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Geology plays a key role in the study of humans, particularly in the subdisciplines of paleoanthropology and archaeology. Anthropologists work closely with geologists and employ geological tools in order to reconstruct aspects of past environmental and ecological contexts from the time of our earliest human ancestors to that of modern peoples.
Extrinsic selective pressures, or those that are derived from a human’s surrounding environment, are revealed through the study of the earth sciences. Anthropologists place the human individual, community, and population back into the environment and attempt to understand how humans interacted with that environment. From the origins of hominins, humans’ bipedal ancestors, to the ascendancy of modern peoples, anthropologists want to know about temperature, aridity and rainfall, landforms, hydrology, and vegetation cover, among many other factors. From these basic environmental indicators, they surmise the kinds of habitats that may have been available and exploited by humans. Geological tools have also been applied directly to human skeletal and artifactual material or associated sediments in modern and ancient settings to address lifeways and determine age of deposits. One of the subdisciplines of geology, paleontology, is concerned with ancient life. Contexts of early humans also include coeval animals occupying the same territories and habitats or exploited as food sources by humans.
Some researchers within the two disciplines assert that the relationship between anthropology and geology is a one-way street; that is, anthropologists use geological tools to address questions of the human condition, but anthropology does not offer anything to the study of geology. In that thread, this research paper recognizes the many geological methods as applied to anthropological questions can be observed as indeed one-way. However, implied in this paper is how anthropology provides relevance to geological data. Certainly, geologists would like to know the history and mechanisms of the earth, but those ideas do not stop there. Geologists also couch that information in the economic, political, (pre)historical, and cultural contexts of humans, all of which are the purview of anthropology and other social sciences.
This research paper is by no means an exhaustive review of geological theory, methods, and applications to the study of humans. Rather it serves as a discussion of some geological tools commonly used to investigate some anthropological questions and suggests further readings.
An Anthropologist’s Use of Geological Theory
Largely credited to the work of James Hutton, the theory of uniformitarianism, also known as actualism, holds that observable earth processes, such as erosion and deposition, occur throughout geological time and are responsible for the landscapes, among other earth forms, present today. That is to say, “the present is the key to the past.” For instance, wind action along a shoreline slowly weathers fine sand grains and transports and deposits the grains in another location. Eventually, a new landform, such as a dune field, is constructed. Uniformitarianism is simplistic in that it evokes the most parsimonious explanations for past geologic phenomena.
Anthropologists using geological methods to reconstruct human environments also follow the theory of uniformitarianism. Archaeological and human paleontological sites are formed by human induced and natural, or nonhuman induced, geological processes. Unraveling where the two meet is essential for understanding how each site formed and aids in the interpretation of what the original environment when humans inhabited the site would have looked like. For example, some of the earliest stone tool use has been documented on the shores of Lake Turkana at archaeological sites of Koobi Fora in northern Kenya. These Late Stone Age sites contain fossil hominins, cutmarked fossil fauna, and stone tools, most of which are deposited in ancient river and lake deposits. Early interpretations painted a picture of hominins living along the lake margin, butchering animal carcasses with stone tools. Anthropologists hypothesized about small groups of nomadic hunters and gatherers using home bases nearly 2 million years ago (mya). Actualistic studies of stone tool and fossil transport in fluvial deposits, largely conducted by Glynn Isaac and his students, shed new light on these early human sites. These researchers found that geological processes, such as erosion and deposition, are also responsible for creating accumulations of artifacts and fossils, and therefore, not all of the so-called sites are the result of humans using home bases and discarding artifacts. Isaac went further to compare the “scatter-between-the-patches” and delineated natural background distribution of broken stone pieces from these clusters of human-made stone artifact assemblages.
Uniformitarianism, however, is used not only by anthropologists to frame the geological aspects of human environments. Actualistic studies have been widely used to interpret the human-induced processes. Lewis Binford proposed the application of middle range theory, first developed by Robert Merton, to archaeological problems. Middle range theory explains patterns of material remains preserved in the past with observable processes performed by humans in the present. These studies encompass such subfields as experimental archaeology and ethnoarchaeology. In one of the earliest Paleolithic experimental studies, Francois Bordes began fashioning his own stone tools from the same raw materials used by Pleistocene humans. He reconstructed each blow to rough rock form and made inferences about the thought processes and foresight that went into making such artifacts by human ancestors. Thus, he employed the dictum “the present is the key to the past” and made analogies from his own toolmaking skills to those of extinct humans. Although Binford disagreed with Bordes’ interpretations of Neanderthal societies based on these experiments and suggested that Bordes look at modern human populations to gain insight into tool function, both anthropologists found explanations in the modern world to make inferences about the past. Zooarchaeologists also use actualistic studies to further interpretations of past human behaviors and lifeways. Robert Blumenschine observed carnivore activity on the savanna grasslands of the Serengeti Plains in order to interpret patterns of PlioPleistocene carnivore tooth marks preserved on herbivore skeletal remains at the early human site of Olduvai Gorge in Tanzania. From observable sequences of carnivore carcass access, he refuted models of hunting by early members of genus Homo and instead demonstrated the capacity for intense scavenging. In sum, anthropologists have adopted geological theory to frame questions about the context of humans as well as human lifeways.
