The words for some animals and trees in the Indo-European languages have common roots, whereas others do not. Similar expressions appear for words like beech, oak, bear, deer, bee and pheasant bird, but there are no such words as elephant, camel, rice and bamboo. The evidence with regard to the homelands of the various animals and trees has helped in pinpointing the origin of the Indo-European languages. The original speakers of the Proto-Indo-European language, the Kurgans, are known by archaeologists to have had a pastoral economy.
Many animals were domesticated, including horses and cattle; but the Kurgans did not have highly developed agriculture. The Kurgans migrated to other parts of Europe and Asia, taking their language with them. Between 3500 and 2500 B.C., Indo-European warriors moved west through Europe and east to Siberia and south-east to South Asia, conquering the previous occupants of the land. The horse was particularly an effective weapon in the conquest.
The Kurgans were evidently looking for additional grasslands for their animals. As in later periods, the conquered people were forced to adopt the language of their rulers in order to survive. While the Kurgans infiltrated a large expanse of territory between Europe and Asia, the territory was by no means governed in a unified manner. The various bands were soon isolated from other members of the tribe. The Indo-European language was disseminated across a wide area but became increasingly differentiated into separate languages as a result of generations of complete isolation.