However, the story changed dramatically during the 1980s when it became possible to look at the genetic (as opposed to purely anatomical) similarities between different species. It soon became apparent that humans are in fact genetically more closely related to the chimpanzees than either of us is to any of the other great apes, though the gorillas come a close second. It is the orang utan – isolated in Asia some 16 million years ago – that is the odd one out. The several species of living African apes (humans, gorillas and chimpanzees) form a single lineage that didn’t split up until a much more modest 6–8 million years ago (Fig. 1.1). We belong not to a separate subfamily within the great apes family, but to the subfamily of African great apes. (Location 53)
The story so far
around 8 million years ago, what was to become the gorilla lineage split off. Some 2 million years later, the lineage that eventually gave rise to modern humans diverged from the human–chimpanzee ancestor (usually known as the Last Common Ancestor, or LCA) and set off on its own evolutionary trajectory. Much later still, around 2 million years ago – at about the same time as the genus Homo made its first appearance in eastern Africa – the chimpanzee lineage split into two, giving rise to the common chimpanzee and the bonobo (or pygmy chimpanzee). Conventionally, taxonomists now refer to the great ape family (including humans) as hominids, while all members of the lineage leading to modern humans that arose after the split with the LCA are referred to as hominins. (Location 70)
Although apes sometimes travel on the ground, they are all, by nature, arboreal species, adapted to shinning up giant forest trees and clambering (or, occasionally, swinging) around in the branches high above the forest floor. What has come to define our lineage – bipedalism – was adopted early on after we parted company with the chimpanzees, presumably in order to facilitate travel on the ground in more open habitats where large forest trees were less common. (Location 85)
specimens that date close to the split between apes and hominins are likely to be ambiguous, and so will be difficult to place with any certainty. (Location 96)
from around 4. 5 million years ago, the number of fossil finds increases dramatically, reflecting a period when the hominin lineage repeatedly split into ever greater numbers of new species. The australopithecines proper had arrived. At times, there may have been as many as half a dozen australopithecine species alive at the same time, albeit mostly in different parts of Africa (Location 103)
The australopithecines did not differ from the modern chimpanzees in terms of brain size. Like them, they were probably frugivores (fruit-eaters) who may have taken a little meat when they could get it. It is possible that they developed stone tools during the later stages (mostly associated with Homo habilis (‘handy man’), now considered to be a late transitional australopith), but at best these tools were quite primitive – much like the hammer stones used by chimpanzees today in West Africa. (Location 108)
The next million and a half years from about 1.8 million years ago are dominated by a single species, Homo erectus, perhaps the longest lived of all the hominin species. Strictly speaking, it is what biologists call a chronospecies, a species that changes through time – not too surprising given its remarkable longevity. It has an earlier phase (Homo ergaster and allies) more or less confined to Africa and a later, larger-brained phase (Homo erectus in the strict sense) mainly confined to Eurasia. This phase of hominin evolution marks the first expansion out of Africa and into Eurasia (perhaps around 1.5 million years ago, or even earlier), and the appearance of the first worked tools (the Acheulian handaxes, named after the first ones found in 1859 at Saint-Acheul in northern France). (Location 112)
Homo Erectus fue la especie homínina más longeva y la primera que salió de África.