Geological Methods in Anthropological Contexts
Anthropologists borrow many tools from the earth sciences to use in reconstructing the contexts of humans. Three subdiscplines presented here are sedimentology, stratigraphy and dating methods, and stable isotope geochemistry. Broadly, these contexts incorporate aspects of time and environment.
The vast majority of modern and fossil human remains over the past 8 million years of evolutionary time are preserved in sedimentary rocks, and thus, anthropologists working with the rock record study sedimentology to understand past and present human contexts. At outcrops, sedimentologists record information about the characteristics of sediments and sedimentary layers. These characteristics either are formed during the initial formation of the layers or may be secondarily altered after deposition. From this information, depositional environments are inferred, such as those produced by river, lake, wind, or soil processes. Traces of human occupation and activity are found in primary deposits and altered sediments.
Primary Sediments and Sedimentary Structures
Primary data collection of sediments and sedimentary rocks is a hierarchical framework from small to large divisions describing individual grains, layers, members, formations, and groups. For anthropologists studying archaeological or human paleontological sites, the scale of the investigation varies with what context questions are being addressed. An archaeologist excavating early human sites with evidence of controlled fire, for instance, may be concerned with the sediments of individual hearths in one layer but also with multiple hearths in many layers across landscapes and possibly through time. Characteristics of grains and layers are discussed.
Sediments and sedimentary rocks are formed from the erosion and deposition of other rocks (igneous, metamorphic, or sedimentary) and sediments. The process is a recycling of older materials into new rocks. Characteristics of sedimentary rocks indicate aspects of the recycling history and tell a story of formation. Sedimentologists use characteristics such as grain size, grain composition and mineralogy, and grain angularity and roundness. Grain size and shape are derived from the manner of weathering and transport. For instance, the larger the grain, the stronger the medium in which it was transported. Wind is capable of transporting relatively small grains compared with water. Moreover, fast-moving water can transport larger grains than slow-moving water. Grain shape indicates how long a grain has been in the transport system. A wellrounded grain has likely traveled a longer distance than an angular grain; its roundness reflects how many times it has struck another grain or rolled on a river bottom. Composition and mineralogy indicate source material from which the grain originated.
Comparable grains with similar histories are segregated into mappable units called layers, or strata. Layer characteristics include thickness, color, fabric and structure, and grain sorting. Layer thickness represents a depositional history and indicates a relative amount of time required to build. Fabric is the manner in which grains are orientated and packed within a layer and indicates water or wind flow (current) direction or lack thereof. Grain sorting is a measure of how similar in size each grain is: A well-sorted layer is composed of the same-sized grains; a poorly sorted layer has many different sized grains. Sorting indicates consistency of transport velocity. Sedimentary structures refer to largescale features and bedforms that reflect aspects of the depositional environments. There are numerous structures, most of which can be classified as bedding, stratification, and bed-plane markings. Bedding and stratification describe the shape of one layer relative to another and include cross-beds deposited at angles from a horizontal plane, parallel beds deposited along a horizontal plane, and massive bedding lacking structure. Ripple marks, or wave-formed sands like those along a near-shore ocean floor, are examples of bedding structures. These examples are forms of sedimentary deposition; however, erosional processes also produce bedforms. Scour marks and channels are missing pieces of sediment, but preservation of these forms indicates the presence of past environments. Bed-plane markings are typically formed on contact surfaces between two layers. As a grain or rock hits the bottom of a river bed, it produces bounce, roll, or skip marks, which are in turn filled by the deposition of the next layer. A mudcrack formed in the sun-dried surface of an ancient lake bottom is an example of a bedplane mark. Bed-plane marks are also produced by biologic processes and result in trace fossils, or ichnofossils. These include footprints, tracks, burrows, rhizoliths (root casts), and leaf imprints. Layer color is derived from grain composition and mineralogy or may be the result of diagenesis, or secondary alteration to the sediments after initial deposition. If groundwater bathes a layer of sand permeating into the spaces between each grain, chemical reactions between the water and elements in the grains may occur. Oxidation of iron, for instance, produces a reddish color on the surface of each grain and results in a layer color different from that indicated by grain composition.
