I read this article, and this sentence for me is particularly interesting:

Some researchers hold that an early Indo-European language was spread by Middle Eastern farmers around 8,000–9,500 years ago (see ‘Steppe in time’). This ‘Anatolian hypothesis’ is supported by well-documented migrations into Europe, where agriculturalists replaced or interbred with the existing hunter-gatherers.

What caused them to migrate? Did the existing hunter gatherer also migrated from middle east millenias before?

Were there any similarities between these two groups?

edit: spelling