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  • From the Ape’s Dilemma to the Weanling’s Dilemma: Early Weaning and its Evolutionary Context – G.E. Kennedy - 2004


    Although humans have a longer period of infant dependency than other hominoids, human infants, in natural fertility societies, are weaned far earlier than any of the great apes: chimps and orangutans wean, on average, at about 5 and 7.7 years, respectively, while humans wean, on average, at about 2.5 years. Assuming that living great apes demonstrate the ancestral weaning pattern, modern humans display a derived pattern that requires explanation, particularly since earlier weaning may result in significant hazards for a child. Clearly, if selection had favored the survival of the child, humans would wean later like other hominoids; selection, then, favored some trait other than the child’s survival. It is argued here that our unique pattern of prolonged, early brain growth—the neurological basis for human intellectual ability—cannot be sustained much beyond one year by a human mother’s milk alone, and thus early weaning, when accompanied by supplementation with more nutritious adult foods, is vital to the ontogeny of our larger brain, despite the associated dangers. Therefore, the child’s intellectual development, rather than its survival, is the primary focus of selection. Consumption of more nutritious foods—derived from animal protein—increased by ca. 2.6 myr ago when a group of early hominins displayed two important behavioral shifts relative to ancestral forms: the recognition that a carcass represented a new and valuable food source—potentially larger than the usual hunted prey—and the use of stone tools to improve access to that food source. The shift in the hominin “prey image” to the carcass and the use of tools for butchery increased the amount of protein and calories available, irrespective of the local landscape. However, this shift brought hominins into competition with carnivores, increasing mortality among young adults and necessitating a number of social responses, such as alloparenting. The increased acquisition of meat ca. 2.6 Ma had significant effects on the later course of human evolution and may have initiated the origin of the genus Homo.

  • Impact of Carnivory on Human Development and Evolution Revealed by a New Unifying Model of Weaning in Mammals – Elia Psouni , Axel Janke, Martin Garwicz Published: April 18, 2012


    The large human brain, long life span and high fertility are key elements of human evolutionary success and are often thought to have evolved in interplay with tool use, carnivory and hunting. However, the specific impact of carnivory on human evolution, life history and development remains controversial. In the study it is shown in quantitative terms that dietary profile is a key factor influencing time to weaning across a wide taxonomic range of mammals, including humans. In a model encompassing a total of 67 species and genera from 12 mammalian orders, adult brain mass and two dichotomous variables reflecting species differences regarding limb biomechanics and dietary profile, accounted for 75.5%, 10.3% and 3.4% of variance in time to weaning, respectively, together capturing 89.2% of total variance. Crucially, carnivory predicted the time point of early weaning in humans with remarkable precision, yielding a prediction error of less than 5% with a sample of forty-six human natural fertility societies as reference. Hence, carnivory appears to provide both a necessary and sufficient explanation as to why humans wean so much earlier than the great apes. While early weaning is regarded as essentially differentiating the genus Homo from the great apes, its timing seems to be determined by the same limited set of factors in humans as in mammals in general, despite some 90 million years of evolution. The analysis emphasizes the high degree of similarity of relative time scales in mammalian development and life history across 67 genera from 12 mammalian orders and shows that the impact of carnivory on time to weaning in humans is quantifiable, and critical. Since early weaning yields shorter interbirth intervals and higher rates of reproduction, with profound effects on population dynamics, the findings highlight the emergence of carnivory as a process fundamentally determining human evolution.

    All the meat-eaters, including ferrets, killer whales, and humans, reached that point of brain development earlier than herbivores or omnivores, the researchers found. They classified humans as carnivores based on the percentage of meat in the typical human diet and despite the moderate meat consumption of Homo sapiens, humans fit the prediction of time to weaning based on fully specialized carnivores.

Meat and Nicotinamide: A Causal Role in Human Evolution, History, and Demographics - 2017 Adrian C Williams and Lisa J Hill

Hunting for meat was a critical step in all animal and human evolution. A key brain-trophic element in meat is vitamin B3 / nicotinamide. The supply of meat and nicotinamide steadily increased from the Cambrian origin of animal predators ratcheting ever larger brains. This culminated in the 3-million-year evolution of Homo sapiens and our overall demographic success. We view human evolution, recent history, and agricultural and demographic transitions in the light of meat and nicotinamide intake. A biochemical and immunological switch is highlighted that affects fertility in the ‘de novo’ tryptophan-to-kynurenine-nicotinamide ‘immune tolerance’ pathway. Longevity relates to nicotinamide adenine dinucleotide consumer pathways. High meat intake correlates with moderate fertility, high intelligence, good health, and longevity with consequent population stability, whereas low meat/high cereal intake (short of starvation) correlates with high fertility, disease, and population booms and busts. Too high a meat intake and fertility falls below replacement levels. Reducing variances in meat consumption might help stabilise population growth and improve human capital.

Meat, NAD, and Human Evolution

Archaeological and palaeo-ontological evidence indicate that hominins increased meat consumption and developed the necessary fabricated stone tools while their brains and their bodies evolved for a novel foraging niche and hunting range, at least 3 million years ago. This ‘cradle of mankind’ was centred around the Rift Valley in East Africa where the variable climate and savannah conditions, with reductions in forests and arboreal living for apes, may have required clever and novel foraging in an area where overall prey availability but also predator dangers were high44–50 (Figure 2). Tools helped hunting and butchery and reduced time and effort spent chewing as did cooking later.51 Another crucial step may have been the evolution of a cooperative social unit with divisions of labour, big enough to ensure against the risks involved in hunting large game and the right size to succeed as an ambush hunter – with the requisite prosocial and altruistic skills to also share the spoil across sexes and ages.52 The ambitious transition from prey to predator hunting the then extensive radiation of megaherbivores so big that they are normally considered immune to carnivores, needed advanced individual and social cognition as humans do not have the usual physical attributes of a top predator.53–59 Adult human requirements to run such big brains are impressive enough, but during development, they are extraordinarily high with 80% to 90% of basal metabolic rate necessary in neonates – this is probably not possible after weaning without the use of animal-derived foods.51,60,61

The Evolutionary Quirk That Made Vitamin B12 Part of Our Diet

It turns out to be a case of bad plumbing. The bacteria that are nice enough to make this nutrient for us live in our large intestine, but we are only capable of absorbing it in our small intestine. Because the small intestine comes before the large intestine in the flow of gastrointestinal traffic, we end up sending the B12 that our gut bacteria produce right to the toilet, rather than absorbing it. What a waste!

Exactly how this glitch in our gastrointestinal functioning came about is largely a mystery. Most primates are herbivorous and indeed all of our fellow apes subsist on a fully or mostly plant-based diet. It is therefore likely that we descend from a long line of vegetarians. During the millions of years our ancestors thrived on plants, they surely were able to capture the vitamin B12 that was being made by bacteria in their guts, or else they wouldn’t have survived. Once our forebears began scavenging meat and bone marrow, they found themselves with a steady supply of dietary vitamin B12, which then grew in abundance when we began to hunt. It must have been during this meat-eating stage in our evolution that we began to absorb B12 in the small intestine instead of the large one. We are now stuck with this odd arrangement, making humans, at least in this very narrow sense, obligate carnivores.


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