Ancient proteins are complex mixtures and the term palaeoproteomics is used to characterise the study of proteomes in the past. [1] Ancients proteins have been recovered from a wide range of archaeological materials, including bones, [2] teeth, [3] eggshells, [4] leathers, [5] parchments, [6] ceramics, [7] painting binders [8] and well-preserved soft tissues like gut intestines. [9] These preserved proteins have provided valuable information about taxonomic identification, evolution history (phylogeny), diet, health, disease, technology and social dynamics in the past.
Like modern proteomics, the study of ancient proteins has also been enabled by technological advances. Various analytical techniques, for example, amino acid profiling, racemisation dating, immunodetection, Edman sequencing, peptide mass fingerprinting, and tandem mass spectrometry have been used to analyse ancient proteins. [10] The introduction of high-performance mass spectrometry (for example, Orbitrap) in 2000 has revolutionised the field, since the entire preserved sequences of complex proteomes can be characterised. [11]
Over the past decade, the study of ancient proteins has evolved into a well-established field in archaeological science. However, like the research of aDNA (ancient DNA preserved in archaeological remains), it has been limited by several challenges such as the coverage of reference databases, identification, contamination and authentication. [12] Researchers have been working on standardising sampling, extraction, data analysis and reporting for ancient proteins. [13] Novel computational tools such as de novo sequencing [14] and open research [15] may also improve the identification of ancient proteomes.
Abelson, Hare and Hoering were leading the studies of ancient proteins between the 1950s and the early 1970s. [16] Abelson was directing the Geophysical Laboratory at the Carnegie Institute (Washington, DC) between 1953 and 1971, and he was the first to discover amino acids in fossils. [17] Hare joined the team and specialised in amino acid racemisation (the conversion of L- to D-amino acids after the death of organisms). D/L ratios were used to date various ancient tissues such as bones, shells and marine sediments. [18] Hoering was another prominent member, contributing to the advancement of isotopes and mass spectrometry. [19] This golden trio drew many talented biologists, geologists, chemists and physicists to the field, including Marilyn Fogel, [20] John Hedges [21] and Noreen Tuross. [22]
Wyckoff was a pioneer in X-ray crystallography and electron microscopy. [23] Using microscopic images, he demonstrated the variability and damage of collagen fibres in ancient bones and shells. [24] His research contributed to the understanding of protein diagenesis (degradation) in the late 1960s, and highlighted that ancient amino acid profiles alone might not be sufficient for protein identification. [25]
Jope and Wesbroek were leading experts in shell proteins and crystallisation. [26] Wesbroek later established Geobiochemistry laboratory at the University of Leiden, focusing on biomineralisation and how this process facilitated protein survival. [27] He also pioneered the use of antibodies for the study of ancient proteins in the 1970s and 1980s, utilising different immunological techniques such as Ouchterlony double immunodiffusion (interactions of antibodies and antigens in a gel). [28]
Ostrom championed the use of mass spectrometry since the 1990s. [29] She was the first to improve the sequence coverage of ancient proteins by combining different techniques such as peptide mass fingerprinting and liquid chromatography-tandem mass spectrometry (LC-MS/MS). [30]
Understanding how ancient proteins are formed and incorporated into archaeological materials are essential in sampling, evaluating contamination and planning analyses. [1] Generally, for ancient proteins in proteinaceous tissues, notably, collagens in bones, keratins in wool, amelogenins in tooth enamel, and intracrystalline proteins in shells, they might be incorporated during the time of tissue formation. [31] [32] [33] However, the formation of proteinaceous tissues is often complex, dynamic and affected by various factors such pH, metals, ion concentration, diet plus other biological, chemical and physical parameters. [34] One of the most characterised phenomena is bone mineralisation, a process by which hydroxyapatite crystals are deposited within collagen fibres, forming a matrix. [35] Despite extensive research, bone scaffolding is still a challenge, and the role of non-collagenous proteins (a wide range of proteoglycans and other proteins) remains poorly understood. [36]
Another category is complex and potentially mineralised tissues, such as ancient human dental calculi and ceramic vessels. Dental calculi are defined as calcified biofilms, created and mediated by interactions between calcium phosphate ions and a wide range of oral microbial, human, and food proteins during episodic biomineralisation. [37] [38] Similarly, the minerals of a ceramic matrix might interact with food proteins during food processing and cooking. This is best explained by calcite deposits adhering to the inside of archaeological ceramic vessels. [7] These protein-rich mineralised deposits might be formed during repeated cooking using hard water and subsequent scaling. [39]
