Red meat and our evolution
The importance of dietary meat and cooking for the evolution of our brain size
Meat, particularly red meat and its fat, is getting bad press, and the health police continue to attack it. Increasingly, plantarian-style diets are encouraged. We don’t hear about ‘meat and veg’ anymore (in my day that was healthy eating), we hear about ‘fruit and veg’ instead. The health, environmental and ideological accusations being levelled at meat are misguided, and I hope to explain why in subsequent posts. This post examines dietary aspects of our evolutionary biology. We ignore our biology at our peril.
What drove our evolution?
Climate and geology, primarily. The period of interest for our evolution begins ~6 million years ago, and it coincides with the onset of a cooling trend in the Earth’s climate. Cooling accelerated about 4 million years ago, and, in addition, swings between warming and cooling (in roughly 40,000-year cycles) started to be superimposed on the decline in average planetary temperature. About 1 million years ago, these swings became greater and longer lasting (100,000 yrs). At the extremes of cooling, which began about 500,000 years ago, global temperature was several degrees cooler and ice-caps covered a third of the planet (as a result, sea levels were so low you could walk across the English Channel).
This climatic instability was related to cyclical changes in the Earth-Sun orbit. There are 3 orbital variances: (1) The elliptical orbit around the Sun (eccentricity) drifts between being more and less circular with a periodicity of about 100,000 years; (2) the tilt of the Earth’s axis in relation to the plane of its orbit (obliquity) changes with a period of about 40,000 years and; (3) the rotation (precession) of both the Earth’s axis and the plane of the Earth’s orbit change with a period of about 20,000 years. However, orbital fluctuations are not thought to have entirely driven climate change, they paced it — at any given time, climatic conditions were mostly determined by earthly factors (such as atmospheric carbon dioxide, or oceanic conditions) — meaning that the same orbital parameters could be associated with different climates. In other words, it’s complicated.
Furthermore, the effect of climate change on habitat was amplified by geology. Current thinking is that our lineage emerged in northeast Africa. This region experienced considerable geological forces between 10 and 5 million years ago, as colliding tectonic plates first caused an upthrust and then a collapse to create the East African Rift Valley (EARV) system (these geological forces continue, and it is expected that NE Africa will ultimately separate from continental Africa). Over the course of these 5 million years, conditions went from a flat terrain and rainforests to a region of mountains, plateaus and rift valleys. The complex topography of the EARV caused the region to be more susceptible to climate forcing. Habitat became highly variable in keeping with orbital-driven climate cycles. Reversibly, savannahs became marshlands that returned to savannahs, large deep lakes formed then dried up, forests expanded and contracted.
These are the factors that drove our evolutionary development. Evolution is forced most strongly by environmental stress, not by stability.
The figure below, from the Smithsonian Institute, uses an isotope of oxygen in ice cores to estimate average planetary temperature over the last 10 million years. Scientists also validate this against other means for estimating temperature. Time runs from right to left. Climate is relatively stable until about 6 million years ago, when it becomes less so and a cooling trend is apparent. There is steady cooling until around 2.5 million years ago, when there is an increase in temperature oscillation (with further cooling). These oscillations become extreme around 0.5 million years ago in the lead up to Homo sapiens. Other aspects of the figure will become clearer as the post progresses.
There is a common vision that we came down from the trees to populate spreading savannahs as bipeds. A sort of ‘Garden of Eden/Expulsion model’. However, it is not thought that this habitat-specific hypothesis is likely, but rather that we evolved by adapting to recurring habitat changes forced by climate and geology — the variability-selection hypothesis.
Some terminology
Taxonomy is a means of grouping, in hierarchical ranks (taxa), organisms with similar characteristics. The main taxa are (with our example in parentheses): Domain (eukaryota), Kingdom (animalia), Phylum (chordata), Class (mammalia), Order (primate), Family (hominidae or hominid), Tribe (hominini), Genus (homo) and Species (sapiens). For this post, Genus (plural Genera) and Species are the most useful taxa to refer to, and I will use the term hominin to refer to members of our Tribe. It is conventional to abbreviate the Genus after its first use — e.g. Homo sapiens becomes H. sapiens.
An overview
Taxonomically, humans did not evolve from an ape — we are apes. We are Great Apes (the common name for the hominid family), along with orang-utans, gorillas and chimpanzees. Gibbons are Lesser Apes, and there are many other primate families (going back ~65 million years). The Great Ape lineage appeared around 16 million years ago and included many species that came and went. Today there are only two surviving species of orang-utans, two of gorillas, two of chimpanzees and just the one of humans.
