The outer trapezoid-shaped bounds in the relationship between δ13CMDE value and δ15NMDE value in Figure 1 are constrained fundamentally by the range of isotope compositions possible at the base of food webs globally26. In the terrestrial biosphere, this range is driven primarily by the established relationships between climate and the δ13CMDE value and δ15NMDE value of primary production27–31. The climate-driven relationship is modulated regionally and locally by edaphic factors such as soil type32 and in some cases by environment-specific phenomena such as the incorporation of low δ13CMDE value carbon from methane oxidation into some lacustrine and wetland food webs33. In the ocean, δ13CMDE and δ15NMDE values at the base of the food web also vary in broadly predictable ways, controlled primarily by global ocean circulation and productivity34,35, modulated at the local scale by interactions with the terrestrial biosphere, for example, through the incorporation of seabird guano into the base of local food webs36.
Additional to these fundamental controls on the stable isotope composition of primary production are local modifications possible through processes specific to particular environmental niches. These include trophic enrichment in both δ13CMDE and δ15NMDE values through complex food web interactions26,37,38 ultimately by the choice between, and manipulation of, the dietary possibilities available to humans, in the context of societal and technological constraints, at a particular time and place10,39–41. All of the space within the outer bounds delineated by the range observed in primary production is occupied to a greater or lesser degree by isotope results from ancient humans, from single individuals to large populations of individuals, indicative of our impressive capacity for adaptive omnivory at the global scale. The observation that the results for modern individuals on a subsistence diet occupies much the same isotope-space as the results for the (much larger) ancient sample group indicates that the conversion of all the results to a ‘modern’ diet in 2010 has successfully enabled the comparison between ancient and modern human diet (Figure 1).
The most striking difference across the entire dataset is the compressed range of modern non-subsistence diets compared to modern subsistence and ancient isotope values. The δ15NMDE values of modern subsistence diets imply consumption across trophic levels from around 1 (plant only) to ~4-5 in populations reliant dominantly on high latitude marine resources (e.g. Greenland), assuming an average trophic enrichment factor of 4‰ (see Methods). Some ancient populations appear to be consuming at even higher trophic levels, though the highest δ15NMDE values >+15‰ relate to sea bird guano fertilization of crops in arid South America42.
In contrast, the modern human range of trophic levels, determined by nitrogen isotope composition (see Methods) is much narrower, from <~1.5 (for vegans and vegetarians) to an average of ~1.75-2.5 for most generally omnivorous populations, aggregated by country (see Methods for calculation). This range is consistent with the global average isotopic human trophic level, as indicated by direct measures of diet, aggregated by country, of 2.21, similar to the trophic level in a natural food web of anchovies and pigs7. In part, the observed compression in the range of modern non-subsistence populations is the result of improvement in diet in developing countries, such that trophic levels in developing countries have increased, thereby converging somewhat with those of developed countries over time7. In part, the observed compression is the result of a general decrease in the trophic level of animal and fish resources extracted for consumption from natural food webs as a result of over-exploitation in recent decades43. However, the largest contribution to the apparent decrease in trophic level over the range observed in ancient populations, is likely due to the disconnection of the majority of the modern human population from complex natural food webs and their replacement by the simpler food webs44 and flatter food chains associated with industrial agriculture and farming45. This has effectively removed these other sources of trophic diversification from the human diet.
In the particular context of the natural nitrogen isotope cycle, modern industrial fertilizer produced from atmospheric nitrogen has a mean δ15N value of -0.2 ± 2‰, whereas natural soils and fertilizers, depending on environment and source have mean δ15N values that range up to +7.1‰46. Industrial fertilizer use has increased rapidly since the 1960s and global demand now exceeds 100 Tg per year47. This fertilizer is used in annual to sub-annual cycles of application and harvest, often in conjunction with irrigation in semi-arid regions once limited to rainfed crop production. Industrial farming has therefore effectively ‘short-circuited’ the suite of longer-term natural soil isotope fractionation processes leading to the higher soil δ15N values that are ultimately reflected in human diets based on natural food webs. Thus, for example, modern human δ15NMDE values from developed countries cover a similar range to the δ15NMDE values of enslaved Africans in the Caribbean in the 17th to 19th centuries, whose diets were dominantly plant-based48.
The equally dramatic compression in the range of modern δ13CMDE values in Figure 1 is a direct consequence of globalization. Thus, C4 staples (e.g. sugar and maize-derived products) and C3 staples (e.g. wheat and rice) are now cultivated well outside of their natural range using irrigation, and shipped across the world. The supermarkets that draw on these global supply chains now have a >50% share of food retailing in countries with a >US$10,000 per capita annual income49. While some regional differentiation between tropical and temperate countries remains in δ13CMDE values (Fig. 1), the modern range in values has reduced to around one-third of that observed across the ancient world.
Given the primacy of climate as the major driver of the broadest global trends in the stable isotope composition of primary production we identify three general ancient ‘environments’ at the global scale (Figure 1). The primary distinction is the separation of regions with an aridity index of >0.5 (sub-humid to humid) from <0.5 (arid to hyper-arid50. ‘Arid C3/C4’ environments span a wide range of temperatures and here include regions with a Mediterranean climate that experience significant seasonal water stress. Humid environments are separated into the warmer regions that potentially contain C4 biomass (Humid C3/C4) and colder regions that contain only C3 biomass (Cold C3), based on the modern distribution of C4 biomass28. Across all three categories some ancient populations also had a variable degree of access to marine and/or freshwater aquatic resources with a stable isotope composition often distinct from local terrestrial resources10,14,37.
