On squeaky voices...

The fact that the early universe gives rise to an 'interesting' abundance of Helium-4, that is, neither zero nor 100%, is a consequence of a delicate coincidence between the gravitational and weak interactions.

There was a tiny window of opportunity for the formation of the light elements at the beginning of the Big Bang. In the first 0.04 seconds it was far too hot for little nuclei not to disintegrate. On the other hand after around 8 minutes the temperature was too low to force the wee nucleons to come in range of the strong nuclear force. So in 14 billion years of history there was only one period of time, just long enough to boil a pan of pasta, during which Hydrogen and Helium could have been formed.

The proportion of neutrons and protons that combined to form either hydrogen or helium is dictated by the various actions of different forces (as alluded to in the quote from Barrow and Tipler above). The maths works out at about 1 He for every 10 H nuclei.

Were the forces not so delicately balanced then we would have 100% Helium or 100% Hydrogen. In the former case life wouldn't be possible (no water, stars that burnt up faster). In the later life would probably have been possible.

Quote Unquote

As we look out into the universe and identify the many accidents of physics and astronomy that have worked together to our benefit, it seems almost as if the Universe must in some sense have known that we were coming.

F Dyson

Cosmic coincidences part 2

Four and a half billion years ago, when the earth was just a wee bairn, it was struck by an object the size of Mars. From what physicists can make out collisions of this magnitude are extremely rare in the universe. When the dust of that collision had settled there were two key outcomes. Firstly the earth had grown a bit bigger, and secondly the debris that was kicked into space coagulated to form the unusually large moon.

Typically moons are very small compared to the planet they orbit, but for earth's satellite body things are very different. This is crucial to maintaining our climate. The size of the moon means that it acts as a gravitational steadying force, stabilising the earth's tilt, and in doing so preventing wild fluctuations in the heat coming from the sun.

Conditions might be bad for complex land-based life if there were no moon and obliquity varied significantly.