Lecture 7 PDF: Hominin Evolution

Lecture 7 , explaining the major developments in hominin evolution. There are several obvious characteristics that make hominins unique from other mammals and even from other primates. And for years, more than a century even, researchers have been trying to understand why these characteristics devel...

Lecture 7 , explaining the major developments in hominin evolution. There are several obvious characteristics that make hominins unique from other mammals and even from other primates. And for years, more than a century even, researchers have been trying to understand why these characteristics developed. In this lecture, we'll examine several of the bigger questions about hominin evolution and the current hypothesis developed to answer them. 4 of the more prominent questions are: why did hominins develop such large brains? Or more importantly, why did we evolve such a high degree of encephalization? Why did we become obligate bipeds? Why did we lose almost all of our body hair coverage? And what was the role of hunting and meat eating in hominid evolution? Let's look at brains first, and let's begin by examining the data trend in increasing brain size. Around 4 and a half 1000000 years ago, we have Ardipithecus with a 350 cubic centimeter brain, followed around 4000000 years ago by australopithecus anemensis with a 370 cc brain. These are both within the range of chimpanzees. Just after 4000000 years ago, we have Australopithecus afarensis, whose average brain size was at the upper range for chimps at 415. Australopithecus africanus and sediba both show a slight increase to an average of 440, followed by our single parenthesis aethiopicus cranium at 410. Then there's a notable increase among the later 2 panthepenes jumping to 525530 respectively. After this, the increase gets pretty dramatic with homo habilis jumping to 650, homogaster to 850, homo erectus to a1000, homo heidelbergensis to 1250, and early members of our homo sapiens lineage and our Neanderthal cousins jumped to 1450. Then there's a slight reduction later in our own lineage to 1,003 150 cubic centimeters. We'll talk about this later in the lecture on modern humans. So since the emergence of the hominin lineage sometime between 45000000 years ago, brain size has increased fourfold. This is a massive increase in size and in a relatively short period of time. Whatever the reason, natural selection has been exerting a lot of pressure on increasing brain size and hominins. So that was the crude data, but this graph better illustrates the changes in brain size from Ardipithecus to us. The horizontal axis is time in 1,000,000 of years, 5,000,000 on the left to recent times on the right, and the vertical axis is cranial capacity in cubic centimeters. The rate of brain increase is notable even among the pre Homo hominin species. But with the appearance of homo, you can see how much more rapid the increase is. However, we cannot just look at raw brain size. We need to express brain size as a function of body size since among most animals, raw brain size is more product of body size and less an accurate reflection of intellect. For most animals, there's a fairly direct linear relationship between body and brain size. Larger bodied animals have larger brains. In order to understand how much larger hominin brains are to other animals and what the differences between hominin brain size might mean, we need to control for body size. The ratio of brain size to body size is expressed as a value called in civilization quotient or EQ. EQ is computed using a formula that holds body size constant. We don't need to worry about the formula itself, but you should be aware that different researchers have developed slightly different formula. They all do the same general thing, but the values produced by different formula are not directly comparable. Encephalization quotient seems to be more informative about levels of intelligence than simple raw brain size, Although, detail of brain structure is also very important. And in fact, we actually can get some good information on the structure of the brains of long extinct species. We get this data from looking at endocranial casts. Like we saw in lecture 4.2 in Egyptopithecus sucesis. We used to get these by doing actual plaster molds of the insides of fossil skulls. Today, we can do the same thing with MRI or CAT scans. The inside of an animal's brain case molds very closely to the outer shape of the brain, right down to where major blood vessels are located. So these endocranial casts are actually a negative image of the detailed shape of the brain itself. They can show this can show us when different major components of hominid brains are developing over our evolutionary history. However, this is something we can't really go into in this class. So for our purposes here, we'll use EQ values as a crude measure of intelligence. This table presents the encephalization quotients of a range of living primates, including us. The first column is a selection of primates, female and male modern humans, chimpanzees, gibbons. Remember that's small ape from Southeast Asia, orangutan, baboon, gorilla, and colobus monkey. The second column is brain size in cubic centimeters centimeters for each species. The third column is average body weight. From these two