How going from eating c3 plants to c4 allowed us to expand and prosper as a species (and have a bigger brain)

How going from eating c3 plants to c4 allowed us to expand and prosper as a species (and have a bigger brain)

If one thinks of C4, a powerful plastic explosive immediately comes to mind (and if it is a freak, perhaps the Nakatomi Plaza in The Crystal Jungle). However, C4s are also a type of plant . And the C3.

This differentiation is established at the physical level. More specifically to the molecular composition of plants, and to specific chemical elements that exist in subtly different forms: isotopes .

Botanical isotopes

Some of these isotopes are stable, while others are radioactive, unstable versions. In nature we find three forms of carbon:

  • Carbon-14 : unstable and radioactive, it is rare, but very useful for archaeologists when it comes to using radiocarbon dating).
  • Carbon-12 : it is most of the carbon in the world, it has six neutrons and six protons in the nucleus).
  • Carbon-13 : it is a heavier but also stable version that has an extra neutron).

When plants photosynthesize, they use energy from the sun to produce a reaction that captures carbon dioxide from the atmosphere and ends up transforming that carbon from the atmosphere into new sugar molecules. The point is that there are several different types of photosynthesis , depending on the chemical pathways used in the process.

Trees and shrubs use a type of photosynthesis that includes the formation of a molecule with three carbon atoms as a first step: botanists call them C3 .

There are plants like some grasses and reeds that photosynthesize creating a molecule with four carbon atoms, called C4 . These types of plants are more efficient at their use of water molecules (so they thrive in more arid environments) and they also get a larger amount of the slightly heavier stable isotope, carbon-13.

That is, if an animal eats many C4 plants, even its bones end up enriched with carbon-13. This information is very important at an archaeological level, as Alice Roberts explains in her book Domesticated :

Chimpanzee diets, for example, are dominated by the leafy C3 plants; their bones do not end up enriched with carbon-13. Our earliest hominid ancestors, about four and a half million years ago, appeared to follow a similar diet of C3 plants. Between four million and a million years ago, the climate was fluctuating, but the landscapes where our ancestors lived were (generally) becoming drier and grassy.

Bigger brain

We know from their carbon-13 enriched bones that they then began to ingest more C4 plants as a result of this change in habitat. Basically more starchy roots and tubers . Eating these hidden but more ubiquitous foods may have helped ancient populations to expand and thrive in new habitats, even in variable and unpredictable environments.

But there is something more important: more starch in the diet perhaps also influenced the size of our brain for good. Although the size increased with the arrival of the regular intake of meat (especially when we began to cook it, that is, pre-digest it to extract more calories when ingesting it), we should not ignore the intake of new vegetables .

Two crucial changes (one cultural and one genetic = would have contributed greatly to releasing the energy locked up in starch. The cultural change was cooking; the genetic change was the multiplication of a gene that produces an enzyme in saliva that breaks down starch ( …) Salivary amylase works much better on cooked starch than on raw, so it is possible that the increase in copies of this gene came on the heels of the adoption of the kitchen.