Our hominini tribe diverged from the chimpanzee tribe (panini) around 6 million years ago, having previously diverged from the orang-utans (pongini) and the gorillas (gorillini). No fossil remains of the last common ancestor of hominini and panini (often referred to as ‘the missing link’) have been unearthed. However, it is presumed to be a large-bodied predominantly arboreal fruit-eating ape that probably supplemented its diet with grubs and insects (as chimpanzees do today).
The hominin’s first 2 million years
The earliest fossil records we have for this period include the genera Sahelanthropus (first appearing ~6–7 million years ago, Chad), Orrorin (~6 million years ago, Kenya) and Ardipithecus (5.8–5.2 million years ago, Ethiopia). The most complete Ardipithecus fossil, nicknamed Ardi (species Ar. ramidus), was a female estimated to have lived between 4.5–4.3 million years ago. She was discovered (in 1994) in the Afar Triangle of northern Ethiopia, in what was a forest environment at the time she lived.
It is noteworthy that today, this region (the yellow ‘triangle’ in the previous EARV map) is isolated and inhospitable. The Dallol depression of the Afar Triangle, 130m below sea level, is a salt bed (kilometres thick), has an active volcano, is toxic from mineral deposits brought to the surface by volcanic hot springs, and experiences year-round world-record heat. The landscape is a badlands, add political instability and palaeo-anthropologists (as well as geologists) need determination to venture there. Such has become the ‘cradle’ of our evolution.
The completeness of Ardi gives insight into early hominin adaptations (dietary fallback foods and tentative bipedalism), driven by a climate-related decline in preferred foods such as fruit.
As preferred foods became scarcer, early hominins had to make greater use of fallback foods such as leaves and stems, and search more widely for them. These fallback foods made the difference between life and death, so natural selection favoured adaptations that helped early hominins find them and get nutrition from them. Thus, natural selection favoured broad flat teeth and powerful jaw muscles to grind and chew pretty much anything resembling food that Ardi could find. In a related adaptation, we see the first signs of a bipedal gait. It is not certain that this was, at first, a primary adaptation for roaming over greater distances for food (preferred and secondary), and the environment in which Ardi was found does not suggest bipedalism evolved in response to increasingly open grasslands. It may have included the advantage of standing on two legs in trees to reach hard to get at fruits (as chimpanzees do today), or to carry these fruits back to a family unit.
Quite a few adaptations have been necessary for our bipedalism to evolve. Over time, it has been necessary to: alter the shape and function of the pelvis (to stabilise the body over each leg when walking, making it unnecessary to lurch from side to side like chimpanzees); develop an S-shaped spine (to move the centre of gravity posteriorly over the feet); alter the neck (to stabilise the head); alter the the foot substantially (including a stiffened arch that enabled propulsion with the forefoot); develop the gluteus muscles (which remain poorly-developed in chimanzees), and; adapt the knees to function differently. The advantage of bipedalism is that it is more energy-efficient than moving around on all-fours (even though it is slower than the galloping gaits of quadrupedal apes). It took a few million years, but Ardi had made a hesitant start.
Nevertheless, Ardi retained features useful for climbing trees, and probably slept arboreally. Her manner of walking was closer to that of chimpanzees than to modern humans, but different to both. Ardi’s diet was still plant-centric and she would have eaten fruit as much as she could, followed by secondary plant materials as needed, while probably supplementing her diet with protein sources such as insects, grubs and small lizards. However, there was no single Ardipithecus diet — these hominins probably ate whatever they could find in the increasingly challenging habitat that they occupied.
Australopithecus (~4 million years ago)
The adaptations apparent in Ardi accelerated about 4 million years ago and gave rise to a new and important genus — Australopithecus, divided into the sub-genera gracile (appearing 4.2 million years ago) and robustus (2.7 million years ago). In some taxa schemes, robustus is given its own genus — Paranthropus.
Just as Ardipithecus had Ardi, Australopithecus also has a famous fossil — Lucy (Au. afarensis, a gracile) that lived ~3.2 million years ago in a marshy environment. As the name of her species suggests, she was also discovered in the Afar Triangle. Remarkably, although they were separated in time by about 1.2 million years, Ardi and Lucy were discovered just 75 km from each other. Lucy was unearthed in 1974 and named after the Beatles hit of the day — ‘Lucy in the Sky with Diamonds’, which the discovery team played all night in celebration. Since then, many fossilised remains of this species, and of at least six other species of Australopithecus, have been discovered in this region and elsewhere in Africa.
