Ancient DNA has been put to another use in mapping the geographical history of disease. The plant pathogen responsible for the nineteenth-century Irish potato famine, Phytophthora infestans (late blight), has been identified by sequencing DNA from museum and herbarium samples of infected potatoes and tomatoes. The ancestral clone of this late blight was believed to have arisen in Mexico and been widespread during the past century. But recently the strain responsible for the potato famine was found to be different, having a South American origin. Late blight remains active today, and if its true history is known, future geographical spread may be predicted. This case also provides further justification for natural history museum collections - as nucleic acid banks that preserve the genetic diversity. But returning to the subject of bringing ancient, extinct animals to virtual life, if the genes can’t help us at present then we must return to the fossils and so-called sub-fossils.
Our supply of sub-fossils begins to dry up in rocks older than 100 million years. But exceptions do exist, and indeed provide some very important evidence in this book. For now, though, it’s back to the genuine, reliable fossils and what they can teach us.
Old bones, new science
Palaeontology has been strong for well over a century. Fossils themselves have provided solid foundations, and the house of palaeontology has risen with walls of equal strength, built with successive blocks of compatible theory. Throughout this construction fossils have always been reliable, but their interpretations - the building blocks - are occasionally flawed. And misconceptions that establish themselves in palaeontological law must eventually be amended. One of these flawed building blocks could exist near the base of the mammalian evolutionary wall.
Most mammals, including ourselves, nurture their developing young within a uterus and are called placentals. Placentals were thought to have originated in the northern hemisphere more than 100 million years ago. Later they spread throughout the globe, and in doing so forced the other two groups of mammals - the egg-laying monotremes and the pouch-bearing marsupials - to retreat. The monotremes (like the platypus) and marsupials (such as kangaroos) retreated towards Australia, their main place of existence today. In fact, the first land-based placentals were believed to have migrated to Australia a mere five million years ago. A nice, neat story that became established in zoological textbooks, an evolutionary classic. But now to throw a spanner in the works - to be precise, a 115-million-year-old jawbone found by a British volunteer working on a beach in Melbourne, Australia, in 1997.
The tiny jawbone, just 16 millimetres long, holds eight of the most controversial teeth ever discovered. Three of the teeth are molars and five are premolars - a characteristic of placentals and not marsupials, which usually have four molars and three premolars. Then there is the shape of the teeth. They are adapted for slicing and crushing food, a feature not found in monotremes. Also, one premolar seems to be departing from the standard triangular form and is almost halfway to becoming the more elaborate form characteristic of molars. Again, this fits with the placental code but not with that of marsupials.
Nearing the outcome of this controversy, the jaw in question has been extrapolated on a computer screen to become a virtual shrew - an insectivorous placental named Ausktribosphenos nyktos, after its discoverer. Could this interpretation represent a crack in an esteemed palaeontological wall? The notion of a placental mammal running around in Australia 115 million years ago would certainly turn our view of mammalian evolution upside down. However, this story has yet to reach a satisfactory conclusion.
A new idea is emerging from the University of California at Berkeley that A. nyktos is neither a monotreme nor a marsupial, and not even a placental, but rather a new group that was either converging with other mammals or running parallel with them, before eventually dying out. The evidence for this theory derives from further refined analyses. It happens that the shape of the depression at the back of each molar is uncharacteristic of placentals.
So when the A. nyktos building block is cemented to the mammalian evolutionary wall, is the construction strengthened or does it fall? For now we must lay down our tools and search for the complete trail of mammalian clues that wait to be unearthed. Maybe some earlier finds would benefit from re-examination. Certainly, modern analytical methods in palaeontology are becoming increasingly refined and sophisticated, and can reveal a surprising wealth of information from even the tiniest portion of a skeleton. And advancements in reconstructing the extinct will become a theme of this chapter. Sometimes it will cause established pillars to fall, and sometimes to rise even higher. But first it is worth pursuing the idea of ancient animals migrating between continents, and how this can be possible considering the immense perimeters of water we see on today’s globe.
The active Earth
The landmasses that form our continents are not static; they are plates of rock that are continuously moving around within the Earth’s crust. These movements are not just horizontal, they are vertical, too. They affect the land both above and below the water. Geological faults are evident at the deepest regions of the ocean, where plates moving in opposite directions eventually tear apart, presenting an opportunity for molten lava from beneath to seep through the gaps formed and into the water. This can be the making of a hydrothermal vent - the black smoker introduced in the previous chapter. On a grander scale, the Hawaiian islands were formed by this very activity. Comparable faults on land can result in the eruption of volcanoes, and the tearing apart at one part of the globe means a crashing together at another part. The Himalayas formed when the Indian plate, once bordered only by sea, crashed into the Asian landmass. This event saw the pre-existing Asian coastline forced upwards, effectively turning the Earth’s plates upside down.