measures, EQ can be computed in the 4th column on the right. You can see that we humans have a much higher EQ than any other living primates by a large margin. We living humans have the highest EQ of any animal that ever lived. This table presents some estimated EQs for a number of extinct species in the hominin lineage with us at the top and chimps at the bottom as points of comparison. This is more or less the order in which they appeared over the course of hominin evolution with the earliest species, Australopithecus afarensis, at the bottom, and that's modern humans at the top. You can see in the right hand column, increasing encephalization has been a very strong trend over the last 4000000 years. This graph is similar to the one we saw 4 slides ago. But here, rather than just looking at increases in raw brain size, we controlled for body size using EQ. The vertical axis is now EQ rather than raw brain size. And now looking at changes in EQ over the last 5000000 years, the earliest hominid species show a significant increase. But again, with the genus homo, the rate increases dramatically. The curve looks even more like an exponential increase. Considering that hominin bodies also started to get significantly larger after 2,000,000 years ago, this trend in brain size is even more impressive. One final thing we might take into consideration besides brain size is trends in hominin body size and stature. This figure is showing hominin stature in centimeters as well as raw brain size in cubic centimeters inside their round heads. What I'd like to point out here is the apparent huge leap in both brain size and stature between Homo habilis and Homo erectus. Some researchers have argued that this is a particularly important event. We'll come back to this argument later. So the question here is why this in civilization trend? It is clear that our large brain and intellect have provided us with major adaptive advantages over our environment and over other species that we might be in competition with for resources. Early researchers simply assumed the big brain was a logical advancement, so didn't spend much time trying to explain it. A large brain was just obviously a huge advantage. However, this explanation would apply to all animals. Why was it among primates and especially hominins that encephalization was so dramatic? Why didn't all animals develop large brains if the benefits are so obvious? Why not earthworms? The reason is brains are very expensive organs. Brains take a lot more energy to run than other organs. So to evolve a bigger brain, a species would have to make other major changes to their diet and anatomy and physiology. Changes that would negatively impact other aspects of their adaptation. For example, if they simply ate more in order to supply the required energy, this would result in increased intergroup and intragroup competition for food resources. And as we saw in lecture 2, it's also really difficult to give birth to babies with big brains. Trying to deal with this by modifying the female pelvis to accommodate babies' larger heads would affect that species' ability to run. And in the case of earthworms, they would get stuck in their holes. Evolving a larger brain is really complicated, So to understand why large brains were selected for in the hominin lineage, we need to examine what early hominin adaptations were like. Currently, the most common theory about why hominins evolve larger brains has to do with the advantages of living in complex social groups, and the abilities required to be able to do this effectively. Currently, most researchers think it was an increased reliance on complex social interaction that selected for increased in civilization in primates in general, but especially in the hominin lineage. This is based on our recognition that group social interaction is more cognitively demanding than most other types of behavior, and the selection was for a larger brain to aid in this interaction. Primates are not the only animals who engage in group social interaction. Wolves have complex social structures, for example. But primate social interaction is much more complex because it involves more complex relationships between the individual members. For a group to work together and for an individual to be able to cooperate successfully within a group, all members need to have a good understanding of everyone's relationship to everyone else. Reading other group members' behavioral signals and playing the politics required to increase your social status and thus increase your access to mates and to resources is very complex. An individual's behavior and their choices of courses of action depend directly on keeping track of specific interactions between themselves and other individuals. For example, whom in your group has shared their food with you? Who has not competed with you for access to food resources? Who has consistently had your back and joined you in alliances against others? And who, on the other hand, has not been keen to interact favorably with you? To successfully navigate life in groups like this, there are 3 levels of increasing complexity that each member must monitor. 