The oldest anatomically modern human remains are beyond the range of radiocarbon dating, and associated deposits lack material suitable for most other dating methods. Consequently, age estimates for early human skeletal material and correlative stratigraphic horizons in southern Africa are frequently based on paleoclimatic correlations to the deep-sea record and extrapolated sedimentation rates, both of which incorporate a number of untested assumptions. Here we focus on one substage of the Middle Stone Age of southern Africa, the Howiesons Poort industry, a distinctive culture-stratigraphic marker in sequences south of the Zambezi. Anatomically modern human skeletal material has been found associated with, or even older than the Howiesons Poort layer in stratified deposits at Border Cave and Klasies River main site. We have dated or bracketed the Howiesons Poort horizon at Border Cave, Boomplaas Cave and Apollo 11 Cave, three stratified cave sites in southern Africa, based on the extent of isoleucine epimerization in associated ostrich eggshells. We conclude that the Howiesons Poort lithic industry is bracketed by limiting dates of 56 and 80ka, and is most likely centered on 66±5ka. Anatomically modern human remains in deeper levels are more than 100ka old, lending support to the hypothesis of an African origin for Homo sapiens.

Variation in the isoleucine epimerization rate and amino acid composition and concentration among and within the three layers comprising the shell of the marine bivalve Chione fuctifraga from the northern Gulf of California was determined in a time series of ten shells, from modern back to 900 yr BP (reservoir-age-corrected radiocarbon age). Differences in epimerization rates of up to 30% were found between various portions of the shells. In samples taken from the middle layer of the shell (two different positions) and the hinge area of the inner layer, D-alloisoleucine/L-isoleucine (A/I) values showed excellent age prediction ability, whereas in samples from the outer layer and the central part of the inner layer, A/I values showed greater variability. Isoleucine epimerization rates were found to differ between sampling positions within both the inner and outer layers of the shell. Average rate differences were also found among layers. The amino acid composition of the three layers is rather similar (with Asp, Glu, and Gly the most abundant amino acids) but variable. Significant differences in amounts of amino acids were found, with the middle layer showing the lowest amounts and the inner layer the highest amounts. Careful choice of sampling position may improve the accuracy of age estimates from amino acid racemization.

GeoarchaeologyVolume 11, Issue 3 p. 189-213 Assessment of integrity and geochronology of archaeological sites using amino acid racemization in land snail shells: Examples from central Texas G. Lain Ellis, Corresponding Author G. Lain Ellis Texas Historical Commission, P.O. Box 12276, Austin, Texas 78711Texas Historical Commission, P.O. Box 12276, Austin, Texas 78711Search for more papers by this authorGlenn A. Goodfriend, Glenn A. Goodfriend Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, DC 20015-1305Search for more papers by this authorJames T. Abbott, James T. Abbott TRC-Mariah Associates, Inc., 3939 Bee Caves Road, Suite C-100, Austin, Texas 78746-6429Search for more papers by this authorP. E. Hare, P. E. Hare Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, DC 20015-1305Search for more papers by this authorDavid W. Von Endt, David W. Von Endt Conservation Analytical Laboratory, Smithsonian Institution, 4210 Silver Hill Road, Suitland, Maryland 20746Search for more papers by this author G. Lain Ellis, Corresponding Author G. Lain Ellis Texas Historical Commission, P.O. Box 12276, Austin, Texas 78711Texas Historical Commission, P.O. Box 12276, Austin, Texas 78711Search for more papers by this authorGlenn A. Goodfriend, Glenn A. Goodfriend Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, DC 20015-1305Search for more papers by this authorJames T. Abbott, James T. Abbott TRC-Mariah Associates, Inc., 3939 Bee Caves Road, Suite C-100, Austin, Texas 78746-6429Search for more papers by this authorP. E. Hare, P. E. Hare Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, DC 20015-1305Search for more papers by this authorDavid W. Von Endt, David W. Von Endt Conservation Analytical Laboratory, Smithsonian Institution, 4210 Silver Hill Road, Suitland, Maryland 20746Search for more papers by this author First published: May 1996 https://doi.org/10.1002/(SICI)1520-6548(199605)11:3<189::AID-GEA1>3.0.CO;2-%23Citations: 12AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat Abstract Detailed racemization analyses were carried out on samples of the land snail Rabdotus mooreanus from archaeological sites at Fort Hood, in central Texas. D-alloisoleucine/L-isoleucine (A/I) values were determined for 260 individual shells from 29 proveniences, including sites in alluvium, colluvium, and rockshelters, as well as burned rock middens. A/I values show a good correlation with radiocarbon age, and so provide reasonably precise estimates of ages. Analyses indicate the presence of redeposited material in a large number of proveniences. These result from sedimentary processes involved in burial of the sites as well as from later disturbance (aboriginal or recent) of site stratigraphy. Because amino acid racemization analyses are relatively easy to carry out, this method lends itself to very detailed chronostratigraphic analyses of archaeological sites, thus permitting assessment of site integrity and assisting in the interpretation of site formation processes. © 1996 John Wiley & Sons, Inc. References Abbott, J. T. (1994). Natural Environment. In W. N. 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In bones not influenced by other diagenetic effects, heating events can be reflected in changes in the relative concentrations of constituent amino acids that alter the collagen-like pattern in bone to a non-collagen-like pattern with an increase in the relative NH, levels. The ratio of glycine (Gly) to glutamic acid (Glu) can be used as an index of a collagen-like or non-collagen-like pattern. For a group of eight bone samples from a southwestern United States Pueblo II period Anasazi site dated to about ad 1100, amino acid composition, as characterized by Gly/Glu values, along with NH3 data, clearly indicated which bone fragments had been subjected to one or more heating events. The use of Gly/Glu and NH, values for the purpose of inferring heating events may not be accurate for bones which have experienced other significant diagenetic effects not related to heating.

