Western Hunter-Gatherers
They were tall.
Caucasian Hunter-Gatherers
They split off WHG by around 43k BCE.
Early European Farmers
Early European Farmers (EEF), First European Farmers (FEF), Neolithic European Farmers or Ancient Aegean Farmers (ANF).
They split from Caucasian Hunter-Gatherers (CHGs) around 23,000 BC.
The EEFs had settled Europe after the worst catastrophes following the Last Ice Age - the Storegga Slides. A continental shelf collapsed & resulting tsunamis drowned an entire landmass (Doggerland) while floods elsewhere in Europe devastated WHG communities. There was also terrible flooding in the Balkans in 6200 BC & the EEF invaded Europe from Anatolia via Greece at the same time.
The early european farmers from anatolia seem to have been a 90-50% replacement (s. europe vs. scandinavia)
Megalith Builders
The Megalith Builders were the result of WHG chieftains overthrowing the decadent EEF chiefs like those of the Linear Ceramics around 4400 BC & subjugating an 1800 year old neolithic civilization. Possibly related to spread of copper-working.
By the time of their destruction the Megalith Builders themselves had been in a centuries long decline from their Golden Age in late 5th & early 4th millennium. Their great realms had likely disintegrated around 3500 BC into smaller chiefdoms engaging in endemic warfare.
IE Invasions
The Indo-European invasions then happened- ending the Megalith Builder Civilization with their urban settlements & leaving much of Europe depopulated for 600 years.