Descriptions of individual grains and layers are used to infer the past environments in which they were deposited or formed. These environments include the physical, chemical, and biological conditions present during initial sediment deposition as well as those conditions that produce postdepositional (diagenetic) alteration. Sedimentary environments are typically interpreted by a method known as facies analysis. Facies analysis makes use of the relationship between observable environmental processes and their resulting sedimentary responses. Processes can be static or dynamic but typically result in predictable outcomes. Facies are a combination of layers with distinct lithological, structural, and biological properties that represent a defined environment. Facies are usually constructed from known sedimentary environments, thus evoking uniformitarianism, but also can be used to theoretically model environments that cannot be observed or are unknown in the present. Depending on the scale of investigation, facies associations are used to interpret large-scale environments, such as fluvial (river) or lacustrine (lake), deltaic or beach, desert or glacial, or smallscale subenvironments within each environment. For instance, within the fluvial setting, facies are used to distinguish between meandering or braided river systems. Ancient soils, or paleosols, represent another identified facies produced from postdepostional alteration of sediments. Recognizing these facies is critical to determining ancient land surfaces that may have been used by humans.
Stratigraphy and Dating Methods
Stratigraphy describes the relationship between strata and is primarily concerned with providing a temporal framework for events in earth’s history. These techniques are essential to understanding the evolution of human ancestors and for examining modern human populations through time. The manner and timing of the formation of sedimentary rocks is explained by a few key stratigraphic principles largely credited to Nicolas Steno in the 17th century. The principle of original horizontality states that sediments are initially laid down horizontally due to gravity. The principle of superposition describes deposition of layers in an upward sequence and thus represents the relative age of one rock layer to another. For example, the rock layer on top of a succession is relatively younger than the one(s) below it. Applying the principle of lateral continuity, Steno observed that these horizontal layers are continuous across the earth’s surface unless disturbed. Sedimentologists and stratigraphers still employ Steno’s principles to assign relative ages and to correlate strata. Dating methods are used to assign relative or numerical ages to sedimentary layers or the biotic materials preserved within and are commonly separated into two categories: relative and radiometric (absolute). Several dating methods are presented below that are commonly used in anthropological contexts.
Relative Dating Methods
Relative dating methods largely stem from the principle of superposition and thus do not yield numeric ages. Rather the methods give an “older than” or “younger than” tie point to compare one human archaeological or paleontological site with another. These methods all produce sequences of relative dating information that can be correlated to other locations or records with established numeric ages. As a result, these methods are commonly used in concert with radiometric dating techniques.
Fossils preserved in sedimentary sequences can be used to assign relative ages if the fossil species is geographically widespread (i.e., laterally extensive) and tightly restricted in one or a few sedimentary units (i.e., temporally constrained). A fossil species that meets these criteria is termed an index fossil. One long sedimentary sequence with a fossil succession can be used to develop a stratigraphic chronology of fossil speciations and extinctions; thus, Fossil A is “older than” Fossil B. Fossil A found in other locations is equal to the time horizon preserved in the long sequence.
Trees in temperate conditions typically grow by annual increments with early and late forming woods (tree rings). These increments can be counted in cross section and used to estimate how many years the tree lived. As trees respond to varying climatic conditions, the increments wax and wane, and thus, a distinct pattern forms at particular times in the region. Coupled with radiometric methods or historical records, the patterns of numerous trees have been compiled to generate a long (banded) time sequence. Wood used by humans for building materials and preserved at archaeological sites can be analyzed for these tree rings and matched to a particular time.
The present configuration of the earth’s magnetic field is normal (indicated by a black strip on a rock), where the magnetic north pole is located at the geographic north pole. In earth’s history, the magnetic field has collapsed and reformed in the opposite configuration, or reversed (white strip), such that the magnetic north pole is was located at the geographic south pole. The orientation of the earth’s magnetic field, either normal or reversed, is imprinted on those rocks and compact sediments with magnetic minerals at the time of formation. Sequences of rocks and sediments all over the world have been sampled in either discrete intervals (e.g., every meter) or cored continuously and analyzed for magnetic orientation, and they display a record of reversals. The result resembles tree rings as a banded pattern (white and black stripes), which can be matched to long paleomagnetic records of known radiometric ages. The phenomenon of magnetic reversals is global and therefore can be used everywhere an appropriate rock sequence is present.
Volcanic eruptions eject ash, pumice, and other products (collectively known as tephra) that can be chemically fingerprinted. The fingerprint is typically unique to a particular eruption of one volcanic edifice and therefore represents a particular time in earth’s history. Tephra is characterized by major, minor, and trace (chemical) elemental and oxide abundances. Tephra layers represent synchronous units that can be traced laterally to compare archaeological and paleontological sites across landscapes.
