African hominin stable isotopic data do not necessarily indicate grass consumption

A series of three papers published in the early PNAS edition of June 3, 2013 (1) evaluate the diet of numerous species of fossilized hominins who lived in Africa from 4.1 to 1.4 million y ago.

These three studies (1) have been interpreted by many in the popular press to suggest that these early hominins were regular consumers of grass and grass seeds (cereal grains), which is an incorrect assumption.

The measurement of ¹³C and ¹²C was made from samples of dental enamel in these extinct hominins, and δ¹³C was calculated relative to a standard value (2). Indeed, δ¹³C values in enamel ultimately indicate the isotopic dietary carbon source, from plants using either the C₃ (trees, shrubs, herbs, and bushes) or C₄ (grasses and sedges) photosynthetic pathways.

A number of fundamental limitations exist with δ¹³C analysis to evaluate diet, because it cannot determine the exact species of either C₃ or C₄ plants that were consumed. Furthermore, δ¹³C values cannot distinguish if the C₃ or C₄ signatures originated from the direct consumption of plants or from the indirect consumption of animals that consumed these plants.

The point in time (∼3.5 Mya) at which the C₄ signature begins to increase occurs simultaneously with the earliest known use (before 3.39 Mya) of stone tools to cut flesh from animal carcasses and to extract marrow from their bones (3). In addition, the gathering of small animals, invertebrates, and fish likely contributed to early hominin diet. Hence, by triangulating this archaeological information with stable carbon isotope data, it is virtually certain that δ¹³C values present in hominin enamel were enriched partially or perhaps mainly from increasing consumption of animals that ate C₄ plants.

Other lines of evidence indicate that early African hominins were increasingly consuming more animal foods during the same time interval (3.5 Mya to 1.5 Mya) that δ¹³C had become enriched. The “expensive tissue hypothesis” suggests a reduction in hominin gut size and metabolic activity and a concurrent increase in brain size starting ∼2.5 Mya that was resultant from increased animal food consumption with higher dietary quality (4), although this hypothesis has been recently disputed. Furthermore, grass leaves and seeds are devoid of arachidonic acid (20:4n6) and docosahexanoic acid (22:6n3), which are necessary structural fatty acids required for the synthesis of brain and neural tissues (5). The conversion of linoleic acid (18:2n6) and α-linolenic acid (18:3n3), which are found in plant foods, to arachidonic acid and docosahexanoic acid, respectively, are inefficient pathways with low product to substrate ratios. Likely candidate animal foods, which simultaneously increased the dietary quality and provided arachidonic acid and docosahexanoic acid, were scavenged de-fleshed long bones (marrow), skulls (brains), and aquatic animals. Other nutrients essential to present day Homo sapiens (and presumably for hominins) are vitamin B12 and iodine, which are scarce or nonexistent in plant foods, but abundant in animal flesh and organs. These foods, along with meats from grazing animals, likely represent the dominant dietary source for the increasing C₄ signature in our African ancestors.