1st, there's your own relationship with every other individual. How do you get along with each of the other members of the group? Secondly, the relationships between each individual and everyone else. And thirdly, these relationships are always changing, typically on a daily basis. In a group of just 100 individuals, this means there are 4,950 individual relationships. You need to keep track of almost 5,000 unique and constantly changing relationships. And as we all know, individual relationships can be amazingly complex on their own. As humans, we recognize this day to day politicking, and for us, it's relatively easy. Not to say it is it is not as stressful. We can also appreciate the extreme complexity of it, and it does take the supercomputer human brain to be able to do this effectively. An important component of any theory that attempts to explain the evolution of a specific characteristic is to frame it in terms of natural selection and explain how it provided an adaptive advantage that would result in that characteristic being selected for. For large brains, its development meant the creation of some problems, like how to give birth to babies with such large heads. So whatever advantage you gave, it had to be really good. In this case, if highly social behavior was the reason for increased in civilization, then what adaptive advantage did this highly social behavior provide that would result in the selection for a larger brain to increase the species' ability to successfully carry out the social behavior. We discussed in the lecture on primate behavior the advantages that living in larger groups provides, increased access to mates, more eyes and noses to find food, and better protection against predators. But these apply to all animals that live in larger groups. What seems to distinguish us hominins from other species, including other primates, is our capability for hypercooperation. That is working closely together in a larger group following a shared plan to carry out a complex task that presumably benefits all members. Hypercooperation is made possible by the uniquely human ability for three things. The first is our ability to know what others are thinking, what's referred to as theory of mind. We're very good at being able to figure out what others are thinking just from their actions and facial expression. Chimps, bonobos, and orangutans are capable of this to some extent, but not like us. Secondly, we are able to put our individual goals and emotions on hold for the sake of the group's goals. Apes have a very difficult time with this, as you saw in the film, Abe's Genius. They have a hard time ignoring their immediate individual needs and desires, even when doing so might benefit them along with the rest of the group. And thirdly, we are keep we are able to understand and appreciate the value of cooperating in achieving a shared goal. This allows us to ignore our own immediate needs and desires for the sake of bigger outcomes. These are some of the most important human traits, and our ability for hypercooperation is likely what has made us so successful as a species. Hypercooperation improves the adaptive success of all members of the group, and group capability and efforts are greater than the sum of their parts. What ten people can achieve together can be much greater than 10 times what an individual can achieve alone. However, these uniquely human abilities that allow us to hyper cooperate require a high level of intellect, which requires a high degree of in civilization. So those early hominins who happen to have slightly larger brains will be better at cooperating with each other and so would be better adapted, have more offspring, and pass on the genes for the larger brain to subsequent generations. Those early hominins who lacked the larger brain would not do as well and not pass their genes on as much. The advantages of hypercooperation would result in natural selection for ever larger brains. It's probably not realistic to separate the development of language from complex social behavior. These two things go hand in hand. But some researchers have argued that the development of complex spoken language outside group social behavior could have been the reason for the encephalization trend in hominins. For example, the development and use of complex spoken language in very specific social interactions like cooperative hunting or in the teaching and learning dynamic between parent and child. However, when we actually examine the evidence for this, it turns out that in fact, hunters working together on a hunt hardly talk at all. Any communication is, for the most part, nonverbal, since avoiding making noise is usually very important. Spoken communication during such hunts is not really necessary anyway, since these hunters have all grown up together and learned to hunt together. They all know how the cooperative hunts work, they know their roles, and they know how to respond to changing circumstances. The role of spoken language in teaching and learning is more plausible as a possible reason for the hominin civilization trend. Language may be an important component of effective teaching and learning, especially for complex abstract ideas. But it's unclear at the moment whether most learning among hunters and gatherers is done simply through observation and trial and error, or if language really is an important part of the process. This is currently a popular area of research. Realistically, the development