homininos evolución homo_erectus
Then, sometime around 500,000 years ago, a new hominin species emerges out of the African ergaster/erectus stock, eventually taking shape as the first archaic humans, Homo heidelbergensis. They mark the beginning of a dramatic explosion in brain size and in the diversity of the material culture. (Location 120)
Neanderthals developed a particular body form that was especially well adapted to high-latitude habitats, and in particular the cold climates that began to engulf Europe and northern Asia as the Ice Age gathered momentum. Their short-limbed, rather heavy build is not unlike that of contemporary arctic specialists like the Inuit (or Eskimo) – designed to minimize heat loss from the extremities. (Location 128)
around 200,000 years ago, populations of archaic humans further south in Africa began to undergo another transformation, giving rise to our own species, anatomically modern humans (AMH) or, to give us our proper scientific name, Homo sapiens. (Location 137)
Using mitochondrial DNA (mtDNA),2 the genetic evidence suggests that the origins of AMH lay in a relatively small population of about 5,000 breeding females who lived around 200,000 years ago. This doesn’t mean that the entire population at the time consisted of only 5,000 females, but rather that only these 5,000 females have contributed to the genetic make-up of all humans alive today. (Location 145)
our species seems to have spread very quickly through Africa, quite rapidly replacing the archaic human populations. How and why they replaced archaics so quickly is rather a mystery, especially given the fact that archaics had successfully occupied Africa (and Europe) for at least 300,000 years by the time modern humans appeared. (Location 151)
Then, around 100,000 years ago, one lineage of modern humans in northeastern Africa began to undergo rapid demographic expansion, and by 70,000 years ago had bridged out of Africa across the Red Sea to colonize the southern coastline of Asia, eventually reaching Australia by 40,000 years ago at latest. (Location 153)
Anatomically modern humans mark an important transition in our story because with them comes culture in a way that had never happened before. The period from about 50,000 years ago marks a veritable sea change in the quality and quantity of weapons, tools, jewellery and artwork of a kind and quality never seen before, not to mention tents, lamps and a host of more substantial gear, including boats. (Location 159)
The Neanderthals remained the undisputed masters of Europe from around 250,000 years ago until 40,000 years ago, when modern humans appeared, like all subsequent historical invaders, on Europe’s eastern fringes out of the Russian steppes, perhaps reaching western Europe only around 32,000 years ago. The two species co-existed until about 28,000 years ago, when the last of the Neanderthal populations died out in the Iberian peninsula. (Location 171)
Why we are not just great apes
The substantive difference lies in our cognition, and what we can do inside our minds. It is this that has given us Culture with a capital ‘C’, culture that produces literature and art. (Location 185)
There are probably two key aspects of culture that stand out as being uniquely human. One is religion and the other is story-telling. There is no other living species, whether ape or crow, that do either of these. They are entirely and genuinely unique to humans. We know they must be unique to humans because both require language for their performance and transmission, and only humans have language of sufficient quality to allow that. What is important about both is that they require us to live in a virtual world, the virtual world of our minds. In both cases, we have to be able to imagine that another world exists that is different to, and separate from, the world we experience on an everyday basis. We have to be able to detach ourselves from the physical world, and mentally step back from it. Only when we can do this are we able to wonder whether the world has to be the way it is and why, or imagine other parallel worlds that might exist, whether these are the fictional worlds of story-telling or para-fictional6 spirit worlds. These peculiar forms of cognitive activity are not trivial evolutionary by-products, but capacities that play – and have played – a fundamental role in human evolution. We shall see why in later chapters. (Location 197)
Humans use music as a mechanism for community bonding in a way that seems to be quite unique. In modern societies, we may often sit listening politely to music in concert halls, but in traditional societies music-making, song and dance are almost indistinguishable and play a crucially important role. (Location 209)
música cita cohesión homininos
the continuous increase over geological time is an illusion, created by pooling specimens from the different species together. Separating the species out gives a pattern that is more suggestive of punctuated equilibria: each new species generates something more akin to a rapid increase or phase shift in brain size when it first appears, and then brain size stabilizes across time. (Location 218)
Each of these triggers a pair of correlated problems for the species concerned: how to fuel the extra costs of these big brains, given that brains are, in energetic terms, exceptionally expensive, and how to bond the ever larger social communities that these large brains were designed to allow. (Location 231)
The real revolution was being able to live in settlements: irrespective of why communities gathered together in fixed settlements, doing so created social stresses that had to be neutralized before the Neolithic could take off. Once this problem had been resolved, however, it opened up the possibility for evolving ever larger communities, and hence eventually the rise of city-states and petty kingdoms and the whole historical development that eventually gave rise to the nation-states of the modern world. Understanding how we managed this transition is thus a key part of our journey. (Location 244)
