Life expectancy was longer century back(over 100 years if excluding war deaths) because of cleaner environment and natural foods compared to modern 60-70 years life expectancy and artificial packed foods. Quantity of population is more today but quality is less because of several scientific equipment comforts like vehicles that are cause of polluted air/Prana(life-force).

[The_Complete_Works_of_Swami_Vivekananda/Volume_1/Lectures_And_Discourses/Steps_Of_Hindu_Philosophic_Thought][1]

> So the Jivas go to heaven, and have a very good time, except now and
> then when the demons give them chase. In our mythology it is said
> there are demons, who sometimes trouble the gods. In all mythologies,
> you read how these demons and the gods fought, and the demons
> sometimes conquered the gods, although many times, it seems, the
> demons did not do so many wicked things as the gods. In all
> mythologies, for instance, you find the Devas fond of women. So after
> their reward is finished, they fall down again, come through the
> clouds, through the rains, and thus get into some grain or plant and
> find their way into the human body, when the grain or plant is eaten
> by men. The father gives them the material out of which to get a
> fitting body. When the material suits them no longer, they have to
> manufacture other bodies. Now there are the very wicked fellows, who
> do, all sorts of diabolical things; they are born again as animals,
> and if they are very bad, they are born as very low animals, or become
> plants, or stones.
> 
> 
> In the Deva form they make no Karma at all; only man makes Karma.
> Karma means work which will produce effect. When a man dies and
> becomes a Deva, he has only a period of pleasure, and during that time
> makes no fresh Karma; it is simply a reward for his past good Karma.
> When the good Karma is worked out, then the remaining Karma begins to
> take effect, and he comes down to earth. He becomes man again, and if
> he does very good works, and purifies himself, he goes to Brahmaloka
> and comes back no more.
> 
> The animal is a state of sojourn for the Jiva evolving from lower
> forms. **In course of time the animal becomes man. It is a significant
> fact that as the human population is increasing, the animal population
> is decreasing. The animal souls are all becoming men. So many species
> of animals have become men already. Where else have they gone?**


  [1]: https://en.wikisource.org/wiki/The_Complete_Works_of_Swami_Vivekananda/Volume_1/Lectures_And_Discourses/Steps_Of_Hindu_Philosophic_Thought