of language was likely closely associated with large group socialization rather than with cooperative hunting or with teaching and learning. Based on modern ethnographic observations among traditional hunting and gathering groups, verbal socialization is most common and most complex at the campsite. That is, within a larger group, in the settled conditions of camp, in close interaction, where silence is not required. This might support the suggestion that language developed to help facilitate the interaction in complex social interactions. Back to our first theory, language would allow individuals to more accurately and directly share information and express desires and intentions, and therefore, socialize and hyper cooperate more effectively. Everyone knows the keys to successful socializing is being in on all the juicy gossip, and gossip has long been of primary importance for us, and its importance shouldn't be underestimated. Nasty gossip is bad. Don't do this. But we all rely on gossip to a large extent to understand our place in our social groups and the state of our our relationships with other members of those groups. We might also add something else here about language. Language could also have developed to aiding complex dissembling. Dissembling is similar to lying. It's trying to create another's perceptions of your thoughts and emotions that run contrary to what you actually think and feel and contrary to what you might be expressing through body language, which is often difficult to control. It's intentionally hiding your true motives, opinions, and feelings in order to fool others for some reason or another. We all practice simple dissembling every day. For example, you're having a bad day, but you don't want to go into a long drawn out explanation about it with every person you cross paths with. You might talk to your close friends about why you feel down. But with everyone else who asks you how it's going, you're going to smile and say fine. It's a bit of a lie perhaps, but simple dissembling like this makes life flow a bit more smoothly. But dissembling, even simple dissembling, is a whole other level of complexity. It involves very complex abstract ideas that require a high level of intellect, and effective dissembling requires complex language. Dissembling with language allows an individual to carry out complex social politics even more elaborately. You can convince members of your group that your intentions run one direction, or they may actually run-in another. In other words, this theory suggests that language allowed people to become politicians. Complex language provides us with other important advantages as well that likely play big roles in effective group adaptation and hypercooperation. Complex language allows us to discuss things that are not present, a herd of animals or another group of people that are not right there in front of you. We we can talk about past and future, not just the here and now. We can discuss abstract concept concepts like kinship systems and religion. And we can store information collectively. Language allows us to lock knowledge into the collective mind of a group. Valuable information that one person has can be more easily spread through a group. This allows groups to adapt more quickly and successfully because it would help lock information and ideas into the collective conscious consciousness of a group. People would not have to be constantly reinventing ways to adapt to changing circumstances. Things like what plants are safe to eat, where important resources are located on the landscape, how to start a fire, how to make better hunting weapons. All these things supercharge culture transmission, that is the passage of knowledge between individuals, and this can effectively increase the rate of innovation of new ideas or technologies within groups. Whatever the reason for the development of complex language, it definitely requires impressive cognitive abilities, and it may be that the adaptive advantage that language represented was strong enough to encourage the development of increased cognitive abilities. However, the problem with the language theory is that the general primate and civilization trend began long before anybody thinks spoken language emerged. Few researchers think that the australopithecines or panthepines had complex language like us. However, maybe the development of language would explain the apparent jump in brain size around 2,000,000 years ago. Our second big question is why hominis became obligate bipeds? Based on the current fossil evidence, it appears that obligate bipedalism appeared around or just before 4000000 years ago, possibly with Ardipithecus ramidus, but they still have prehensile feet. We definitely have obligate bipedalism with the early australopithecines at 4000000 years ago. Obligate bipedalism was undoubtedly preceded by habitual or facultative bipedalism, like that practiced by chimps and bonobos. But bipedalism obviously provided some adaptive advantage in the hominin lineage and was strongly selected for until it became obligate. It became the dominant locomotion for hominins. First off, we need to recognize that there are some serious issues with obligate bipedalism. Compared to most quadrupeds, bipedalism is a slow and awkward form of locomotion. Most quadrupeds