Studying 1,600-year-old cotton from the banks of the Nile has allowed scientists to find what they believe is the first evidence that punctuated evolution has occurred in a major crop group within what can be seen as the relatively short history of plant domestication.
The findings offer remarkable insight into the dynamics of agriculture in the ancient world and could even help today’s domestic crops face challenges such as climate change and drought.
Researchers, led by Dr Robin Allaby from the School of Life Sciences at the University of Warwick, examined the remains of ancient cotton at Qasr Ibrim in Egypt’s Upper Nile using high throughput sequencing technologies. The site of Qasr Ibrim, is located about 40 kilometres from Abu Simbel and 70 kilometres from the modern Sudanese border on the east bank of what is now Lake Nasser it was occupied for more than 3,000 years, only being abandoned in the mid 19th century.
A native crop
Using this latest generation of DNA sequencing technology the team were also able to confirm that seeds recovered from the site – dating to around 400 CE - were of the G. herbaceum, variety which is native to Africa, rather than G. arboreum, which is native to the Indian subcontinent.
This is the first time such technology has been used on ancient plants and also the first time the technique has been applied to archaeological samples in such hot conditions.
For archaeologists, the results shed light on agricultural development in the ancient world as there has long been uncertainty as to whether ancient Egyptians had imported domesticated cotton from the Indian subcontinent, as had happened with other crops, or whether they were growing a native African variety which had been domesticated locally.
The findings that the Qasr Ibrim seeds were of the G. herbaceum variety, native to Africa, rather than G.arboreum, which is native to the Indian subcontinent, represents the first molecular-based identification of archaeobotanical cotton to a species level.
Dr Allaby said the findings confirm there was an indigenous domestication of cotton in Africa which was separate from the domestication of cotton in India.
“The presence of cotton textiles on Egyptian and Nubian sites has been well documented but there has always been uncertainty among archaeologists as to the origin of these. It’s not possible to identify some cotton varieties just by looking at them, so we were asked to delve into the DNA.
He concluded, “We identified the African variety – G. herbaceum, which suggest that domesticated cotton was not a cultural import – it was a technology that had grown up independently.”
The results showed that even over the relatively short timescale of a millennia and a half, the Egyptian cotton showed evidence of significant genomic reorganisation when the ancient and the modern variety were compared.
The researchers also studied South American samples from sites in Peru and Brazil aged between 800 and nearly 4,000 years old.
However investigation at a genetic level of the closely-related G.Barbadense from the sites in South America showed a genomic stability between the two samples, even when separated by more than 2,000 miles in distance between the Peru and Brazil samples and 3,000 years in time.
This divergent picture points towards what is termed punctuated evolution – long periods of evolutionary stability interspersed by bursts of rapid change – having occurred within the cotton family.
Dr Allaby said: “We think of evolution as a very slow process, but as we analyse more genome information we can see that there’s been a huge amount of large-scale proactive change during recent history. ”
“Our results for the cotton from Egypt indicate that there has been the potential for more adaptive evolution going on in domesticated plant species than was appreciated up until now.
More importantly for todays agricultural stresses, Dr Allaby continued, “Plants that are local to their particular area will develop genes which allow them to better tolerate the stresses they find in the environment around them. It’s possible that cotton at the Qasr Ibrim site has adapted in response to extreme environmental stress, such as not enough water.”
Past Horizons. 2012. "Ancient Egyptian cotton: A secret revealed". Past Horizons. . Posted: April 5, 2012. Available online: http://www.pasthorizonspr.com/index.php/archives/04/2012/ancient-egyptian-cotton-a-secret-revealed
S. A. Palmer, A. J. Clapham, P. Rose, F. O. Freitas, B. D. Owen, D. Beresford-Jones, J. D. Moore, J. L. Kitchen, R. G. Allaby. Archaeogenomic Evidence of Punctuated Genome Evolution in Gossypium. Molecular Biology and Evolution, 2012; DOI: 10.1093/molbev/mss070