Chemotaxonomic and aminostratigraphic use of planktonic foraminiferal shell amino acid compositions assume that recovered amino acids are from organic material originally produced by the living foraminifer. Deep sea planktonic foraminiferal shells that were bleached prior to amino acid analysis show evidence of a labile, amino acid-containing organic fraction which may include contaminating adsorbed material. The labile organic matter composition and amount have been inferred by comparing amino acid recoveries from samples cleaned only by ultrasonication in buffered sodium hexametaphosphate or methanol, to those additionally bleached with sodium hypochlorite. The labile fraction is chemically accessible to bleach, contains an unknown proportion of adsorbed contamination, and has a different amino acid composition than the chemically inaccessible fraction.The amino acid composition of the labile material differs between Globigerinoides sacculifer, Globorotalia tumida, and Globorotalia inflata samples from different core samples. Labile material is not always present: amino acids were removed by bleaching late Pliocene Globorotalia inflata samples from DSDP 588-4-1, but not by bleaching late Pliocene samples of the same species from DSDP 588-5-1. Bleached late Pliocene Globorotalia inflata samples from DSDP 588-4-1 also had a higher D-allo-isoleucine to L-isoleucine ratio (aIle/Ile) than unbleached samples.The amount of labile material removed varied from 0.6 to 1.3 mumole/g calcite. Amino acid recovery from bleached samples with the labile fraction removed ranged from 1-2 mumole/g calcite. Labile fraction amino acids losses thus represented 23%-42% of unbleached total recoveries. The amino acid composition of the chemically inaccessible fraction retained a species specificity among the three planktonic foraminiferal species examined. Removal of labile, possibly exogenous, material by oxidative treatment is recommended prior to amino acid analysis of all foraminiferal shell samples for aminostratigraphy and chemotaxonomy, so that only amino acids from the chemically inaccessible organic material are compared.

A study was conducted to determine the feasibility of using differences in the natural abundance of dietary 13C as a means of in vivo labeling. Four female pigs were fed a C3 plant diet (barley, soybean meal and alfalfa) and four were fed a C4 plant diet (corn and corn gluten meal). The pigs were continually fed the diets from 8 wks of age to maturity and through pregnancy until they had themselves produced offspring. The resulting piglets were sacrificed at various time intervals (0 to 165 days) after switching either the piglets or their mothers' to the opposite diet. Changes in the delta 13C levels in samples of milk and tissues following the diet change were monitored by isotope ratio mass spectrometry. The differences in the delta 13C content of the two diets was 12.9 parts per mil (/1000). After piglets born to a sow fed the C3 diet were switched at one day of age to a surrogate sow fed the C4 diet, tissue delta 13C levels of the piglets were nearly identical to the diet of the surrogate sow by 11 days while those switched from C4 to C3 grew more slowly and differed from the diet by 2-5/1000. When the mothers diet was switched, 15-20 days later the delta 13C in the milk had changed by only 2-3/1000, indicating that the body rather than the diet was the primary source of carbon in the milk. At 28 days after the piglets diet was switched at weaning, the tissue delta 13C had changed by 8.4/1000 (C4 to C3) and 2.9/1000 (C3 to C4). The delta 13C level in the liver changed more rapidly than in the muscle or fat and the rate of change was also influenced by the size and growth rate of the pig.

D/L aspartic acid values from a 350-year time series of annual growth bands of a living colony of the coral Porites australiensis show a very regular pattern of increase with age. The initial rate of racemization is extraordinarily rapid (0.6% per year) but slows in older growth bands to 0.04% per year (4% per century). The skeletal proteins show progressive hydrolysis with increasing age, with free aspartic acid comprising 16% of the total aspartic acid in the 350-year-old band. The proteins are unusually rich in aspartic acid (nearly 50 mol%). The relative weakness of the peptide bonds formed by aspartic acid moieties is probably responsible for the rapid hydrolysis and consequent rapid racemization of aspartic acid. Racemization analysis provides a means for checking for sections of missing bands in corals and for screening of prospective samples for U-Th dating.

Aspartic acid from an HC1 hydrolyzed portion of 20–25th Dynasty Egyptian bone gave a D/L value of 0.28. Various peptide and molecular weight fractions separated before hydrolysis from another aliquot of the same bone portion yielded D/L aspartic acid values ranging from 0.09 to 0.68. Higher molecular weight and higher content of hydrophobic amino acids are factors leading to lower D/L aspartic acid values. Insoluble, high molecular weight polypeptide residues showed very low D/L aspartic acid values of 0.09. We propose that particularly stable peptides be isolated and characterized and then used for comparison with similarly isolated peptides from other fossil bone samples for purposes of age estimation.

