Download `The Importance of Dietary Carbohydrate in Human Evolution

Our recent article in The Quarterly Review of Biology titled ‘The Importance of Dietary Carbohydrate
in Human Evolution’ has been criticised by some members of ‘The Paleo diet™’ community, who
appear to see it as a direct attack on their dietary beliefs. We take this opportunity to clarify its
hypothesised link to the modern ‘Paleodiet™’ concept.
Firstly it is important to note that ‘The Paleo diet™’ and the academic study of the diets of Paleolithic
humans are distinct concepts. ‘The Paleo diet™’ is a trademarked popular dietary regime proposed by
Dr Loren Cordain; the trademark was registered March 10th 2009. It recommends higher meat /
protein consumption, low carbohydrate / low glycaemic index foods, moderate to higher fat intake
dominated by monounsaturated and polyunsaturated fats with balanced Omega-3 and Omega-6 fats,
higher potassium and lower sodium intake, a net alkaline food load, and increased intake of vitamins,
minerals, antioxidants, and plant phytochemicals. The diet is based on assertions about the nature of
the diets of our Palaeolithic ancestors, and highlights in particular the role of long chain fatty acids in
the evolution of the human brain.
The academic study of the diets of Palaeolithic humans is an on-going area of research that attempts
to integrate information from anthropology, archaeology, nutritional sciences, genetics, physiology
and various other fields. Despite significant progress, considerable uncertainties remain about the
nature of Palaeolithic human diets, the importance of key nutrients for brain size increases over the
last 3 million years, and the extent to which we have adapted biologically and culturally to dietary
changes since the advent of agriculture. While this fascinating field of study has the potential to
inform on the relationship between diet and health, it is currently best described as ‘hypothesis-rich;
data-poor’.
Our article – which was subjected to rigorous academic review – proposes a testable hypothesis that
over the last 800,000 years our ancestors met the considerable and sustained glycemic energy
requirements of our expanding brain, reproductive and other tissues, through a combination of
carbohydrate (particularly starch) consumption, the use of fire for cooking, and the evolution of
multiple copies of the salivary amylase gene, AMY1. This hypothesis is supported by a wide range of
previously published work and integrates evidence from diverse scientific disciplines, including
genetics, nutrition, archaeology, anthropology, human physiology and carbohydrate chemistry.
Critically, our hypothesis is falsifiable (a cornerstone of modern science) in that makes a clear
prediction that the widespread adoption of fire for cooking is correlated with the evolution of multiple
copies of the salivary amylase gene and accelerated increase in brain size.
Our paper does not consider the merits or otherwise of ‘The Paleo diet™’ for modern humans. Nor
does it claim that carbohydrates were a ‘magic bullet’ in human evolution; we recognise that other
nutrients may also have played a critical role. What our hypothesis does do is put carbohydrates back
on the Palaeolithic table.
Here we clarify a few key points raised by members of ‘The Paleo diet™’ community:
1. At no point have we claimed that meat or fat was not important. However, the need for DHA
– which is one of the main arguments for meat consumption for brain growth – is available in
other dietary sources. Equally, the importance of DHA may have been overemphasised, due
to the abundance in the modern Western diet of omega-6 fatty acids, which inhibit a key
enzyme step in the conversion of ALA to DHA.
2. Humans have adapted to dietary shifts after the Palaeolithic era ended. For example,
mutations causing lactase persistence – which allow adults to consume large quantities of
milk – have become widespread over the last 5,000 years and show among the strongest
signatures of recent positive natural selection in the human genome. ‘The Paleo diet™’
community is correct in stating dairy products were not available in the Palaeolithic; there
were no domesticated animals to provide these. But once domestication occurred, we rapidly
evolved this trait and it clearly gave many of our ancestors a considerable selective advantage.
3. Hunting wild animals is far less secure than collecting plants from a known source. Without
the behavioural ability to exploit plants for nutritional purposes hominins would have had
insecure and dangerously randomised access to their essential daily nutrition, which would be
based entirely on their ability to hunt.
4. Evidence for fish and shellfish, an important component of ‘The Paleo diet™’, occurs in a
small number of sites in South Africa and the Mediterranean during the Middle Palaeolithic.
However, there is little evidence for the widespread integration of marine products into the
human diet until the Mesolithic, a short period after the end of the Palaeolithic.
‘The Paleo diet™’ is an artificial construct that has limited demonstrable overlap with what we can
confidently state our Palaeolithic ancestors ate. The Palaeolithic period lasted for at least 2.6 million
years, incorporated several species of ancestral hominins, including modern humans, and covered a
wide-range of environments and ecologies; from equatorial rain forests to Arctic tundra. There is no
single ‘Palaeolithic diet’.
Knowledge of actual Palaeolithic diets does have the potential to increase our understanding of the
relationship between diet and health today, and as we state in our paper, “to a first order of
approximation – our physiology should be optimized to the diet that we have experienced during our
evolutionary past.” But Palaeolithic diets are still very poorly characterised as so little evidence
survives from most of this extremely long time period. In the absence of robust quantitative evidence
it is perhaps tempting for some to assert as fait accompli that we ate a specific diet during the
Palaeolithic era, particularly when a profit motive is involved. Meanwhile, the scientific community is
slowly and cautiously building up a clearer picture of ancestral diets, and we hope our paper makes a
meaningful contribution to that endeavour.
Karen Hardy
Jennie Brand-Miller
Katherine D. Brown
Mark G. Thomas
Les Copeland