And of course the extinct genes are, barring a few exceptions, subject to theoretical fabrication.
Fossils, on the other hand, are factual. They are literally hard facts that we cannot ignore. Around a decade ago, molecular sequences pointed to a Cambrian explosion that occurred way back in the Precambrian. The fossil record, which places an Early Cambrian label on the grand event, was thus contradictory and appeared to be standing in the way of progress. But palaeontologists stood firm, reminding us that fossils were not optical illusions. When 350-million-year-old rocks are split to reveal the fine details of a bony fish, then bony fish did swim in Earth’s waters 350 million years ago. When rocks formed under similar conditions, but from 550 million years ago, are consistently found without bony fish, eventually we must conclude that bony fish did not exist during this time. However, it would be equally foolish to ignore the genetic evidence, and indeed by reconciling the fossils and the genes a true picture of the Cambrian explosion has been painted. But whichever way they are looked upon, fossils are precious to the study of evolution. And they certainly justify a chapter of their own in this book, where the subject of fossils will surface again during discussion of seemingly unrelated topics.
It was the role of fossils in revealing the paths taken by evolution which contributed heavily to the previous chapter. The main purposes of this chapter are to expose the tricks used in creating this knowledge, but also to demonstrate that fossils have much more to say. The history book, ‘The History of Life’, conceptualised here contains two-dimensional pages. The next task is to pump blood into the flattened veins of fossils and let them spring from the pages, so ancient animals can be seen doing what they once did. The application of engineering, physics, chemistry and biology can indeed transform a load of old bones into a virtual 3D world, perhaps millions of years old, where animals run, fly, gallop, burrow, eat and avoid being eaten.
Fossils can add some surprising details to the past, and they will provide considerable hard evidence towards the Cambrian enigma that this book attempts to solve. The individual cases in this chapter will provide a flavour of palaeontology in the twenty-first century, and constitute tools for the evolutionary trade. The art of Sherlock Holmes and modern forensic science will be reconciled with that of dinosaur specialists and religious artists. Fossil leaves will be employed to aid the palaeo-meteorologist. The technology of car designers will bring 400-million-year-old ‘worms’ and arthropods back to virtual life on the computer screen. And the biology of living organisms and principles of Scuba diving will help to solve the ‘ammonite mysteries’. But to begin I will ask the question: ‘What, exactly, is a fossil?’ The answer to this is not so obvious, especially when the remains of some extinct species are so ‘fresh’ they can literally be brought back to life.
The youngest fossils
I have a colossal, antiquated book on the fauna of Earth. It is entitled Knight’s Pictorial Museum of Animated Nature and is now in its seventh generation within my family. Between the heavy, morbid black covers exist brief descriptions, biological data and woodcut illustrations for thousands of species. Some of the illustrations are quite primitive, especially the unnatural poses of monkeys quite clearly based on stuffed museum specimens. The kangaroo drawings appear like those made by the first Europeans to reach Australia, and the story is similar for the American buffalo. A quick glimpse of a very unfamiliar form can result in a reconstruction with a more familiar form in mind. A buffalo could become cow-like, and a kangaroo could acquire some of the features of a hare. Here lies a lesson in fossil reconstructions - extrapolation can be risky, at least beyond a reasonable point. Crocodiles may be the closest living relative to certain dinosaurs. Although it may be safe to infer a similar scale-like skin texture, as we can confirm from recent finds of fossil skin, the sluggish quadrupedal form with a belly that scrapes the ground is probably a characteristic of the crocodile only. Yet pioneers of dinosaur reconstructions depicted the Diplodocus with its belly scraping the ground. That’s fine - we need mistakes from which to learn (and mistakes are everywhere in science). Nowhere is this principle of extrapolation more dangerous than in the colour of extinct animals, as will be demonstrated later in this book.
Figure 2.1 Butterworth’s 1920s illustration of Diplodocus walking, crocodile-style.
Knight’s Pictorial Museum also contains information on fossils. At the interface of the living and fossil species lies the dodo, an animal we know so much about through the written accounts of seventeenth-century travellers who descended on its native Mauritius, yet it has been extinct since at least the time of Knight’s Pictorial Museum. But an even more detailed account of behaviour is given for the great auk and Tasmanian tiger, both of which, distressingly, appear in the section of living animals. The great auk and Tasmanian tiger are now extinct.
