It hasn't. It's actually a lot more tangled than most people would guess.

Here's one study that's been done.

From this, for example, Southern Europeans such as the Romans were rather Mideastern. More interestingly, this happened millenia even before Rome.

One such quote:

Southern Europeans had more of their genetic ancestry from the ancient farmers, with up to 90 percent of the DNA of Sardinians tracing back to these early European immigrants.

These farmers who came from the Near East brought new capabilities to Europe, domesticating animals including pigs and cows, growing crops including types of wheat, barley, peas and lentils, and using obsidian sickles for harvest.