homo sapiens desafío favorite comunidad cita transición mentalización asentamientos
The way ahead
The approach that I will adopt here is very different. It exploits our much improved understanding of how primates allocate time to the various core activities (feeding, travel, rest, social bonding) that are crucial to their ability to survive in a particular habitat. This approach builds on a series of time budget models that we have developed for a number of monkey and ape species – models that allow us to predict exactly how much time an animal should devote to each of these core activities in a particular habitat. The key issue here is that the length of the working day is limited (we all sleep at night) and all these core activities have to be performed within the waking day. The fact that we are dealing with a biological system here is a crucial advantage: in biological systems, changes in one component cannot happen without having knock-on consequences elsewhere in the system. A species cannot increase its brain or body size without affecting the time it has to spend feeding, and it cannot change its feeding time allocation without affecting the time available for other equally crucial activities like travel or socializing. In a word, the numbers have to add up. And this gives us a powerful tool for exploring species’ responses to changing circumstances. (Location 271)
Originally proposed by the psychologists Andy Whiten and Dick Byrne as an explanation for the fact that primates have much larger brains for body size than all other mammals, the social brain hypothesis has since come to be an explanation for the correlated differences in cognition and sociality between primate species. The crucial feature of this hypothesis is that it provides a quantitative equation relating brain size to social group size. The fact that this relationship is extremely robust and almost independent of any direct effects due to ecology means that we have a way of predicting typical social group size in fossil species. This provides us with two key insights into time budgets. One is the fact that, because brain size predicts group size, we can determine how much extra time is needed for bonding larger groups; the other is that an increase in brain size has to be fuelled by additional foraging time. The question we ask for each species is simply this: how did they accommodate these additional demands on their time? And if their time budgets were already stretched to their limit, what novel solutions did they find to make the extra time they needed available? (Location 281)
What we have to explain
This, then, will be an exercise in detection. We have the crime scene before us in the archaeological record, tantalizingly imperfect as every crime scene always is. Our task is to try to infer what happened where, when and why. The social brain hypothesis and the time budget models provide us with a forensic toolkit that we can apply rigorously to each stage in the unfolding story. We shall proceed, like all good detectives, by trying to fit the bits of the jigsaw together. Because our forensic toolkit is quantitative (the numbers must add up in a time budget model), we can’t just make the jigsaw pieces fit into some arbitrary pattern that happens to suit our particular predilections. (Location 346)
Primates are, above all, intensely social, and their peculiar kind of bonded relationships allows them to form groups that remain stable and coherent over time. (Location 376)
A primate group is an implicit social contract (it is a collective solution to the problem of predation), and social contracts are always susceptible to being broken by freeriders – those who take the benefit of the contract but don’t pay the costs, thereby benefiting doubly at everyone else’s expense. (Location 387)
contrato_social free_rider predación evolución simios cita
In primates, the direct costs are borne mainly by the females, because the stresses of living in a group – the casual jostlings in cramped conditions, the occasional conflicts over access to food or safe refuges – impact on female menstrual cycle endocrinology and lead to infertility. These stresses tend to accumulate down the dominance hierarchy simply because lower-ranking animals have more individuals who can harass them. When the stresses mount up, they block the normal menstrual hormones, and the result is an anovulatory menstrual cycle – a cycle in which ovulation does not occur, and the female loses a chance to conceive. Every time this happens, her lifetime reproductive output (and hence her fitness) is reduced by a fraction. The rate at which this form of temporary infertility accumulates down the dominance hierarchy is sufficiently steep that, in some species, a female ranked tenth in her group can be completely infertile. (Location 391)
Defusing the stresses of group-living
Monkeys and apes neutralize the stresses created by living in groups by forming coalitions that buffer their members against harassment. (Location 402)
Although other neurotransmitters and neuroendocrines such as oxytocin – the so-called ‘love hormone’ – are also released during close social interactions and play an important role in mammalian sociality, the evidence points to a unique role for endorphins in the maintenance of close relationships in anthropoid primates. Grooming and other forms of light social touch (stroking, cuddling) trigger activation in a special set of neurons (the unmyelinated afferent c-tactile fibres) that respond specifically to light touch on the skin and convey these sensations directly to the brain. (Location 407)
Psychologically, endorphin release is experienced as a mild opiate ‘high’, light analgesia, pleasure and calmness. It seems that this is intimately involved in the processes for forming the kind of deep attachments that we find in anthropoid primates (including, of course, ourselves). (Location 413)
