Pleistocene extinctions

One of the biggest questions that has emerged from our investigation of Pleistocene time is whether it was climate change or human intervention that wiped out the magnificent beasts of the Ice Ages. The mammoth, woolly rhino, giant cave bears, giant deer, sabre-tooth cats and herds of wild horse, cattle, bison and reindeer that covered the Americas and Eurasia and were so evocatively figured by our ancestors in cave paintings, rock drawings and portable carvings have mostly disappeared, but what caused their demise? Did they just fall by the wayside, victims of climate change, or were they pushed to extinction by our forebears? There are many other ideas about the possible causes of these extinctions, including a current fashion for invoking pandemics of disease. Nevertheless, however uncomfortable it might seem, the front-runners are still those skilled modern human hunters who first colonised the Earth beyond Africa. Climate change or possibly a combination of human intervention and climate change is the second main option.

You might reasonably think that it should not be too difficult for scientists to answer this question but it is surprisingly complex. There is no doubt that the Pleistocene so-called megafauna of large and medium-sized mammals of the northern hemisphere suffered an extinction event after the latter part of the Ice Ages. Over the last few tens of thousands of years a significant proportion of the megafauna has gone and the process is not finished yet. This time interval might seem distinctly protracted, but on a geological time scale it is still very quick.

To begin with, the primary suspect was actually climate change. The idea was that drastic changes in vegetation had a major effect on the large numbers of plant eaters, which were dependent on fairly specific food types such as the grasses on which the mammoths, horse, cattle and bison depended. With warmer and wetter climates at the end of the last glacial, the vaste cold steppe grasslands were gradually overgrown by encroaching scrub, herbs and eventually forest, leading to severe reductions and terminal decline of the grass eaters. There was a knock-on or cascade effect. Populations of top carnivores such as the big cats, which depended on the plant eaters, consequently crashed as well. But what of the role of our ancestors, who we know were hunters and serious meat eaters (see p. 384)?

The Americas seemed to provide an ideal test of the problem because of the late arrival of modern humans in the continent, either around 12,000-14C years ago (around 14 ka calendar years, see p. 412) or perhaps, as we have seen, much earlier, around 22 ka. Could the megafaunal extinction event be chronologically tied to the arrival of these hunters? Of all the big beasts of the ice ages, the mammoth has attracted most attention and there is plenty of fossil evidence for the existence of various species of mammoth and other elephant relatives in the Americas. Their huge bones tend to fossilise quite well and their massive cheek teeth are particularly resistant to subsequent (post-mortem) destruction.

The discovery of many animal bones at a number of American sites seemed to support the idea that the early human occupiers of the continent were ruthlessly efficient big-game hunters. It has been claimed that the remains of whole family groups or even herds are present at some of these sites. However, it can be very difficult to prove whether the animals did actually belong to single groups and were slain by a single 'mass kill', or whether they were 'recruited' more gradually over a longer period of time.

A number of American sites preserve the remains of groups of mammoths ranging from 5 to over 100 individuals. Some of these sites (e.g. Waco, Texas, dated at c. 28 ka) have been shown to be the result of natural mass mortality (for example drowning in flood waters) before humans arrived on the continent. Others, such as Freisenham Cave, are clearly natural accumulations (for example a sabre-tooth cat den, dated at 20-17 ka) formed over a considerable time. But there are also a number of sites that are younger than 13 ka and preserve human artefacts along with the mammoth remains.

Much of the linkage between the animal remains and humans comes from stone tools associated with the mammoth bones, but these do not necessarily prove that the two were exactly contemporaneous, let alone that the humans killed the animals. Even the common occurrence of cut marks on the bones does not prove that the humans hunted down and killed the animals, although they do strongly suggest a close association in time. Cut marks on the bones are usually associated with the act of butchering or defleshing a carcass, but the animal may have already been dead or dying. Killing a healthy mammoth with spears was not easy; it was a lot safer to take on beasts that have already been killed by other predators or are dying from natural causes. Very rarely is there any obvious cause of death, although there are a few cases where sharp worked stone points have been found embedded in animal rib cages.