are faster and more agile than us, which, especially in the African savannah, would have made us poor predators and easy prey. We would not be able to outrun large predators, especially the early small bodied hominins, which is probably why they lived mainly in heavily wooded environments where they could scamper up a handy tree when necessary. Another problem is that our bipedal skeleton evolved from a basic quadrupedal skeleton. Quadrupedal skeletal structure has been evolving for this purpose for over 300000000 years. It has a very stable structural design, much like a suspension bridge, the front limbs and hind limbs acting like the bridge pylons, and the backbone acts like the suspension cables that span the uprights and support the heavy road bed, or the muscles and guts in the case of the quadruped. We hominins have retained much of that basic quadrupedal structure, but we turn it up on end where it no longer works as it evolves to. This has resulted in a high occurrence of chronic back problems, slipped discs, and pinched nerves. Lower back problems are a very common occurrence among people today, and back pain is second only to the common cold among reasons for clinical visits. Trying to explain hominin bipedalism has a long history. Even Darwin contributed to the discussion back in the mid 1800. Over the decades, quite a number of theories have been formed to explain the emergence of bipedalism. We'll just look at several of the more prominent ones. The classical explanation is that walking on hind legs freed up our front limbs for tool use. This is the explanation that Darwin proposed, and this theory has a lot to support it. However, a current problem for some researchers is that if obligate bipedalism developed by 4000000 years ago, it predates the oldest known stone tools by over half a 1000000 years. However, many researchers expect that we will eventually find evidence that the use of stone tools extended back much further in prehistory than current evidence suggests. But there's also no reason to assume that tool use began with just stone tools. Chimpanzees and hunters and gatherers use wood and other organic materials for tools as well. As we discussed in the last lecture, organic materials do not last in the archaeological record, so you will never know when the use of wood for tools began. It may be that simple wooden tools were being used as early as sat lanthus jadensis, 7000000 years ago. Still on the topic of tool use, it's also worth considering the impressive throwing abilities of us humans and the importance of throwing in hunting and gathering adaptations over the last 2000000 years. And, also, consider the extreme dexterity of the human hand. Ours is the most dextrous of all the primate hands. And imagine for a second the incredibly important role that it has played and and still plays in human adaptation, making fire and manufacture manufacturing warm clothing, for example. Could such Dexter's arms and hands have developed simply as byproducts of some other selection for obligate bifidelity? Depending so much on making and using handheld tools and being a quadruped are not compatible developments. Tool use is still a very strong candidate for explaining the development of obligate bipedalism. Our second theory has to do with bipedalism and thermoregulation. Thermoregulation is the maintenance of body core temperatures within a healthy range. We know that there have been major climatic changes in East Africa over the last several 1000000 years, a shift from warmer and wetter to drier conditions. Forests and woodlands shrank and were replaced by more open grassland savannah. We saw this in the sites in East Africa, like Olduvai Gorge. We looked at it in lecture 5. For many years, it was thought that this major climatic shift occurred around the same time that bipedalism emerged among early hominins. It is thought that with the loss of their original forest biome, early hominins would be forced to start moving into more open habitats where they would be more exposed to the hot African sun. Under these conditions, thermoregulation would have become an important adaptive issue. And in the early 19 nineties, Peter Wheeler argued that the need to avoid overheating in these open Savannah conditions would explain bipedalism. Wheeler argued that bipedalism provided 2 important advantages to hominins living in more open exposed environments. 1st, it reduced the surface area of the body that is directly exposed to the overhead sun. When we stand up, rather than the entire back, shoulders, and neck being exposed to the to the sun, Only the top of the head and the shoulders are directly exposed. Secondly, it moves much of the body mass up away from the ground. Air temperature is slightly cooler above the first meter above ground level. Standing up moves a significant portion of the body mass into this slightly cooler zone. Also, above this meter, air movement increases significantly, which also helps to keep the body cooler even in very hot environments. By standing up and walking bipedally, hominins would stay cool and become much more effective at exploring savanna resources. Unfortunately, for this theory, it has become quite clear that bipedalism well predates this climatic change. The major changes to a drier, more open environment occurred after 2 