Organic (containing carbon) biomolecules like proteins are prone to degradation. [40] For example, experimental studies demonstrate that robust, fibrous and hydrophobic keratins such as feathers and woollen fabrics decay quickly at room temperature. [41] [42] Indeed ancient proteins are exceptional, and they are often recovered from extreme burial contexts, especially dry and cold environments. [43] [44] This is because the lack of water and low temperature may slow down hydrolysis, microbial attack and enzymatic activities. [31]
There are also proteins whose chemical and physical properties may enable their preservation in the long term. The best example is Type 1 collagen; it is one of the most abundant proteins in skin (80-85%) and bone (80-90%) extracellular matrices. [45] It is also mineralised, organised in a triple helix and stabilised by hydrogen bonding. [46] Type 1 collagen has been routinely extracted from ancient bones, leathers, and parchments; these characteristics may contribute to its stability over time. [47] [48] Another common protein in the archaeological record is milk beta-lactoglobulin, often recovered from ancient dental calculi. [49] Beta-lactoglobulin is a small whey protein with a molecular mass of around 18400 Da (dalton). [50] It is resistant to heating and enzymatic degradation; structurally, it has a beta-barrel associated with binding to small hydrophobic molecules such as fatty acids, forming stable polymers. [51] [52]
Given that proteins vary in abundance, size, hydrophobicity (water insolubility), structure, conformation (shape), function and stability, understanding protein preservation is challenging. [12] While there are common determinants of protein survival, including thermal history (temperature/time), burial conditions (pH/soil chemistry/water table) and protein properties (neighbouring amino acids/secondary structure/tertiary folding/proteome content), there is no clear answer and protein diagenesis is still an active research field. [1]
Generally, proteins have four levels of structural complexity: quaternary (multiple polypeptides, or subunits), tertiary (the 3D folding of a polypeptide), secondary (alpha helices/beta sheets/random coils) and primary structure (linear amino acid sequences linked by peptide bonds). [53] Ancient proteins are expected to lose their structural integrity over time, due to denaturation (protein unfolding) or other diagenetic processes. [54]
Ancient proteins also tend to be fragmented, damaged and altered. Proteins can be cleaved into small fragments over time, since hydrolysis (the addition of water) breaks peptide bonds (covalent bonds between two neighbouring alpha-amino acids). [55] In terms of post-translational modifications (changes occur after RNA translation), ancient proteins are often characterised by extensive damage such as oxidation (methionine), hydroxylation (proline), deamidation (glutamine/asparagine), citrullination (arginine), phosphorylation (serine/threonine/tyrosine), N-terminus glutamate to pyroglutamate and the addition of advanced glycation products to lysine or arginine. [56] [12] Among these modifications, glutamine deamidation is one of the most time-dependent processes. [57] Glutamine deamidation is mostly a non-enzymatic process, by which glutamine is converted to glutamic acid (+0.98406 Da) via side-chain hydrolysis or the formation of a glutarimide ring. [58] It is a slow conversion with a long half-time, depending on adjacent amino acids, secondary structures, 3D folding, pH, temperature and other factors. [59] Bioinformatic tools are available to calculate bulk and site-specific deamidation rates of ancient proteins. [60] The structural manifestation of these chemical changes within ancient proteins was first documented using scanning electron microscopy (SEM). Type-1 collagen protein fibrils of a permafrost-preserved woolly mammoth (Yukon, Canada) were directly imaged and shown to retain their characteristic banding pattern. These were compared against type-1 collagen fibrils from a temperate Columbian mammoth specimen (Montana, U.S.A.). The Columbian mammoth collagen fibrils, unlike those of the permafrost-frozen woolly mammoth, had lost their banding, indicating substantial chemical degradation of the constituent peptide sequences. This also constitutes the first time that collagen banding, or the molecular structure for any ancient protein, has been directly imaged with scanning electron microscopy. [47]
Palaeoproteomics is a fast-developing field that combines archaeology, biology, chemistry and heritage studies. Comparable to its high-profile sister field, aDNA analysis, the extraction, identification and authentication of ancient proteins are challenging, since both ancient DNA and proteins tend to be ultrashort, highly fragmented, extensively damaged and chemically modified. [1] [61]
However, ancient proteins are still one of the most informative biomolecules. Proteins tend to degrade more slowly than DNA, especially biomineralised proteins. [32] [62] While ancient lipids can be used to differentiate between marine, plant and animal fats, [63] ancient protein data is high-resolution with taxon- and tissue-specificities.
