How did the availability of domestic plants and animals explain why literacy, and steel weapons developed earliest in Guns, Germs, and Steel?

According to Jared Diamond in Guns, Germs, and Steel, the availability of domestic plants and animals explains why empires, literacy, and steel weapons developed earliest in Eurasia because this availability led farming to develop in Eurasia before it developed anywhere else.  Farming, Diamond says, allows empires, literacy, and steel weapons to arise.

For Diamond, farming is the key to power and wealth.  Without it, a society remains at the hunter-gatherer stage and cannot become strong and rich.  This is the case because only farming societies can build food surpluses and because farming societies are much more likely to be sedentary than hunter-gatherer societies.

A developed farming society can produce a great deal of food.  Importantly, not all of its people need to be involved in obtaining food.  This means that there are people in the society who are free to do other kinds of work.  These people can invent and work with technology such as the technology needed to develop steel weapons.  They can develop writing and keep written records.  They can act as priests in a religion that supports their ruler and their society.  They can act as soldiers.  All of these things make it more likely that farming societies will develop literacy and steel weapons and will eventually gain empires.

So why does the availability of domestic plants and animals lead to farming?  Diamond says that farming arises first in those areas where there are suitable plants and animals.  People in such areas are much more likely to find plants to give them protein, oil, fiber (for cloth) and other things that they need.  If there are fewer domesticable plants, they may only find a few of these things and farming will be harder to develop.  People in such areas will be able to have animals that will help them in a variety of ways.  The animals will provide protein.  They will provide fertilizer in the form of their manure.  They will provide power to pull plows and other devices that will make farming easier.

In these ways, the availability of domestic plants and animals (which Eurasia had more than any other continent) will lead to the development of farming which will, in turn, allow for the development of steel weapons, literacy, and empires.