Given the importance of grooming in creating and maintaining coalitions, it is perhaps not surprising to find that time devoted to grooming correlates with social group size in Old World monkeys and apes (Location 415)
in the most social primates, the number of animals groomed decreases as group size and grooming time increases. Both across species and within species, the cliquishness of grooming (i.e. how broken up the group is into small, semi-independent grooming cliques that mostly groom among themselves) increases as group size increases. This seems to be because, as the stresses increase with group size, it becomes more and more essential to ensure that one’s coalition works as reliably as possible, and so animals increasingly concentrate their grooming effort on their most important social partners to the exclusion of casual grooming. (Location 419)
One rather crude measure that we do have for both monkeys and ourselves is how much time we devote to each of our friends. Our studies indicate that time spent in face-to-face interaction is important in maintaining the quality of a relationship: when we asked people to assess their own relationships on a simple 1–10 scale (1 being ‘rather neutral’, 10 being ‘I love them dearly’), we found that the ratings correlated very strongly with the frequency of interaction. Indeed, the emotional quality of friendships (but not that of relationships with family members) declines precipitously when friends no longer see each other as much as they have previously done. (Location 436)
How cognition underpins sociality
Theory of mind gets its name from the fact that it defines a state in which individuals ‘have a theory of mind’ – that is, they have an informal theory or belief about what having a mind is like. Practically speaking, this means that they recognize that other individuals have minds like their own. Formally, mentalizing involves the capacity to use words like believe, suppose, imagine, want, understand, think and intend. Philosophers of mind refer to these kinds of words by the general term intentionality, meaning the capacity to take an intentional stance or view. (Location 452)
Humans are not born with theory of mind, though they probably soon become self-aware (first order intentionality). Children acquire full theory of mind at about five years of age, and then gradually progress up through the levels until they reach normal adult competence at fifth order intentionality sometime in their teens. At that stage, humans can handle statements like: ‘I believe that you suppose that Peter wants Susan to believe that Edward intends [something or other].’ While our studies of normal adult humans suggest that there is some variation around this (most adults vary between fourth and sixth order), the average is consistently fifth order. Only about 20 per cent of the adult population can do better than fifth order. (Location 466)
teoría de la mente desarrollo capacidad homo sapiens cita favorite
The social brain really is genuinely costly, and, since the volume of neural matter recruited to handle the higher orders of intentionality is proportional to the level of intentionality at which the subject is working, species that need to be able to work at higher orders of intentionality will need bigger brains. It is, of course, significant that it is precisely these frontal parts of the brain that have evolved most recently in anthropoid primates, and are largest in those species that are socially most complex. They are also the last parts of the brain to myelinate (acquire the fatty sheaths that allow neurons to work efficiently), reflecting the fact that a great deal of social learning and neural adaptation is involved in learning the skills needed to navigate our complex social worlds. (Location 480)
evolución ontogenia intencionalidad costo cognición vínculo cita
The orders of intentionality play a particularly significant role in the story told in this book because they provide a quantitative index of the cognitive differences between modern humans and other primates. In one sense, it doesn’t actually matter that we don’t really understand what mentalizing is in cognitive terms, the fact is that it provides a simple, reliable linear scale of social cognitive complexity. Pretty much everyone agrees that most if not all mammals (and certainly most monkeys) are first order intentional: monkeys understand the contents of their own mind, and are aware that they have beliefs about the world. There is some experimental evidence to suggest that great apes (specifically, orangs and chimpanzees) can just about cope with second order intentionality (formal theory of mind) – they are certainly not as good as five-year-old children (who pass these tests with ease), but they are about as good as four-year-olds who are just beginning to get the hang of the idea. And at the other end of the scale, our data show that normal human adults are fifth order intentional. If we plot these performances against frontal lobe volume, we get a surprisingly tight linear relationship (Fig. 2.4), suggesting that neural volume in these brain regions is directly related to mentalizing competences in a way that mirrors the relationship we found within species in our neuroimaging studies of adult humans. (Location 491)
Primate social evolution
The really important finding, however, is that there is no route out of monogamy: once a species has opted for this state, it seems that it can never escape from it. In effect, it appears that monogamy is a kind of demographic and cognitive sink, probably because the cognitive demands of monogamous partnerships are so great that once the brain has been rewired to accommodate them, this cannot easily be undone. Monogamy requires the male and female to be very tolerant of each other, but at the same time very intolerant of all other members of their own sex. For this reason, monogamous primates always end up as territorial, with each monogamous pair occupying its own exclusive territory. This intolerance of same-sex individuals is quite unusual in mammals outside of monogamous species and makes it very difficult for several individuals of the same sex to live together, especially once they become reproductively active at puberty. (Location 533)
