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Department of Health and Exercise Science Colorado State University Fort Collins, CO 80523 E-mail: cordain{at}cahs.colostate.edu
Department of Biochemistry University of Sydney Sydney, NSW 2006 Australia
Departments of Radiology and Anthropology Emory University Atlanta, GA 30322
Department of Food Science Royal Melbourne Institute of Technology University GPO Box 2476V Melbourne, VIC 3001 Australia
Dear Sir:
Our analysis was based on data derived from historically studied hunter-gatherers (Homo sapiens); hence, it may be inappropriate to assume, as Cunnane did, that Paleolithic hunter-gatherers would have maintained identical nutritional patterns characteristic of modern hunter-gatherers. During the Paleolithic period (a time period extending from roughly 2.6 million y ago until 10000 y ago), 3 hominid genuses (Australopithecus, Paranthropus, and Homo) that encompassed 11 separate species were simultaneously present (1). There is abundant evidence suggesting that the hominid's diet was not static but, rather, evolved and varied throughout the Paleolithic period, depending on the species and the ecologic niche that was exploited. A common nutritional element of those hominid species that eventually led to anatomically modern humans was the inclusion of more energy-dense animal foods in their diet (2, 3). There is little or no fossil evidence to indicate that animal foods derived from the aquatic environment played a significant role in the diet of either early or later hominids until the Upper Paleolithic (35000–40000 y ago) period (4, 5). The fossil record shows that invertebrate shell refuse piles (middens) and fossilized fish remains associated with hominid occupation sites did not appear until the Upper Paleolithic period, concurrent with the technologic advent of hooks, lines, weirs, nets, and barbed spears (4, 5). Consequently, the high fish consumption (median: 26–35% of energy) we showed for 229 historically studied hunter-gatherers likely would not have been representative of Early (2.6 million y ago until 250000 y ago) and Middle (250000–40000 y ago) Paleolithic hominids. Hence, fish, shellfish, and other shore-based foods likely would have played a minor role in providing nutrients, including essential fatty acids, that were crucial for the rapid hominid brain expansion that occurred during the Early Paleolithic.
In regard to our estimation of the mean plant-food macronutrient profile (62% carbohydrate, 24% fat, and 14% protein), we clearly included vegetables in our estimates. Tubers, roots, bulbs, leaves, and flowers are plant-food categories and are included in Table 3 of our article; these categories would subsume such modern vegetables as potatoes (tubers), radishes (roots), onions (bulbs), lettuce (leaves), and broccoli (flowers). These food categories accounted for 29.3% of our entire wild plant food database and have a mean energy density of 4.18 kJ/g. As we mentioned in our article, hunter-gatherers collected plant-food species not randomly but in a fashion predicted by optimal foraging theory that would tend to maximize the ratio of energy capture to energy expenditure. Lipid-rich seeds and nuts (mean energy density: 13.14 kJ/g) would have been selected preferentially over vegetable foods when available. Hence, our weighting of the plant-food database in Table 3 of our article reflects the preferential foraging of these fat-containing plant foods by hunter-gatherers.
In regard to the physiologic protein ceiling, we agree that Early Paleolithic hominids such as Homo habilis—because of their small size (male: height = 132 cm, weight = 37 kg), lack of effective weapons, and limited behavioral sophistication—would have been unsuccessful hunters of large herbivores and hence would have had only occasional access to "copious amounts of meat" as well as abdominal organs and depot fat. For the same reasons, these diminutive hominids would also have had little success in confrontational scavenging and stealing prey from large, carnivorous predators. The fossil record indicates that the passive scavenging of the abandoned and defleshed long bones and skulls of herbivores with their intact contents of marrow and brain would have represented the primary large animal food source for early ancestral humans (6, 7). Hominids did not become successful hunters of large game until the Middle to Upper Paleolithic period.
The evolution of a large metabolically active brain in our species required food sources that were energetically dense (2, 3) and that contained docosahexaenoic acid (22:6n-3) (8). Although East African freshwater fish are good sources of 22:6n-3 (549 mg/100 g), they are a poor energy source (498 kJ/100 g) (9) and are less energetically dense than is a mixture of wild, edible plants (540 kJ/100 g) consumed by hunter-gatherers (10). Scavenged marrow is a rich energy source (3289 kJ/100 g) and scavenged brain is a more concentrated source of 22:6n-3 (861 mg/100 g) than is East African freshwater fish. Because scavenged marrow is a more highly concentrated energy source (3289 kJ/100 g) than is freshwater fish (498 kJ/100 g), the energy return versus the energy expenditure for scavenged marrow bones would have far exceeded that available from the manual capture of freshwater fish. Furthermore, because the energy-protein ratio in African ruminant marrow (477 kJ/g protein) is almost 20 times greater than for African freshwater fish (27 kJ/g protein), fish consumption would have been constrained by the physiologic protein ceiling, whereas marrow consumption would not have been. Thus, when the option was available, scavenged marrow and the brain that was concurrently present in the skull of the defleshed skeleton would almost always have been chosen over active capture of either fish or aquatic invertebrates. Taken together, the data indicate that scavenged marrow from ruminant long bones would have represented the concentrated energy source required for hominid brain evolution and that the brains of scavenged skulls would have represented the predominant source of 22:6n-3.
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