August 2, 2012 | 6
In conversation we say someone is haunted by the past. Evolutionarily we are all haunted by many pasts, pasts buried in each of our cells, organs or actions. Each bit of you has antecedents, half-dry clay into which natural selection’s ruthless cleaver has carved. Your mitochondria—those whirring motors of energy in your cells—are the descendents of ancient bacteria and bear their marks. Your lungs are the descendents of fish lungs; your arms are modified fins. Your diet, of course, has antecedents too. But while we can consider the evolution of our arms by looking at the fossils of ancient fins, nearly everything we know about what we once ate, we know indirectly.
The elements left in in ancient teeth and bones can reveal crude measures of the diets of our ancestors or their kin. We can also look to the diets of our living relatives for more detailed insight. Living apes are not our ancestors. They have changed since the time our two kinds were one. Yet, the common ancestor from which both modern apes and modern humans descend was probably less like us and more like them. So what do the modern apes—and in particular our closest relatives the chimpanzees and bonobos—eat? Plants. Yes, plants. But what kind or how many or how? Recently, a new study by David Watts at Yale University and colleagues reconsiders the answer for chimpanzees in Kibale National Park in Uganda. From the perspective of our modern human diet, eight features of the chimpanzee diet in Kibale stand out.
1-Diversity—In Kibale chimpanzees were found to have consumed no fewer than 102 species (and perhaps many more) of plants either in the form of fruit or leaves. How many species are in your fridge? I did a quick count and found fewer than fifty plant species in my entire local farmer’s market.
2-Figs—Nearly half of all of the food consumed by chimps appears to be one or another kind of figs, fruits of the Ficus trees. Fig trees produce delicious, nutritious fruits in large numbers. Chimps are not the only animals to eat figs. Fruit bats love figs, as do many birds and as do the other apes. When living in the same region, gorillas eat fewer figs than chimps, but gorillas do eat figs, as do orangutans. The majority of the carbon atoms in the average American have been said to come from corn; it is possible more than half of the carbon atoms in chimps come from figs.
[Image 1. A chimpanzee giving into the temptation to eat a fig in Kibale, as chimpanzees do very often. Photo credit: Alaine Houle]
3-Clumps—Chimps eat many kinds of fruit throughout the year, but any given fecal sample is likely to include just one or two species of fruit. In essence, chimps eat bananas and apples all day one day and oranges and pears all day the next. In an ideal world, the perfect diet of a chimp might be different. But fruits themselves are patchy. Chimps must eat what is “in season” and/or tree which, with the exception of a few continually fruiting plants, is something different every month or week.
4-Meat. Partially lapsed vegetarians, chimps eat meat. Seven kinds of primates, including their favorite, red colobus monkeys (Procolobus rufomitratus tephrosceles) are on the menu as are three other mammal species. But most chimps don’t eat such meaty treats often. Three percent of the average chimp diet comes from meat. On average, nine days a year are meat days for chimps. But because chimps don’t share perfectly, most chimps probably gets less than this. Bonobos appear to eat even less meat than chimps. A recent study of the carbon and nitrogen isotopes in bonobos placed bonobos at a trophic level slightly lower than one antelope species.
5-Variety—In considering our ancestral diets, we tend to imagine our ancestors ate “ideal” diets for their bodies. No chimps eat an “ideal” chimp diet. Different chimps eat different diets as a function of what they find, just as our ancestors would have eaten different foods depending on just where they lived as well as other factors. If there are no fig trees, chimps do not eat figs. If there are no monkeys or duikers, chimps don’t eat mammal meat. Studies of chimp diets based on carbon and nitrogen isotopes have found some chimps appear to eat exclusively plants, whereas some individuals feed a little higher on the food chain. Some species must eat a specific mix of foods to survive; chimps are not one of those species.
6-Termites and ants (and their chitin)—Some chimps are good at eating termites, honey bees and driver ants. I’ve eaten both ants and termites, but I suspect that makes me weird rather than in touch with my ancestry. Interestingly, most of my friends seem to have eaten ants and termites too (which, friends, makes you weird too). Ants and termites have chitinous exoskeletons. No one seems to know if chimpanzees have any special ability to digest chitin. But there is more than chitin to these insects, particularly if one can find a fat and juicy queen.
7-Other–A fair number of items I do not think of as food are eaten by chimps and other apes. Chimps at Kibale ate dirt. Gorillas eat wood and the occasional stone. For apes more of the forest is edible than one would guess.
8-More figs—OK, there were only seven things. But figs, I’ll reiterate, are everywhere in the life of chimps, bonobos and other apes. Figs. Figs. Figs. We tend to think of the megafauna as being the group of species that shaped humanity, but it seems as reasonable to postulate we were shaped by figs. Like the fruits themselves, we grew out of their trees.
You are unlikely to eat like a chimpanzee eats. If you are the average American, you eat more meat and more simple sugar2. You eat differently because of choices you make and choices our societies have made (e.g., to produce huge quantities of the foods that most simply satisfy our ancient urges). You also eat differently because the species around you are different, unless you happen to own a greenhouse specializing in tropical African trees. But even if you were to abandon agricultural food and move into a forest in Tanzania you would still not eat exactly like a chimpanzee. By most reports the food chimpanzees eat tastes bad, at least to humans, (though, one hopes, not to chimpanzees). By some accounting the food chimpanzees eat is also insufficient to keep a human alive and fertile.
How is it that we might taste the same forbidden fruit the chimps eat and find it inedible or even just unpalatable? I will return to the answer, next week. There is though at least one food that chimps frequent, gorillas and orangutans all frequent and that you can easily eat. Figs. The figs these apes eat are, by most reports, rather tasty. So eat a fig and think of the ancestors. Figs are common and clumpy and you can sit in a big patch of them and eat until you are fat with their seed.
Interestingly, while I’ve described chimps and bonobos as largely vegetarian, most figs have animals inside them. Figs are pollinated by specialist wasps. When the wasp pollinates a fig, she dies inside beside her eggs. If the fig is uneaten long enough, the new generation of wasps matures and fly away to find more figs. If it is eaten early, the wasps are eaten. They are the meat at the heart of every fruity bite of life, the temptation we could not quite describe. Yet, while nearly all figs are pollinated by wasps, the most common variety of domestic figs is an exception. Figs were among the very fist plants domesticated. Eventually, these domesticated varieties were managed so heavily some forms became able to reproduce clonally, one mother plant giving birth to identical daughters. The fig you eat is all woman. It is civilizations dangling, delicious, unrequited fruit. Eat it and think of your ancestors, but know you are missing something when you do as is always the case when we try to bite in to the truth of the past3.
1-And that eating some ancestral diet on its own will not make us healthy. Our ancestors did not eat diets perfectly in tune with their body. Rather, they took the best advantage of the foods around them they could in light of their bodies which, like ours, were whittled by evolution out of more ancient forms and so flawed, complicated, and filled with tradeoffs.
2-Which makes me feel bad for eating animal crackers now while I drink my late night coffee. Guilt, that is another evolutionary story.
3-Arrgh. I was trying to write a short blog post and here I am at 1500 words, adding a few more as I finish up this note.