Radiometric Dating Methods
Radiometric dating methods make use of the radioactive decay of chemical elements in various materials (e.g., bone, shell, minerals) to calculate the numeric age of human skeletons directly or to infer the age of humans associated with the measured material. Chemical systems used for dating are specific to time intervals because each chemical element decays at a particular rate. Half-life refers to the time required for half of a radiometric element (parent material) to decay to a daughter element.
The radioactive isotope of carbon (14C) is found in several different kinds of organic materials (e.g., bone, wood, shell, paleosols) and thus is useful in many anthropological contexts. Living organisms incorporate 14C into their tissues by physiologic processes, such as photosynthesis and respiration, in equilibrium with the atmosphere. When an organism dies, 14C within its tissues is converted to nitrogen (14N). Since 14C decays to 14N at a known constant (half-life: 5,370 years), time since death can be calculated by measuring the amount of 14C remaining relative to 14N in preserved tissues. Due to the relatively short half-life, radiocarbon methods are applicable to time intervals younger than approximately 50,000 years ago.
Potassium-Argon and Argon-40/Argon-39
Radioactive potassium (40K) decays to argon (40Ar) at a relatively slow rate (half-life: 1.3 billion years), and therefore, the method is typically used to date human fossil contexts older than 100,000 years. This method is restricted to minerals that contain ample amounts of K (e.g., feldspar, sanidine) with negligible 40Ar at the time of formation. Minerals are dated by measuring how much daughter product (40Ar) has been produced by the decay of parent material (40K). A slight variation on the method (40Ar/39Ar) uses a smaller amount of material and can measure single crystals. Anthropologists employ 40K/40Ar and 40Ar/39Ar to date rock and sediment layers associated with fossil materials rather than the fossils directly.
Various daughter products of 235U and 238U with different decay constants are used to date materials (e.g., coral, enamel, mineral) spanning a few years to 350,000 years old. Uranium decay in equilibrium produces an amount of daughter product equal to that decayed from parent material. If the system is disturbed, the balance between parent and daughter material is offset, and thus, the time since disturbance can be calculated. The method requires that either (1) the measured material formed without prior parent product, or (2) the equilibrium of parent to daughter has been disturbed. With regard to the latter, the initial amount of parent and daughter must be known.
Luminescence refers to light emission by a mineral (e.g., quartz, feldspar) when heated (TL: thermoluminescence) or exposed to light (OSL: optically stimulated luminescence). The dating method requires that a material (e.g., bone, tooth, artifact) has been buried and thus not exposed to light and heat since that individual was alive or since the artifact was used. Burial exposes the material to radioactive elements contained within the sediments, which contributes free electrons to the material’s crystal lattice. The amount of light emitted from the material represents burial duration. The method requires known or no residual luminescence from the time before burial and no disturbance since burial.
Stable Isotope Geochemistry
Using stable isotope geochemistry in anthropology rests on the principle that a human’s skeleton and preserved soft tissues represent aspects of the individual’s life. In addition, geological materials preserved at an archaeological or human paleontological site (e.g., shell, soil carbonates, animal bone, and teeth) can be used to reconstruct portions of that past human environment.
Stable isotopes do not decay or decay so slowly that the process is undetectable in contrast to radiometric isotopes. Lighter isotopes of a single chemical element, or those with the lowest atomic masses, have higher kinetic energy and are readily incorporated into chemical reactions. The heavier isotopes of the same element, or those with the highest atomic masses, move more slowly and are less likely to be integrated into reactions. These elements behave predictably in nature, and thus, comparing ratios of heavy to light isotopes in different materials can be used to reconstruct environmental conditions. Stable isotopes are analyzed by mass spectrometry. Mass spectrometers have three primary components: ion source, magnet, and ion detector. Once the preparation systems convert the solids to gases, the gases are introduced into the ion source, which removes an electron and creates a positively charged ion. The ions are accelerated and directed along a flight tube toward the magnet. The magnetic field causes the ions to bend to varying degrees based on atomic mass. Detectors at the end of the flight tube count the number of ions of each atomic mass and charge number. These results are compared with a standard and the ratio of heavy to light elements is given in delta notation calculated by the equation δ18O(o/oo) =
(18O/16Osample−18O/16Ostandard) / (18O/16Ostandard) × 1,000, using oxygen as an example and reported in parts per thousand (per mil, ‰). Generally, three stable isotopic systems are employed to answer questions about past human lifeways: oxygen, carbon, and nitrogen, and more recently, trace isotopes, such as strontium.
