Step 1: Introduce Al-Biruni:
Al-Biruni (973-1048 CE), a Central Asian scholar who accompanied Mahmud of Ghazni's campaigns, studied Sanskrit and Indian society, recording his observations in 'Kitab-ul-Hind' (Tarikh-al-Hind).
Step 2: State his description of the varna system:
He described the four varnas -- Brahmana, Kshatriya, Vaishya and Shudra -- explaining their traditional occupations (priesthood, warfare/rule, trade, and service respectively) and noting the belief that each varna originated from a different part of Brahma's body.
Step 3: Note his comparative and critical observations:
Al-Biruni compared the Indian varna system to social hierarchies in ancient Persia, and while he tried to explain it objectively, he was critical of the notion of ritual pollution/untouchability practised against those considered outside the four varnas (the antyajas).
Final Answer:
\[ \boxed{\text{Al-Biruni described the four varnas and their functions, comparing them to Persian social hierarchy, and criticised untouchability}} \]