Harvest of the Seasons is the second episode of a thirteen-part documentary series called The Ascent of Man written and presented by Jacob Bronowski. The series reached wide audiences for Bronowski’s highly intellectual but simple and convincing analysis. Much of his long monologues were unscripted and were a source of inspiration for creating a template for Carl Sagan’s hugely sucessful Cosmos series. The motto used by Bronowski “A personal view” became “A personal voyage” in Sagan’s. The episode provides an account for the beginning of agriculture and domestication of grasses but says very little about the genetic background of this key historical event. Throughout the remainder of the text, I will try to supplement this fascinating story with recent genetic findings in a domesticated African grass called sorghum which is the 5th most important cereal crop worldwide.
Grasses that we trample under our feet played an immensely important role in human history. In terrestrial environments they form a very large fraction of the base of the food web. We have evidence for use of grasses in human diet as far back as 105 thousand years. For instance, take the Mongolian Empire as an outstanding example. Ghengis Khan rose to power in 1206, after he united Mongolia’s tribes behind him, and died in 1227. How could a man of humble origins managed to lead an army that claimed more territory in 25 years than the Romans conquered in 400? Dendrochronological (tree ring) data show that from 1208 to 1231 Mongolia enjoyed a string of wetter-than-usual years which was longer than any other such period in the past millennium. A mild and wetter climate lasting a generation would have provided richer grazing for grasses thrive in humid conditions. More grass means more horses, and thus provide ample power for the empire’s armies. Imagine a hypothetical logistic situation where all the powerful tanks and jetfighters of an army have unlimited access to fuel wherever they go. Therefore there’s more to the charisma of the Genghis Khan. His strategic genius might have been crippled if the climate had brought him no fluffy clouds full of rain. This what probably happened to Mayan’s maize-dominated (another species of grass) agricultural civilization. A century before Genghis Khan their thriving empire collapsed tragically due to a decline in rainfall.
Harvest of the Seasons starts with an ethnographical narrative of Bakhtiyari people which was the subject of an iconic 1924 documentary called Grass. Bronowski’s “civilization can never develop on the move” is an accurate statement for describing the constraints of nomadic way of life. A sedentary population sustained by agriculture can enable accumulation of goods and specialized skills. But then, how did agriculture start? What were the factors that made a sedentary way of life more profitable to that of the nomad (or hunters and gatherers)? Certainly agriculture was simply impossible to gain hold under ice age conditions. In this documentary Bronowski reflects a rather outdated view of the 70s about the origins of agriculture. He sees agriculture as a sudden “revolution”. On the contrary, based on genetic and archeological evidence we now know that the process was a protracted gradual evolution. Selection imposed on wild plants transformed them into domesticated varieties over long periods of time lasting a few thousand years.
Bronowski then takes us to the Pre-Pottery Neolithic village of Jericho. At the time of this documentary, excavations on another key neolithic site Çatalhöyük was rather rudimentary. Although Jericho is much older Çatalhöyük is a denser and city-like settlement and has been providing us very important clues on trajectory of domestication process including charred remains of wild seeds on which we can observe signs of domestication.
The natural tendency for a wild plant is to spread its seeds as far and wide as possible so that next generation will have chance to grow on diverse habitats. Ripe seeds of wild plants separate from the mother plant very easily: They shatter. Therefore, for early foragers harvestability must have been a high priority. Non-shattering seeds that are most easily gathered would be planted and so pass on the non-shattering trait to future generations. Over many generations plants with non-shattering seeds increased their frequency even though it was a major handicap and they wouldn’t be able to survive in nature if humans didn’t help.
Non-shattering forms have been observed in many domesticated plants such as rice, wheat, maize and sorghum. These varieties arise mostly through a mutation in a type of gene called transcription factor which also control expression of other genes in a cascading manner.
In sorghum for example, non-shattering seeds in domesticated variety (Sorghum bicolor) is conferred by a mutation in a transcription factor called WRKY. In the video below sections of the both wild (right) and domesticated (left) sorghum WRKY transcription factors are shown. Conserved WRKY DNA binding domains are visualized as ball and stick representations on both models. In wild sorghum (S. propinquum) first 144 amino acid is shown in red. In domesticated sorghum this section is deleted due to a mutation that moves the start codon to a later position. First beta sheet in wild sorghum (red arrow) intercalates between 5th and 6th beta-sheets (green arrows) forming a ribcage-like structure supported by strong polar interactions (yellow interrupted lines). Model also predicts a zinc finger domain among Cys-72, Cys-77, His-58 and His-104 residues (orange).
In domesticated sorghum lack of first beta-sheet disrupts the integrity of the rest of the beta-sheets. There are a total of 17 polar interactions in wild sorghum molecular model. Among them 15 maintain the B-sheet orientation. Remaining two interactions are with WRKY domain and may be playing the central role in DNA binding.