The feet and the skin from the beak of a dodo are preserved in natural history museums in London and Oxford. A great auk in its entirety can be seen stuffed in a penguin-like pose in London, and complete Tasmanian tiger specimens, which survived to see the twentieth century, are more common. Maybe there are many more cases like these. We are living in the harshest extinction event of all, which highlights the growing importance of natural history museum collections. One day I became interested in the colour of stick insects, and while I was exploring the entomological cabinets of the Australian Museum in Sydney, my attention was drawn to a giant specimen from Lord Howe Island in the Pacific. Unfortunately my request for a loan of this fragile specimen was rejected on the grounds that it was collected over a hundred years ago and was the last of its kind. But can we classify specimens that contain their original, organic parts as fossils, even though their species are now extinct? Maybe the age (relatively youthful) of the specimens (in geological terms) in these particular cases provides a strong bias against a fossil categorisation - our not-to
o-distant relatives could have collected them.
The question as to what defines a fossil becomes more interesting when the subject derives from a more distant epoch. The first mammoth appeared 150,000 years ago, into the second to last Ice Age. The mammoth spread through northern Asia, America and Europe, sharing its environment with giant ground sloths, sabre-toothed cats and bighorned bison. Precisely 20,380 years ago, one individual, 8-ton, 11-foot-tall male mammoth died on the frozen plains of Siberia at the age of forty-seven, thirteen years short of the average life span of a mammoth. If ancient animals are considered in this way, they become animals that once lived, rather than animals that are now extinct. To effectively bring an extinct animal back to life is a palaeontological goal, but in the case of the mammoth we have evidence well beyond the norm.
Much is known of the mammoth’s lifestyle through discoveries of ancient human cave dwellings. Piles of bones and tusks belonging to mammoths have been found alongside stone-pointed spears, suggesting that humans were mammoth hunters. And they were probably significant mammoth hunters. Numerous Ice Age caves have been discovered with pigments preserved - primitive paintings depicting scenes of large-scale mammoth hunts. These pictures have even prompted theories that the mammoth was the first species to be wiped out by humans. Maybe if we had earlier had preserved specimens of the mammoth we could have conducted forensic examinations to discover the extent of hunting with spears. Well, now we have one.
One day in 1997, a nine-year-old Russian boy from the Zharkov family set out to hunt reindeer in the frozen wastes of Siberia. All seemed quite normal until an unusual whitish object came into view against the blue horizon. That object became a pair of objects as the boy approached, and soon they could be identified with accuracy. Protruding from the frozen ground, or permafrost, were the tusks of a mammoth - the individual that, it would transpire, had died 20,380 years earlier. Such a sight was familiar to the rest of the Zharkov family - mammoth tusks no longer make the news in Siberia - but there was something different about this particular find. The Zharkovs brought these tusks to the attention of the scientific world because they were attached to a block of ice with signs of flesh and thick tufts of fur. That made scientists sit up and listen. The first country to secure funding for a mammoth autopsy was France.
Two years later, in 1999, an unusual operation by French Arctic scientists began. A Russian helicopter was employed to raise a huge, cubic block of permafrost, complete with massive tusks projecting. Within this block, it so happened, was a complete mammoth, almost perfectly preserved in its icy tomb. The mammoth, initially frozen to -50°C by searing winds, was airlifted 200 miles to the city of Khatanga. If the sight of that alone did not create some amusement, the event that followed certainly did - for six weeks scientists stood around the mammoth defrosting it with hairdryers. But the scientific team had the last laugh when they became the owners of a museum-quality mammoth specimen, complete with DNA. The hairdryers not only warmed the ice but also dried the skin and muscles, thus aiding preservation.
At the moment, the French team is conducting a thorough forensic examination on the 20,380-year-old mammoth carcass with the aim of determining the cause of death. This could provide support for the theory of a human-driven extinction, or supply evidence towards an alternative idea that the species succumbed to malnutrition following a dramatic climate change. A spear would leave its telltale impression in frozen flesh, but maybe not so in a skeleton. A skeleton also would provide little evidence of malnutrition. So there are certainly limitations to interpreting the past using only the bones, but, as we will see, we have further tricks up our palaeontological sleeve.
As to whether this mammoth could be considered a fossil, the answer is really not so important. Here the original organic material is preserved, like the skin and bones of Egyptian mummies. In the true tradition of a fossil, a carcass is entombed within a material of some description before decomposition by microbes takes over. This can happen via sedimentation, when mineral particles falling out of the water blanket the carcass on their way to forming the sediment or substrate that constitutes the sea floor. Then minerals from within the substrate replace the organic material. The precise forms of the ‘replacement’ minerals become different from those in the substrate; thus the fossil is separated and easily identified from the surrounding matrix. But sometimes only part of a newly deceased carcass becomes fossilised and the remaining organic material is preserved unaltered. Since this balance can shift in either direction, it is academic whether we apply the term fossil to an ancient specimen with 1 per cent replacement minerals and 99 per cent organic material, but not to a specimen with 100 per cent organic preservation. Either way the dead animal has left its mark for palaeontologists to find.
