I would like to point out that relative components of ancient diets can be estimated utilizing stable carbon isotopic analysis (i.e. the 13C/12C ratio). 13C/12C analysis has been used to determine presumed dietary intake of Australopithecus robustus from 1.8 -1.0 million years ago (1). The stable carbon isotope ratio (13C/12C) distinguishes the relative proportions of plants utilizing either the C3 or C4 photosynthetic pathways. During photosynthesis, C3 plants fix CO2 into a 3 carbon molecule, while C4 plants fix CO2 into a 4 carbon molecule. C3 plants tend to have lower 13C/12C ratios than do C4 plants because 13C (the heavier isotope) is not as readily incorporated into the tissues of these plants as C4 plants. Grasses are almost exclusively C4 plants whereas trees, shrubs and forbs are more generally C3 plants. The carbon isotopes (13C and 12C) from plants are incorporated into the bones and teeth of the animals who consume these plants. Grass eating herbivores and the carnivores feeding on them can then be identified by the higher C13/C12 ratio they maintain in their bones and teeth. Lee-Thorp et al.'s data (1) suggest Australopithecus robustus consumed a mixed diet containing both C3 and C4 foods. Although, the interpretation is more difficult, strontium calcium ratios (Sr/Ca) have also been used to estimate dietary components of ancient diets (2). I would like to respond to Steve Meyers comments regarding the fat level in the paleolithic diet. As I have mentioned in a previous post, there is considerable controversy regarding the macronutrient (PRO/FAT/CHO %) content of pre-agricultural diets. Clearly, there was no single "paleolithic diet" and its macronutrient content varied according to latitude, season and local environmental conditions. Boyd Eaton's widely quoted estimate (3) suggests a breakdown of 37% protein, 41% carbohydrate and 22% fat. This is based upon an estimation of 65% of derived calories emanating from plant foods and 35% of total calories emanating from animal foods. Boyd used Richard's Lee analysis of the Ethnographic Atlas (4) to arrive at the 65/35 ratio; however Carole Ember (5) has pointed out that Lee incorrectly tabulated the data and that in 77% of the societies examined in the Atlas, plant derived foods represented less than 50% of the calories. Carefully studied recent hunter-gatherer societies (Hiwi, Inuit, Ache, Pygmy, !Kung) show a mean value for the calories derived from animal foods to be 59% (6). If one throws out the high value for the Inuit (96% animal food) and the low value for the !Kung (33%), the value would show that 56% of the average hunter-gatherer diet was derived from animal foods. Thus, the average macronutrient breakdown would probably show a slightly lower CHO % and slighly higher protein intake when compared to Boyd's estimate. If fatty portions of the animal being consumed were preferentially eaten and lean not consumed, then the fat % could have been significantly higher. This clearly would have had to be true for populations living in northern latitudes wherein the plant food intake would have been seasonally restricted or eliminated. In northern latitudes wherein no plant foods were consumed, expecially during winter months, the dietary intake was solely reliant upon animal protein and fat. Speth (7) has pointed out that the maximal protein intake cannot constitute more than 50% of total calories (when there is no CHO available) and probably cannot exceed 35% for long periods of time unless CHO is available. High dietary protein induces diarhea, lethargy and eventual death and was called "rabbit starvation" by early trappers, explorers and others forced to eat only lean meat. The famous Stefansson experiment at Bellevue Hospital (utilizing an all meat diet) showed that when only meat and fat were available, self selected diets contained an 80% fat/ 20% protein mix. Although many of the animals that were hunted by paleolithic humans have become extinct and we will never know the total carcass lipid content, many prey animals survived to modern times. Cave drawings as well as taphonomical analyses of human occupation sites indicate animals such as ibex, horse, european elk, deer and other cervids were widely hunted. Data from our laboratory as well as data presented by Speth and colleagues (7) shows that cervid total body fat content can range from 6 to as high as 15% . It is quite likely that large northern pleistocene mammals may have had similar if not greater values. Remember that there is significant seasonal variation in these numbers and certain African ruminants have been reported to have total body fat percentages of less than 1%, which clearly must approach physiological limits. Average body fat % of wild ruminants tend to be higher and 4-6% seems to be a pretty good overall estimate, based upon the few whole body analyses reported in the literature (7). REFERENCES 1. Lee-Thorp JA et al. Diet of Australopithecus robustus at Swartkrans from stable carbon isotopic analysis. J Hum Evol 1994;27:361-72. 2. Sillen A et al. Strontium calcium ratios and strontium isotopic ratios of Australopithecus robustus and Homo sp. from Swartkrans. J Hum Evol 1995;28:277-85. 3. Eaton SB et al. Paleolithic nutrition revisited: A twelve year retrospective on its nature and implications. Eur J Clin Nutr 1997;51:207-16. 4. Lee RB. What hunters do for a living, or how to make out on scarce resources. In Lee RB, DeVore I, (eds). Man the Hunter. Chicago: Aldine, 1968:30-48. 5. Ember CR. Myths about hunter gatherers. Ethnology 1978;17:439-48. 6. Leonard WR et al. Evolutionary perspectives on human nutrition: the influence of brain and boyd size on diet and metabolism. Am J Hum Biol 1994;6:77-88. 7. Speth JD et al. Energy source, protein metabolism, and hunter-gatherer subsistence strategies. J Anthropological Archaeology 1983;2:1-31.