sexta-feira, 22 de junho de 2018


HOW CAN WE USE FOSSILISED FAECES TO INVESTIGATE WHEN THE EARLIEST HUMAN OCCUPATION OF NORTH AMERICA OCCURRED? Dr Helen Whelton, postdoctoral research assistant at the School of Chemistry, University of Bristol

The question of how, when and why people first settled the Americas has been a subject of intense debate which continues to the present. There are two schools of thought, the ‘Clovis First’ and ‘Pre-Clovis’ theories. The ‘Clovis First’ theory hypothesises that the Clovis people were the earliest humans to colonise in North America via an ice-free corridor over the land bridge between Asia and America, around 13,500 years BP (before present). Evidence of ‘Pre-Clovis’ human occupation (dating to 14,500 years B.P.) in North America has been found at a number of archaeological sites across the Americas extending as far as southern Chile.

The Paisley Caves, series of 8 caves, located in south central Oregon, USA, are one of the key sites in the debate surrounding the peopling of the Americas. Various artefacts such as stone tools and animal bones have been found at the site along with coprolites (fossilised faeces) and hearth features which provide evidence of human occupation. The Paisley Caves support the ‘pre-Clovis’ theory of human occupation and is associated with a different cultural group known as the Western Stemmed tradition. DNA analysis of coprolites from the Paisley Caves has identified the earliest human occupation at 14,300 years B.P. This date, relying on 14C dating of associated faunal bone collagen (a structural protein) has been criticised due to potential contamination and uncertainties over stratigraphic integrity.

To identify the earliest human occupation at the Paisley Caves we can study the compounds preserved within coprolites found in the sediments. Coprolites contain a suite of lipid (fat molecules) biomolecules; these biomarkers are organic compounds which have a definitive biological origin and are an invaluable source of palaeobiological and palaeoecological information. The lipid biomarkers preserved in coprolites are reflective of the organisms’ digestive processes, their digestive tract microbes and local vegetation. These in turn can tell us more about the producer of the faeces, their digestive physiology and consumed diet.

The two groups of biomarkers which enable us to determine the producer of faecal material are 5β-stanols and bile acids.

1. 5ß-stanols are saturated analogues of animal and plant sterols (e.g. cholesterol, campesterol and sitosterol) with a specific (stereochemical) configuration and are produced in the digestive tract by bifidobacteria. The ratios of the 5ß-stanols produced in the gut of carnivores, herbivores and omnivores differ due to their diet and physiology, enabling them to be distinguished.

The second group of compounds, bile acids, are produced by animals to assist with the breakdown of fats in the diet and to regulate cholesterol levels. Primary bile acids are formed in the liver and are modified in the digestive tract by bacteria to form secondary bile acids. As with 5ß-stanols certain taxa (classification group) produce characteristic suites of bile acids.

Once the origin of coprolites has been determined as human compound-specific 14C dating of the 5β-stanols can be used to precisely date the human presence in the cave. This work, linking archaeology with chemistry, is ongoing at the School of Chemistry, University of Bristol.

Figure1 : Examples of faecal biomarkers, 5β-stanols and bile acids, formed from their precursors in the human gut.

Dr Helen Whelton is a postdoctoral research assistant at the School of Chemistry, University of Bristol. She read her BSc at Kent in Forensic Science and her PhD in Chemistry at Bristol. In between she worked in industry as an analytical chemist. During her PhD, she focused upon the diet and mobility of prehistoric humans during the Neolithic in northern Greece. She is currently investigating the timing and nature earliest human occupation of North America through the analyses of faecal biomarkers.

Nenhum comentário:

Postar um comentário