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Following is my opinion and open for discussion. Manav (Man), Danav (Demon), Vanar (monkey/tribal)& Devta (God) were not four species but four civilizations.  

Devtas were most advanced civilization. They lived on Northern most part of Bharat, mostly in mountain area. They had advance weapons like Brahmastra, Puspak Viman, Chariot  etc. They were resourceful. Everyone wanted favours from Devtas.

Manavs were less developed than Devta. Lived in the villages, towns and cities on the plains of North Bharat. Well-developed administrative and educational system. Good relationship with Devtas and Vanars.

Vanars. Not monkey but tribals who lived in forest. Less advanced than Manav but had some intellectual like Hanuman. Lived life like animals that is why called Monkeys/Vanar.

Danav, not demons but rebellious of the society. These were the people who rebel against the rule and regulation of the civilized society. Created their own settlement and system. Settled in Southern part of Bharat. Also had presence throughout the land. Killed or gave hard time to all other three.

No wars between Devta ,Manav & Vanar but Danav always at war with other three for dominance.

Ravan, the king of Danav was a worshipper of Shiv and so were Ram & Hanuman

By the time of Mahabhartha, it is the story of mainly Manav(s) and very little presence of Danav, except Hidimba, Indr Devta  giving divine Shastra to Karan & Arjun & Eklavya, a lone tribal boy.

At the beginning of Kaliyuga only Manav remained in cities and tribal in forest.