To date, ancient peptide sequences have been successfully extracted and securely characterised from various archaeological remains, including a 3.8 Ma (million year) ostrich eggshell, [32] 1.77 Ma Homo erectus teeth, [64] a 0.16 Ma Denisovan jawbone [65] and several Neolithic (6000-5600 cal BC) pots. [7] Hence, palaeoproteomics has provided valuable insight into past evolutionary relationships, extinct species and societies.
Generally, there are two approaches: a digestion-free, top-down method and bottom-up proteomics. Top-down proteomics is seldom used to analyse ancient proteins due to analytical and computational difficulties. [66] For bottom-up, or shotgun proteomics, ancient proteins are digested into peptides using enzymes, for example trypsin. Mineralised archaeological remains such as bones, teeth, shells, dental calculi and ceramics require an extra demineralisation step to release proteins from mineral matrices. [1] This is often achieved by using a weak acid (ethylenediaminetetraacetic acid, EDTA) or cold (4 °C) hydrochloric acid (HCl) to minimise chemical modifications that may introduced during extraction. [67]
To make ancient proteins soluble, heat, sonication, chaotropic agents (urea/guanidine hydrochloride, GnHCl), detergents or other buffers can be used. [1] Alkylation and reduction are often included for cysteine to disrupt disulfide bonds and avoid crosslinking. [68]
After demineralisation, protein solubilisation, alkylation and reduction, buffer exchange is needed to ensure that extracts are compatible with downstream analysis. Currently, there are three widely-used protocols for ancient proteins and gels (GASP), [69] filters (FASP) [70] and magnetic beads (SP3) [71] can be used for this purpose. Once buffer exchange is completed, extracts are incubated with digestion enzymes, then concentrated, purified and desalted.
For non-mineralised archaeological materials such as parchments, leathers and paintings, demineralisation is not necessary, and protocols can be changed depending on sample preservation and sampling size. [6]
Nowadays, palaeoproteomics is dominated by two mass spectrometry-based techniques: MALDI-ToF (matrix-assisted laser desorption/ionisation-time-of-flight) and LC-MS/MS. MALDI-ToF is used to determine the mass-to-charge (m/z) ratios of ions and their peak patterns. [72] Digested peptides are spotted on a MALDI plate, co-crystallise with a matrix (mainly α-cyano-4-hydroxycinnamic acid, CHCA); a laser excites and ionises the matrix, then its time to travel a vacuum tube is measured and converted to a spectrum of m/z ratios and intensities. [73]
Since only peak patterns, not entire amino acid sequences of digested peptides are characterised, peptide markers are needed for pattern matching and ancient protein identification. [72] In archaeological contexts, MALDI-ToF has been routinely used for bones and collagens in a field known as ZooMS (zooarchaeolgy by mass spectrometry). [2]
LC-MS/MS is another widely used approach. It is a powerful analytical technique to separate, sequence and quantify complex protein mixtures. [74] The first step in LC-MS/MS is liquid chromatography. Protein mixtures are separated in a liquid mobile phase using a stationary column. [75] How liquid analytes interact with a stationary phase depends on their size, charge, hydrophobicity and affinity. [76] These differences lead to distinct elution and retention time (when a component of a mixture exit a column). After chromatographic separation, protein components are ionised and introduced into mass spectrometers. [77] During a first mass scan (MS1), the m/z ratios of precursor ions are measured. Selected precursors are further fragmented and the m/z ratios of fragment ions are determined in a second mass scan (MS2). There are different fragmentation methods, for example, high-energy C-trap dissociation (HCD) and collision induced dissociation (CID), but b- and y-ions are frequently targeted. [78]
Search engines and software tools are often used to process ancient MS/MS data, including MaxQuant, Mascot and PEAKS. [79] [80] [81] Protein sequence data can be downloaded from public genebanks (UniProt/NCBI) and exported as FASTA files for sequencing algorithms. [13] Recently, open search engines such as MetaMorpheus, pFind and Fragpipe have received attention, because they make it possible to identify all modifications associated with peptide spectral matches (PSMs). [82] [83] [84]