The vast majority of ancient results from regions where C4 vegetation does not naturally occur plot along a broad diagonal from terrestrial δ13CMDE values of -25 to -28‰ and δ15NMDE values below +10‰ (e.g., Western Europe) towards a progressively more marine-influenced diet indicated by δ13CMDE values of >-20‰ and δ15NMDE values >+10‰ (e.g. Greenland and Alaska). At δ15N values <10‰, δ13CMDE values up to ~-23‰ can still indicate a purely C3 diet influenced by combinations of trophic enrichment as a result of meat consumption and the adoption of agricultural innovations such as manuring51,52 along with natural variations in discrimination by C3 plants, particularly between trees (forest) and C3 grasses (pasture) and associated with climate, soil type32,53 and land use54. The scatter of points to higher δ13CMDE values >~-23‰ with relatively low δ15NMDE values reflect a variable degree of consumption of introduced C4 crops such as millet in prehistory, particularly in Eastern Europe and the Caucasus as well as individuals migrating from locations with a C4 component in the diet in the last few centuries.
The ancient results for the Humid C3/C4 grouping scatter over a relatively narrower range of δ15NMDE values between 0 and +10‰, but span a wide range from purely C3 (southern Japan and eastern US), to almost exclusively C4 (Southern China and Mexico). The reliance on millet (a C4 crop) in China is evident in the concentration of analyses with δ13CMDE values above -15‰, and δ15NMDE values below +5‰. The data for the humid C3/C4 grouping as a whole implies that diet across this global range is within a maximum of 3.5 isotope-calculated trophic levels. This is consistent with modern subsistence diets from populations in the environments represented in this category from the Amazon (mixed plant and fish-based diets) to Africa (mixed plant and meat). The grouping also includes island populations in the Pacific Ocean indicating that it is not possible to differentiate a marine from a purely terrestrial diet in the absence of other evidence.
The arid C3/C4 grouping covers the entire δ13CMDE isotope dietary space from exclusively C3 plant-based (e.g. montane central USA) to largely C4-based (e.g. central Chile). In some populations in coastal Peru δ15NMDE values exceed +15‰ but these very high values have been attributed to the addition of seabird guano fertilizer to crops42. For coastal populations, the utilization of marine resources may explain the extension to higher δ15NMDE values compared to the humid C3/C4 grouping. For populations in more arid environments it is likely that the increase in plant δ15NMDE values generally observed in arid environments is a contributing factor to the comparatively high δ15NMDE values observed in many samples in this group. In addition, it may be that in particularly hyper-arid environments such as montane central Peru, where plant biomass is not an abundant resource, populations rely to a greater extent on higher-trophic-level animal resources for their diet. As with the humid C3/C4 grouping it is not possible to uniquely identify an aquatic component to diet across much of the arid C3/C4 range in the absence of other information.
The approach adopted here, and represented conceptually in Figure 2 moves from a local archaeological framing, where interest is primarily in the proportion of potential food items in the diet of ancient individuals in that area, to a global context where humans can be placed in an ecological framework as part of, but increasingly able to manipulate, complex natural food webs. A growing body of anthropological research also examines the human place in food webs worldwide. Humans have been shown, through their hyper-omnivory and prey switching ability, consumed a wider variety of organisms than any other taxon in their respective systems4,55. Modelled food webs that include humans indicate the aggregate trophic position of humans ranges from up to ~5 for offshore food webs in the Aleutian Islands4 to ~2.3 for modern Indigenous populations in the deserts of Western Australia55. These trophic positions were both determined using the Short-Weighted Trophic level calculation, which allows an estimate of feeding strength from binary interactions within modelled food webs56. The compilation of full ecologically realistic model trophic webs is laborious and therefore there are few available. However, the broad comparability of the Short-Weighted trophic level inferences drawn from the modelling approach, with the isotope approach presented here, is encouraging.
Thus, it becomes possible to conceive of the stable isotope composition of archaeological remains as an integrated signal of human utilization and manipulation of their local food web55,58. Re-casting the archaeological results into their modern diet equivalent composition then allows comparison with the much larger and more detailed datasets available for plants and fauna in modern ecosystems (Figure 2). Direct comparisons of spatially (or temporally) distinct archaeological isotope datasets can be relatively easily achieved for data collected from humid C3 dominated terrestrial environments (as in Figure 1 above) because there is limited scope for natural variation in the isotope baselines in these environments. The extension of this approach to other, and particularly arid, environments will require a finer-grained, spatially explicit (isoscape) understanding of regional variability in isotope baselines in relation to climate and soil variables than is presently available59.
Human populations in prehistory bolstered their resilience by being able to prey switch within complex natural food webs58. Comparison of the ancient and modern isotope results presented here suggest that modern human food webs have become dramatically compressed as a result of the ongoing expansion of industrial agriculture and pastoralism at the expense of natural ecosystems. In turn, this is resulting in a cascade of ‘rewiring’ to remaining natural terrestrial and marine food webs globally60 that can reduce complexity, and therefore the resilience, of global ecosystems in the face of accelerating environmental change43–45,61.