Oxygen, in the form of water, can be ingested and precipitated into tissues such as bone, enamel, and dentin. Oxygen can also be precipitated from water to form invertebrate shell material or carbonate nodules in soils. Oxygen isotopes are all incorporated into the skeletal material of organisms or soil carbonates according to the isotopes’ relative abundances in the water source. Due to a greater mass difference, the ratio 18O/16O is measured and reported as δ18O values. The differences in δ18O values of water sources result from differential evaporation rates of the two isotopes. The lighter isotope, 16O, is preferentially evaporated from water into the atmosphere resulting in an enrichment of the heavier isotope, 18O, in the water. Water temperature also changes δ18O values, and therefore, paleotemperatures can be estimated based on known relationships in biological tissues or soil carbonates formed in equilibrium with water. Oxygen isotope analysis detects systematic variation when individuals obtain water from different sources.
Carbon occurs in organic and inorganic phases of plant and animal tissue, and as carbonate in soils. In plants, carbon isotopes reflect one of three photosynthetic pathways: C3, C4, and crassulacean acid metabolism (CAM). C3 plants use the Calvin-Benson (3-carbon) cycle and consist of trees, shrubs, and moist climate-adapted grasses. C4 plants use the Hatch-Slack (4-carbon) cycle and include hot and dry climate-adapted grasses and marine vegetation. CAM plants switch from the 3- to 4-carbon cycle due to diurnal requirements and comprise the succulents. C3 plants prefer the lighter isotope (12C) and therefore have lower 13C/12C ratios or δ13C values. C4 plants do not discriminate between the two stable isotopes as strictly as C3 plants, and as a result, have higher δ13C values. CAM plants in effect display the full range of δ13C variability. In an individual, carbon isotopic ratios represent the photosynthetic pathway of ingested plant material with additional fractionation, or change in isotopic ratio, by means of metabolic activity as the organism digests the plant material. For instance, an organism that consumes corn or other C4 plants (e.g., sorghum, sugar cane, and tropical grasses) will have high δ13C values, whereas one that eats C3 plants (e.g., legumes, rice, wheat, trees and shrubs) will have low δ13C values. Carbon isotopic analysis can also distinguish marine and terrestrial floral and faunal dietary sources. Marine sources yield similar ranges of δ13C values to those of C4 plants. It is important to note, however, that δ13C values do not differentiate between an animal consuming plant material directly and a carnivore eating an herbivore that has consumed plant material. For this reason, carbon is a useful tool for reconstructing the ingestion of plant foods but lacks resolution in interpreting protein consumption.
Nitrogen is common in organic systems and provides trophic level information, and by extension, protein consumption. The lighter isotope (14N) is more easily incorporated into metabolic processes, such as ammonia excretion, resulting in enrichment of organism tissue in 15N. At the base of the food pyramid, nitrogen-fixing flora, such as legumes, are typically lower in 15N. These and other primary producers supply herbivores with low starting δ15N values, resulting in isotopic distinctions between herbivores feeding on legumes versus nonnitrogen fixers. As individuals consume protein, they incorporate the food δ15N values, resulting in a stepwise function up trophic levels. Additional trophic spaces in marine ecosystems give rise to higher δ15N values relative to terrestrial and freshwater ecosystems.
Strontium is a trace element found in numerous lithological and biological systems. Strontium isotopes are heavy elements and cannot be discriminated by physiological processes. As a result, plants and animals incorporate strontium isotopic ratios (87Sr/86Sr) from the bedrock on which they obtain food and water. All isotopes of strontium are stable and do not undergo radioactive decay; however, 87Sr is derived from the radioactive decay of 87Rb. Since the abundance of nonradiogenic 86Sr in a mineral does not change with time, 87Sr/86Sr ratios are determined by rock type and age. Old metamorphic rocks contain high 87Rb concentrations and therefore have high 87Sr/86Sr ratios. Young volcanic rocks, on the other hand, have little 87Rb and low 87Sr/86Sr ratios. Marine limestones reflect the 87Sr/86Sr composition of seawater at the time of formation.
Since strontium isotopes are indicators of where an individual obtained food and water, the method can be used to infer mobility and sedentism. In terrestrial ecosystems, 87Sr/86Sr ratios reflect the geologic substrate, or bedrock, from the landscape where plants and animals obtain nutrients. Soils and vegetation incorporate 87Sr/86Sr ratios from underlying bedrock. Ground and surface waters dissolve and integrate strontium from surrounding substrates, often mixing strontium from various sources. Organisms feeding on vegetation and drinking local water sources record bedrock 87Sr/86Sr ratios in appositionally forming tissues, such as dental enamel and bone. Mammalian teeth are formed at different times in an individual’s life, and therefore, several snapshots can be measured isotopically by sampling throughout the dental arcade and, more recently refined, by subsampling growth intervals of individual teeth. Place of origin and movement across bedrocks is then inferred by variable 87Sr/86Sr ratios sampled from teeth with different eruption times.
