The Dawn of Human Culture (32 page)

Our readers sitting as jurors must still reach a verdict, and if we have presented our case capably, they will agree that anatomically modern Africans became behaviorally modern about 50,000 years ago and that this allowed them to spread to Europe where they rapidly replaced the Neanderthals. They will probably also accept the likelihood that modern behavior allowed modern humans of recent African descent to replace non-modern people in the Far East, although in this instance, we as prosecutors would understand if they asked for more evidence. Their only serious reservation, roughly akin to reasonable doubt in the legal system, may concern our argument for what prompted the emergence of modern human behavior about 50,000

years ago. The crux here is logic and parsimony, not evidence, and with the full sweep of human evolution in mind, we would appreciate feedback on just how persuasive our logic is.

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APPENDIX

PLACING ANCIENT SITES

IN TIME

To lay observers, it may appear as if human fossils and artifacts are the main facts of human evolution, and they are obviously vital to it. However, they would lose much of their value if they could not be arranged in time or “dated.” We introduced key dating techniques at relevant points in previous chapters, but given the importance of the topic, we pull them together in this brief appendix. They can be broadly divided between “relative” and “absolute (or numerical)” methods.

Relative methods are ones that allow objects to be arranged from younger to older (or vice versa) without specifying precisely how old any given object is. The most obvious relative dating method is the principle of stratigraphic superposition, which states that all other things equal, the deeper the rock layer in which an object occurs, the older the object is. When this principle is carefully applied in the field, it allows specialists to construct the sequences of animal communities 09 Appendix.r.qxd 1/29/02 5:07 PM Page 275

Appendix | 275

and artifact assemblages that have existed through time within a given region. Animal fossils or artifacts can then be used to determine the antiquity of a site with respect to others, even when the site contains only a single layer. Thus, in Africa, the particular species of elephants, horses (zebras), or pigs that occur in two ancient human fossil or archeological sites often suffice to determine whether one site is older, younger, or perhaps the same age as the other. In eastern Africa, the time ranges of fossil species or species groups have often been determined in years, and this has allowed age estimates in years for significant australopith and other sites in southern Africa where the same species or species groups occurred. The use of fossil species to arrange sites in time, and in some cases to estimate how old they are in years, is often known as “faunal dating,” and it is by far the most widely applied relative dating method in paleoanthropology.

Absolute dating methods are ones that provide age estimates in years. Since sites dated in years are automatically arranged in time with respect to others, absolute dating methods may be regarded as especially precise variants of relative dating. In paleoanthropology, the most important absolute methods rely on the decay of naturally occurring radioactive isotopes (varieties of elements). So far, the two most informative and reliable methods are based on the decay of radiocarbon (carbon-14) and radiopotassium (potassium-40). Radiopotassium decays into argon, and the method is thus commonly known as the potassium/argon technique. We introduced the potassium/argon and radiocarbon techniques on pp. 44 and 209 respectively, and Figure A.1

presents the approximate time range that each method covers and the materials to which it is routinely applicable. Application in each case is limited partly by the absence of suitable materials in many sites and 09 Appendix.r.qxd 1/29/02 5:07 PM Page 276

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| THE DAWN OF HUMAN CULTURE

Approximate Range in Years Before Present

100

10,000

1,000,000

1000

100,000

10,000,000

Method

Suitable Materials

the practical gap

wood, charcoal,

Radiocarbon

between the lower

shell

limit of radiocarbon

and the upper limit of

potassium/argon

volcanic rock,

Potassium/argon

meteorites

flowstones

(stalagmites &

Uranium-series

stalactites),

coral

windblown sand,

Luminescence

burnt flint

Electron Spin

tooth

Resonance

1000

100,000

10,000,000

100

10,000

1,000,000

FIGURE A.1

The time ranges covered by the absolute (or numerical) dating methods that are important to paleoanthropology.

partly by the possibility of contamination from older or younger materials that were introduced into a site during or after burial. The absence of volcanic materials, for example, precludes the use of potassium/

argon dating at ancient southern African sites, while the possibility that minute amounts of more recent carbon have contaminated many ancient samples makes it difficult to obtain reliable radiocarbon ages beyond 25,000 to 30,000 years ago, even when materials suitable for dating are present.

There is the further problem that except in unusual circumstances, the potassium/argon method cannot produce reliable ages younger than about 200,000 years, while the radiocarbon method is limited to roughly the last 50,000 years. In practice then, there is a gap 09 Appendix.r.qxd 1/29/02 5:07 PM Page 277

Appendix | 277

in time of about 150,000 years that the two methods cannot cover (Figure A.1). The most dependable technique for filling the gap is the uranium (U-) series method that we described on p. 249. This is based on the radioactive decay of uranium and its daughter products Thorium (Th) and Protactinium (Pa), but its applicability is limited by the rarity of suitable target materials in ancient human sites. The Electron Spin Resonance (ESR) and luminescence methods that we introduced on pp. 124 and 164 respectively are more widely applicable, and they have provided many interesting and oft-cited ages.

However, in general, the results of each method depend heavily on unverifiable site-specific assumptions about the history of radioactivity in the burial environment or in the dated object, and their reliabil-ity in many instances is thus questionable.

Finally, it is sometimes possible to use the known history of past shifts in the direction of Earth’s magnetic field to estimate the age of a site in which the deposits record one or more shifts in past direction. We introduced such paleomagnetic dating on pp. 66—67. Similarly, specialists can sometimes provide an age estimate by comparing the sequence of glacial/interglacial alternations that a site records to the dated sequence that has been firmly established from deposits on the deep sea floor. This method works best for sites that formed within the last 700,000 years or so, and it generally requires deposits that accumulated continuously (without major breaks) and that have been partially dated by another method such as potassium/argon or radiocarbon. Such “climatic dating” is particularly effective for determining whether a site formed during the last interglacial period, between roughly 127,000 and 71,000 years ago, or during the last glacial period, between about 71,000 and 12,000 years ago.

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THE FIRST TRUE HUMANS

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