Guns, Germs and Steel is a landmark Pulitzer prize winning book by Jared Diamond published in 1997. It is a very successful non-racist whirlwind tour of human history. Throughout the book historical and geographical roots of global inequality and resource distribution is explored. This first episode is dedicated to plant and animal domestication in different geographical settings. You can find a great encapsulation of this episode published in Nature Magazine in 2002. There’s also a simplified narrative of the book by PBS.
Diamond captured the essence of animal domestication superbly. However as it happens to every documentary domestication of plants needs an update. Diamond lumps plant domestication traits such as nutrition, productivity, harvestability and storability as if they were selected by our ancestors simultaneously. However recent genetic findings in major crops that have been domesticated independently in different parts of the world tell us a very fascinating convergent story.
The natural tendency for a wild plant is to spread its seeds as far and wide as possible so that next generation will have more chance to grow on favorable 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.
Grasses played an immensely important role in human history. In terrestrial environments they form a very large fraction of the base of the food web. In fact, we have evidence for use of grasses in Human diet as far back as 105 thousand years. In all major crops, especially in grasses domestication started with this single trait of non-shattering. Today if a combine harvester is able to collect crops with minimal losses that is because our ancestors tamed this character first.
Genetic mechanism behind non-shattering seed 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 the African grass 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.