Emergence of Agriculture

From Science Daily today, a report reevaluating the history of human
agricultural practices through genetic evidence.

This is especially interesting, as I have claimed that PIE started spreading
and splitting much earlier than usually hypothesized.

Best Regards

Arnaud

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Emergence Of Agriculture In Prehistory Took Much Longer, Genetic
Evidence Suggests

ScienceDaily (Sep. 19, 2008) - Researchers led by Dr Robin Allaby of
the University of Warwick's plant research arm Warwick HRI have
found evidence that genetics supports the idea that the emergence of
agriculture in prehistory took much longer than originally thought.

Until recently researchers say the story of the origin of
agriculture was one of a relatively sudden appearance of plant
cultivation in the Near East around 10,000 years ago spreading
quickly into Europe and dovetailing conveniently with ideas about
how quickly language and population genes spread from the Near East
to Europe. Initially, genetics appeared to support this idea but now
cracks are beginning to appear in the evidence underpinning that
model

Now a team led by Dr Robin Allaby from the University of Warwick
have developed a new mathematical model that shows how plant
agriculture actually began much earlier than first thought, well
before the Younger Dryas (the last "big freeze" with glacial
conditions in the higher latitudes of the Northern Hemisphere). It
also shows that useful gene types could have actually taken
thousands of years to become stable.

Up till now researchers believed in a rapid establishment of
efficient agriculture which came about as artificial selection was
easily able to dominate natural plant selection, and, crucially, as
a consequence they thought most crops came from a single location
and single domestication event.

However recent archaeological evidence has already begun to
undermine this model pushing back the date of the first appearance
of plant agriculture. The best example of this being the
archaeological site Ohalo II in Syria where more than 90,000 plant
fragments from 23,000 years ago show that wild cereals were being
gathered over 10,000 years earlier than previously thought, and
before the last glacial maximum (18,000-15,000 years ago).

The field of Archaeobotany is also producing further evidence to
undermine the quick development model. The tough rachis mutant is
caused by a single recessive allele (one gene on a pair or group of
genes) , and this mutant is easily identifiable in the
archaeological specimens as a jagged scar on the chaff of the plant
noting an abscission (shedding of a body part) as opposed to the
smooth abscission scar associated with the wild type brittle rachis.

Simply counting the proportion of chaff types in a sample gives a
direct measure of frequency of the two different gene types in this
plant. That study has shown that the tough rachis mutant appeared
some 9,250 years ago and had not reached fixation over 3,000 years
later even after the spread of agriculture into Europe was well
underway. Studies like these have shown that the rise of the
domestication syndrome was a slow process and that plant traits
appeared in slow sequence, not together over a short period of time.

Genome wide surveys of crops such as einkorn and barley that in the
past that have suggested a single origin from a narrow geographical
range, supporting the rapid establishment view, have long been in
conflict with other gene studies. The most notable conflict is in
the case of barley for which there is a large body of evidence that
suggests more than one common ancestor was used in its development.

These challenges to the fast model of agricultural development need
a new model to explain how and why the development was so slow and
demonstrate why artificial selection of just one plant type does not
have the expected quick result. This computer model has now been
provided by Dr Robin Allaby and his team at the University of
Warwick, the Institute of Archaeology, University College London,
and Manchester Interdisciplinary Biocentre has outlined the new
mathematical model in a paper published in Proceedings of the
National Academy of Sciences USA 2008 and in a summary article in
the Biologist (2008 55:94-99).

Their paper entitled The genetic expectations of a protracted model
for the origins of domesticated crops used computer simulations that
showed that over time a cultivated population will become
monophyletic (settle into one stable species) at a rate proportional
to its population size as compared various gene variations in the
wild populations. They found this rate of change matched closely the
3000 years it took the tough rachis mutant to become established.

Ironically, this process is actually accelerated if there is more
than one

wild source population (in other words if attempts at domestication
happen more than once) because any resulting hybrid between those
domesticated populations then has a heightened differentiation
compared with either one of the wild populations of the two parent
plants.

This mathematical model also more supportive of a longer complex
origin of plants through cross breeding of a number of attempts at
domestication rather than a single plant type being selectively bred
and from a single useful mutation that is selectively grown quickly
out paces the benefits natural selection

Dr Robin Allaby says:

"This picture of protracted development of crops has major
implications for the understanding of the biology of the
domestication process and these strike chords with other areas of
evolutionary biology."

"This lengthy development should favour the close linkage of
domestication syndrome trait genes which may become much more
important because linked genes will not be broken up by gene flow -
and this makes trait selection and retention easier. Interestingly,
as more crop genomes become mapped, the close linkage of two or more
domestication syndrome genes has been reported on several
occasions."

"This process has similarities to the evolution of 'supergenes' in
which many genes cluster around a single locus to contribute to one
overall purpose."

"We now need to move this research area to a new level.
Domestication was a complex process and can now be viewed more
legitimately as the paragon of evolutionary process that Darwin
originally recognized. There are many interacting factors involved
that we know about operating on a wide range of levels from the gene
to the farmer and climate - the challenge is to integrate them into
a single story."

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