The adaptations apparent in Lucy seem to have resigned her to dietary diversity rather than using it as a fallback position, partly because fruit was becoming ever more difficult to find. Thus, there were further developments in the teeth and jaws for chewing (molars became bigger again, and flatter for grinding while jaws further strengthened), and there are indications of a more habitual terrestrial bipedalism. The strongest evidence for upright walking over ground comes from the Laetoli footprints (Tanzania), left by a group of individuals that walked across a wet ash plain around 3.6 million years ago.
An important food-related development for this genus was digging. Their hand shape was conducive to grasping sticks, which would have required little skill to select or modify for more efficient digging (even today’s chimpanzees use sticks for poking insects out of logs or termite mounds). This gave Lucy and her kind access to a new food source — tubers, bulbs and deeper roots — together known as underground storage organs (USOs). Although they required time and effort to dig for, and knowledge of the plants that had them, USOs provided several survival advantages — they were generally high in starch and therefore a good energy source, they stored water even in dry times, they could be less fibrous than many of the fruits or fallback foods available at the time, and they tended to be available all year-round. Overall though, Lucy retained much in common with Ardi — she may have slept in trees at night, retained a large gastrointestinal system for digesting plant matter, and had limited cranial capacity for the brain because of the need for large oral cavities and teeth for grinding food.
Australopithecus may have represented a natural limit to the hominin line, because there is only so much time you can spend every day chewing tough fibrous plant food of minimal nutritional value. Indeed, natural selection went further down this path with the later robustus, developing even bigger teeth and stronger jaws, however, it was an evolutionary dead-end. The hominins were at a watershed.
Homo
About 2.5 million years ago, the climate had cooled enough to herald the beginning of a series of Ice Ages, but more important, the climate became even more unstable, with warm interglacial periods. As natural selection accelerates in the presence of habitat forcing, a new genus started to emerge in response to this challenge — Homo. It is most likely that this genus was an evolutionary development of the gracile Australopithecus.
After the grand-sounding tongue-twisters Ardipithecus and Australopithecus, using the name Homo for our genus seems uncharacteristically demure. For clarification, ‘Homo’, in the present context, comes from the Latin, and means ‘human’, whereas the prefix ‘homo’ (as with homogeneous or homosexual) comes from the Greek, and means ‘same’.
Nine species of Homo have been identified, however, the most significant developments were occasioned by H. habilis, H. erectus, H. heidelbergensis, H. neanderthalensis and, H. sapiens. The developments can be further summarised as tool manufacture, food processing, hunting, cooking and culture. Each species contributed to one or more of these factors. The overarching driver was an increase in energy availability in the diet, particularly from animal sources — the hominins were getting nowhere masticating plants. Homo, from its inception, was a carnivorous omnivore. The organ that benefited most from this new energy availability was the brain.
i) Homo habilis
The first Homo species of significance was H. habilis (meaning handyman), and appeared around 2.3 million years ago (persisting until around 1.4 million years ago). It mostly had an Australopithecus body, but its hand was different and becoming more recognisably modern. The habilis hand’s most important feature was that it combined power and precision — the thumb was getting stronger and the fingers shortening. This enabled technological advances in the manufacture of stone tools, and signalled the beginning of the ‘Stone Age’ (or Lower Palaeolithic).
Using stones as implements was nothing new, and even today chimpanzees use stones to crack open nuts. However, habilis was more advanced, deliberately fashioning flints for cutting. These could be used to hack meat from a carcass, and habilis marked an increase in meat consumption, presumably from scavenging. As well, heavier stone implements could break open bones or skulls for marrow or the brain. Broad flat stones were used for pounding and tenderising both meat and fibrous plant materials. These food processing techniques decreased the need for laborious chewing, in turn selecting for smaller teeth and jaws, while the increase in nutrient availability from animal sources selected for a smaller gut. The hominins had headed in a new direction.
ii) Homo erectus
The species H. erectus entered the fossil record ~1.9 million years ago. This was a walking/running humanoid. Erectus had long legs, short arms, narrow waist, small gastro-intestinal system, smaller teeth, larger brain and further changes to feet, spine and neck to walk and run more efficiently. Erectus is our first recognisably human-like ancestor.
