Modern Europeans descended from 3 major migrations of prehistoric people. Hunter-gatherers first arrived in Europe ~37 TYA. Farmers began migrating from Anatolia into Europe 9 TYA. Bringing their own families and lifestyle, these immigrants did not initially intermingle much with the local hunter-gatherers.
By the 3rd millennium, many Europeans had settled into towns and villages with trade routes between them. As humans are generally unhygienic, plague easily spreads when populations concentrate. So it did across Europe 5,700 years ago. By 5,400 ya the towns had depopulated.
The Yamnaya swept into a weakened western Europe 5000 to 4800 years ago. Yamnayans were hunter-gatherers and nomadic herders in their native lands of the Caucasus steppes.
There must have been a genocide. ~ Danish archaeologist Kristian Kristiansen
The Yamnaya that rapidly radiated through Europe were young men on horseback: axe-wielding warriors with conquest on their mind. Yamnayan men bred with the European women they encountered, fostering a fresh European race and dramatically altering European culture.
Europe was not alone in taking a Yamnaya impression. Yamnayans also invaded India, where they became known as Aryans.
The ancestral tongue from which all of today’s 400 Indo-European languages spring emanated from the Yamnaya.