Complex sentence: Some older scholarship has speculated that Attacotti were an aboriginal, pre-Indo-European substratum of the population of northern Britain, an unfounded thesis that has generated much conjecture concerning this "race", for the most part at the level of pseudo-history.
Split into simpler sentences: Some older scholarship has speculated that Attacotti were an aboriginal, pre-Indo-European substratum of the population of northern Britain. This thesis has generated much conjecture concerning this "race", for the most part at the level of pseudo-history.