While it is likely that habilis scavenged, there is evidence that, as well, erectus was soon hunting live animals such as deer. Remarkably, he was armed only with clubs or wooden spears that could neither be thrown accurately nor penetrate a hide from a distance. The success of these hunts depended on persistence. Deer galloped faster than H. erectus could run, however deer (like most quadrupeds) depend on panting to cool down their bodies (they cannot sweat through their hides). It is not anatomically possible to pant and gallop at the same time — the lungs are compressed and expanded in time with forelimb-hindlimb movement during the gallop and so can’t operate at panting rates (there are other issues as well). So it is necessary for the deer to stop in order to pant, or risk overheating. Erectus would persist at his own pace, tracking the deer until he caught up, forcing the deer to gallop again before it had a chance to cool its core temperature. Erectus placed further pressure on its prey by hunting in the heat of the African day, taking advantage of his capacity to cool down by sweating from all over his skin (like us). Eventually, persistence wins and the deer collapses from exhaustion and heat. In that state, it is possible for erectus to approach the animal for the final kill with his basic tools. Persistence hunting has been documented even in modern tribal societies of the early 20th century. It has been suggested that the reasoning needed for animal tracking (deductive and inductive) was an evolutionary precursor to scientific thinking.
This strategy (and presumably others such as herding animals over cliffs) gave erectus a choice supply of animal nutrients, rather than relying on the unpredictable availability of a scavenged carcass. It also gave erectus access to the ‘best’ parts of the animal that would otherwise have already been consumed by the original predator and other intervening scavengers. Now, erectus could drink the blood, eat tender and highly nutritious internal organs like kidney and liver and heart, and preferentially select high fat (and calorie-dense) portions of meat. Erectus was still eating raw food, but a fresh kill would be safer to eat than a scavenged carcass, and he still had stone food-processing technology to improve nutrient yield. All of this gave erectus access to nutrient-dense food, superior to plant matter, that could empower further natural selection, particularly the enlargement of brain size. Over the course of its presence in the hominin line, H. erectus doubled its brain size. Albeit at the excruciatingly slow rate of about 1ml every 10,000 years.
Erectus was successful with these strategies, the population increased, and the species was the first hominin to disperse out of Africa. Quite quickly in fact. As soon as 1.8 million years ago it was in Eastern Europe, and by 1.6 million years in Asia and 1.2 million years in southern Europe. It has been estimated that an annual population increase of just 0.5% would have been sufficient to drive that degree of dispersal over those time-frames. Erectus persisted as a species until as recently as a few hundred thousand years ago.
iii) Archaic Humans and Homo heidelbergensis
The term archaic human covers a number of species that elaborated on erectus. This phase of Homo evolution is relatively recent, around 600,000 years ago. One of the most significant advances of this era, appearing around 500,000 years ago, was the sharp stone spear point. By now, technical advances meant that very much sharper, and even triangular, stone flints could be fashioned, bound to wooden shafts and thrown at prey from a distance. The thrown flints were sharp enough to penetrate hides, and the embedded flint would lacerate the animal as it ran, causing blood loss, muscle damage, weakness and collapse. Hunting got that bit more effective. Correspondingly, anatomical changes were selected for that favoured accurate throwing (as opposed to the hurling or tossing of chimpanzees), and there were further adaptations to bipedal running. The brain continued to enlarge (and the gut shrink) in parallel with increasing nutrient availability in the diet.
A second momentous development for archaic humans was the use of fire — the ultimate food processing technique. While there is sporadic evidence that erectus may have taken advantage of naturally-occurring fire (as early as 800,000 years ago), by-and-large their diet (and that of all earlier species) was a raw one. However, by ~400,000 years ago, there is good evidence that archaic humans were regularly cooking their food. This marked another stepwise increase in nutrient availability, because cooking is a powerful form of pre-digestion. It meant that the same food could yield higher calories and nutrients. There is a modern misconception that raw food is more nutritious than cooked food, however, it is not the density of nutrients that matters but rather their bio-availability — cooking breaks down cell walls and makes nutrients more available for digestion. It also meant that food was safer to eat. It was a significant advance.
The species of archaic human most relevant to us was H. heidelbergensis, so named because the first fossil of this species was uncovered (in 1907) near Heidelberg, Germany. However, the species didn’t originate there, it evolved from erectus in Africa, around 500,000 years ago. Like erectus, it too dispersed out of Africa into Europe and Asia, competing with erectus (which in turn may have evolved into other species, such as the denisovians in Central Asia).
