Monday, February 11, 2013

Experts on the evolution of human nutrition

Want to eat a diet that mimics that of our Paleolithic ancestors? It might be a little more complicated than what the popular books say.

The fact is, there was never one Paleo Diet; it's more likely there were hundreds of them and that they were continually changing and broadening over evolutionary time.

That was the overarching message of an impressive lineup of experts on ancient human diets at a symposium entitled "The Evolution of Human Nutrition" organized by the Center of Academic Research and Training in Anthropogeny (CARTA) at UC San Diego on December 7, 2012.

Now, I'm happy to report, the videos of a few of the talks have been made available (embedded in this post below). You can also read what other folks on Twitter had to say about the event using the #CARTAsymp hashtag in my Storify story.

President of the Wenner-Gren Foundation for Anthropological Research Leslie Aiello, who chaired the symposium, expressed some amusement over folks who aspire to the great lengths of trying to live like cavemen in the big city.

However flawed their premise, she noted, the gaining interest into the diet of our ancestors was one to be welcomed. After all, it could lend clues into current causes of epidemics of obesity, type-2 diabetes, and cardiovascular disease.

What we still don't know

Aiello gave a refreshing perspective on the history and complexity of studying ancient human diets. Back in 1995, she and Peter Wheeler were the first to propose the "expensive tissue hypothesis" -- one that proposed that we gained our energy-hungry brains by sacrificing our large guts.

Yet as early 2011, Aiello explains, even the well-founded theory she and Wheeler had proposed was challenged. Ana Navarrete, Carel P. van Schaik, and Karin Isler of the University of Zurich found no evidence of a brain-gut tradeoff among any other mammals.

Navarrete and her colleagues now have a competing theory: that larger brains came as a result of our abilities to store body fat. Among mammals, they suggest, those with larger brains acquire them by sacrificing adipose tissue for energy storage. But humans "buck the trend," Aiello said.

But there are so many other variables that could have also played important roles -- perhaps even billions of them.

The relationships that hominins had with microbes at the time might have contributed to adaptations, according to biological anthropologist Steven Leigh of University of Illinois, Urbana-Champaign. Microbes in the intestine continue to contribute for up to 7 percent of our daily energy supply in form of short-chain fatty acids and they also supply valuable B vitamins and hormones.

Leigh's preliminary findings are that microbial communities can change drastically depending on where primates live. Rainforest monkeys, for example, harbor more species of microbes in comparison to those living in semi-deciduous forests or in captivity. Changes affecting early humans may have factored into adaptations over evolutionary time.

What we may know  

Current hunter-gatherers offer a "window" into how early humans lived and ate, according to Alyssa Crittendon. But the nutritional anthropologist and behavioral ecologist of University of Nevada, Las Vegas, adds, "Hunter-gatherers are not living fossils. They are contemporary modern populations just like you and I."

Based on her research analyzing energy values of foods eaten by the Hadza of Tanzania, Crittendon said hunter-gather diets can vary widely depending on plants and animals of the season. During the dry season, for example, Hadza get most of their energy from meat. In the wet season, the energy contribution shifts dramatically to plant foods, especially berries.

Honey is Hadza's most preferred food being made up of 80 to 95 percent sugar and they have learned to obtain it more regularly through unusual means. A bird aptly named the greater honeyguide has learned to lead Hadza men to beehives in exchange for a share of the honeycomb. It is an extraordinary partnership of food sharing.

A look into the past is also provided through the diets of other primates such as chimpanzees, according to Peter Ungar of University of Arkansas. For example, judging from the jaw structure of chimpanzees one can find similarities with that of Ardipithecus ramidus.

This direct ancestor was likely to have eaten fruit, soft leaves, and soft legumes as chimpanzees are fond of doing today (depending on their region).

But the best evidence available on ancient human diets is from fossil teeth, Ungar adds. Clues from tooth size, shape, and structure can be combined with dental microwear and enamel chemistry.

Summarizing evidence from teeth "food prints," Ungar shares:

  • Australopithecus likely ate mostly soft or tough foods like fruits, and leaves in a mixed setting 4 to 2 million years ago, but could handle a fairly broad diet. Around 2.5 may, there was a fork in the evolutionary road.
  • Our Paranthropus cousins had more specialized teeth and likely had different diets in different places. Some ate tough savanna foods like grasses and sedges; others had harder items such as nuts and seeds in a mixed setting. 
  • Our early Homo ancestors had less specialized teeth and likely had a broader or more variable diet including both savanna and forest resources, along with some meat. 

