In the last quarter of the 19th century, it took 1,700 hours of labor to purchase the annual food supply for a family. Today it requires just 260 hours, and it is likely that by 2040, a family’s food supply will be purchased with about 160 hours of labor.
from Robert Fogel Nobel Prize winning economic historian via Greg Mankiw
Americans now spend around 10% of their income on food. Those living on a $1 a day in the poorest countries in the world spend between 50%-70% on food. Fogel cites that in the US late 19th century it was close to 90%.
This makes me think of the recent Planet Money (NPR's econ show) podcast on 12th century economics (listen here you may want to skip the first 1.5 minutes)
As Phillip Daileder the historian notes in the podcast most of economic progress can be thought of as the result of increasing food productivity.
On this subject my favorite recent work is this paper by Nunn and Qian claims that almsot 20% of the world's population growth is due to the spread of the potato.
So next time you go out with 4 of your friend. You should order some fries and thank the fries because without potatoes there would be one less person at the table.