evolución cita monogamia favorite mamíferos
Given that monogamy has a very specific evolutionary history, the question of why monogamy should have evolved as a social and mating system needs to be considered. Over the years three different explanations have been offered for monogamy in mammals. These are: (1) the need for biparental care (two parents are needed to raise big-brained offspring), (2) mate-guarding by males (when females are so widely dispersed that a male cannot defend more than one at a time, he sticks with one female in order to ensure that he at least fertilizes her when she becomes receptive, and at the same time prevents other males from getting a look-in), and (3) infanticide risk (a female locks on to a male in order to use the male as a ‘hired gun’ or bodyguard to defend her against other males that might harass her and/or kill her offspring). (Location 543)
Infanticide risk has long been recognized as a serious problem for primates: because primates’ large brains slow down their reproductive rate (because it takes a long time to grow a large brain), a male who takes over a female from another male may have to wait a year or more before being able to sire his own offspring. However, if the male kills the female’s current infant, she will return to breeding condition immediately (the same is true of women who lose an unweaned infant, by the way)3 and the male can get started on his reproductive career straight away. (Location 548)
evolución infanticidio incentivo perverso
Mate-guarding due to females being in dispersed ranges is certainly the explanation for the evolution of monogamy in most mammals, but not, it seems, in primates. (Location 562)
Gorillas have a star-shaped social structure, with the male at the centre and the females forming the arms of the star: each female grooms with the male, but females rarely groom with each other. (Location 577)
The fundamental point is that as social group size increases, females will face increasing stresses and males will be forced into competition with each other. If they cannot find solutions that defuse these constraints, they will not be able to occupy new kinds of habitats or evolve into new species with larger brains. The inevitable result will be extinction when climate change results in dramatic reduction in its favoured habitats. Since we know that they did survive, we can be certain that they must have solved these problems somehow. This brings us to our two fundamental principles that underpin the story of human evolution: brain size and time budgets. Brain size determines social group size as a response to environmental conditions, and group size and environmental conditions between them impose demands on time budgets that must be satisfied if a new evolutionary step change is to be possible. (Location 583)
the framework that I will be using to explore human evolution consists of two key elements, the social brain hypothesis and the time budget models. (Location 593)
The social brain hypothesis
most mammals and birds, the social brain hypothesis appears as a relationship between brain size and the mating system, with monogamously pairbonded species having significantly larger brains than species that mate polygamously or promiscuously, and especially so if monogamy involves lifelong pairbonds. We think this is probably because longlasting pairbonds are cognitively much more demanding than the more casual relationships of species that mate promiscuously. Pairbonded individuals have to be able to factor their partner’s interests into their decisions on what to do; they need to be able to negotiate between their respective needs to find some appropriate compromise – in effect, a primitive form of mentalizing, even if it is not full-blown theory of mind. (Location 600)
In primates, and perhaps a small number of other mammal families (notably elephants and the horse family), the social brain effect is converted into a quantitative relationship between a species’ brain size and the average size of its social groups. (Location 610)
simios cerebro evolución proporción grupo
The social brain relationship manifests itself in primates as a cognitive limit on social group size that gives rise to a correlation between social group size and brain size (Location 616)
neuroimaging studies compared individuals within the same species, thus demonstrating that the social brain hypothesis applies not only at the species level but also within species at the individual level. (Location 620)
the relationship is really one between behavioural complexity and brain (or neocortex) size, with group size itself being an emergent property: the number of relationships an individual can maintain depends on the complexity of its social behaviour, which is in turn dependent on its cognitive abilities (and hence brain size). (Location 623)
This is because most of these are constraints on brain growth. Brain tissue can only be laid down at a constant rate during development, so if you want a bigger brain, you have to take longer to produce it. There are no short cuts. This means that, at least in mammals, you have to have longer periods of gestation and lactation if you want to evolve a larger brain. And since a computer is no use without software, you also have to have a longer period of socialization (essentially the time between weaning and the start of reproduction) in order to allow the brain to fine tune its ability to deal with all the subtleties of the dynamic, constantly shifting social world. (Location 642)