Alison Brooks of George Washington University adds that Neandertal teeth, as well as their large caloric needs, indicate that they too must have eaten plant foods.

The microfossil evidence reveals starchy grains that also suggest Neandertals cooked and boiled their food. Now it's up to archaeologists to find pots, Brooks said.

That Neandertals ate plants is despite what the nitrogen isotope data show, anthropologist and research archaeologist Margaret Schoeninger of UCSD said.

It's necessary to rethink the nitrogen isotope data, Schoeninger said, because all humans -- whether subsisting on plants or animals -- and some plant themselves return a signal as a "top predator."

Cooking may also play in as a factor, she suggested.

What we know were crucial for modernity and leaving Africa: Cooking, food sharing, division of labor

Understanding that the introduction of fire was a major precursor to modern humans has long been argued by Primatologist Richard Wrangham of Harvard to be an essential part of better studying early human diets.

He presented new data showing that cooked carbohydrates and proteins also contribute more calories than when raw. The data support that cooking could have helped lead to expansion of large brains while at the same time reducing gut size.

Clues into what early humans ate in a setting can also be gathered through study of decisions that might our ancestors may have made based on "return on investment" scenarios, anthropologist Mary Stiner of University of Arizona said.

For example, hunting a large or small animal each can take a lot of effort, yet the larger one yields a higher return and is preferable. Comparatively, some plant foods represent greater or lower effort with higher or lower yields compared to others depending on investment into obtaining them, processing them, and/or cooking them.

It would make sense, she reasons, that cooking, food sharing and division of labor were crucial for humans to insulate infants and children (which need a great deal more attention, compared to those of other primates) from variations in food supply.

The likelihood is that each of these human characteristics -- cooking, food sharing, and division of labor -- were apparent at the time of our direct ancestor Homo erectus around 1.89 million and 143,000 years ago.

Homo had evolved to be "eating machines," Schoeninger said in an earlier discussion on the topic at Eleanor Roosevelt College (video embedded below).

Schoeninger noted that it was H. erectus that first left Africa and needed fire to stay warm. On their journey, Schoeninger suggests, it was likely that they subsisted on animal foods during the winter and on seed heads from grasses (cereal grains) as familiar food when traveling into Asia and Europe (because of unfamiliarity with fruits and vegetables of other regions).

Agriculture and population health decline

By 15 to 10,000 years ago, modern humans had taken over the landscape and were already shifting from foraging to farming, which represented a major transition in the human diet, Schoeninger said.

The shift was was one from a varied, broad diet that changed with seasons to be replaced with one that included specific plant foods such as cereal grains as a dietary staple, according to anthropologist Clark Spencer Larsen.

This reliance on single foods such as grains as staples with characteristically low nutritional quality, along with more availability of food and more sedentary lifestyle, he said, was the start of health decline as evidenced by study of bones and teeth of the era.

Barry Bogin, a biological anthropologist of Loughborough University, UK, has witnessed this sort of population health decline first-hand while studying the Maya who in recent years have made the switch from their traditional varied diets to ones that are more modern and containing a great deal more animal products, cereal grains, and added sugars.

Unfortunately, Maya children will ultimately suffer the most from the dietary transition with higher risk of obesity and chronic diseases such as type 2 diabetes.

Judging from the history, isn't it understandable that folks would be compelled to change their diets (and propose that others change theirs) to one that is relatively more similar to that of our hunter-gatherer ancestors?

But when asked about dietary advice (including what food groups to be consumed and to not be consumed) that can be gathered from an extensive knowledge of evolution of the human diet, Ungar simply shrugs and responds, "I tell people to go see a nutritionist."

Videos from CARTA

Steven Leigh, Peter Ungar, Alison Brooks, and Margaret Schoeninger:

Mary Stiner, Alyssa Crittenden, and Steven Leigh:

"Eating Machines" - Margaret Schoeninger from Feb 20, 2012 at Eleanor Roosevelt College: