April
2007
Science background0
How photosynthesis tells us what our ancestors ate
1 We can learn an incredible amount about the nature and behaviour of plant, animal and even human life in the distant past by studying the ratio of carbon-13 to carbon-12 in their fossils. It works like this (explanation originally written for older kids):
2 One of the most important gases in the atmosphere is carbon dioxide.
3 Green plants absorb carbon dioxide from the atmosphere and release oxygen.
4 This is called photosynthesis.
5 It is the source of all food on Earth and of the oxygen we breathe.
6 The carbon in carbon dioxide occurs in two slightly different forms: carbon-12 and carbon-13.
7 These are called isotopes of carbon.
8 The carbon-13 isotope makes up 1% of the carbon in the atmosphere.
9 The other 99% of carbon in the atmosphere is carbon-12.
10 So you might expect the carbon in plants and trees to have these same proportions.
11 But photosynthesis is a complex and inefficient process.
12 It slightly prefers the lighter carbon-12.
13 So the carbon trapped in plants and trees has a greater fraction of carbon-12 than the atmosphere.
14 And a lower fraction of carbon-13
15 Now it gets slightly more complicated:
16 Normal photosynthesis is even more inefficient in hot, dry conditions.
17 So a slightly different form of photosynthesis appeared in some tropical plants around 65 million years ago.
18 Tropical grasses, sugar-cane and maize (often called corn) use this newer method of photosynthesis.
19 These are called C4 plants.
20 The vast majority of green plants and trees continue to use the old method.
21 These are called C3 plants.
22 C4 plants prefer carbon-12 to carbon-13, just as C3 plants do.
23 But they don’t prefer it quite as strongly.
24 So C4 plants end up with a slightly greater fraction of carbon-13 than C3 plants.
25 This means that the carbon trapped in tropical grass (a C4 plant) on the savannah, has a greater fraction of carbon-13 than does the carbon trapped in the forest trees (C3 plants).
26 The last steps come from noticing that the carbon in plants that are eaten goes to build bodies.
27 So the carbon isotope proportions of hair, nails, teeth and bones reflect the carbon isotope proportions of the plants that have been eaten.
28 They still reflect those fractions after the animal or human are long dead.
29 So a scientist sitting in a lab in Utah can tell what an early human was eating by studying pieces of his 2 million-year-old tooth.
30 The scientist can even tell what the early human was eating at different times of the year by studying different parts of the tooth.
31 Notice that whether the early human was eating tropical grass or animals grazing on that grass, the carbon isotope proportions will show that he or she had a C4 diet.
32 Note: This whole method works because both carbon-12 and carbon-13 are stable isotopes.
33 This means they are not radioactive and so don’t change as time passes.
34 Carbon-14, which is used in carbon dating, is not stable – it is radioactive and does change with time.
35 But neither carbon-14 nor radiocarbon dating has anything to do with the science of this story.