Brains are very expensive both to grow and to maintain. An adult human brain consumes about 20 per cent of our total daily energy intake, but represents only about 2 per cent of our body weight – so it consumes about ten times more energy than we would expect for its mass (and that is just to keep it alive and doesn’t include the costs when it is busy sorting out what we should do). (Location 648)
The prefrontal cortex also appears to be associated with the ability to inhibit impulsive responses (technically, to inhibit prepotent responses) so as to allow the animal to postpone a reward until later rather than grab what it can now – something that humans are, relatively speaking, particularly good at. Being able to postpone a reward in this way is crucial to living in large, bonded social groups because this is only possible if each of us is willing to forgo some of our immediate selfish desires in order to allow everyone else to have a fair share of the cake. Species with larger frontal lobes are better able to control their impulses – to suppress the red mist that leads to an angry response when you are affronted by someone else’s behaviour. (Location 664)
Humans and the social brain
the social brain hypothesis provides us with a precise equation for predicting social group size from brain size. (Location 671)
when estimating human group sizes. Interpolating the neocortex ratio for modern humans into the ape equation gives a predicted group size of approximately 150. (Location 676)
In modern armies, for example, the smallest unit that can act on its own is the company, and this has an average size of almost exactly 150 (with a range of about 120–180). (Location 693)
When a community isn’t quite a community
The ethnographic evidence tells us that hunter-gatherer camp groups (bands) are not, in fact, the fundamental units of human social organization, because they are actually quite unstable: their membership changes over time on a scale of months as individuals or families decide to join or leave. The important point is that when a family does join another camp group, it invariably joins one whose members belong to the same 150-member community (bonded community or clan); they rarely join camps from a completely different community (unless, of course, they are under extreme ecological duress or have been banished). (Location 781)
The social brain relationship specifically identifies the 150 layer as the grouping for modern humans that is equivalent to the natural groups of other monkeys and apes. We can always identify other grouping layers as being important, as different groups of researchers have done, but these layers are not actually all the same kind of thing: they represent very different kinds and qualities of social relationship, and, as we shall see later, they serve very different functions for us. (Location 796)
Structural complexity in primate social systems
We analysed the distribution of grouping sizes in the ethnographic dataset using the mathematics of fractals to search for repeated patterns in the data. This revealed that these social layers have a very distinct scaling signature: each layer is three times the size of the layer inside it. For our hunter-gatherer dataset, this yields a series of layers at 50, 150, 500 and 1,500: three camp groups of around 50 individuals make up a bonded community (or clan), three bonded communities make up an endogamous community (or mega-band) and three endogamous communities make up an ethnolinguistic unit (or tribe). (Location 813)
fractal estructura comunidad homo_sapiens vínculo
we also found the same scaling ratio in the hierarchical structure of the social systems of other mammal species that live in complex societies (chimpanzees, baboons, elephants and orcas), suggesting that this pattern may be widely characteristic of mammals that have complex social systems. (Location 822)
The capacity to maintain these multi-level social systems may well depend on a species having sufficiently developed social cognition to be able to manage several grouping levels at the same time, and hence a large enough brain to support the mentalizing capacities required to do this (Location 840)
In a very nice set of field experiments, Thore Bergman and Jacinta Beehner showed that baboons can integrate knowledge about an individual’s kinship relationships with knowledge about their dominance ranks and keep the two dimensions separate in their minds. Less intellectually well-endowed species like colobus monkeys would not be able to do this, and as a result their groups are structured around the simpler principle of dominance rank, and this probably explains why their groups are smaller. (Location 844)
simios cognición parentesco cita favorite comunidad jerarquía capacidad
we each have a very distinctive signature, rather like a fingerprint, in how we distribute our social capital (time and emotional effort) across the individuals in our extended social networks – so much so that this pattern is preserved even when there is considerable turnover in network membership (as happens when someone moves away from their home town). (Location 855)
Why time is so important
One of the particularly comforting findings from this family of models is the fact that we only need to know three climate variables to be able to predict time budget components for each of these groups of primates. These are rainfall, temperature and some index of seasonality. (Location 904)
Primates bond their social groups by grooming each other. As a result, social grooming increases linearly with group size across the primates (Fig. 2.1). The fact that the relationship between social time and group size is more or less linear provides us with a simple rubric for determining how much time animals ought to allocate to grooming if they want to live in groups of a certain size. (Location 911)
