Lone Survivors (7 page)

So to go farther than a relative date, we need physical clocks that will tell us how far back some rocks were laid down, how long it is since an animal or plant died, or when an event happened, such as the heating of clay or flint. Many of these clocks measure time using the natural radioactive decay of isotopes.
Isotopes
are distinct atoms of substances, such as argon or carbon, that have different atomic weights (because they contain different numbers of particles called neutrons). An example of such a technique is
potassium-argon dating
, which can be used on volcanic rocks. Potassium partly consists of an unstable isotope called potassium-40, and this isotope gradually changes over many millions of years into the gas argon. When there is a volcanic eruption, the liquid lava or hot ash contains a small proportion of potassium-40, and when the lava or ash cools and solidifies, this unstable isotope of potassium begins to change into argon, such that half of it decays into argon about every 1.25 billion years (this is its half-life). Provided the volcanic eruption was sufficiently energetic to drive out any previous argon gas (usually a reasonable assumption), and provided any newly formed argon gas remained trapped in the volcanic layer once it set hard, the amount produced can be used as a natural measure of time since the volcanic rock was deposited. In one of the first and most famous applications of this technique to archaeology, lava at the base of the site of Olduvai Gorge in Tanzania was shown to be about 1.8 million years old. This caused a sensation in 1960 because it indicated for the first time just how ancient the artifacts and humanlike fossils in Bed I at Olduvai might really be, doubling their expected age at a stroke. A more recent development from potassium-argon dating is to use the decay of argon-40 to argon-39 instead, since this can be used to date single crystals of volcanic rock with much greater accuracy over the time span of human evolution.

The most famous physical dating method is
radiocarbon dating
, based on an unstable form of carbon. The method relies on the fact that radiocarbon (an isotope of carbon called carbon-14) is constantly produced in the upper layers of the Earth's atmosphere by cosmic radiation acting on the element nitrogen. This unstable form of carbon gets taken up into the bodies of living things, along with the much more common, and stable, carbon-12. However, when the plant or animal dies, no more carbon-14 is taken in, and the amount left begins to break down by radioactive decay, such that the amount present halves about every 5,700 years—a very much shorter time span than that of potassium-argon dating. So measuring the amount of carbon-14 left in, say, a piece of charcoal or a fossil bone allows us to estimate how long it is since the plant or animal concerned was alive.

In 1949 the American chemist Willard Libby and his colleagues first applied it to a sample of acacia wood from the tomb of the pharaoh Zoser (who lived nearly 5,000 years ago). Libby reasoned that since the half-life of radiocarbon was close to 5,000 years, they should obtain a carbon-14 concentration of about 50 percent of that found in living wood, and this was confirmed. That work, and much that followed, earned Libby a Nobel Prize in 1960. The method cannot be used on very ancient materials because the amount of carbon-14 left behind is too small to measure accurately, and hence radiocarbon dating becomes increasingly unreliable beyond about 30,000 years ago. Moreover, the assumption of constant production and uptake of carbon-14 is now known to be only an approximation, due to past fluctuations in cosmic rays and changes in the Earth's atmospheric circulation—thus scientists talk of dates in
radiocarbon years
rather than real (calendar) years.

This means that other methods are needed to cross-check (calibrate) the accuracy of radiocarbon dates. Several methods have been particularly useful for dates in the last 10,000 years or so, and all of them require the counting and dating of annual layers. The first uses tree rings (dendrochronology) and builds up overlaps in patterns of growth rings from timbers preserved in buildings, boats, or natural deposits, in order to establish a long sequence where the age assessed from the wood is compared with a radiocarbon date obtained on rings within the wood. A comparable method uses varves (annually deposited layers in the bottom of deep lakes), where spans of time can be measured through counting varves, and also by radiocarbon dating of plant or animal residues within the varves. Yet another method uses radiocarbon dates obtained within annual layers of ice, and this can be taken even farther since trapped bubbles of gas in the ice preserve a snapshot of the composition of the atmosphere when a particular layer was deposited. Beyond these methods, very ancient trees preserved in bogs in New Zealand hold the promise of accurately calibrating radiocarbon to beyond 40,000 years, while ancient coral terraces can be dated both by radiocarbon and by uranium-series dating (discussed later in this chapter), giving a cross-check between independent physical dating methods, each with different assumptions.