Luckily, a new, sophisticated application of isotope analysis can help answer a number of these problems. Measurement of variation in carbon, oxygen and strontium isotopes from the enamel of mammoth cheek teeth allows differentiation between mass kills that took place over a short period of time and longer-term accumulation of the remains of individual animals. Carbon isotopes can reflect differences in the plant material consumed, oxygen isotopes track local climates and variations in strontium isotopes reflect soil types. The greater the variation in the isotopes, the less likely are they to have lived and died together.

The technique has been tested out at two prehuman American sites in Texas - Waco (c. 28 ka), where the remains of some 23 mammoths and a single camel have been found, and Freisenhahn Cave (c. 20-17 ka), with over 100 individuals. At Waco most of the skeletons are articulated and the animals must have died where they are found, within a single layer of silty clay. The bones are well preserved and show little sign of post-mortem disturbance, suggesting that they were rapidly buried soon after they died. Age analysis shows that they were predominantly adult females and juveniles along with one adult male, a sex and age profile typical of living groups of elephants in which the adolescent and older males separate from the females and babies until one or more of the females comes into oestrus and is sexually receptive to adult males, or at least a dominant male who can exclude any rivals.

By comparison, the Freisenhahn Cave site contains a large variety of bone material including bison, horse, peccary, tapir, dog and large cats, as well as mammoth teeth. Radiocarbon dating shows that accumulation continued over a period of some 3000 years. Detailed analysis of the jumble of dismembered bones shows that the site was a sabre-tooth cat (Homotherium serum) den. Parts of carcasses were dragged back to the cave by the big cats so that they could be consumed without having to constantly protect their food from the attentions of other predators and scavengers.

The carbon isotopes from the Waco site were tightly clustered, whereas those from Freisenhahn showed a significant level of variance. This supports the interpretation of the Waco site as a natural 'mass kill', which probably resulted from a group of closely related animals being caught and drowned together by a single flash flood. In contrast, the cave accumulation consists of unrelated animals being 'recruited' one by one over a long period of time.

The same techniques have also been applied to sites where the association of Clovis hunter artefacts with mammoth remains has been used to argue for accumulation by human-generated mass kills. A recent study tested three Clovis sites (Blackwater Draw, New Mexico - 6 animals, 13-11 ka; Miami, Texas -$ animals, 11.4-10.$ ka; and Dent, Colorado - 13 animals, 11.2-10.9 ka).

Surprisingly high levels of variability in each of the isotope systems indicate that the accumulation of mammoth remains at all three sites was not the result of mass kills, but rather represents slow accumulation over a number of years. From this evidence it seems that the Clovis hunters of the Great Plains did not slaughter entire family groups of mammoths but rather hunted or scavenged individual animals. However, there are still some other Clovis sites, such as Lehner and Murry Springs in Arizona, which need to be tested before we can say the same for all known 'mass' mortalities linked to the Clovis hunters.

But this result does not necessarily discount the idea that human hunters were not entirely responsible for the extinction. It just may not have been as dramatic as television and hype-driven reconstructions would prefer. Modelling of the viability of declining prey populations suggests that even small numbers of mobile hunters can, within centuries, reduce large herds of slow-reproducing megaherbivores such as the mammoth (and of course bison within historical times) to unsustainable population numbers.

In recent years the global scenario for this Pleistocene extinction has been broadened to include the southern hemisphere. The timing of extinctions of the megafauna of Australia and New Zealand convincingly supports the idea of a human hunter-driven event. For instance, the unusual flightless avifauna of New Zealand, including the moas, survived until the 'late' arrival of humans around AD 1000.

Thousands of Years Ago

10 ka Beringian connection submerged

11.2-io.9 ka Dent, Colorado mammoth site 11.4-io.5ka Miami,Texas mammoth site

Folsom people

13-11 ka Blackwater Draw, New Mexico ' Clovis Pe°Ple mammoth site

• arrival of modern humans in North America ? • Meadowcraft rock shelter 15 ka ? . Monte Verde, chile

| climate warming ? • Bluefish Cave artefacts sea levels rising coastal route ice free i8ka glacial maximum

20 ka 20-17 ka Freisenham Cave mammoth site climate cooling closure of ice corridor within N. America ? arrival of modern humans in N. America

| falling sea levels

25 ka

28 ka Waco, Texas mammoth site

30 ka • arrival of first modern humans in northern Siberia | rising sea levels

30 to 10 thousand years.

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