De novo sequencing is also possible for the analysis of ancient MS/MS spectra. It is a sequencing technique that assembles amino acid sequences directly from spectra without reference databases. [85] Advances in deep learning also lead to the development of multiple pipelines such as DeNovoGUI, DeepNovo2 and Casanovo. [86] [87] [88] However, it may be challenging to evaluate the outputs of de novo sequences and optimisation may be required for ancient proteins to minimise false positives and overfitting. [1]
While palaeoproteomics is a useful tool for a wide array of research questions, there are some analytical challenges that prevent the field from reaching its potential. The first issue is preservation. Mineral-binding seems to stabilise proteins, but this is a complex, dynamic process that has not been systematically investigated in different archaeological and burial contexts. [12] [32]
Destructive sampling is another problem that can cause irreparable damage to archaeological materials. Although minimally-destructive or non-destructive sampling methods are being developed for parchments, bones, mummified tissues and leathers, it is unclear if they are suitable for other types of remains such as dental calculi, ceramics and food crusts. [102] [103] [104]
It is equally difficult to extract mineral-bound proteins due to their low abundance, extensive degradation, and often strong intermolecular interactions (hydrogen bonding, dispersion, ion-dipole and dipole-dipole interactions) with mineral matrice. [105] Ancient proteins also vary in preservation states, hydrophobicity, solubility and optimum pH values; methodological development is still required to maximise protein recovery. [106] [107]
Ancient protein identification is still a challenge, because database search algorithms are not optimised for low-intensity and damaged ancient proteins, increasing the probabilities of false positive and false negatives. [12] There is also the issue of dark proteomes (unknown protein regions that cannot be sequenced); approximately 44-54% of proteins in eukaryotes such as animals and plants are dark. [108] Reference databases are also biassed towards model organisms such as yeasts and mouses, [109] and current sequence data may not cover all archaeological materials.
Lastly, while cytosine deamination (cytosine being converted to uracil over time that causes misreadings) has been widely used in the authentication of aDNA, there are no standardised procedures to authenticate ancient proteins. [61] [110] [111] This authentication issue is highlighted by the claim identification of 78 Ma Brachylophosaurus canadensis (hadrosaur) and 68 Ma Tyrannosaurus rex collagen peptides. [112] [113] The lack of post-translational modifications and subsequent experimental studies demonstrate that these sequences may be derived from bacterial biofilms, the cross-contamination of control samples or modern laboratory procedures. [114]
Despite significant analytical challenges, palaeoproteomics is constantly evolving and adopting new technology. Latest high-performance mass spectrometry, for example, TimsToF (trapped ion mobility-time-of-flight) in a DIA mode (data independent acquisition) may help with the separation, selection and resolution of ancient MS/MS data. [1] Novel extraction protocols such as DES (Deep Eutectic Solvent)-assisted procedures may increase the numbers and types of extracted palaeoproteomes. [115] Identification tools are also improving thanks to progress of bioinformatics, machine learning and artificial intelligence. [116]
The proteome is the entire set of proteins that is, or can be, expressed by a genome, cell, tissue, or organism at a certain time. It is the set of expressed proteins in a given type of cell or organism, at a given time, under defined conditions. Proteomics is the study of the proteome.
Proteomics is the large-scale study of proteins. Proteins are vital parts of living organisms, with many functions such as the formation of structural fibers of muscle tissue, enzymatic digestion of food, or synthesis and replication of DNA. In addition, other kinds of proteins include antibodies that protect an organism from infection, and hormones that send important signals throughout the body.
Tandem mass spectrometry, also known as MS/MS or MS2, is a technique in instrumental analysis where two or more stages of analysis using one or more mass analyzer are performed with an additional reaction step in between these analyses to increase their abilities to analyse chemical samples. A common use of tandem MS is the analysis of biomolecules, such as proteins and peptides.