Deserts have not always been areas of desolation. Coral reefs have not always existed where they are found today. In the scheme of global history deserts and coral reefs can be associated by their geography - they may share the same geographic coordinates. The Great Basin area of Nevada, Utah and California forms one of the most significant deserts in the United States. Yet in the rocks of higher altitudes can be found fossilised ecosystems that existed some 510 million years ago - underwater. Corals, moss animals, arthropods and many other animal phyla are represented in abundance after their otherwise shallow graves were entombed forever by a fateful wave of mud. The mud turned to stone and fixed the life forms forever, but in different geographic locations through time. Their graves were transported with the movement of the Earth’s plates. First they were lifted out of the water, then high into the air, until they are exposed on a mountainside today. In fact the Burgess Shale fossils embarked on a similar journey. But the Earth’s plates continue to move, and maybe in another 100 million years these fossils will travel full circle and return to their watery origins.
Reconstructing ancient environments
The conception of a mobile Earth’s crust is known as plate tectonics. This subject becomes extremely important when contemplating the original environment of a fossilised animal. A point on the Earth’s crust can change its longitude and latitude, and it is perhaps the latitudinal change that is the more significant. That is the movement most responsible for a climate change. So a fossil found in a hot desert need not belong to a hot desert dweller. But the biggest clues to a fossilised animal’s precise environment can be found in the surrounding fossils, particularly the plants. Aquatic plants are quite distinct from their counterparts on land. But Cambrian life was exclusively marine, so in order to improve on Cambrian biology we should distinguish further than between simply land and aquatic environments. And if one looks even more closely at the entire fossil community, one really can be more specific. The presence of photosynthetic algae in a fossil assemblage indicates that this community lived under reasonable levels of sunlight, placing the extinct environment within the photic zone - between the ocean surface and around 90 metres in depth. Similarly, biology can be inferred from fossilised land-based organisms.
For instance, we are beginning to map the fine variations in the external skeletons, or exoskeleton, of living beetles.
Beetle exoskeleton is effectively constructed of thin layers, laid down parallel to the outer surface. If the individual layers are relatively thick and corrugated, then the beetle can withstand high temperatures. If there are many pores in the exoskeleton then wax can be secreted to prevent it drying out. A combination of both characteristics indicates an adaptation to deserts. Beetles from temperate climates tend to possess flat layers in their exoskeletons, where all the layers are thin except for a very thick outer layer which provides physical protection. The exoskeletons of cold-adapted and aquatic beetles are different again. So the structure of beetle exoskeleton could be considered an indicator of temperature or other properties of an environment. But can this theory be applied to the geological past? Interestingly, well-preserved fossilised beetles exist with their exoskeletons intact, such as those from the twenty-five- to thirty-million-year-old fossil site at Riversleigh in Australia. Maybe further information on the original Riversleigh environment really can be deduced from its beetles. And the potential for linking fossil anatomy with ancient environments has been bolstered by a study on plant leaves.
For over a century it has been known that increased levels of carbon dioxide in the atmosphere lead to an increase in temperatures. Similar conclusions were more recently drawn from analyses of air trapped in ice cores 420,000 years old, an age where temperatures are known. Unfortunately, studies on ice cores are restricted to the last half a million years. So to link carbon dioxide to some of the really important events in Earth’s biological history, new ways of tracking the history of this gas are needed. One ingenious new way could be to use our extensive collections of fossil leaves.
Plants require carbon dioxide for photosynthesis. The gas is taken up through valve-like pores that occur on the surfaces of leaves. It is understood that the past 200 years have witnessed increased carbon dioxide levels as a result of industrial fossil fuel consumption. It is also known that plants have responded to this increase by producing fewer pores on their leaves. In fact there is a distinct inverse relationship between the concentration of carbon dioxide in the atmosphere and the density of pores on leaves. And now this relationship has been exploited by a palaeontologist in possession of fossil leaves from Ginkgo trees and their like, up to 300 million years old.
Within the collection rooms of the University of Oregon, dust was blown from the stacks of ancient leaves, which overlap in the fossil record, and the proportion of pores was determined. This resulted in a complete count of pores over the past 300 million years. From the pore counts, the levels of carbon dioxide have been predicted over this vast period. In turn, 300 million years of atmospheric temperature has been discerned. Impressive work, proving again that it can be worthwhile waking the sleepy, forgotten museum collections.
Marine geochemical data accurately predicts the temperature of the more recent part of geological history - and this matches the predictions from pore data. To test the predictions further back in geological time, one can turn to the record of sedimentation and the oxygen-isotope record of marine fossils. The oxygen-isotope data shows only trends in temperature over the past 300 million years, but the peaks and troughs do conform well with those from the pore data. Both data sets infer that periods of low carbon dioxide prevailed between about 296 million and 275 million years ago, between thirty million and twenty million years ago, and during the past eight million years. And the sedimentary record of glacial deposits in high-latitude regions indicates comparable trends. The periods of low carbon dioxide do appear to coincide with the periods of cool, ‘ice-house’ modes of Earth’s climate history. But the pore data is the most useful because it has the finest resolution. This information could be invaluable when considering the cause of extinctions of ancient fauna that occurred at precise moments in geological time. Global warming or cooling could indeed push animal chemistry beyond critical barriers.