Geological Applications to Anthropological Questions
The geological methods presented above among many others have been used independently or in conjunction to answer critical anthropological questions and have expanded knowledge of the human lineage. As a result, many anthropologists have become experts in the various geological specialties and apply geological tools to modern and fossil human contexts with regularity.
Site Formation and Fossilization Processes
Human archaeological sites, whether they are composed of artifacts, ecofacts, structures, features, or skeletal material, typically involve or are preserved in the rock record. As Waters (1992) articulates, sediments and soils are the matrix of the archaeological site. Anthropologists apply principles of sedimentology and stratigraphy to unravel how humans used these sites in the past and how the sites in whole or in part changed through time until discovery. In the context of the site, sedimentological information and facies associations are intertwined with human activities.
South African Cave Site Formation
From Raymond Dart’s first discovery of Australopithecus africanus and recognition of its ancestry to humans, anthropologists made inferences of ancient lifeways, hunting capabilities, and history of violence. “Man the Hunter” was the prevailing view garnered from ethnographic studies of modern hunter-gatherer populations, which were constructed as a primitive society and a window into past human behavior. Cultural anthropologists have long since deconstructed these linear models of cultural evolution; however, geological applications by paleoanthropologists also aided in shifting paradigms. Australopithecus and other mammalian fossils excavated from cave deposits in South Africa were thought to be the result of interpersonal violence and hunting activities. C. K. Brain conducted detailed analyses of these cave layers using sedimentological and stratigraphic methods. He deciphered the timing of layer accumulations and found connections to surface processes. Brain reinterpreted the cave as a natural sink of bone accumulation rather than a home base used by early hominins. Moreover, based on his observations on modern leopard behavior (i.e., actualistic studies), he found that early humans were not violent hunters at all but just another mammal on the landscape hunted by saber-toothed cats.
Time Averaging of the Human Fossil Record
Similar to archaeological sites, the human fossil record forms by an interaction between geological and biological processes during formation and after deposition. Fossil accumulations represent a mixture of several time scales that culminate to produce a finite sedimentary unit. One fossil skeleton represents one individual with a particular life span, whereas a sedimentary layer full of many fossil skeletons may represent one coeval population, several generations, or more than one species. Paleoanthropologists separate these timescales with sedimentological and stratigraphic methods and actualistic studies of fossilization processes. Kay Behrensmeyer has verified time averaging within the human fossil record with modern studies in savanna environments in Amboseli National Park, Kenya. On her “bone-walks” she records the manner in which animal carcasses enter the fossil record and how fossilization processes vary significantly with respect to sedimentary matrices and geomorphological features.
Tempo of Human Evolution
Principles of stratigraphy and various dating methods provide the critical temporal framework for the origins and extinctions of modern humans and ancestral human species. Early discoveries of primitive and derived human forms in stratigraphic sequence painted a broad-brushed picture of linear and gradual evolution. Over the past two decades, paleoanthropologists have demonstrated a more complicated and branching pattern of human evolution. Below are three examples of how geochronological data informed anthropological interpretation and overturned established paradigms.
Evolution of Early Homo
The rise of the human genus Homo, represented by fossil material discovered principally by the Leakey family in the Turkana Basin in Kenya and Olduvai Gorge in Tanzania, was long reconstructed as gradual and linear. Homo habilis, or handy man, dated to 2.5 million years ago (mya), was placed at the beginning of the human family tree. H. habilis possessed a slightly larger brain size compared with members of Australopithecus, and moreover, was associated with rudimentary stone tools. Homo rudolfensis, discovered on the shores of Lake Turkana (formerly Lake Rudolf), was dated to 1.9 mya and possessed a larger brain than H. habilis. In the same location, Homo erectus was discovered and dated to 1.8 mya. Corresponding with the model of linear evolution, H. erectus indeed showed a larger brain and body size than H. rudolfensis. Additional fossil finds led to morphological comparisons that indicated a complex pattern of speciation; however, the linear model persisted until recent years in the scientific community largely due to the handful of dates. Tephrochronological work in the Turkana Basin by Frank Brown and Craig Feibel and 40K/40Ar measurements by Ian MacDougall in the 1980s and 1990s provided a detailed sedimentological and stratigraphic framework for all paleontological collecting areas. In 2007, Fred Spoor and colleagues reported a new specimen of Homo erectus that predated the last appearance of H. habilis and H. rudolfensis in the region. Based on the established geochronology, early members of genus Homo are now interpreted to have lived during overlapping time periods. As a result, these species do not represent an ancestral line of humans, but rather sympatric species competing for resources.
