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Kromdraai Faunal dating_proteonomics

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ProposalDescription: 

This research is been conducted as part of the Wisdom Teeth project (https://sites.google.com/york.ac.uk/wisdom-teeth/home) which aims to make use of the recent breakthrough made in using protein degradation to date tooth enamel. Applying it to regions where the palaeoenvironmental record can help us understand the sensitivity of Africa’s mammalian fauna to climate change. In doing so, it will provide a new, more accurately dated record for the African Pleistocene and Late Pliocene, unlocking insights into our own evolutionary history.

Expanded_Motivation: 

Amino acids are the building blocks of proteins, which are found in all living tissues and can be preserved in fossil biominerals such enamel or shells. Intracrystalline protein degradation (IcPD) dating relies on the predictable breakdown of proteins and amino acids within the closed system environment of a given biomineral, to give a direct estimate of the age. IcPD analysis of molluscan material from Europe has shown that IcPD dating covers at least the last 2.5 Ma, and thus is applicable to the whole of the Quaternary Period (Penkman et al., 2011; 2013). Additionally, recent IcPD analysis of teeth from a range of genera from European deposits suggests enamel IcPD may be able to date material further back in time, potentially into the Pliocene (Dickinson et al., 2019; Cappellini et al., 2019; Welker et al., 2020). However, the range for which it is applicable is highly dependent of the temperature history of the samples and thus in warmer climates the applicable dating range may be shorter. Most amino acids can exist in 2 forms which are non-superimposable mirror images of each other (Figure ), designated left-handed (laevo, L-form) and right-handed (dextro, D-form). In living organisms, proteins are almost exclusively made from the L-form. However after death, a spontaneous reaction (called racemization) starts to occur. This leads to a progressively increasing proportion of the D-form in direct relation to the time elapsed, until the D and L forms are present in equal quantities. Depending on the amino acid, this process can take thousands or millions of years and therefore is applicable over Quaternary timescales. The extent of amino acid racemisation (AAR) in a sample is recorded as a D/L value, and its age can thus be determined based on (a) which amino acid it is, (b) the species being analysed, and (c) a baseline reference framework of comparative data from independently dated sites (an aminostratigraphy). There are several measures of protein degradation that can be used to estimate the age of a fossil, such as extent of peptide hydrolysis and changes in amino acid composition but the most time sensitive process of degradation used for age estimation is AAR. Protein degradation consists of a series of chemical reactions that are dependent not only on time, but also on environmental factors (e.g. pH, availability of water), which can confound the time signal. These difficulties in AAR’s early applications have led to a focus on analysing ‘closed-system’ protein from fossil samples (Towe, 1980; Brooks et al., 1990), where the fraction of protein analysed is physically or chemically shielded from the environment and the difficulties associated with contamination, leaching and environmental factors are circumvented (Penkman et al., 2008). Tooth enamel has been found to contain a fraction of intra-crystalline amino acids that exhibit closed system behaviour, meaning that the extent of AAR within this fraction is solely time and temperature dependent (Penkman et al., 2008; Dickinson et al., 2019). The rate of breakdown towards D/L equilibrium in the intra-crystalline fraction is still affected by temperature, so comparative frameworks need to be applied from regions with a broadly similar temperature history, which is currently being developed for South Africa through a NERC-funded Wisdom Teeth Project. Analyses are routinely undertaken on the total hydrolysable amino acid fraction (THAA, which includes both free and peptide-bound amino acids), and often also on the free amino acid fraction (FAA, produced by natural hydrolysis). The method involves removing a small sample of enamel (~30-50 mg) and then removing all other dental components (e.g. dentine) from the sample. This is done using a precision drill with a small abrasive drill bit. The drill should be kept on the slowest setting to reduce the impact of the drilling on the specimen. The samples are then powder using an agate pestle and mortar and bleached to remove all sources of external contamination and to isolate an intracrystalline fraction (Dickinson et al., 2019). Two subsamples are then taken from the bleached material: one is used to analysed the free amino acid (FAA) content, the other the total hydrolysable amino acid (THAA) content. The THAA fraction is treated with strong acid to hydrolyse the peptides and proteins. After the inorganic mineral has been removed, the samples are analysed by RP-HPLC to determine the amino acid composition and ratio of D and L amino acid. It is expected that the project will generate relative age estimates for the material analysed and that these results will be published alongside other data from material from South Africa. Upon publication, amino acid data are now being archived by NOAA and are freely available at http://www.ncdc.noaa.gov/paleo/aar.html.

ApplicationDate: 

Saturday, May 14, 2022 - 08:17

CaseID: 

18566

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