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In this month’s ‘Science Notes’, we are examining amino acid geochronology (AAG) – a dating technique that can be used to help date the whole Quaternary period, from c.2 million years ago up to the present day.
Recently, a team of researchers, led by scientists from Leibniz-Zentrum für Archäologie in Germany, used this technique to redate the Schöningen ‘Spear Horizon’ – the world’s oldest wooden hunting assemblage – which was discovered during excavations between 1992 and 2008 at Landkreis Helmstedt in Lower Saxony, Germany. The collection consists of nine spears, one lance, and six double-pointed sticks, as well as 1,500 flint artefacts interspersed with the butchered remains of more than 50 horses. Found along the shoreline of a former lake, it is believed that these artefacts were deposited during repeated hunting episodes.
When the site was first discovered, it was thought to date to 400,000 years ago. This was later revised to 300,000 years ago after thermoluminescence (TL) dating was used to date heated flint from the site, which provided an age of 321,000 years ago ±14,000 years. A similar maximum date of c.300,000 years ago was confirmed by optically stimulated luminescence (OSL). Both the flint and peat used for each dating method, however, were found in layers stratigraphically below the Spear Horizon finds, meaning that these are maximum dates of the assemblage.
To obtain a date from the spear layers themselves, the researchers turned to amino acid geochronology. Nearly 50 sediment blocks from the original excavations had been kept in storage, and from them the team managed to extract four opercula (the little ‘trap door’ that closes snails into their shells) from Bithynia tentaculata – a type of freshwater snail. AAG on the intra-crystalline fraction of opercula is well established and an ideal candidate for obtaining an age estimation.
AAG is a technique centred around the fact that, within fossils, there is a specific, time-dependent breakdown of proteins, and that this breakdown can be measured to ascertain the date of a fossil. All but one of the 20 amino acids that occur naturally in organisms have a ‘chiral centre’, meaning that they can be found in two chemically identical, mirror-image forms. While an organism is alive, all proteins are comprised of the ‘left-handed’ forms of amino acids. Once an organism dies, however, a spontaneous reaction, or racemisation, occurs by which the left-handed amino acids (L) are spontaneously converted to their right-handed (D) counterparts, and vice versa, until they reach equilibrium. This degradation occurs over a predictable length of time, which varies by amino acid, so that by measuring the D-to-L ratios present in a sample an age estimate can be established.
In the closed system found in the intra-crystalline fraction of opercula, the D/L values of aspartic acid/asparagine (Asx), glutamic acid/glutamine (Glx), alanine (Ala), and valine (Val) can reliably be used for dating purposes. Additionally, since serine (Ser) is unstable and decomposes into several products, including alanine, the concentration of Ser to Ala can be used as another indicator of age. These values obtained from each sample were then correlated with the marine oxygen isotope record to constrain their age.
The results showed that the Schöningen ‘Spear Horizon’ is much younger than originally thought, dating to c.200,000 years ago. This younger age in no way diminishes the significance of the site, however, as the preservation of the spears along with the sheer number of horse remains is unmatched in the archaeological record. The site also adds greatly to our growing knowledge of the Middle Palaeolithic. In particular, this revised age means the archaeological remains at the site are now more in keeping with other sites of this age; previously, a date of 400,000 or even 300,000 would have meant that the spears were made by an early hominin, such as Homo heidelbergensis, and such a large assemblage of weapons alongside butchered remains would have been a unique find for that period. Now we know that the site was probably made by Neanderthals, and is more consistent with other Middle Palaeolithic sites where hunting a single prey was favoured, although which prey Neanderthals chose to hunt varied: for example, aurochs at Biache-Saint-Vaast, France; rhinoceros at Taubach, Germany; and horse at Zwoleń, Poland. As emphasised by the researchers in their recent paper, published in Science Advances (https://doi.org/10.1126/sciadv.adv0752): ‘To achieve this level of routine success, hunting forays at Schöningen and other mono-specific sites must have been wholly collaborative and group-minded efforts motivated by a communal set of objectives.’
Text: Kathryn Krakowka / Image: with kind permission of the Niedersächsisches Landesamt für Denkmalpflege (NLD)
