Happy Turkey Day
Carl Sagan famously said, "We are all star-stuff." That's true, and it's thanks to the efficiency of hydrogen fusion.
The universe is mostly hydrogen. To produce more complex elements—in particular, the ones that make life possible—there has to be a way to get those other elements from hydrogen. The universe does it with stars, which really are just very large balls of hydrogen, assembled through gravitational attraction. The pressure of this gravitational attraction is so strong that nuclear reactions start to occur, and hydrogen is transmuted into helium through fusion.
The amount of energy released in this process is given by Einstein's famous equation E = mc2. But only 0.7 percent of the hydrogen initially present actually becomes energy. Expressed as a decimal, this number is 0.007. This is the efficiency of hydrogen fusion, and the presence of life in the universe is very sensitive to this number.
One of the first steps in the fusion of hydrogen is the production of deuterium (heavy hydrogen) and this would not happen if the efficiency of hydrogen fusion fell below 0.006. Stars would still form, but they would simply be large glowing balls of hydrogen. If the efficiency of hydrogen fusion were 0.008 or higher, then fusion would be too efficient. Hydrogen would become helium so quickly that the hydrogen in the universe would be used up. Since each water molecule contains two atoms of hydrogen, it would be impossible for water to form. Without water, life as we know it could not exist.
https://www.popularmechanics.com/flight/g163/13-most-important-numbers-in-the-universe/