Unlocking the Past Read online

  Further inland, conditions are still sufficiently dry to conserve the bodies of animals farmed in the hills. These include the preserved llamas and alpacas to which we shall return in Chapter 6, preserved for over 1,000 years below the house floors of the prehistoric Peruvians who had offered them up in sacrifice. So pristine were the coats of animals buried in this way that the colour and quality of their wool could be studied as if the animal had only just died.

  The persistence of such organic remains has not gone on entirely without water, which is never completely absent. All organic materials start by containing at least some water, and even a seed that feels dry may be composed of more than 10 percent water by weight. Just as there are organisms that can unlock the oxygen from seemingly oxygen-free remains, there are also organisms that can release the water from seemingly dry organic materials. Even in our hottest, driest deserts, decay is not entirely curtailed. Just as John Parkes’s deep ocean sediments display one aspect of the resilience of life on earth, another aspect of this resilience can be found within the pans of salt that accumulate on parching desert surfaces.

  As those salt deposits form, the highly saline pools from which the water is evaporating do contain life. A bloom of highly salt-tolerant bacteria endows those pools with a reddish hue. They are probably the most salt-tolerant organisms alive, manufacturing their water from long-chain organic molecules. Bill Grant and his colleagues at Leicester set out to look for these specialized organisms beneath the surface, where the salt lakes had turned to solid rock. Deep in the ground, in rock salt laid down 260 million years ago, he found living, reproducing bacteria. They were pursuing their incredibly slow life-cycles imprisoned in tiny vacuoles of hyper-saline solution, in a world as seemingly static and hostile as that encountered by John Parkes a kilometre below the sea-bed.

  In comparison with these extremes, the shallow terrestrial sediments into which archaeologists probe will always be relatively hospitable to the organisms of decay. We may excavate objects whose decay has been severely curtailed, but not fully arrested. If this is not much in evidence from a visual inspection of what look like intact objects, it is often betrayed by the state of the molecules within.

  burning up and breaking down

  Many of the molecules that make up life take the form of a chain. A sequence of smaller molecules, all rather similar to each other, make up the links. The end result is a sizeable molecular filament that can serve a variety of functions. It can be a strengthening device, like proteins such as collagen and keratin, and carbohydrates such as cellulose. It can be one of the cell’s molecular ‘machine tools’, for example the enzyme proteins and RNA. Or it may be like DNA, a filament of coded information, a blueprint from which life’s instructions are read. All of these long-chain molecules, or polymers, are vulnerable to the kind of biological breakdown that has been slowed, but not stopped, in the extreme contexts described above. Severely curtailing that biological breakdown is however insufficient in itself to protect the molecules. Even when biological activity is minimal, these long-chain molecules can still end up in a sorry state. This is because biology is not the only thing breaking them down. Chemistry too will do the damage.

  The two principal chemical villains are oxidation and hydrolysis. Oxidation eventually leads to a kind of burning up of organic molecules, transforming them into small molecules such as carbon dioxide and water. Where things survive at all, it is because oxidation has been at most a partial process, attacking only the most vulnerable and exposed sections of large biomolecules. Much biomolecular archaeology now involves working with molecules that have been thus disfigured by oxidation. Their analysis entails working back from this molecular debris to the pristine form. The other major chemical villain, hydrolysis, literally means ‘breakage by water’, and that is precisely what happens to these long-chain molecules. Even very small quantities of water can begin to break the links that make up the chain. In the course of time, the long-chain molecules in dead organic tissue get shorter and shorter. The strengthening proteins lose their strength and elasticity, and the informative DNA loses its code.

