What chimpanzee predatory behavior can tell us about early human diets

Among primates, we humans are unique in how much meat we eat. On average we eat 10 times as much meat as chimpanzees, who eat the most meat among wild apes. And, unlike any other primate, humans specialize in eating big-game animals (larger than ourselves) like reindeer and mammoths. 

Because of how much meat humans eat, a few major questions are under discussion among biologists and anthropologists: What role did meat play in human evolution? How much meat did human ancestors really eat early on?

Cutmarks on bones, unfortunately, don't say much about whether meat was eaten once a day, once a week, or once a month. But could a few clues into early human diets be gleaned from the extensive field research into the predatory nature of wild chimps?

Biological anthropologist Craig Stanford says he gained a research window into studying chimpanzee meat-eating because Jane Goodall, a committed vegetarian, found the chimp's brutality too morally repugnant and awful to watch. He has long held the view that wild chimp hunting can help us learn more about why humans themselves began eating meat around 2.5 million years ago.  

At the Ancestral Health Symposium at UCLA on Aug. 5, Stanford made the point that because wild chimp meat eating varies a lot depending on geographical region and habitat, it's likely that the same was the case for early humans. In fact, there's even a huge variance in the amount of meat that modern hunter-gatherers eat now -- some like the Ache are largely carnivore and others like the Kitava are more herbivore.

"If the diets of our early ancestors who were humans vary as much as diets of great apes [and modern hunter-gatherers] today, that might tell us something important and interesting,'" Stanford said.

Does meat matter so much nutritionally for chimps?

Meat eating variability among regions and communities is significant for chimps. For example, chimps at Gombe have well recorded predatory patterns, eating an average of about 65 grams a day during the peak of their dry season. 

But chimps at the Budongo in Uganda eat almost no meat at all probably because the colobus monkey (a favorite) doesn't occur in that forest, although other monkey species do. 

"The significant thing is that there's no evidence that not eating meat at all for these chimps has a nutritional or reproductive effect," Stanford said. "They don't eat it and it doesn't seem to affect them."

Seasons and travel

During the dry season, chimps that do eat meat have a great deal more of it. Seasons are also thought to have been a major factor that influenced hominin meat eating. The predominant view is that meat-eating was at its peak during the dry season when other foods weren't available. 

However, even this view is challenged by scientists such as Harvard primatologist Richard Wrangham. He believes humans were more likely to eat meat in the wet season when they could afford burning calories to catch prey. They also could do with the risk of not catching anything.

One thing that chimpanzees can tell us is that the early humans who did eat plenty of meat probably traveled far and wide to obtain it. As an arboreal species, chimps spend most of their time in the trees traveling the equivalent of what is a half marathon a day in search of their meals.

The majority of what they get are ripe fruits, although the kind of fruits that are not recognizable in the grocery store, Stanford said. The types of fruits chimps enjoy are surprisingly bitter, fibrous and lacking in sweetness.

When chimps hunt, they do it cooperatively. However, despite their efficiency, they end up with little meat. For example, a typical baby colobus monkey, generally the size of the kitten, is shared among up to 10 chimpanzees. Stanford said chimps are nearly Machiavellian in their "sharing," which may include fighting, stealing, and even bartering sex for share of the flesh as large as the "fraction of a steak."

Meanwhile, gorillas who live on the same landscape as chimps are massive and sedentary. They live mainly on the ground. They do eat fruit when they can get it, but rely on "fall back" low-nutrient, low-calorie foods like foliage and leaves. They survive easily enough without competition from the chimps. 

"Gorillas are seen largely as the cows of the primate world because they're big, they don't move very fast, they spend most of their time on the ground, and they spend most of their time grazing," Stanford said.

Meat-adaptive genes

In fact, gorillas are not at all adapted to eating meat. Even if you put an antelope directly in front of a huge gorilla, Stanford said, a gorilla won't touch it. It's also known that feeding eggs or meat to zoo gorillas leads to heart disease. They just aren't physiologically equipped to handle the food. 

Chimpanzees, on the other hand, have a physiology more similar to humans. They immediately take advantage of any chance to eat meat. They can also live on a diet almost entirely of meat. What this tells us, Stanford said, is that clearly, at some point in history, mutations arose that gave chimps and humans the ability to live on a diet with plenty of meat. 

In one study, Stanford said he and his colleague Caleb Finch argued that humans had meat-adaptive genes. To test his hypothesis, he fed rhesus monkeys, colobus monkeys and chimps a commercially prepared meal with a pre-set amount of cholesterol. He showed that you had to feed dramatically more to a human to get serum cholesterol levels to spike. The other primates given the same quantities to incur 

"We're relatively immune to the harmful effects of cholesterol," Stanford said, likely due to an evolutionary change for humans. Did these meat-adaptive genes, perhaps, come at a time when humans also had to travel a great deal?

Other interesting variables

One other interesting correlation Stanford found may sound sexist to some. The largest hunting party sizes of chimps appeared during periods of time when the females were ovulating. The ovulating females were like "magnets" -- exciting the males, driving them to create hunting parties. The large parties then led to greater success in the hunt.  

This I found particularly interesting because it reminds me of an argument I read once in a book by Leonard Shlain. He theorized that meat was used as barter from men who wanted sex from females. He also argued that meat must've been especially important for human women because of their unique menstruating patterns among primates. 

What Stanford suggests is similar in that meat eating may have great implications in the "social and political arena" of chimps. 

A few key takeaways

There is still a lot to be understood about meat eating and its role in early human diets. But if there's one thing that we can learn is to avoid generalizing an ideal, Edenistic, pre-Neolithic diet of 10 kya. There probably wasn't one, but many, and probably changed over time. 

In fact, Stanford said, there's little reason to pick a point in time and say, 'Here's where our digestive system really came into being and should inform us about how we eat today.' Homo sapiens are a much older species -- so, if looking for a specific time period to inform on diet, why not pick 150 kya or 200 kya? This is the time when humans are thought to have first emerged from Homo ergaster. 

Regional variation among chimps and modern hunter-gatherers show that meat eating and macronutrient and micronutrient ratios vary depending on the season from one geographical region to the next. So, the question really should be: How many "paleo diets" were there? 

See video of Stanford's talk below. 

"Great Apes and the Evolution of Human Diet" by Craig Stanford, PhD from Ancestry on Vimeo.

Here's a video of chimps capturing a colobus monkey and killing it (not for the faint of heart).