The significance of H. heidelbergensis is that this species, in Europe, evolved into H. neanderthalensis (the neanderthals), while those that remained in Africa evolved into us — H. sapiens. Both of these developments began independently at around the same time, ~200,000 years ago.
iv) Homo neanderthalensis
The neanderthals (the ‘th’ should be pronounced ’t’) are named after the Neander valley in Germany (‘thal’ is German for someone who lives in a valley), where its fossilised remains were first identified in the 1800s.
The neanderthals have a certain popular reputation: Dull-witted, brutish, short-lived and failed pre-sapiens — the archetypal Stone Agers. All of that is wrong. In particular, we (sapiens) did not evolve from the neanderthals, we evolved in parallel with them on different continents (Africa and Europe). Furthermore, the neanderthals were as advanced as us at the time — they lived at low population densities as hunters and gatherers, were toolmakers, hunted large animals using spears, made fires and cooked food, lived socially in tribal groups and had large brains (in fact the neanderthal brain was larger than the sapiens). There is reason to think that neanderthals could be alive today if it were not for H. sapiens.
There was one interesting distinction between us and the neanderthals though. We left symbolic cave art everywhere we went. There is no cave art that has ever been attributable to neanderthals. So, we may have been behaving the same, but we were thinking different — we were running different software.
v) Homo sapiens
As recently as ~50,000 years ago (Upper Palaeolithic), something dramatic (and unexplained) occurred amongst the African sapiens — there was a surge in technology. Weapons became more lethal and effective, adding to the sources for nutrient-dense animal food in the diet. Specialised stone tools appeared that were sharper and thinner and better suited to killing birds and small mammals, nets were manufactured for trapping, the bow and arrow appeared, and fishhooks and harpoons diversified the diet into seafood. As well, bone tools were developed that could be used to ‘sew’ hides together, camps became more complex and semi-permanent.
As sapiens became better hunters, their population increased modestly, but sufficiently to disperse out of Africa, possibly in more than one wave (only the last of which was sustained). The exact timing and model of this dispersal is subject to alternative theories. There is also evidence supporting a partial dispersal back into Africa.
These were critical times — climatic conditions were posing major challenges and sapiens needed their wits to survive (more so the neanderthals in a harsher climate). It is estimated that the sapiens population that finally established itself in the present-day Middle East, before dispersing to Europe and Asia, may have been just a few thousand breeding pairs (and not many more remained in Africa). This explains our present-day genetic homogeneity, despite differences in phenotype (appearance). As a species, we are ‘homo’ in both the Latin and the Greek senses of the term.
Sapiens reached Europe by 40,000 years ago, and in just 10,000 years they had driven the neanderthals to extinction (the neanderthals withdrew to a small region in southern Spain, from which they could retreat no further and died out). H. sapiens had the same deleterious effect on all other species of Homo across all inhabited continents. There is much said about what makes our species different. Customarily, this includes our culture, reasoning and many more traits that we feel superior about. However, what rarely gets mentioned is our propensity for genocide (inter- and intra-species), unique in the animal kingdom on the scale and ruthlessness that we practise it.
Before exterminating the neanderthals, some sapiens interbred with them. This may have given early European sapiens access to genes suited to their new environment. It also means that some of your genes, if you are of European descent, are neanderthal. The neanderthal genome has been fully sequenced (from fossil remains), and about 2% of your genes are neanderthal. A kind of atonement, perhaps.
Interbreeding has led some to propose that we, and the neanderthals, are sub-species of sapiens rather than separate (albeit closely-related) species, since different species are not meant to be able to interbreed. With this thinking, the neanderthal is classified as Homo sapiens neanderthalensis, while we are H. sapiens sapiens.
Where does this get us?
Our tribe, hominini, have been on a 6 million year journey driven by habitats made variable by a capricious climate and amplified by geological circumstances. This epic was built around essentially two developments: making do with secondary plant foods and being better at seeking them out (the Ardi-Lucy phase, enduring 3.5 million years), and; getting better at carnivory and growing larger brains on the energy surplus (the Homo phase, 2.5 million years and counting, tentatively though). Further, this second phase was facilitated by food processing with stone tools (H. habilis), hunting rather than scavenging (H. erectus), cooking (H. heidelbergensis) and further technological and cultural breakthroughs (H. sapiens).
It was all possible because of energy and nutrients. Animal sources are greater in both of these than plant sources, in particular, animal fat was plentiful and had over twice the energy per gram than plant carbohydrates (or pure protein), and animal internal organs are amongst the most nutrient-dense foods that our ancestors had access to (eggs and seafood are others).