  What these two major chemical processes have in common is that they are very sensitive to heat. An ancient body exposed in the hot desert sun may superficially look as intact as one buried beneath the glacial ice, but such similarities in appearance are often illusory. The desert body may be brittle and crumbly to the touch, while the ice corpse may retain some of its flexibility, a sign that in the latter the long elastic protein filaments are at least partially intact. Even a few degrees of difference in temperature can make a major impact on the state of biomolecules, because of how temperature affects chemical reactions. Variation in temperature is not one of the factors to which archaeologists paid great heed before biomolecules came on the scene. It is now seen as an important variable affecting preservation. The world’s coolest regions, towards the north and south of the globe, and at high altitudes, have a lot to offer biomolecular archaeology.

  the plywood principle

  Let us look a little closer at the partial nature of biomolecular preservation. Imagine a sheet of plywood. Viewed straight on, it looks like any other sheet of wood–the only pattern being that of its grain. It is only along the edge of the plywood that we can see its layered form, from which its great strength is derived. Now imagine a similar sheet in which all the alternate layers have decayed. Viewed straight on it looks the same. It is only when we flex the sheet and it snaps in two that we discover that its strength has gone.

  A number of preserved organic materials are rather like this. Their strength comes from a partnership between two different molecules, minutely intermeshed to confer upon the tissue a strength that neither molecule could achieve alone. Sometimes both those materials will survive the biological and chemical effects of time. More often, one survives rather better than the other.

  disappearing bodies

  One clear example can be found in the final remains of a human or animal body placed beneath the ground. Skeletons have been central to the search for ancient molecules, partly because they provide access to our own species, and partly because bone is a widespread and familiar archaeological material. It is of one of nature’s plywood analogues, deriving its strength from a coupling of distinct molecules, a mineral and a protein. The mineral is the alkaline substance hydroxy-apatite, which is very vulnerable to any acid solution in the immediate vicinity of its burial. The protein is collagen, and this is vulnerable to both chemical and biological attack. A cold, dry, alkaline sediment provides the best medium of preservation, acid sandy soil probably the worst. A burial in the latter may retain nothing except the most resilient part of the skeleton, the teeth, together with a slight stain betraying where the body once rested. Between these two extremes there is a range of possible states. Even within a bone that looks intact from the outside, a host of subtle chemical transformations may have taken place.

  These transformations begin as soon as the body dies. The skeleton itself is alkaline, but the flesh around it is mildly acid. This is something with which the living body can cope but, after death, the soft and hard tissues enter into chemical combat. The ancient Egyptians understood this conflict, and one of the first things they would do in preparing a mummy was to take out the large soft organs, conserving them separately.

  The combat also has a biological dimension. Soon after death, the body’s own recycling processes go into overdrive. Organs such as the liver, kidneys and pancreas have a particularly large number of lysosomes–cellular bodies whose function is to clean up dead tissue that appears within the organ. As these lysosomes begin to break the soft tissue down, the acid juices and enzymes so released eat into the bones, allowing bacteria and fungi to gain a foothold. Once the bone becomes porous in this way, it has lost its first line of defence, a compact exterior, and speedily follows the flesh back into the biosphere.

  A seemingly inert mineral object thus retains a considerable vestige of its biological origin
. Even the ancient bone’s own mineral strength may itself be partly secondary, the result of lime migrating in from the soil, to replace what biology and chemistry have taken away. Other factors may influence its chance of survival: how much flesh was still attached, whether it had been cooked, and how quickly it was buried. The consequent variation in preservation is enormous. Sometimes nothing is left, sometimes the bone survives but is fragile and biscuity, and sometimes it is tough enough to hold. In other cases, however, the processes of preservation can take a body in a quite different direction, when it is more than just the teeth and bones that survives. In places as far apart as the Andes mountains, the lowland peat bogs of north Europe and the permafrost of central Asia, flesh, hair, even internal organs can survive for thousands of years. In each of these cases, some combination of low temperature and the exclusion of water or oxygen has arrested their decay.

  The terms ‘mummy’ and ‘mummification’ are used for bodies that have been preserved by rapid drying. The words come from the Persian for bitumen, which was sometimes used to preserve dead bodies. What all mummies have in common is that they owe their survival to rapid desiccation. The connective tissues, such as skin, hair, gut and tendon, are the most easily conserved, and they are what many natural mummies are composed of–a surprisingly lifelike hairy hide draped loosely over what survives of the skeleton. The gut is the most durable internal organ, which is why the most frequent internal analysis is of stomach contents and the unfortunate victim’s last meal. Other organs, such as muscle, brain and intestine, only survive in the most rapidly desiccated mummies of all.