and a half 1000000 years ago, long after obligate bipedalism developed in hominins. And we now know that most of the early obligate biped hominins, like the early australopithecines, were living mainly in woodlet environments. So this hypothesis doesn't work anymore. Our third theory has to do with how efficiently bipeds use energy. Most quadrupeds are faster than bipeds. Even an elephant can easily outrun the average person, and some species like cheetahs and gazelles are much faster than even the fastest humans. All the large animals that we might want to hunt for food can easily outrun us, and perhaps more importantly, so can the animals that want to eat us. This simplified graph is meant to illustrate typical energy usage by a large quadruped like a lion. The horizontal axis at the bottom indicates how efficiently energy is being used. When in motion, the more efficient the use of energy means fewer calories are being expended per distance covered. Low efficient energy use means using lots of calories up without traveling very far, and highly efficient energy use means covering long distances using few calories. All animals, whether quadrupeds or bipeds like us, use energy just standing still. It takes some energy going to our various muscles to keep us from falling in a heap on the ground. When quadruped quadrupeds start moving from the slowest walk to the fastest fastest run, they must expend some energy on both bearing their body weight and producing forward momentum. Whether walking or running, they need to push off with their feet to create forward momentum. But as they move from a standstill to a walk, their efficiency of energy use drops, and this continues when they are running at low velocity, when they are jogging. But when they start sprinting, they use energy very efficiently. They still cannot sprint for long periods because even at efficient use levels of energy use, they will still use up their energy energy reserves quickly, but they can cover relatively long distances very quickly on the calories they do use. Compare this to an obligate bipeds use of energy. For us, obligate bipeds, it's essentially the opposite. We become more efficient as we start to walk and then reach a jogging speed. This is because of a big difference in how we achieve forward momentum. We do this by simply leaning forward and letting our legs swing forward like pendulums to catch us. We do not have to push off, so we don't use much more energy walking or jogging than we do standing still. We can achieve low velocity for momentum with very efficient use of energy. When standing still or walking slowly, we use energy rather inefficiently. And if we run very fast, we do need to push off with our trailing foot, and so we also tend to use energy very inefficiently. We are not designed to run at high speeds. We use up available energy very inefficiently if we do. However, for low velocity running and covering long distances, bipelism is energy efficient. It appears that we have evolved to be joggers. Our third big question is, why do we humans have such reduced body hair cover? Why have we become essentially naked apes? We are clearly less hairy than most mammals. What do we know about this? Unlike brain size and bipedalism, it's difficult to know anything about evolutionary changes in hair coverage because the evidence of it doesn't survive in the fossil record. Hair doesn't preserve. But there are some things we do know. First, we are not the only mammals with limited body hair coverage. Elephants and rhinoceroses also lack body hair coverage, but they have thick skin to protect them from the sun. We have very thin skin. The second thing is that we actually have a similar number of body hairs as the great apes, chimps, bonobos, gorillas, and orangutans, somewhere around 5,000,000. What's different about us is that the majority of our body hairs are very short and very fine, which makes us appear to have fewer hairs. With many people, their body hairs are so small and so fine that they really don't appear to have any, so we are effectively naked. It is also worth noting that there's quite a range of variability in human body hair coverage, more than we tend to acknowledge. Some modern humans actually have quite thick body hair. And body hair thickness also is likely a very plastic trait. It probably changes easily in response to natural selection, so it can change relatively quickly in response to different environmental conditions. Hominids living in different regions may have had notably different body hair coverage. However, it is definitely true that compared to most mammals, we, recent humans, do have much more exposed exposed skin. So when do we think hominins lost their body hair coverage? Currently, researchers think there's a strong link between this reduction in body hair coverage and the development of full body sweating, another uniquely human trait, and that both developed in concert as a thermoregulation mechanism to deal with living in very hot open environments. Sweating or transpiration is when the body moves heat heated water from its insides out onto the surface of the skin, or it can evaporate quickly. And unlike other mammals, we humans have evolved whole body transpiration, which is much more effective at dissipating excess body heat than the method most