Why do all living beings absorb the same proportion of carbon 14? I understand that all living beings have the same proportion of carbon 12 to carbon 14 and that this is an assumption used to carry out carbon dating but I do not understand how this is possible. As isotopes are not evenly distributed, how can it be that the proportion of carbon 14 is the same in all living beings?
 A: "All living beings have the same proportion of C12 and C14" is the simplified rule, used to explain carbon dating.  In practice a very large number of correction terms have to be applied because it isn't just that simple.  Radiocarbon dating is calibrated against other sources of aging information, like tree rings.  Once calibrated for an area, it can be used for dating.  However, the general principle holds because most terrestrial plans and animals share their carbon with the large atmospheric reservoir, and that is reasonably constant, as far as things go.  For instance, it is highly unlikely you will find two wolves from the same era in the same general location with different isotopic ratios.
The wikipedia page on Radiocarbon dating goes into many of these.  If any particular source of isotopic shift has you particularly interested, I encourage asking a second question focused on that!
A: Land animals get their carbon from plants. Green plants get their carbon from the atmosphere. Most of them have the same mechanism for that (chlorophyl), so that would not be expected to differ between trees and grass.
It is conceivable that isotope ratios would be a bit different in water plants (because that carbon did not come from the atmosphere) and in blue-green algae (because of the different chemical pathway). Isotope ratios in land animals and sea animals are probably a bit different.
There are also the carbonate shells of sea animals where the carbon does not come from photosynthesis (I think). Those isotope ratios may be different from internal skeletons.
So this is a bit of a biology question.
