All planets prior to their formation are part of their parent stars, like the nine planets of our solar system were part of the Sun. After parting from the Sun, it took millions of years for our Earth to cool and become solid. The Earth is hollow inside, with a tiny white-dwarf "sun" at its center. Most of the other planets in this universe are hollow inside. And most of the planets are inhabited too, because the basic purpose of planets is to support life - as that of stars is to support planets. On a planet, both the outside surface and the inside surface, or any one of them, could be suitable for life. For our Earth, both the surfaces support life - in fact, the "inside world" (referred to as Pataal Loka in Hindu scriptures) is more suitable for life as is protected from outside natural calamities.
Presently, the twenty eighth chaturyugi of the seventh manvantar is in its last phase. The Earth was formed in the second manvantar, while the Moon was formed in the third. Continents came out of the oceans in the fourth manvantar. Vegetation was born in the fifth, animals in sixth and humans at the beginning of the current seventh. So, human life on Earth is roughly 120 million years old.
At first the humans were born by asexual means - in fully grown-up and knowledgeable state - in the Meru Parvat region in Himalayas (Tibet). They further carried on the chain of life through sexual means.
At the birth of every cosmos the complete knowledge of God is transmitted in wave-form and spreads out with the expanding universe. This knowledge was received by the foremost of the earliest humans - the four rishis (sages) - through meditation, and given to the humankind in the form of the four Vedas.
Population increased, Vedic knowledge spread, and human society flourished. Since the very beginning, humanity has taken two paths, the Devas and the Asurs, the believers in God and the worshippers of nature, the good and the evil. The society got divided along these two paths. The Deva or Aryan culture believes in scientific development supported by equal spiritual enlightenment for the ultimate betterment of humanity. On the other hand, the Asur culture believes in extreme materialism and considers the nature to be all powerful, leading to usage of science for destructive purposes.
The fight for supremacy between the two cultures, between good and bad, follows a sinusoidal path, and so do peace and wars, and the scientific and spiritual development of humanity. Today it's the asur culture that is on top, but the fight is still on.