mammal mammals have evolved, panting. All mammals have similar skin physiology, which includes two main types of excretive glands. Sebaceous glands, which secrete oil out onto hairs, and eccrine glands, which secrete sweat, which is almost entirely water. Both play a role in thermoregulation, but the effectiveness of sebaceous glands is limited. Humans have more eccrine glands, 2 to 5000000, which can produce up to 12 liters of sweat a day. This is a much more effective cooling system than other mammals have and gives us a huge advantage in hot environments. We hominins could avoid overheating much more effectively than other mammals living in hot environments. We could be active when other animals have to be dormant or risk hyperthermia, dangerous overheating. We don't know exactly when these changes in body hair coverage and increased transpiration evolved, but some researchers like Nina Jablonski have argued that this hyperefficient thermoregulation physiology must be tied to the move from forested regions to more open environments and developed along with modern body proportions. Our especially long legs would have greatly improved our walking and running abilities as we moved across open exposed terrain. All this corresponds to the appearance of Homer Gaster around 2000000 years ago. Peter Wheeler has also shown that there are thermoregulation benefits to having a larger body in hot environments. In hot environments, larger bodies lose water content at a lower rate than smaller bodies because larger bodies have less skin relative to body mass, so fewer sweat glands relative to their body size. So in very hot environments, a small bodied hominin, like a 35 kilogram Australopithecus afarensis, will lose about 2.4% of its body weight in water. While in the same period, a larger body hominin, like a 90 kilogram homargaster, will only lose 1.7% of their body weight in water. This does not seem like a big difference, but it means that the Homerangaster individual can forge much further and for much longer across the landscape, although they have to find water to replenish their body's moisture reserves. This works even better with tall, narrow bodies like we'll see shortly with Homerangaster or Erectus. Unlike obligate bipedalism, the appearance of large bodies with tall, narrow frames does coincide in time with the with the shrinking forests and increasing grassland in Africa around 2,000,000 years ago. Whenever hairlessness did develop, it would have resulted in hominins having increased exposure of skin to sunlight, more specifically, to ultraviolet radiation. This is very dangerous and would result in increased frequency of skin cancer, which means these hominins would require the evolution of some some sort of skin protection. This turns out to be increased melanin production. Melanin is a dark brown or black protein that accumulates in the skin, so increased melanin results in darker skin. This is why groups of people who have been living in equatorial regions for generations have darker skin, And it means that as hominins lost their hair cover, they would have evolved to the dark skin typical of modern human populations that have been evolving to live in very sunny regions. And one last thing that brings 2 of our previous discussions together. It turns out that together, being good joggers and having a very effective thermoregulation system in hot environments has provided us with another advantage over other large mammals. We can actually outrun large quadrupeds in hot dry environments if we pace ourselves. Quadrupeds will die of heat exhaustion if they run too long in hot environments, whereas we will not. We are well evolved for covering large areas in hot dry regions. Some traditional hunter gather groups like the Kunsan in South Africa and the Hadza in East Africa are known to run down large game animals like kudu or eland over long distances. This is called persistence hunting. If you'd like to see persistence hunters in action, you can simply search for persistence hunting on YouTube or click on the link in Canvas. And finally, what about hunting and the role of meat eating in hominin evolution? Traditionally, researchers associated hominid adaptations with hunting big game animals. Early hunting methods would certainly have been simple, but it was assumed that early hominins did intentionally hunt animals for food. Hunting has long been strongly tied to visions of our early hominin ancestors. Over much of the 20th century, there was a certain view of early man the hunter, lacking the claws and the big teeth, of other predators, but still successfully hunting much bigger animals than ourselves with just their courage and a sharp spear. To early 20th century researchers, hunting readily explains 2 of the major trends in hominin evolution that we've talked about already. It is argued that obligate bipolars evolved to free up the hands from the manufacture and use of hunting weapons. This is just a more specific version of the tool use hypothesis and still does make some sense. And it was also argued that the increased in subtilization resulted from the need for better communication to be successful hunters. We've already seen that this hasn't held up to scrutiny. In fact, 20th century ideas were dominated by the hunting hypothesis. The idea that the development