An opinion from Utrecht University in the Netherlands agrees that concentrations of carbon dioxide will emerge as the main factor of temperature and climate during the past 500 million years. But it is warned that plenty of other ingredients would have been added to the climate cauldron at different times and to different extents in the past. Changes in the configuration of continents, topography such as mountain building and ocean circulation can all have a profound influence on climate. But there are also planetary factors to consider, such as changes in the Earth’s orbit or the angle of its axis, as well as solar brightness. Any of these elements could have affected atmospheric temperature and played an indirect role in major evolutionary events over the past 500 million years. But then there are suspects other than temperature which have the potential to induce macro-evolutionary events. Because the enigma central to this book lies beyond the 500 million years in question here, I can afford, fortunately, to leave this problem to others. Nevertheless, this has been a nice demonstration of how fossils can indicate past climates and ultimately help to reconstruct ancient environments. Now we can continue along the palaeontological path and consider the animal inhabitants of those environments and the marks they have left behind.
Palaeontology - the first forensic science
The word fossil derives from the Latin, meaning something dug up. Until the eighteenth century, any unusual object dug out of the ground was known as a fossil. In medieval Europe crystals such as amethyst and ancient man-made arrowheads were considered fossils. To North American Indians, dinosaur bones were thought to be the bones of giants that once populated the Earth. But what were they supposed to think? Today we are able to travel globally and enjoy the benefit of international knowledge. We are all familiar with tigers, elephants, emus, sharks and crocodiles even if we are unlikely to meet them all in their natural environments. But what might have been the thoughts of ancient Greek adventurers as they first set foot in Egypt, to be confronted by a crocodile? Such a creature would have been no more alien to the ancient Greeks than a dragon is to us today. Maybe Greek mythology was not so unbelievable in 500 BC. Or maybe thoughts of evolution were formulating in the minds of disparate, ancient people, as they were in those who lived nearer to the Darwinian age. Of course, any such thoughts must have been kept to the individual’s own self, and the safer option of mythology formulated.
Amon was an ancient Egyptian god often represented as having the body of a man but the head of a ram. The ammonoids were a group of molluscs, long extinct, related to the octopus and squid. They possessed shells that were often coiled spirally and are found commonly as fossils today. As their nomenclature suggests, fossil ammonoid shells, or ammonites, were thought to be the horns of Amon. Admittedly, some ammonites do look like rams’ horns, but ammonites can also resemble the shells of a living marine animal also related to squids - the nautilus. So can one employ the nautilus to bring the ammonoids back to virtual life? This is a significant question and, before jumping to any premature conclusions, it is worthwhile examining the techniques available to perform such a feat, which include the forensic methods for reconstructing human images. These methods have even been employed to reconstruct the most famous image of all - the face of Jesus.
Researchers have devised what is said to be the closest possible likeness of the historical Jesus, producing an image far removed from centuries of convention. The skull of a Jewish man from a first-century burial and the latest forensic techniques were combined to create a virtual image that challenges the stereotype in use in art since the Renaissance.
Before the second century Judaic tradition upheld a ban on the pictorial representation of God. Thus only the symbolic representation of Jesus could be depicted, bestowing the form of a fish or a lamb. St John’s Gospel includes the statement ‘I am the good shepherd’, and in the earliest figurative representations of Jesus he was portrayed as the Good Shepherd. Later, when Christianity replaced the Roman Empire, Jesus was boldly illustrated as the King of
Heaven, and gained the features of the stereotypical Roman aristocrat - he appeared older, more authoritative and beardless. But the Byzantine Church always preferred the bearded Jesus, and so this image became the standard everywhere. Hence a pillar of credibility was constructed that, like those of palaeontology, has proved difficult to topple. Giotto and Raphael among others continued with the bearded tradition that has remained popular up to the present day.
In 2000, road construction workers unearthed a group of skeletons in Jerusalem. Israeli archaeologists studied the alignment of the graves and the artefacts in the surrounding earth to conclude that the burial site was first-century Jewish. All of the skulls found were quite distinct from others of different ages and of different regional tribes. One skull was selected as being a good representative of the group, and typical of the kind of person that would have lived in Jerusalem in the first century AD.
Skulls determine the shape of a face, including the eyebrows, nose and jawline. To bring the Jerusalem skull to virtual life, it was handed over to a forensic expert in England, at Manchester University. Strips of clay were layered upon a plaster cast of the skull, in the proportions known from human postmortems. (This method was employed successfully to identify the remains of a King’s Cross fire victim in London in 1987, and can generally boast a 70 per cent success rate for similar identifications. In that case the head still had skin, but the skin colour, and colour and style of the hair, remained in question.)
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