Peptide mass fingerprinting (PMF), also known as protein fingerprinting, is an analytical technique for protein identification in which the unknown protein of interest is first cleaved into smaller peptides, whose absolute masses can be accurately measured with a mass spectrometer such as MALDI-TOF or ESI-TOF. The method was developed in 1993 by several groups independently. The peptide masses are compared to either a database containing known protein sequences or even the genome. This is achieved by using computer programs that translate the known genome of the organism into proteins, then theoretically cut the proteins into peptides, and calculate the absolute masses of the peptides from each protein. They then compare the masses of the peptides of the unknown protein to the theoretical peptide masses of each protein encoded in the genome. The results are statistically analyzed to find the best match.
Citrullination or deimination is the conversion of the amino acid arginine in a protein into the amino acid citrulline. Citrulline is not one of the 20 standard amino acids encoded by DNA in the genetic code. Instead, it is the result of a post-translational modification. Citrullination is distinct from the formation of the free amino acid citrulline as part of the urea cycle or as a byproduct of enzymes of the nitric oxide synthase family.
Deamidation is a chemical reaction in which an amide functional group in the side chain of the amino acids asparagine or glutamine is removed or converted to another functional group. Typically, asparagine is converted to aspartic acid or isoaspartic acid. Glutamine is converted to glutamic acid or pyroglutamic acid (5-oxoproline). In a protein or peptide, these reactions are important because they may alter its structure, stability or function and may lead to protein degradation. The net chemical change is the addition of a water group and removal of an ammonia group, which corresponds to a +1 (0.98402) Da mass increase. Although deamidation occurs on glutamine, glycosylated asparagine and other amides, these are negligible under typical proteolysis conditions.
Stable isotope labeling by/with amino acids in cell culture (SILAC) is a technique based on mass spectrometry that detects differences in protein abundance among samples using non-radioactive isotopic labeling. It is a popular method for quantitative proteomics.
Genome-based peptide fingerprint scanning (GFS) is a system in bioinformatics analysis that attempts to identify the genomic origin of sample proteins by scanning their peptide-mass fingerprint against the theoretical translation and proteolytic digest of an entire genome. This method is an improvement from previous methods because it compares the peptide fingerprints to an entire genome instead of comparing it to an already annotated genome. This improvement has the potential to improve genome annotation and identify proteins with incorrect or missing annotations.
A peptide sequence tag is a piece of information about a peptide obtained by tandem mass spectrometry that can be used to identify this peptide in a protein database.
Mascot is a software search engine that uses mass spectrometry data to identify proteins from peptide sequence databases. Mascot is widely used by research facilities around the world. Mascot uses a probabilistic scoring algorithm for protein identification that was adapted from the MOWSE algorithm. Mascot is freely available to use on the website of Matrix Science. A license is required for in-house use where more features can be incorporated.
A tandem mass tag (TMT) is a chemical label that facilitates sample multiplexing in mass spectrometry (MS)-based quantification and identification of biological macromolecules such as proteins, peptides and nucleic acids. TMT belongs to a family of reagents referred to as isobaric mass tags which are a set of molecules with the same mass, but yield reporter ions of differing mass after fragmentation. The relative ratio of the measured reporter ions represents the relative abundance of the tagged molecule, although ion suppression has a detrimental effect on accuracy. Despite these complications, TMT-based proteomics has been shown to afford higher precision than Label-free quantification. In addition to aiding in protein quantification, TMT tags can also increase the detection sensitivity of certain highly hydrophilic analytes, such as phosphopeptides, in RPLC-MS analyses.
Protein mass spectrometry refers to the application of mass spectrometry to the study of proteins. Mass spectrometry is an important method for the accurate mass determination and characterization of proteins, and a variety of methods and instrumentations have been developed for its many uses. Its applications include the identification of proteins and their post-translational modifications, the elucidation of protein complexes, their subunits and functional interactions, as well as the global measurement of proteins in proteomics. It can also be used to localize proteins to the various organelles, and determine the interactions between different proteins as well as with membrane lipids.
Shotgun proteomics refers to the use of bottom-up proteomics techniques in identifying proteins in complex mixtures using a combination of high performance liquid chromatography combined with mass spectrometry. The name is derived from shotgun sequencing of DNA which is itself named after the rapidly expanding, quasi-random firing pattern of a shotgun. The most common method of shotgun proteomics starts with the proteins in the mixture being digested and the resulting peptides are separated by liquid chromatography. Tandem mass spectrometry is then used to identify the peptides.
