Biology · Evolution · NEET
No. This is the most common mix-up. The Big Bang theory only explains the origin of the universe (the huge explosion, expansion, cooling, and formation of galaxies). The origin of life came much later, on the early Earth, through chemical evolution. In NCERT, Big Bang is the background context; the origin of life is a separate topic that follows it.
NCERT gives two numbers you must not swap. The universe is almost 13.8 billion years old. The Earth was formed about 4.5 billion years ago in the solar system of the Milky Way galaxy. Remember: universe first (13.8), Earth much later (4.5), both in billions of years.
Hydrogen and Helium formed sometime after the explosion, once the universe expanded and cooled. They did not exist at the very first instant. These light gases later condensed under gravitation to form galaxies. So the order is: explosion first, cooling, then Hydrogen and Helium.
The single explosion made the universe expand, and because it expanded, the temperature came down. This cooling allowed Hydrogen and Helium to form and then condense under gravity into galaxies. Without expansion and cooling, matter could not clump into galaxies and stars.
After the universe forms, gases condense under gravitation to form galaxies. Galaxies contain stars and clouds of gas and dust. Inside the Milky Way, Earth formed about 4.5 billion years ago with no atmosphere at first — that early Earth is where the origin-of-life story begins.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
About 13.8 billion years ago. This is the age of the universe used in NCERT Class 12 Evolution.
About 4.5 billion years ago, in the solar system of the Milky Way galaxy. Do not confuse this with the age of the universe (13.8 billion years).
Hydrogen and Helium formed first, after the universe expanded and cooled. Oxygen came much later, on early Earth, when UV rays broke water vapour into Hydrogen and Oxygen.
The gases (Hydrogen and Helium) condensed under gravitation and formed the galaxies of the present-day universe. Galaxies contain stars and clouds of gas and dust.
No. Big Bang explains the origin of the universe. The origin of life is a separate chemical-evolution story that begins on the early Earth after Earth formed.
