The Neolithic


Paleodental evidence suggests that after the divergence of early hominids from the hominoid ancestral line, a gradual increase in consumption of harder and/or more abrasive foods occurred - most likely nuts and seeds. This change probably increased intake of vegetable fat (mostly non-serum-cholesterol raising in nature) and would, perhaps, have facilitated overall access to food energy, but probably had little other nutritional effect. About 2.5 million years ago, however, there is evidence that animal foods began to occupy an increasingly prominent place in our ancestorís subsistence. Decreased molar size, less mandibular and cranial robusticity, and alterations in incisor shape all suggest greater emphasis on foods requiring less grinding and more tearing, such as meat.


An increasing proportion of meat in the diet would obviously have provided more animal protein, a factor perhaps related to the stature increase which appears to have accompanied the transition from Australopithecines through Homo habilis to H. erectus, (McHenry, 1992) but greater availability of animal fat was probably a more important dietary alteration. Even crude Oldowan stone tools would have allowed early humans access to brain and marrow from a broad range of animals obtained by scavenging or hunting - including some species larger than those from which chimpanzee hunters preferentially extract brain tissue and marrow fat. These and other carcass fats were probably prized by the early hominids as they are by recently-observed modern human hunter-gatherers. (Steffanson, 1960) More animal fat in the diet meant not only additional energy, but also a source of ready-formed long chain PUFAs, including AA, DTA(docosatetraenoic acid (DTA, C22:4, w-3), and DHA. These three fatty acids together make up over 90% of the long chain PUFA (i.e. the structurally significant and biochemically active fat) found in the brain gray matter of all mammalian species. (Sinclair, 1975)


A cardinal feature of human evolution has been development of increasing brain size: Homo sapiensí cranial capacity is thrice that of Australopithecus afarensis. A prime selective force driving this increase was almost certainly the complex nature of social interactions among early hominids. There is, however, no a priori reason to assume that such interactions, at first, differed much from those of chimpanzee and gorilla ancestors. Social complexity was thus a necessary, but insufficient, selective pressure acting to increase brain size. Another necessary factor was probably adequate dietary substrate to allow formation of brain tissue. (Crawford, 1992) The limiting raw materials, AA, DTA and DHA, could have been provided by animal tissues as hunting and/or scavenging activities assumed greater importance in human subsistence. (Eaton, 1998) Increasing complexity of interpersonal and social interactions together with availability of animal tissues - to provide the necessary structural lipid - constituted a unique psychonutritional nexus which may explain human brain expansion.


Dramatic nutritional, anatomic, and behavioral changes accompanied hunting and scavenging: division of labor by sexes, increased day range, reduced sexual dimorphism, decreased gut size, greater stature and increasing brain size. (Leonard & Robertson, 1994; Aiello & Wheeler, 1995) However, it is important to recognize that these changes were gradual. H. habilis evolved into H. erectus over the course of several hundred thousand years and this rate of cultural change, in all likelihood, allowed for concomitant genetic modification. The behavior, subsistence and biological characteristics of anatomically modern Late Paleolithic humans differed strikingly from those of H. habilis, but these changes developed over a 2.5 million year period. In contrast, subsequent comparably dramatic changes in both behavior and subsistence have occurred with far greater rapidity, almost certainly overmatching the capacity of genetic evolution to keep pace.


The Neolithic