It lies within a zone of so-called Mediterranean climate.

The wild ancestors of many Fertile Crescent crops were already abundant and highly productive.

It includes a high percentage of hermaphroditic "selfers", that is, plants that usually pollinate themselves but that are occasionally cross-polinated.

Those were some of the advantages that the Fertile Crescent's flora arfforded the first farmers: it included an unusually high percentage of wild plants suitable for domesticaton.

When the domesticated animals and plants reached other places, the climate wasn't the same. Therefore, the spread of farming was slowed down.

Points on the same latitude had the same climate, so the the domesticated animals and plants didn't have to adapt to new climates. Therefore, the spread of faming could be faster.

The first stage is illustrated by dozens of diseases that we now and then pick up directly from our pets and domestic animals.

In the second stage a former animal pathogen evolves to the point where it does get transmitted directly between people and cause epidemics.

A third stage in the evolution of our major diseases is represented by former animal pathogens that did establish themselves in humans, that have not (not yet?) died out, and that may or may not still become major killers of humanity.

The final stage of this evolution is represented by the major, long-established epidemic diseases confined to humans.