Botanists have a very tough time plotting the evolution of citrus fruits. They’ve been fiddled with by humans so much the past few millennia it’s impossible to figure out which are the “original” fruit and which are the “cultivated” varieties. The trouble is citrus of one type is so easily grafted onto another. You can basically plaster an orange bud onto a kumquat tree to get a…well, what would you call that? But then the trees themselves don’t cooperate much either, since they spin off hybrids of their own so readily.
Best guess is that the lemon was the result of an intentional hybridization of a lime and a citron tree. Where that exactly happened (or when) is a mystery, though again best guess: somewhere in Southeast Asia, well before the time of Christ. Eventually lemons spread along the Silk Road eastward, reaching the Middle East, well, again no one knows when. But our word “lemon” is derived from the Persian word “lima”, so it’s safe to say lemons first entered our hemisphere that way.
From there they moved further West, exactly how is again a mystery. Some say the Romans ate lemons having come upon them via their Eastern conquests. Others claim they were an Arab fruit introduced to Europe by returning Crusaders. But then returning Crusaders are every lazy food historians go-to guys when it comes to explaining how Middle Eastern foods ended up in the West. My personal guess is that lemons (or some close variation) were present around the Mediterranean well before then. But then who the heck am I?