The TCA cycle can be used for the complete oxidation of organic carbon in respiring organisms. This is obvious by observing that one full "turn" of the cycle releases (one, two, three, four, six) molecules of (acetyl-CoA O2, CH4, CO, CO2).

2, CO2

Explanation:

Acetyl CoA (a 2 carbon compound) enters into the TCA cycle and combines with oxaloacetate (4C compound) to form a six-carbon intermediate called citrate. Further reactions include the release of one CO2 during decarboxylation of Isocitrate into alpha-Ketoglutarate and release of the second CO2 during decarboxylation of alpha-Ketoglutarate to form succinyl-CoA. In this way, the 2 carbon atoms that enter the TCA cycle in the form of acetyl CoA are released in the form of 2 molecules of CO2.

the fossil record provides evidence of a common ancestor to many species.