Additionally the fossilisation process itself can occur in varying degrees of complexity. The outlines of bones only can be saved as fossils. But then sometimes the skin, organs and internal parts of bones can be entered into the fossil record too. When the fine detail is preserved, physical information can be extracted equally from a true fossil or preserved organic material. We know that mammoth tusks were optimally strong due to their construction. The stacks of thin, corrugated layers of alternating material provide greater strength and toughness than do either thick layers of alternating materials or stacks of thin layers that are flat in profile. Plywood and corrugated iron are strong for these reasons, respectively. This information can be extracted from both truly fossilised remains and original organic specimens. Less well-preserved fossils, on the other hand, bear only the outline of tusks, providing information on their size and shape only. But there is one important difference between the well-preserved fossils and organic remains, and one that is showing signs of great scientific potential - the preservation of nucleic acids.
The boundary between organic remains and classical fossils becomes increasingly fuzzy when the organic subjects are seventy million years old. Surely remains this old must be considered fossils? Insects that lived seventy million years ago have been preserved in amber, in all their organic glory. Flies coming to rest on tree trunks today occasionally find themselves sticking to the yellowish sap that seeps through bark. The more the flies struggle, the further into the sap they sink, quicksand-style. Seventy million years ago, flies came to a similarly sticky end. The sap eventually hardened and entombed the flies for ever. This hardened sap, called amber, provides a barrier to microbes and chemicals, so the organic material of the fly remains unchanged. Embarrassed by the age of these specimens, palaeontologists have coined the term ‘sub-fossil’ for such nonconformists. The flies in amber, particularly the blood-sucking mosquitoes, are also the most famously controversial group to be considered for the preservation of nucleic acids.
Some of the more spectacular dinosaurs lived seventy million years ago and were probably the victims of mosquitoes. The idea that dinosaurs could be brought back to life based on dinosaur blood preserved within ancient mosquitoes is now a distant one. What rained on this particular parade was contamination - the apparently ancient DNA from dinosaurs was in fact recent DNA, from a contaminant within the molecular lab. Now it is generally believed that nucleic acids cannot survive such periods of millions of years. But the methods planned for converting genomes into a living, breathing T. rex have been retained.
Microbiologists routinely revive 10,000-year-old microbes from Antarctica. Living microbial spores can be dispersed by the wind, and some have the misfortune to land on the ice of Antarctica, where the spore cells immediately become dormant. They shrink in size and shut down all metabolic activity.
The Russian station at Vostok is one of the most uninhabitable in Antarctica, situated at the very centre of the continent. At Vostok the ice is drilled and cores are removed, and the ice at the bottom of a core can be up to 500,000 years old. In 1988 an American microbiologist found spores locked in part of a core containing ice 200,000 years old. Miraculously, on warming the spores live bacteria emerged, which could be
cultured as if 200,000 years had never elapsed. This signals hope of reviving other nucleic acids up to a similar age, and is viewed as an unconditional green light by the mammoth team.
The French owners of the 20,380-year-old mammoth have high hopes of extracting DNA from the frozen cells and cloning a new mammoth - one that walks and does the things a mammoth did, things we would like to know. One Japanese scientist likes the idea so much that he is scouring Siberia and Alaska for a frozen mammoth of his own. Similarly, nucleic acid from an appropriately preserved Tasmanian tiger (‘thylacine’) pup has been extracted, with cloning intentions, at the Australian Museum. Here, cloning methodology, which utilises the closest living relative as a surrogate mother, is under investigation. In addition to cloning such old DNA, pitfalls to consider include its compatibility with chromosomes from a different species, followed by the acceptance of a foreign embryo by a surrogate mother. Then, if the cloning is successful, scientists must aim to avoid sterile creations. Mules, for instance, are almost invariably sterile because they amass an odd number of chromosomes - thirty-one from the donkey parent plus thirty-two from the horse parent. The sixty-three chromosomes in the mule’s body cells divide randomly into thirty-one or thirty-two in the reproductive cells. When two mules mate, the pairs of gametes are so unevenly matched that the chromosomes simply cannot pair up. But if novel cloning methods did succeed on the Tasmanian tiger, a new science would dawn. And in any case, sequences from ancient nucleic acid would be useful to fit into those evolutionary analyses that otherwise rely on predictions when dealing with creatures that are extinct.
In The Blink Of An Eye Page 7
