What is the fine-grained topic of the following text: Inhabited by various indigenous people, including Chukchi, Koryak, Yukaghir and Siberian Yuit, among many others, Russians penetrated the region in 1640s, pushing eastward from the Urals in search of commercially profitable fur bearers, sable (marten) above all. Only return the topic.
Russian exploration and colonization of indigenous lands in Siberia for fur trading in the 1640s.