Dispersal of Homo erectus From Africa
Anthropologists conceived early dispersal of Homo erectus from Africa to Asia as a technological advancement to new resources occurring only after the first appearance of the Acheulean tool kit. Even without numeric dates, researchers claimed that H. erectus did not inhabit Asia prior to 1 mya. In 1994, Carl Swisher and colleagues dated volcanic material associated with early H. erectus specimens (Mojokerto) by 39Ar/40Ar and paleomagnetic methods on the island of Java. Previous estimations placed H. erectus on Java by 500,000 years ago; the new date, 1.8 mya, was significantly older and led to a controversy within the anthropological community. Many claimed that H. erectus was incapable of dispersing with only the primitive Oldawan tool kit. Others claimed the provenience of the hominin fossils was questionable. The dates remained debated until 2000 when H. erectus specimens were discovered in the Republic of Georgia and were dated by many geologists, including Swisher, by 39Ar/40Ar, paleomagnetic, and biostratigraphic methods to be 1.7 million years old. Although some anthropologists remain skeptical about the plausibility of early dispersal, geochronological data provide evidence for an alternative view.
Rise and Longevity of Homo floresiensis
The 2003 discovery of the small human specimen on the island of Flores stunned the anthropological community, and researchers are still debating about if the finds represent a new species, Homo floresiensis, or a pathological individual. Anthropologists relied on geological tools to determine the context, age, and, possibly more important to the debate of new species or pathology, the longevity of the small-bodied forms. The most complete specimen of the so-called Flores hobbit was initially dated to 13,000 years ago by radiocarbon and luminescence methods. This date was surprising because it meant that these dwarfed individuals were coeval with normal-sized humans inhabiting the region and must have had watercraft/ raft technologies. Another mandibular (jawbone) specimen was discovered showing the same unusually small proportions, but it was radiocarbon dated to 18,000 years ago. Overall, several specimens have been recovered and span the interval from 38,000 to 12,000 years ago. The debate over the identity of these dwarfed individuals continues, but because of geochronological evidence, anthropologists must incorporate thousands of years within their explanatory models.
Human Diet and Mobility
Stable isotope-geochemical methods revolutionized studies of human diet and mobility. Prior to these analyses, information about diet and movement was inferred from time averaged and taphonomically altered plant and faunal remains, artifact assemblages, and materials. Geochemical analyses of skeletal material provided an independent test of other methods, but moreover, offered diet and movement information about the individual. Over the last three decades, stable isotope analysis has been rigorously developed and has now become a standard measure of human materials from numerous archaeological and paleontological sites. Here are three examples of geochemical approaches to anthropological questions of diet and mobility.
Introduction of Corn to the New World
Age calculations of North American archaeological sites using radiocarbon methods assumed a constant 12C and 13C abundance in plant tissue; however, due to different photosynthetic pathways, dates on corn yielded systematic errors. Researchers realized that plants with different photosynthetic pathways are distinguished by stable carbon isotopes as discussed in the carbon isotope section above. It wasn’t long before anthropologists applied these geochemical findings to questions about the origins of corn cultivation and spread throughout the New World. John Vogel and Nicholas van der Merwe (1977) isotopically analyzed human skeletons from New York State and demonstrated that “you are what you eat.” Dale Hutchinson, Lynette Norr, and colleagues have documented the mosaic pattern of corn domestication in southeastern North America integrating skeletal indicators of health, disease, and pathology. These early investigations paved the way for numerous isotopic studies into diet and the rise of agriculture.
Early Hominin Diets and Mobility
Workers from the Light Stable Isotope Laboratory at the University of Cape Town, South Africa, obtained carbon and oxygen isotopic data from several fossil hominin specimens that indicate aspects of early diet and mobility. Matt Sponheimer and Julia Lee-Thorp found that Australopithecus africanus and A. robustus were encroaching on the more open habitats and may have subsisted on a mixed diet of vegetal matter and animal protein. Lee-Thorp and colleagues interpreted the close isotopic results of carnivore and hominin species indicating hominins as prey, although she demonstrated that sabertoothed cats might not have been the culprits as C. K. Brain initially thought. However, Lee-Thorp cautions that carbon isotopes alone may be insufficient in answering questions about trophic space. Andrew Sillen and colleagues applied strontium isotopes to determine mobility of A. robustus across the karst landscape of South Africa. Sillen and later Sponheimer found significant differences between male and female specimens, and those specimens suggest that robust australopithecines (paranthropines) may have had male dispersal patterns similar to that of modern gorillas.
Neanderthal Hunting and Mobility
Groups led by Herve Bocherens and Michael Richards have applied nitrogen, carbon, and strontium isotopic methods to well-preserved Neanderthal specimens from Europe. Although to date few individuals have been analyzed, these studies suggest that Neanderthals consumed high levels of herbivorous prey and may have traveled long distances to obtain dietary resources. Moreover, these dietary interpretations challenge the previous notion that Neanderthals were inefficient foragers and hunters.