Energy is important because a brain needs a lot of it. In our present form, the brain weighs about 2% of a healthy-weight body but consumes ~25% of its available energy. So, we could only have expanded our brain size with plentiful dietary energy. There were other consequences. Our intestines became smaller because our food was easier to digest, so carrying around a big vat for fermenting plant material was no longer required. The gut is also energy-demanding, so a smaller gut can make more energy available to the brain (a gorilla’s gut consumes ~60% of its available energy). Females began to wean their infants earlier because there was food for the infant. This meant the female was able to reproduce more frequently, and populations could expand (chimpanzees breastfeed for about twice as many years as we do). Plentiful energy also meant we could take more time to develop our brains, during childhood and adolescence, and the social experiences acquired during this time were probably crucial for the brain’s functional maturity. We also lived longer — hominins living past their reproductive age could still be fed, while usefully contributing to the upbringing of their offspring’s children. There is an idea that our ancestors’ lives were constantly threatened by starvation and deprivation, however, it is more likely that much of the time they did rather well for themselves, accidental death notwithstanding.
So finally, all of this can be summarised thus: carnivory probably made us.
Which is the point of this post, we now seem to be worried about the very thing that made us (and more particularly our brains) possible.
Further, an evolutionary argument for a plant-centric diet would mean ignoring the evolution of our genus entirely — not just our species, but our genus. We would need to look back at least 3 million years to the now extinct australopithecus, or even earlier to ardipithecus. Even so, you can be confident that they supplemented with foods from the animal kingdom if they could.
I haven’t delved into more recent developments, such as the agricultural revolution that started about 12,000 years ago. This post is already too long. However, it is clear that this plant-centric revolution resulted in a significant decline in public health and a shortening in lifespan. Diseases, such as heart disease, that we associate with modern living, became apparent. The Egyptians are the best-documented example — their diet was a good match for current guidelines. Population health declined, but birth-rates increased. These societies (and others) survived and expanded with this tradeoff.
Coda
The study of evolution can have only so much to say about diet. The specifics of what our ancestors ate cannot inform our modern eating practice — at the very least the food we have today is vastly different (and has itself evolved, or been re-engineered). However, it is telling us that a plant-based diet is not likely to be a healthy diet over the longer-term, because to eat that way is in denial of our evolutionary biology.
An afterthought — The chimpanzee
They are our closest living Great Ape relatives. We share a remarkable 98–99% of our DNA with the chimpanzee, so I don’t want to leave this post without a brief look at their diet and behaviour too.
The modern chimpanzee survives as two species, the common (or robust) chimpanzee (Pan troglodyte) and the rarer pygmy chimpanzee (Pan bonobo), both of which evolved from our last common ancestor, just as we have.
Chimpanzees are mostly arboreal, but they can be terrestrial during the day where they move around on all-fours although they can stand and waddle short distances. They feed mostly on fruit, supplemented with insects and grubs, and occasionally eggs. In times of hardship, they will eat leaves, stems and other secondary parts of plants. But, they are also hunters. While they will eat birds and small mammals, their preferred prey is an old-world monkey, the red colobus, that they share habitat with.
Chimpanzees pursue their prey as a loud, screeching mob, each with different hunting roles (ambushers, blockers, chasers etc). The red colobus will put up a fight or, being smaller, seek refuge on thin upper-canopy branches that can’t bear a chimpanzees weight. Alternately, chimpanzees might ambush a female and snatch her infant (they have been documented doing this with unsuspecting human mothers too, partly eating the infant or child). They also practice infanticide with their own young. While chimpanzees will get some nutritional benefit from meat and offal, it seems that the main function of this behaviour is social cohesion (the prey is shared in a hierarchal way but often not fully eaten — it’s not so much about the food).
As disturbing as this behaviour may seem to us, it’s nothing like genocide. Still, red colobus numbers are under threat in some parts of Africa — the chimpanzee is, after all, our closest living relative.
Climate
Can we learn anything from the climatic variability during our evolution, that can inform our current situation? I think not, other than that the planet is capable of extremes. Past climate variability has happened over time-scales of 20–100 thousand years. Our current climate forcing has occurred over just a couple of hundred years. Therein lies the concern. The plot below shows greenhouse gas concentrations over just the last 2,000 years. Try and spot the industrial revolution and its aftermath.
