The Science of Food

The Ship of Theseus is the name given to a thought experiment first related by Plutarch around the beginning of the second century. In this story the ship Theseus rescued the children of Athens with was kept by the people of Athens and used every year in a pilgrimage to Delos. Over time, and many journeys, parts of the ship were replaced so that there came a time when every part of the ship had been replaced, it no longer contained any of the original parts. As told by Plutarch this prompted Greek philosophers to pose the question: is it still the same ship and if not when did it stop being the same ship?¹

I’m not proposing an answer to this question, I’m no philosopher, but I do want to point out that you can also consider the Ship of Theseus as an allegory for the human body. Everyday our cells get turned over; cells die and new cells are born in a continual cycle of renewal². If we dodge the identity questions posed by Greek philosophers we can say that just like Theseus’s ship we are constantly replacing parts of our body. The raw material for this, the atoms that we use to build our organic molecules, comes, mostly, from our food. We are literally what we eat.

Some new research published in the journal Science last week takes advantage of this to draw some interesting conclusions about our evolution. The authors wanted to demonstrate how changes in the eating behaviour of some of the earlier ancestors of Homo sapiens, specifically the consumption of grasses, led to changes in their morphology. An evolutionary process like this is called ‘behavioural drive’ and Darwin himself gave an example of this in birds, suggesting that in some species female mating behaviour could rely on male plumage. When hens start selecting males based on plumage you eventually end up with peacocks.

Now you may be asking how can scientists possibly work out how our ancestors 2 million years ago were behaving? And this is where the Ship of Theseus allegory comes in. If our ancestors started eating graminoid plants (grasses), because the atoms of this food would be incorporated into their bodies then we would expect their fossils to have a similar chemical makeup as the grass that they are eating.

To test this we can use isotopes. The isotopes of an element, say carbon, are atoms that have the number of protons you’d expect for that element but different number of neutrons. For example, ¹²C, the most common form of carbon, has six neutrons but ¹³C has seven. The different isotopes have the same chemical properties but their mass is different, more neutrons more mass, so they can be detected using something like mass spectrometry.

Researchers can work out if our ancestors were eating grasses by looking at the ratio of ¹²C to ¹³C in fossils. This works because some plants have a slightly different photosynthetic pathway that has a preference for the lighter ¹²C. Most plants, known as C3 plants, don’t use this pathway so they incorporate both isotopes of carbon into glucose during photosynthesis. But some plants, the C4 plants, do and grasses are among them. So organisms that eat grasses will have more ¹²C in relation to ¹³C than organisms that eat C3 plants³, reflecting the preference that C4 plants have for ¹²C.

The authors of this paper were able to determine the dietary behaviour of our ancestors by looking at how the ratio of ¹²C to ¹³C changed over time in the fossil record. Eating grasses exerts selection pressure on dental morphology, to extract the maximum energy from grasses you want good molars that can grind up tough fibrous parts of the grass. So by measuring molar length in the fossil record the authors could see if a shift to eating grass led to a change dental morphology and how long after the behavioural shift that took place.

So what did they find? Well from about 4 million years ago there was an increasing preference for eating grasses with a corresponding and lagging development of longer molars. About 2 million years ago, though, this flipped and our ancestors began to display less preference for grasses and, sure enough, about 700,000 years later there was a change in the length of our molars, they got shorter. This did show a clear example of behavioural drive, we stopped eating grasses and our teeth changed, but then the question was what exactly were our ancestors eating if not grasses?

Well, similar to the carbon isotope story, you can use can also use oxygen isotopes to get some insight into ancient behaviour. Because ¹⁶O is lighter it evaporates more readily from above-ground parts of plants, so if you eat a lot of leaves you’ll have more ¹⁸O in your body in comparison to ¹⁶O. Underground parts of the plant get their water from the groundwater and there is less evaporation so they have more ¹⁶O. Just like carbon isotopes in grasses the ratio of ¹⁶O to ¹⁸O will be different depending on what part of a plant you are eating. Turns out some of our early ancestors had more ¹⁶O in comparison to ¹⁸O. There are a few explanations for this but the most likely is we started to eat tubers, the underground parts of plants. Tubers are a characteristic of C3 plants, not C4, so this also explains the decline in C4. We switched form grasses to tubers.

I’ve simplified things a lot but this is a really interesting study. It illustrates an example of behavioural drive in early hominin diet, it shows that behavioural plasticity might have been our evolutionary super power (you need to be amenable to change to change) and it suggests that a shift to eating calorie dense underground bulbs was a key moment in our survival as a species. Pretty good for just looking at some isotopic differences in the fossil record.

Footnotes

1. Hobbs, a few thousand years later added to the question. Asking if all the parts were used to make another ship, what is the status of that ship? I’m not going to attempt an answer to this question either. ↩︎
2. Some quick googling suggests we replace 98% of our atoms every 1-2 years but I couldn’t find any good sources so it deserves a more rigorous look. ↩︎
3. Animals that eat animals that eat C4 plants will also have an isotopic makeup richer in ¹²C. In this case the behaviour was being compared to a morphological change that was the result of eating grasses so we don’t need to worry about that too much. ↩︎