Despite the fact that these models are based on only a handful of climate variables, they turn out to be extremely good at predicting where species can and cannot live. We have tested the models by using them to predict the known biogeographic distribution of the various species on a continent-wide scale. We do this by dividing the continent, say Africa, into a matrix of small squares 1o latitude by 1o longitude, determining the relevant climate profile for that square (usually from large-scale climate models), and then using this with the equations from the appropriate model to predict group sizes in the squares and, finally, comparing the resulting distribution with where the species actually lives. (Location 920)
Chimpanzees are only with us today because they can exploit a form of fission–fusion sociality to offset the high costs of travel that they face. Fission–fusion social systems are ones in which the community can split up into smaller foraging parties during the day. This allows the animals to reduce the time demand for travel drastically. Were chimpanzee communities forced to forage as a single large group, they would run out of time almost everywhere that they currently live. (Location 938)
evolución estructura eficiencia alimentación grupos chimpancés
by the time these footprints came to light, we had had half a century’s worth of australopithecine fossils to play with. Some 54 years earlier in 1924, the South African anatomist Raymond Dart was searching through a box of fossils that had been found by quarrymen near the small town of Taung when he came across the broken skull of what he at first thought was a fossil baboon. Closer examination revealed something completely new: it was neither monkey nor ape, but a seemingly primitive form of hominin. The fossil came to be known as the Taung child, the first of the australopithecine ‘ape-men’ to be discovered. The number of fossils discovered in sub-Saharan Africa increased exponentially over the ensuing decades, giving us an ever more complex picture of our ancestral tree over this first crucial period of our evolutionary history. However, the main consequence of these discoveries in southern Africa was to reorient our entire picture of human ancestry from the long-held assumption that we had evolved in Asia and place Africa firmly centre stage as the cradle of human evolution. Since then, nothing has emerged from the archaeological record to change our minds on this. (Location 987)
Who were the australopithecines? (Location 1011)
The most striking aspect of the australopithecines as a group is not so much how they differ from the other great apes, but how they do not differ from them. First and most importantly, their brains were not much larger than those of the chimpanzees (see Fig. 1.3) and, aside from the shift in habitat and their large molar teeth, the only substantive evolutionary development that distinguishes them is bipedalism (Location 1019)
the foramen magnum is underneath the skull so that the head is held balanced above the erect spine. In all quadrupedal monkeys and apes, the foramen magnum is at the back of the skull so that the head can look forwards while walking on all fours; when we walk on all fours, we find it difficult to do anything but stare at the ground. (Location 1032)
The australopithecines’ world
baboons, like all Old World monkeys, can process unripe fruit, but apes cannot. This obliges apes to travel further in search of food patches offering ripe fruit, and so causes their travel time requirements to explode as foraging group size increases and habitat quality deteriorates, (Location 1069)
Was bipedalism the solution?
By lengthening their legs compared with apes, early hominins might have made it possible to travel further per stride, and so use less time (and energy) than great apes do to travel a given distance (or, alternatively of course, travel further in the same amount of time). (Location 1110)
Physiological studies of locomotion show that, in terms of oxygen consumption, bipedalism is slightly less efficient than quadrupedalism for chimps, mainly because they have to walk with bent knees. This is because their elongated ischia (the wings of the pelvis to which the leg muscles attach) get in the way when they walk bipedally. The remodelling of the hominin pelvis into the bowl shape that we now have allows our legs to swing without obstruction, enabling us to stride with straight legs. In addition, modern humans gain a 75 per cent advantage in energy savings over chimpanzees when walking bipedally due to a number of unique anatomical adaptations to the hip and foot that provide elastic propulsion. The cartilages that link the bones of the arched human foot, for example, act like a spring and store energy that gives extra propulsion on each stride. (Location 1112)
Robert Foley and Sarah Elton modelled the energetic costs of bipedal vs quadrupedal travel in trees and on the ground and concluded that bipeds gain an advantage over quadrupeds when more than about 65 per cent of the time is spent on the ground. The switch to a more terrestrial lifestyle thus seems to have been critical. (Location 1120)
One additional advantage to bipedalism is cooling. In relatively open habitats, the body of a quadrupedal animal absorbs more sunlight than a bipedal one because standing upright means that only the top of the head and the shoulders are exposed to the sun, especially during the middle period of the day when the sun is overhead and at its hottest. (Location 1131)
favorite beneficios homininos bipedalismo cita
Modern humans have two features that are not only unique among the primates but also seem to be directly related to this heat-load problem, namely the loss of fur over most of the body (other than the head and, to a much lesser extent, shoulders – the areas of the body most exposed to the sun at midday) and a greatly increased capacity for sweating (we have many times the number of eccrine sweat glands in the skin than all other primates except for baboons, the only other terrestrial open country species). (Location 1140)
The key point here is that sweat evaporating off fur just cools the tips of the hair and not the skin underneath; to benefit from evaporative cooling of sweat, the animal has to be naked. (Location 1145)