Top-down proteomics is a method of protein identification that either uses an ion trapping mass spectrometer to store an isolated protein ion for mass measurement and tandem mass spectrometry (MS/MS) analysis or other protein purification methods such as two-dimensional gel electrophoresis in conjunction with MS/MS. Top-down proteomics is capable of identifying and quantitating unique proteoforms through the analysis of intact proteins. The name is derived from the similar approach to DNA sequencing. During mass spectrometry intact proteins are typically ionized by electrospray ionization and trapped in a Fourier transform ion cyclotron resonance, quadrupole ion trap or Orbitrap mass spectrometer. Fragmentation for tandem mass spectrometry is accomplished by electron-capture dissociation or electron-transfer dissociation. Effective fractionation is critical for sample handling before mass-spectrometry-based proteomics. Proteome analysis routinely involves digesting intact proteins followed by inferred protein identification using mass spectrometry (MS). Top-down MS (non-gel) proteomics interrogates protein structure through measurement of an intact mass followed by direct ion dissociation in the gas phase.
Bottom-up proteomics is a common method to identify proteins and characterize their amino acid sequences and post-translational modifications by proteolytic digestion of proteins prior to analysis by mass spectrometry. The major alternative workflow used in proteomics is called top-down proteomics where intact proteins are purified prior to digestion and/or fragmentation either within the mass spectrometer or by 2D electrophoresis. Essentially, bottom-up proteomics is a relatively simple and reliable means of determining the protein make-up of a given sample of cells, tissues, etc.
Quantitative proteomics is an analytical chemistry technique for determining the amount of proteins in a sample. The methods for protein identification are identical to those used in general proteomics, but include quantification as an additional dimension. Rather than just providing lists of proteins identified in a certain sample, quantitative proteomics yields information about the physiological differences between two biological samples. For example, this approach can be used to compare samples from healthy and diseased patients. Quantitative proteomics is mainly performed by two-dimensional gel electrophoresis (2-DE), preparative native PAGE, or mass spectrometry (MS). However, a recent developed method of quantitative dot blot (QDB) analysis is able to measure both the absolute and relative quantity of an individual proteins in the sample in high throughput format, thus open a new direction for proteomic research. In contrast to 2-DE, which requires MS for the downstream protein identification, MS technology can identify and quantify the changes.
John R. Yates III is an American chemist and Ernest W. Hahn Professor in the Departments of Molecular Medicine and Neurobiology at The Scripps Research Institute in La Jolla, California.
In the field of cellular biology, single-cell analysis and subcellular analysis is the study of genomics, transcriptomics, proteomics, metabolomics and cell–cell interactions at the single cell level. The concept of single-cell analysis originated in the 1970s. Before the discovery of heterogeneity, single-cell analysis mainly referred to the analysis or manipulation of an individual cell in a bulk population of cells at a particular condition using optical or electronic microscope. To date, due to the heterogeneity seen in both eukaryotic and prokaryotic cell populations, analyzing a single cell makes it possible to discover mechanisms not seen when studying a bulk population of cells. Technologies such as fluorescence-activated cell sorting (FACS) allow the precise isolation of selected single cells from complex samples, while high throughput single cell partitioning technologies, enable the simultaneous molecular analysis of hundreds or thousands of single unsorted cells; this is particularly useful for the analysis of transcriptome variation in genotypically identical cells, allowing the definition of otherwise undetectable cell subtypes. The development of new technologies is increasing our ability to analyze the genome and transcriptome of single cells, as well as to quantify their proteome and metabolome. Mass spectrometry techniques have become important analytical tools for proteomic and metabolomic analysis of single cells. Recent advances have enabled quantifying thousands of protein across hundreds of single cells, and thus make possible new types of analysis. In situ sequencing and fluorescence in situ hybridization (FISH) do not require that cells be isolated and are increasingly being used for analysis of tissues.
Zooarchaeology by mass spectrometry, commonly referred to by the abbreviation ZooMS, is a scientific method that identifies animal species by means of characteristic peptide sequences in the protein collagen. ZooMS is the most common archaeological application of peptide mass fingerprinting (PMF) and can be used for species identification of bones, teeth, skin and antler. It is commonly used to identify objects that cannot be identified morphologically. In an archaeological context this usually means that the object is too fragmented or that it has been shaped into an artefact. Archaeologists use these species identification to study among others past environments, diet and raw material selection for the production of tools.
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