Syntax. Part I. Use of the cases. Lesson 3. Use of case to denote animate direct object. Синтаксис. Часть I. Употребление падежей. Урок 3. Употребление одушевленных прямых дополнений.

Many animate nouns must be put in the genitive case when they are used as direct objects. This usage arises from the fact that in most type of noun the nominative and accusative forms have come to coincide.
Given the flexibility of Russian word order, clauses in which both subject and object are animate could be ambiguous were the grammatical forms of subject and object to remain undifferentiated. By marking the object by use of the genitive form, which in all categories of noun is distinct from the accusative, a speaker avoids confusion as to which noun is subject and which is object.