Scientists
studying 1,600-year-old cotton from the banks of the Nile have found
what they believe is the first evidence that punctuated evolution has
occurred in a major crop group within the relatively short history of
plant domestication.
The
findings offer an insight into the dynamics of agriculture in the
ancient world and could also help today’s domestic crops face challenges
such as climate change and water scarcity.
The
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.
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 countries.
The
site is located about 40 km from Abu Simbel and 70 km from the modern
Sudanese border on the east bank of what is now Lake Nasser.
They also studied South American samples from sites in Peru and Brazil aged between 800 and nearly 4,000 years old.
The
results showed that even over the relatively short timescale of a
millennia and a half, the Egyptian cotton, identified as G. herbaceum,
showed evidence of significant genomic reorganization when the ancient
and the modern variety were compared.
However
closely-related G. Barbadense from the sites in South America showed
genomic stability between the two samples, even though these were
separated by more than 2,000 miles in distance and 3,000 years in time.
This
divergent picture points towards punctuated evolution – long periods of
evolutionary stability interspersed by bursts of rapid change—having
occurred in the cotton family.
Dr
Allaby said: “We think of evolution as a very slow process, but as we
analyze 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.
“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.
“This
insight into how domesticated crops evolved when faced with
environmental stress is of value for modern agriculture in the face of
current challenges like climate change and water scarcity.”
For archaeologists, the results also shed light on agricultural development in the ancient world.
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
study’s 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.
“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.”
Archaeogenomic evidence of punctuated genome evolution in Gossypium
Source: University of Warwick
