In The Two Towers, Aragorn observes that the fallen Uruk-hai at Amon Hen were not like any breed of Orc he has seen before.
Being stronger, they looked down upon and often bullied and cowed other kinds of Orcs. These original Uruks were of Sauron's breeding, but Saruman bred his own, making further improvements like resistance to sunlight, and giving them different armour and weapons. The race of Uruk-hai, described as very large black orcs of great strength, first appeared about the year 2475 of the Third Age, when they conquered Ithilien and destroyed the city of Osgiliath.