Reconstructing Human Environments
Geological methods provide tools for reconstructing the environmental contexts of modern and extinct humans. As discussed above, excavation of archaeological sites involves geological expertise to infer site formation processes; however, geological tools can also be used to infer aspects of the surrounding local, regional, and global contexts with respect to climate, resource distribution, and geomorphology present during the humans’ tenure. Two examples are presented that demonstrate how geological data were used to show the influence of the environment on human cultural and morphological change.
Drought and the Collapse of the Maya Civilization
In 1995, David Hodell, Jason Curtis, and Mark Brenner conducted oxygen isotopic analyses and mineralogical studies of lake cores from Central America and Haiti and reconstructed paleoclimate and paleoenvironmental conditions in these regions for the Holocene. The lake sediment records illustrated the gradual shift from cold and dry conditions from the start of the Holocene—coinciding with the last glacial period—to warm and wet conditions reaching a maximum between 7,000 and 4,000 years ago. They also detected a relatively small-scale dry phenomenon from roughly 2,000 to 3,000 years ago previously undetected by deep-sea core operations. At the height of the dry period, Hodell et al. reported a 200-year drought in the Yucatan Peninsula. Radiocarbon analysis of pollen showed that the drought temporally coincided with the collapse of the Maya civilization. Since that time, other researchers have found similar patterns of environmental change and drought. Although archaeologists disagree as to the ultimate cause(s) of societal collapse, the reconstructed context suggests that severe environmental conditions may have been a significant factor.
Grassland Spread and Early Human Evolution
Since Darwin first suggested grasslands played a key role in shaping human evolution, researchers have been drawing temporal associations between environmental change and the rise and extinction of hominins. Darwin explained increased range with living and evolving in open habitats. Paleoanthropologists proposed that hominins were arid-adapted species and took their beginnings in a savanna environment. Several researchers have linked climate to grassland spread to frame hominin adaptations including the rise of bipedality, dietary change, and technological advancements. Although the proposed hypotheses of adaptations involve complex interactions of the environment with behavioral and morphological underpinnings, they all rely on the presence of grasslands.
Evidence for grassland spread was largely determined by carbon and oxygen isotopic geochemistry by Thure Cerling and colleagues. Cerling and others documented a global shift in photosynthetic pathway from C3, used by many low- to midlatitude plants including grasses, to C4, used mostly by tropical grasses, beginning in the Miocene. Cerling and others isotopically analyzed suites of mammalian enamel and ancient soil carbonates from fossilbearing sediments, and they reconstructed grassland versus woodland distribution in hominin environs. Evidence to date from many fossil hominin locales illustrates that grassland spread occurred in a mosaic pattern that is both temporally correlated and out of sync with hominin speciation and extinction events. Local and regional tectonic regimes, paleogeography, and paleohydrology, among other factors, rather than climate alone influenced the timing of grassland distribution.
The future likely holds a closer relationship between geology and anthropology, although possibly one where anthropology can be useful to geological pursuits rather than one that is strictly characterized by an anthropologist’s use of geological tools. Two new directions are briefly mentioned here: The first illustrates a recent application of geological methods by forensic anthropologists, which has become a new and widespread undergraduate major in the United States. The second direction is how anthropological knowledge and tools may serve geological interests.
Geological tools have recently been applied to forensic anthropology and crime scene investigations. When human remains are discovered in sedimentological contexts (e.g., earthen burials), sediments and sedimentary features can prove useful to determine manner and timing of deposition. Recently, stable isotopic methods have been applied to individuate unidentified human remains. Strontium isotopes, in particular, identify a person’s place of origin based on geologic bedrock.
Impact of Climate Change on Humans
Departments of earth sciences have become increasingly invested in studying past global climate in order to predict how and when global climate change will affect humans in the future. Geologists are commonly asked to weigh in on political and social decisions about global warming. Anthropology has much to offer geology with respect to perspective, information, and methods in both living and past human groups that can be used to address how climate (and environmental) change will impact humans today.
Methods from the field of geology are critical to advancing understanding of human lifeways and evolution in the past and present. Geology’s theory of uniformitarianism has shaped anthropology’s use of actualistic studies and the development of archaeology’s middle range theory. Sedimentology offers tools to infer site formation and fossilization processes. Stratigraphy and dating methods provide the temporal framework for the human lineage from the earliest members to yesterday’s cultures. Over the past three decades, stable isotope geochemistry has been applied to anthropological questions of diet and subsistence, mobility and sedentism, and environmental influences on speciation and extinction.
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