Carbohydrates, Isotopes, and Study of Ancient Diets

The outcome of the photosynthesis processes is ultimately similar in all green plants carbon dioxide from the atmosphere is taken up by the plants, where it reacts with water to form carbohydrates and oxygen the carbohydrates are assimilated by the plants while the oxygen is released to the atmosphere (see Textbox 53). Extensive studies have shown that the conversion of carbon dioxide and water into carbohydrates in different plants may follow, however, one of three different photosynthetic pathways, which are usually referred to as the C3, C4, and CAM paths. Each type of plant follows just one of these three pathways. 

More than 95% of the plants of the world follow the C3path, whereby the carbon dioxide is initially incorporated, by the plants, into intermediate compounds made up of three atoms of carbon - which is the reason these plants are known as C3 plants. Only about 1% of all plants, including maize, millet, sorghum, and sugarcane, follow the C4 path, incorporating carbon dioxide into intermediate compounds made up of four atoms of carbon - which is the reason these plants are known as C4 plants. The remaining 4% of plants, mostly succulents (cacti), follow the third, CAM, path, which has no bearing on the discussion that follows. 

When plants are consumed as food by herbivorous animals, the isotopic signatures in the plants are passed on to the consumers. Therefore, provided the isotopic signatures of C3 and C4 plants are known, determining the isotopie signatures in the tissues of herbivorous animals enables one to determine the relative amounts of C3 and C4 plants that the animals consumed as food, and to reconstruct their diets. Moreover, since carnivorous and omnivorous animals, including humans, feed on herbivorous animals as well as on plants, determining the isotopic signatures of the isotopes of carbon in tissues of ancient animals and humans makes it possible to elucidate the components of their diets. 

Also the stable isotopes of nitrogen, like those of carbon, are fractionated when nitrogen is incorporated into the plants. Determining the isotopic ratios between nitrogen-15 and nitrogen-14 in animal remains also reveals information on the diets of ancienf animals and humans (White 1999 de Niro 1987). 

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