Comparisons so far suggest that radiocarbon dating, while not exact over the last 40,000 years, is quite reliable, although sometimes off by as much as 10 percent. Unfortunately, one of its least accurate phases covers the demise of the Neanderthals and much of the spread of modern humans around the world—hence the need to further refine radiocarbon dating or supplement it with other methods wherever possible, as I will explain later in this chapter.

Many technical improvements have been made in radiocarbon dating procedures since Libby's initial work. For example, he analyzed solid carbon, while later techniques convert the carbon to gas or dissolve it in solvents. The early methods also required large sample sizes to detect radiocarbon decay, so that important artifacts or bones had to have large chunks sawn off them to attempt a date—permission for which was understandably often refused by concerned museum curators. Luckily, from about 1977, the method of
accelerator mass spectrometry
(
AMS
) has increasingly taken over, and this counts individual atoms of carbon-14 directly, rather than measuring their radioactivity. So now only milligram-sized samples are needed, allowing the dating of relics as precious as the Shroud of Turin, the Dead Sea Scrolls, the Alpine iceman “Ötzi,” and the Ice Age art of the Lascaux and Chauvet caves.

A good example of the enhanced power of radiocarbon dating came when four colleagues and I investigated one of the enduring mysteries of the Paleolithic record of Britain. Representations of Ice Age art are extremely rare in Britain, and two of the only examples known (or claimed) are from Robin Hood Cave in Derbyshire, found in the 1870s, and from the town of Sherborne in Dorset. Both showed a rather similar profile of a horse engraved on a flat fragment of bone. While the Derbyshire example was discovered by prehistorians in a cave alongside Paleolithic artifacts of appropriate age (about 14,000 years old), the “Sherborne bone” was discovered in 1912 by schoolboys from the local public school, in the vicinity of a quarry from which no comparable material had ever been reported. Serious doubts were soon raised about the authenticity of the Sherborne discovery, but direct radiocarbon dating could not have been contemplated when application of the method would probably have destroyed most or all of the object. The advent of AMS dating at Oxford University allowed us, in 1995, to drill a tiny sample from it and date the bone to about six hundred years old, while microscopic studies of the engraving showed that it was probably carried out quite recently with a metal implement, rather than a flint tool. This result was in line with suggestions from one of the staff at Sherborne fourteen years after the “discovery” that a boy had probably copied the engraving from an illustration of the Robin Hood specimen in their school library, in order to play a joke on their science teacher!

But even AMS dating is not perfect, since it finds and produces a date from whatever radiocarbon is in the sample; even a small amount of contaminant radiocarbon can greatly affect an age estimate, especially when the sample is 30,000 or 40,000 years old, and only a tiny fraction of its original radiocarbon is still there. Fortunately, new preparation procedures such as
acid-base-wet oxidation
(
ABOX
)
dating
for charcoal samples and
ultrafiltration
for bone are largely overcoming the problems of contamination in dating Paleolithic materials and are giving increasingly trustworthy determinations. The advantages provided by ultrafiltration were very well demonstrated through the redating of bone samples from Gough's Cave in Cheddar Gorge, Somerset. This is one of Britain's most spectacular tourist caves but also one of our most important Upper Paleolithic sites. Excavations spread over more than one hundred years have revealed quantities of stone artifacts together with human and animal bones representing its late Ice Age inhabitants. Revised radiocarbon dating has now shed further light on the nature of the human presence here, and on the timing of the return of people to Britain after a period of Ice Age abandonment lasting about 10,000 years. Prior to this new research, it was uncertain when occupation took place and how different parts of the archaeological story fitted together, but it now seems that Gough's Cave was one of the first sites to be used by hunters of wild horses and red deer when people returned to Britain after the peak of the last glaciation.