Molecular level 14C dating is the isolation of specific classes of molecules for their 14C dating by accelerator mass spectrometry (AMS). Complex matrices such as fossil bone are difficult to date due to their extreme chemical heterogeneity. By isolating individual amino acids, contaminants (humates) are removed and crystalline amino acids result.

Pigs were reared in laboratory pens on controlled diets that consisted of either 100% C3 plants or 100% C4 plants. Carbon and nitrogen isotopic compositions of the diets, and the resulting pig products, purified collagen and muscle tissue, were measured to determine isotopic fractionation during growth and metabolism. Total collagen from pigs grown on C3 diets was enriched in 13C by 3·2‰ and in 15N by 2·2‰, whereas that from pigs reared on C4 diets was enriched in 13C by 1·4‰ and in 15N by 2·3‰. In addition, fractionation between pigs and their diets was determined at the molecular level on individual amino acids separated by ion exchange chromatography. The carbon isotopic compositions of separated amino acids from the C3 and C4 diets were transferred to amino acids in bone collagen. For nitrogen, the isotopic compositions of all nonessential amino acids were enriched in 15N relative to those amino acids in the diet. Threonine, an essential amino acid, behaved oppositely, in that its isotope ratio (δ15N) was depleted by an average of 6‰ from the δ15N of the whole collagen. Similar isotopic patterns were analysed in collagenous amino acids extracted from field specimens that included both herbivores and carnivores; marine animals and terrestrial animals; and C3 and C4 feeders. Amino acids from two fossil bones, a bison (4500 years old) and a whale (70,000 years old), recorded the same isotopic signals as modern collagen. The ubiquity of these isotopic patterns at the molecular level suggests that distinct biochemical mechanisms control the metabolism of amino acids in animals rather than random synthesis.

Eggshells of the African ostrich ( Struthio camelus ), ubiquitous in archeological sites in Africa, have been shown by laboratory simulation experiments to retain their indigenous organic matrix residues during diagenesis far better than any other calcified tissue yet studied. The rate of L-isoleucine epimerization to D-alloisoleucine follows reversible first-order kinetics and has been calibrated for local temperature effects and used to estimate the age range of stratified archeological sites. Age estimates are consistent with radiocarbon dates from several stratified archeological sites. With adequate calibration, this technique can provide accurate ages to within 10 to 15 percent for strata deposited within the last 200,000 years in the tropics and the last 1,000,000 years in colder regions such as China.

Accelerator mass spectrometry (AMS) radiocarbon dates fail to provide conclusive evidence that all New World human fossils are younger than approximately 11,000 yr. Because fossil bones vary widely in preservation, their radiocarbon dates are not equally accurate. Molecular-level radiocarbon dating, which used individual amino acids to assess fossil diagenesis, revealed that dates on known-age, noncollagenous bone were underestimated by at least 2000 to 9000 yr. The significance is that >11,000-yr-old fossil bones with poor preservation would yield Holocene and not Pleistocene radiocarbon ages, regardless of what chemical pretreatment or 14C counting method was used. Irreplaceable evidence for Pleistocene-age fossils in the New World could be lost if the diagenesis of fossil bones is not evaluated before the bones are radiocarbon dated. In contrast, radiocarbon ages for collagenous fossils can be determined more accurately if 14C is measured in several individual amino acids that are isolated from collagenous bone protein. Molecular-level radiocarbon dating will greatly improve not only the accuracy of chronologies for human migrations and animal extinctions, but of all late Quaternary chronologies that are based upon the 14C dating of fossil proteins.

The concentration of free amino acids (FAA) increases in three species of bivalves, Tagelus plebeius, Rangia cuneata, and Phacoides pectinatus, taken from ever increasing depth in the upper 72 cm of a core from a mesohaline, sandy-bottom site in Copano Bay, Texas. Analysis of museum shells of known age indicates that [FAA]* increases with shell age (time-since-death) as the proteinaceous matrix gradually breaks down. Estimation of relative age (one shell older than another) in shells less than a few hundred years old is readily accomplished using a measure of [FAA]. The estimation of absolute age (actual time-since-death) depends on an independent calibration. Museum shells of known age offer one possible method. The glycinelalanine ratio decreases with time either because the rate of alanine production increases or because glycine diffuses out of shells more rapidly. Hence [glycine] is a useful estimator of age for relatively young shells and [alanine] or [glutamic acid] for older shells. Age estimates from calibration curves using museum specimens of T. plebeius and R. cuneata and independent estimates based on sediment accumulation rates agree for shells taken from the Copano Bay site. Measuring [FAA] provides a method to appraise the degree of time averaging in modern death assemblages because the relationship of shell age and burial depth can be determined. [FAA] changed little in shells below 72 cm in the core indicating that an increase in time averaging may have occurred below that depth, whereas, in the upper 72 cm, changes in death assemblage