What’s significant about carbon isotopes?

Carbon isotopes are used to investigate the diet of our ancestors; but what’s so special about them in the first place?


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If you’ve read much about archaeology you’ve probably encountered the idea of “stable isotope analysis.” For the uninitiated, this is basically the scientific principle of you are what you eat. There are slight variations of elements – known as isotopes – that are almost identical, but not quite. These small differences mean they accumulate in different places, plants and things. For example, different carbon isotopes accumulate in different plants; and thus in people who eat different plants.

Carbon isotope

The differences in isotopes present in different feeding patterns

Specifically, carbon isotopes can tell us whether someone was eating C3 (most cereals and trees) or C4 (grasses and weeds) plants. Derek asks what the significance of all that is:

I often see remarks about C3 and C4 plants.  Reading around (OK – just Wikipeia!) gives a technical definition.  But what is the significance (if any) of an animal mostly eating C3 or C4?

And since it’s Wednesday, I’m going to answer Derek’s question. There isn’t any significance to the carbon isotope content of an animals’ food.

Thanks for asking Derek. Remember; if you also want your question about human evolution answered you can use the “contact me” form at the top of the page.

carbon isotope native american

The spike in carbon isotope levels in Native American populations; indicating when they invented farming

Ok; I suppose I’ll elaborate a bit.

The diet of an animal is of course hugely significant to our understanding of its evolution. Carbon isotopes are a great tool for studying the diet of animals; so can reveal significant results. However, there isn’t anything particularly special about the specific plants carbon isotope analysis examines. It isn’t as though C4 plants are superfoods, and the shift to them marks a key point in our evolution. They aren’t inherently special.

For instance, Native Americans mostly ate C3 plants until they domesticated maize; a C4 plant. Thus carbon isotope analysis can help pinpoint the origin of farming; a hugely significant event. Yet there isn’t anything inherently special about maize.

Or how carbon isotopes revealed that our close relative Australopithecus sediba had a very versatile and flexible diet. This included bark. Again, bark isn’t that special, but the flexibility this reveals is.

I hope this makes it all a bit easier to understand. And I’ll repeat: if you also want your question about human evolution answered you can use the “contact me” form at the top of the page.

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