  The very earliest Egyptian mummies were dried by exposure to hot arid winds. By the second millennium BC, mummification had developed into a complex and specialized service catering to the elite. The whole process took around ten weeks, beginning with the removal of internal organs. The emptied corpse was rinsed in wine, then packed with salt. The salt’s strong drying effect would eventually produce the stabilized raw material for cosmetic improvement and preservation.

  Other societies, as far apart as Australia, China and North and South America, leave similar legacies of desiccated mummies, sometimes through natural drying, at other times with artificial assistance. In the South American Andes, both the natural and the assisted forms of mummification were widespread. The mountain range and its adjacent coastal strip offer a wide spectrum of very arid sites, and a wide range of temperature regimes. Beneath their soils was found a considerable collection of preserved bodies that have formed the basis of a number of molecular analyses. Among them are the South American Chinchorro mummies described earlier. Above them in the Andes, at elevations of 15,000 feet or more, another series of naturally mummified human bodies has been found. Many are around 500 years old and are believed to have been offered as sacrifices according to Inca traditions, to appease the mountain gods after earthquakes, eclipses and droughts. At these high altitudes, preservation is affected not just by dry conditions, but also by low temperatures.

  Low-temperature preservation of bodies is found not only at high altitudes, but also at extreme latitudes. Thirty years ago, two brothers stumbled upon this low-temperature effect, while out hunting wild birds in Greenland. They came across the bodies of six women and two children, their flesh and clothing still intact. Subsequent scientific examination revealed the stomach and small and large intestines. The soft tissues were shriveled but, in one woman at least, the lungs, heart, liver and gall bladder were discernible. They were partially rehydrated and, within the heart, valves, cavities and arteries could be seen. Within the lungs, lenses of soot could be discerned, the consequences of inhaling smoke from the blubber lamp. A date was also established for the bodies. The natural refrigeration of the Greenland environment had conserved these fine details in the flesh of people who had died five centuries earlier.

  crumbling wood

  Plants display a similar array of modes of survival–from complete disappearance through to uncanny resemblance to the living form, thousands of years after death. While lacking a bony skeleton, many land plants do become woody or ‘lignified’ in many of their tissues, and it is often this woody tissue that persists, thanks to another ‘molecular plywood’, one that brings together cellulose and lignin. Cellulose is the substance that gives ‘backbone’ to big plant and small plant alike. Even a blade of grass would flop without the cellulose shell around each cell. Lignin is the back-up support for really tall plants, and gives them their characteristic woody texture. The combined strength of cellulose and lignin is what allows tree trunks to reach hundreds of metres in height. Rather as with bone, the archaeological wood recovered by archaeologists often has only one member of the molecular partnership in good condition. That is the lignin, which is more resistant than cellulose to biological and chemical attack. In the ancient timber within the Somerset peat, the lignin survives, but the cellulose is gone. Above the permanent water table, soil fungi would also digest the lignin, but they cannot survive without air. Some bacteria can, but are only able to digest the cellulose, which is why it selectively disappears. With only one member of the partnership intact, the strength of the ancient wood has gone. Only the watery matrix is buoying it up. As that water disappears from the ancient timbers that archaeologists bring to the surface, the cells within the wood finally release the strength they have clung on to for millennia under the ground, and begin to collapse. The outer layers of many seeds are composed of a similar cellulose/lignin mix, and these two may be reduced with time to lignin alone, while the starchy interior breaks down even more speedily.

  the colour of time

  The living world is full of different greens, highlighted with reds, yellows, purples and countless other colours besides, but in the archaeological record all those natural colours merge towards the dark leathery colour of time. In the waterlogged layers of the Somerset Levels, a leaf occasionally retains something of its green hue, birch bark may still be silver, and beetle carapaces may retain their iridescent blue. But like the skin of the bog bodies, the predominant colour is brown. Something similar is encountered in desert sites. The remarkably preserved 4,000-year-old wheat grains from Akhenaten’s city, Tel Amarna, may retain most of the physical features of fresh grain, but their colour too has darkened. Fresh wheat grains are light amber in colour. These were like roasted coffee, and other desiccated objects are similarly transformed. In wet and dry deposits alike, organic remains tend gradually to darken.

  In the case of food plants, these changes are well understood. The reason that the Egyptian grains look like roasted coffee beans is that a short spell in an oven has a very similar effect on a seed to a very long spell at normal temperatures. In both cases, two of the seed’s molecular components, the carbohydrates and proteins, have interacted to produce ‘Maillard products’, normal and desired products of cooking, that lend aroma, a crust and a dark colour. As well as breaking apart, molecules are in some cases binding together to generate new molecular structures, of which the Maillard products are examples.

  Another case of cross-linking is the process we call tanning. When leather is tanned, new bonds are formed between the existing protein chains, giving the whole thing added strength and flexibility. This is achieved by adding tanning agents. Nature also yields its own tanning agents, one of which is released by the mosses that go to make up peat. Many of our bogs are simply an enormous accumulation of old Sphagnum moss, piled up upon itself without decay, smothering and pickling other organisms as it develops. One of its breakdown products, sphagnan, is a tanning agent, and its presence causes some of the bog bodies trapped in peat to end up with a fine leather as their skin.

  In bogs with a lot of wood rather than moss, a group of compounds called polyphenols can similarly tan organic remains.

  Maillard products and tanning processes may help a tissue remain intact, but they are not good news in the search for molecules. All this recombination and cross-linking is a nightmare for the analytical chemist. The hunt for
ancient molecules always resembles the proverbial search for needles in a haystack. Science has become remarkably adept at separating chemical needles from chemical hay. They can be persuaded to segregate according to differences in their solubility, their mobility as gases, or their tendency to cling to some other, introduced molecule that acts as a kind of magnet to pick out the needles. These sorting processes have been fashioned into elegant automated procedures that are central to the search. However, the situation is made far more complex when the needles start forming bonds with the hay, at precisely the same time that the internal bonds holding needles and hay intact are themselves weakening.

  When the search for molecular information from archaeological sites began, the general assumption was that if archaeological tissue was sufficiently intact that cellular structure could be seen, then intact molecules might be expected. We have learnt to be considerably more circumspect. Organic tissue clearly has a marked ability to retain its visual form, even after the molecules within have greatly changed their form. They may have shortened, oxidized, recombined with other molecules, entered the tissue or left it. In amongst this turmoil, however, highly informative molecules do survive and are the basis of the continuing hunt. Before following the hunt’s course, though, we have to address the most basic question of all.

  why does any of this survive?

  A conflict resides within the picture painted above. It is clear that archaeological deposits, in terms of their molecular dynamics, are nowhere inert. They are far too shallow to elude the profound reach of biological breakdown, in which physical and chemical processes drag large and complex molecules back into nature’s melting pot. We have become more, rather than less, aware of the intensity of nature’s recycling process. How is it that the ubiquitous forces of degradation and decay fail to recapture these rather shallowly buried remnants of our past? Those remnants are far from consumed. They may be in a poor state, and pose enormous challenges to the molecular archaeologist, but they are there to be recovered. The truth is, we still do not really understand why decay is partial, except that it invariably is. Not that nature is by any means inefficient. Each year, something like 99.9 percent of all living material is disassembled into small enough molecules to feed a new generation of organisms. That still leaves one part in a thousand that accumulates. In a century, those annual increments have amassed to a tenth of the size of the active biosphere. After a millennium, the accumulation rivals the size of the biosphere itself. On the geological time scale, vast banks of fossil carbon become a major element of the Earth’s crust. Many of those accumulating fractions–but by no means all–reside in rather particular deposits, such as those below the permanent water table. Moving back above the waterlogged levels to the active soil below the turf, with its pots, bones, seeds and teeming agents of decay, even here complex molecules survive. They survive within the hearts of the seeds and bones, and in the porous interior surfaces of the fired clay of which the pots are composed. Some would claim that they even survive on the smooth surface of a stone tool. They are there in tiny quantities, and subject to all of the transformational problems outlined in this chapter, but they are there.