Very approximately, before the universe was 1 trillionth of a second old, there was only a soup of quarks present in the universe. Then at about 1 thousandth of a second or so, the universe became cool enough that quarks could combine to form various nucleons such as protons and neutrons and mesons. As the universe continued to expand and cool, the energies of the colliding particles decreased still further so that more and more quarks could combine into neutrons and protons, but the short lived mesons and muons began to decay away. The ratio of protons to neutrons changed until by 1 second after the Big Bang, there were about 5 protons for every 2 neutrons. At this time the temperature was 10 billion degrees. After a few minutes, the protons and neutrons could fused together to form helium nuclei, and the remaining unmatched protons eventually became the hydrogen nuclei. But at the same time the cosmos was growing rapidly less dense, so after 10 minutes or so it was nearly impossible for these high-speed nucleii to foind each other anf fuse into elements heavier than lithium. This is why there is a 'cosmological' abundance of hydrogen, helium, deuterium and lithoum that is the same value in all parts of the visible universe we can measure today.

From this point on, the destiny of a single proton depended on chance. There seems to be little intergalactic hydrogen gas, so the chance was very good that eventually a single proton would find itself a part of some galaxy. And since most of the mass of a galaxy is in its stars, the proton would probably take up residence inside a star after about a few billion years after the Big Bang. Because planets only represent 1 part in about 1000 of the mass of a star, and assuming that stars with planets are common, there is a good chance that, say, 1 proton in 100,000 might end up inside a planet in the guise of one of its constituent elements. Beyond this point, its pretty well unknown what happens next. Most of the elements in planets are likely to be simple gasses of carbon dioxide, ammonia, hydrogen, nitrogen. And rocky planets have silicates, iron and so on. Life at the bacterial level seems pretty hardy, but in terms of mass is 1 part in 100 billion billion of the mass of a planet so the probability that a particular proton from the Big Bang winds up inside a specific cell is probably 1 part in 10^30 or lower.

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