Monday, December 13, 2010

Slowdown of the Neolithic Transition in the Levant?

Overlooking the Sea of Galilee         

Based on extensive archaeological work, the 2008 Zeder paper on early domestication (see previous post) gives some estimates of where and when domestication of goats, sheep, cattle and pigs occurred as well as dates for the arrival in other Fertile Crescent areas.  A map in the paper illustrates the location of domestication as being on the Taurus-Zagros Arc:

  (Link)

From the points of domestication, domesticated goats, sheep, cattle and pigs were brought southward into the Southern Levant. 

Of particular interest are the dates for the introduction of domesticated goats.  Travelling from the westernmost tip of goat domestication on the upper Tigris River 11,000 years ago, it takes 1,400 years to reach the northern tip of the Levantine Corridor.  However, goat domestication then travels simultaneously from the area of the Natufian site Abu Hureira to the Southern Levant.

From the archaeological record, we know that the Late Natufian culture, with their distinctive exposed semi-subterranean houses, moved northward into the Syro-Arabian desert, reaching Abu Hureira, starting approximately 14,500 years before present:  "The climatic improvement after 14,500 B.P. seems to have been responsible for the presence of more stable occupations in the steppic and desertic belts.  Groups moved into areas that were previously uninhabited, from the Mediterranean steppe into the margins of the Syro-Arabian desert.  Others came from the Nile valley, creating an interesting social mosaic."(link, p. 161)  The "Syro-Arabian desert in the east accommodated only small Natufian occupations due to both their lower carrying capacity and the presence of other groups of foragers who exploited this vast region." (Ibid. p. 162). 

Abu Hureira at the northern end of the Levantine Corridor

One possible explanation for the rapid transmission of goats from the area of Abu Hureira to the Southern Levant is that the Natufian culture adopted goat domestication from others in Abu Hureira and then immediately carried it southward. 

For the domestication of sheep, cattle and pigs, the Zeder 2008 paper describes dates for the region of origin and destination in the southern Levant.  From these, it is possible to estimate speeds for the dispersal of these livestock breeds.   Since there are only two points for sheep, cattle and pigs, it isn't possible to know if the dispersal sped up or slowed down during the trip.

From the Isern and Fort paper, we have a rough idea of the speed of advance of European Neolithic farmers as a function of the population density in the area of advance. 

This raises the question as to whether there was a similar slowdown with the Neolithic expansion southward into the southern Levant.  We know that today the ADMIXTURE population components "West Asian", "Southern European" and "Southwest Asian" are distributed in an approximate normal distribution, indicating the phenomena of absolute density regulation and space competition. (link)  However, a lot can happen in 10,000 years, so we cannot directly infer that Neolithic Fertile Crescent populations experienced space competition.

It is not difficult to plot the speed of the advance for goats, sheep, pigs and cattle from the point of domestication to the southern Levant.  In the case of goats, the distance used is from the westernmost point of domestication in the Zagros to Abu Hureira.  The other distances are calculated from the point of domestication to Jericho.  These speeds (horizontal lines) are plotted with the c(m1) and c(m4) functions derived for the European Neolithic advance (Isern and For), to obtain c/cmax and c:

Note that values for cmax, D, a and T are replicated from Isern and Fort.

Goats and sheep seem to be the slowest travelers at about 0.3 km/year  Pigs are a little faster and cattle are veritable race horses, travelling at 0.4 km/year. 

Considering that Levant shepherds traversed mostly open woodland and steppe, these speeds seem very slow indeed.  What's more, we know that since sheep and goats were the first to reach the Levantine Corridor, they were on the leading edge of the Neolithic advance.  If there was already a population in the Levant such as Natufian hunters, it would be the leading edge of the front that would be the slowest.  That appears to be the case for sheep.  Pigs and cattle, who arrive 500 to 1000 years after sheep, are faster.

It is also notable that the Levant livestock speeds intersect the European m4 test function at approximately the distance actually travelled by Fertile Crescent shepherds and farmers from the point of domestication to Jericho:  about 700 kilometers.

The goat data lend to the idea that Natufian Hunter-Gatherers adopted goat shepherding in the area of Abu Hureira 9600 years ago and very quickly introduced it in the Southern Levant. 

Because we don't have a midpoint date and position for domestication of sheep, pigs and cattle, it is more difficult to infer whether it was Natufians or Northern Fertile Crescent farmers who brought sheep, pigs and cattle southward.  It does seem that the southward advance was slow going, especially for the first wave of sheep herders, suggesting some kind of resistance.