1/ We are just after Big Bang, Universe is made of simple atoms: hydrogen and helium. These atoms are made of two sorts of particles: protons and neutrons. One or two proton(s) plus zero, one or two neutron(s), nearly!, per atom.

2/ These simple atoms gather!

3/ These regroupings of atoms provoke...ignitions, they begin to burn due to nuclear forces, "First Generation stars" are born, only made of simple atoms Hyrogen and Helium.

4/ In these stars, complex atoms are produced. That means atoms made of more than one or two proton(s) and/or neutron(s), carbon, iron, gold and even...uranium atoms are being born!

5/ First generation stars explode! In these explosions, complex atoms are scattered through Universe ( of course all "First generation stars" do not explode together at the same time, but we simplify here to make the understanding easier ).

6/ Complex atoms are scattered anywhere in Universe!

7/ complex atoms are gathering!

8/ These regroupings of atoms provoke...ignitions again, they begin to burn due to nuclear forces again!, "Second Generation stars " are born,  made of complex atoms Hyrogen, Helium, carbon, iron etc...
Planets are born too when regroupings don't concern enough atoms to provoke ignitions.
Earth with any living forms existing nowadays on it ( we too!) are made of these complex atoms.

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What's wrong?

The problem is between step 6 and step 7!
How complex atoms thrown through Universe after an explosion can gather again? It's not logical! After an explosion, broken pieces of the body which has exploded don't gather! On the contrary they move away!
( You can try, bursting a balloon if you want! )
To gather them again You need to use a strong force which can go through long long distance: gravity! The lonely one!
To gather pieces of these explosions, you have to change G, the gravitational constant...you have to increase it!
 

Conclusion: There have been several change of the gravitational constant G through the Universe's story!
One 65 millions years ago provoking extinction of dinosaurs, and one other billions years ago
allowing "second generation stars and planets" formation!
 

Amazing no?