In the book Guns, Germs, and Steel, what does living on the same latitude mean and why was it important?

Jared Diamond, in the book Guns, Germs, and Steel, attempts to explain why certain groups of people developed civilization earlier than others. He traces the dissemination of food production from nine locations in the world where plant and animal domestication happened at the earliest point of history.  He surmises that food production spread on a deliberate east-west axis along the same lines of latitude.  Moving food production technologies was more difficult when crossing lines of longitude.  The spread of crop and livestock species can spread easier across the same latitude because the days are of the same lengths. Also, the seasons are the same along the same latitude.  This makes it easier to transfer planting and agricultural knowledge than when moving north and south.  
http://www.jareddiamond.org/Jared_Diamond/Guns,_Germs,_and_Steel.html