Tuesday, June 18, 2013

Early human evolution and the myth of the Paleo Diet

hominin looking out on savanna

One of the most visible fad diets lately (at least where I live) is the so-called Paleo Diet.  If you haven't heard of it, you can think of the Paleo Diet as pop evolutionary psychology for foodies. Essentially, the idea is that our bodies are adapted to the diet of our "pre-agricultural, hunter gatherer ancestors", and that we should try to mimic this diet as much as possible, because it is more natural.  

The basic idea is sensible enough, right? I mean, to the extent that the diet recommends doing away with the heavily processed carbs which are modern technological innovations, I think we can all get on board (leaving aside critical questions regarding poverty and food access for the moment).  However, the picture gets a little less clear when we dig deeper into questions like "which ancestors are we talking about" and "what did those ancestors actually eat"? That's where the science comes in.  

caveman eating burger and fries

In the June 3rd issue of PNAS, there were three important papers on the stable isotopic evidence for early hominin diet, especially hominins in East Africa. These are dense technical papers that provide invaluable direct evidence regarding the diets of our early ancestors.  I won't try to summarize these papers, I just want to pick out three broad points that they drive home for me:
  1. Apes eat mostly ripe fruits and leafy greens, but human ancestors shifted their diets away from ape-like diets very early on.
  2. The diets of human ancestors are notably variable when compared with other animals.  
  3. Some foods that our ancestors have probably eaten for millions of years would be forbidden by the so-called Paleodiet. 
Point 1: While the early hominin species A. anamensis had a carbon isotope signature dominated by C3 resources (Cerling et al, 2013), it is clear that by the time A. afarensis (Lucy's species) came around, they were eating lots of C4 (Wynn et al, 2013).  This is interesting, because it means that a dietary transition away from ape-like diets dominated by C3 vegetation occurred really early in human evolution (around 3.4 Ma) in a species that most researchers agree is directly ancestral to us.  This dietary shift seems really interesting when you consider that even chimpanzees that routinely live in open savanna environments don't eat much C4 vegetation (Schoeninger et al, 1999).  Sponheimer and colleagues (2013) verify that this shift occurred in multiple species through time, and that there is a weak trend towards more C4 as time goes on.  

Point 2: Human ancestors had remarkably varied diets!  Wynn and colleagues (2013) show that different A. afarensis individuals had isotope values that ranged from the neighborhood of committed browsers nearly (but not quite) up to that of committed grazers. Clearly early hominins were eating lots of different things.  This is consistent with a previous study suggesting that different species of the genus Paranthropus were eating remarkably different diets, even though their jaw anatomy is extremely similar.

Point 3: Stable isotope analysis can't tell us exactly which C4 plants hominins were eating.  But, a likely candiate C4 food item for early humans are underground storage organs of certain C4 plants. These starchy roots and tubers would be gritty and fibrous, but would include ample carbohydrates and water.  The idea that  early hominins relied on these so-called underground storage organs (USOs) goes way back in paleoanthropology (Hatley and Kappelman, 1980) and had a bit of a revival in the last decade (Laden and Wrangham, 2005).  It is ironic that practitioners of the paleodiet swear off starchy tubers like potatoes when they have likely been an important part of hominin diets for the last 3.4 million years!!!

Conclusion: It is really hard to know what our earliest ancestors ate. The best science is starting to paint a picture, though and it is clear that leaving behind the ape-like diet of leafy greens and fruits was an integral part of human evolution from very early days.

If we were to consider recent ancestors (say in the last 50,000 years), it would be clear that hunting and gathering human populations have made a living from every kind of diet you can imagine (think near vegetarians on one extreme and eating tons of whale blubber on the other hand)! It is far from clear what it means to "eat like a caveman" and this idea has much more to do with selling diet books than it does with the science of figuring out what our ancestors actually ate. Thankfully, we have lots of careful scientists doing the difficult task of figuring it out!

References:


Hatley T, and Kappelman J. 1980. Bears, pigs, and Plio-Pleistocene hominids:a case for the exploitation of belowground food resources. Human Ecology 8:371–387.

Laden G, and Wrangham R. 2005. The rise of the hominids as an adaptive shift in fallback foods: Plant underground storage organs (USOs) and australopith origins. Journal of Human Evolution 49:482–498.

Schoeninger MJ, Moore J, and Sept JM. 1999. Subsistence strategies of two“ savanna” chimpanzee populations: the stable isotope evidence. American Journal of Primatology 49:297–314.


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