This transformation was achieved by the dating specialist Tom Higham and the archaeologist Roger Jacobi, using ultrafiltration pretreatment on animal bones butchered or worked by the Stone Age humans, and on the remains of the humans themselves. Previously, the radiocarbon dates obtained had only made it possible to tie occupation down to a span of about 1,500 years. Now, much greater confidence can be ascribed to dates that show almost all the Upper Paleolithic material in the cave accumulated over as little as two to three human generations, centering around 14,700 years ago. Interestingly, this date corresponds precisely to a dramatic warming of climate recorded in the composition of annual layers of ice in Greenland. These archives suggest that the previously ice-covered Atlantic Ocean defrosted in about five years. Among the material dated at Gough's were bones of several of the humans, some of which show patterns of cut marks interpreted as evidence of cannibalism. Before, it had been thought that these might have belonged to a more recent phase of activity than the one associated with the horse and deer hunting, but we now know they were precisely the same age. Thus the animals, and the people who preyed on them, represented some of the first colonizers of Britain after the peak of the last Ice Age. As the climate rapidly warmed, herds of horses and deer must have migrated across Doggerland, now submerged under the North Sea, and the hunters followed.

A much older British fossil that I have been involved in studying was found in 1927 at Kent's Cavern, in southwest England. After its discovery, the anatomist Arthur Keith described this fragment of upper jaw as a modern human, but it had to wait another sixty years to achieve further fame, when it was one of the first fossil humans to be dated by the radiocarbon accelerator at Oxford. The estimated age of about 35,000 years made it one of the oldest modern humans in Europe; subsequent detective work on the Kent's Cavern archives by Roger Jacobi suggested that it could date from even earlier. So, in 2004, we decided to borrow the specimen from Torquay Museum and restudy it, using every scientific approach we could muster. The team I assembled involved researchers such as Erik Trinkaus and Tim Compton, specialists in CT and ancient DNA (techniques that I will discuss in chapters 3 and 7), curators and conservators, and Higham and Jacobi. Careful examination and CT modeling confirmed Erik's hunch that one of the teeth had been glued back into the wrong socket; a new reconstruction was made, allowing the sampling of the tooth roots for ancient DNA and ultrafiltered accelerator dating. Sadly, both of those attempts ultimately failed, but accelerator dating of animal bones found around the fossil indicates that its real age is some 40,000 years, and it may record an early spread of modern humans to western Europe.

Other physical dating methods that can be applied to fossil and archaeological materials beyond the limits of radiocarbon dating have also been developed or enhanced in the last twenty years. These include
uranium-series
(
U-S
)
dating
, which is based on the radioactive decay of different forms of uranium. Accumulation and measurement of the so-called daughter products are possible in substances like stalagmites and corals. The former has been very useful in cave sites, while the latter has been used to examine past changes in sea levels around tropical and subtropical coasts and, as mentioned already, to check the accuracy of radiocarbon measurements. One of the holy grails of dating has been to get uranium decay methods to work on fossil bones. However, this has proved notoriously difficult because, in contrast to stalagmites and corals, which are essentially sealed after deposition, bone continues to be open to the accumulation or loss of uranium (for example, as groundwater percolates through it). This means that its physical clock can run very erratically. Nevertheless considerable progress has been made recently, and I will discuss some of the results as applied to the Broken Hill fossils of
Homo heidelbergensis
in chapter 9.

A number of other methods depend on the fact that crystalline substances such as sand grains, flint, or the enamel of a tooth store up changes in electrons within their crystal structure from the radiation they receive from their surroundings, once they are buried. The amount of change (corresponding to radiation damage) can be measured from the accumulated energy released in the sand or flint when treated with a laser beam (
optically stimulated luminescence
, or
OSL
) or by heating (
thermoluminescence
, or
TL
), while in tooth enamel, the accumulated changes in the electrons can be detected using microwave radiation (
electron spin resonance
, or
ESR
). For any of these methods to work, the radiation signal must first be set at zero—for example, when a tooth begins to grow (ESR)—or set back to zero when the previous signal is wiped out as sand grains are bleached by exposure to the sun, or when flint or clay is strongly heated in a fire (luminescence). Provided the rate of subsequent accumulation of radiation damage in the material can be estimated from the environment in which it was buried, the length of time it has been in the ground (for example, in a Cro-Magnon fireplace or a Neanderthal butchery site) can be estimated.