Why HCO?

The question is, why are you primarily made out of hydrogen, carbon and oxygen? Wouldn't some other combination do as well? Hydrogen is understandable, right. It's the lightest and most common element in the universe. So it makes sense that you've got a lot of hydrogen in you. But carbon and oxygen?

Carbon has an atomic number of 6. Hydrogen has an atomic number of 1. Why jump all the way to 6 instead of using one of the elements between? Let's look at them in order. Number two is helium. Helium is a noble gas and doesn't really react all that much. So it's unlikely that you would have much helium bound into your molecules. Number three is lithium. Lithium nuclei verge on unstable, so they get broken easily. For that reason, there's not much lithium around in the solar system. Number four is beryllium, another rare element that's produced primarily through cosmic rays punching heavier nuclei and knocking out protons. It appears in stellar nucleosynthesis, but gets fused away rapidly. Number five is boron. Boron is only produced through cosmic ray spallation, so it's very rare.

Then we get to number six, carbon. Carbon is the fourth most common element in the universe, and can form a massive amount of different compounds. It's produced in stellar nucleosynthesis by the triple-alpha process where two helium nuclei fuse into a highly unstable beryllium nucleus, followed by a third helium nucleus fusing with the beryllium to produce a carbon nucleus. Additionally, if a fourth helium nucleus fuses with the carbon nucleus, they produce oxygen. If carbon gets fused with a hydrogen nucleus instead, the result is nitrogen. Much of the carbon gets further fused into neon, which breaks up into helium and oxygen in neon-burning conditions.

So you get the five most common elements: hydrogen, helium, oxygen, carbon and nitrogen. Helium is not very reactive, but the other four are. And so you too are made up of hydrogen, oxygen, carbon and nitrogen, in that order (by atomic count).

The funny way to think about it is that plants and animals are solids made out of gases. Burn hydrogen and oxygen to get water, add in some carbon dioxide and nitrogen and bake in sunlight.

[Sources: Wikipedia]