of hunting can explain many of the trends in hominin evolution and behavior. This included both anatomical and physiological changes, like the reduction in the size of our canines, argued to be a response to the increased role that hunting tools took on. Tools replaced teeth. It was argued that hunting led to the increase in body size that we'll see with the appearance of homo ergaster and erectus. It was argued that a larger body would allow hunters to be more effective at hunting larger prey species, and it would be an advantage in competing with other large like lions or hyenas. In fact, there was an increase in body size in early homo species as we'll see in the next unit that corresponds to the move from arboreal to terrestrial adaptations. This could very well be associated with increased hunting, but it's also the case that as we discussed earlier, there are other reasons why hominins evolve larger bodies. Something moving away from forested regions into open environments would free hominins to become larger as a small body is really an advantage in arboreal living. A small individual can move much faster through the trees, use smaller branches, and climb higher. And, also, as we just saw, a larger body provides increased thermoregulation advantages in hot open environments. It was also argued that major behavioral changes that were argued to be common among modern hunters and gatherers could be the result from a reliance on hunting early in hominin evolution. The early develop development of hunting was seen as the source of traditional hunting and gathering divisions of labor based on sex. Men hunted, and women stayed back at camp and had babies and foraged for plant foods. While it is the case that among hunting and gathering groups, men typically do most of the hunting. There isn't such a clear division of labor. Women do sometimes hunt, and men do forage for plant foods. Another traditional view was that prior to hunting, individual hominins simply fended for themselves getting food as is the case with nonhuman primates. It does seem to be true that sharing does not play a major role in other primate species. Generally, individual monkey snapes find their own food. However, sharing is very important among hunting and gathering societies. There's a strong ethic against ownership and individualism. Researchers argued that when early hominins began hunting large animals, much of the group's food was coming in as large packages of meat. So hunters were obliged to share with those who had stayed back in camp looking after the children. Maybe there is still some logic to this old idea. The hunting hypothesis further argued that if the women in early hominin groups were heavily dependent on meat supplied by men, this could explain the evolution of 2 very human traits. The first is the constant sexual reception among human females. Unlike other mammals, including nonhuman primates, except perhaps bonobos, human females are not only receptive to sex around the time of ovulation. Humans generally have sex regardless of ovulation cycles. And secondly, concealed ovulation and monogamy, as was mentioned in the unit on primate behavior, unlike all of the nonhuman primates, except for vervet monkeys, modern human females have not evolved overt physio physiological signals that indicate they are ovulating. Without any overt overt signals, the males of the group would not know which females were ovulating at any one time. The hunting hypothesis argued that this would tend to keep each male attentive to a single female since he wouldn't be distracted by other females who are clearly ovulating. And he also wouldn't know when the female he was bonded to might be able to be successfully impregnated. Concealed ovulation would limit male male competition and encourage more cooperation and perhaps even encourage monogamy. In the 19 sixties 19 seventies, researchers recognized problems with this traditional emphasis on the importance of hunting and meat eating in common adaptations. Primatologists studying nonhuman primates in the wild noted that their diets seem to be composed entirely of plants and insects with meat rarely being included at all. So if we were going to use them as analogs for our earliest hominin ancestors, we'd have to rethink how important we thought hunting was. And researchers studying modern hunters and gatherers noted that among most groups, meat typically made up only about 25% of the diet with the other 75% being edible plants. This is highly variable variable from group to group, and in some regions, it was even more. It is also noted that around 75% of the diet, mainly plants and small animals, was collected almost entirely by women. It also seemed that scavenging meat from other predators' kills often contributed a significant proportion of the meat consumed by some groups. This didn't really fit well with the romantic view of man the hunter. Scavenging likely did play a big role in early adaptations more than had traditionally been thought. And in fact, act active hunting may have been quite rare among the earliest hominins. These are the views among researchers starting in the 19 sixties and continuing into the 19 nineties. But more recent primate and hunting and gathering research has swung the thinking back. We know now that the hunters and gatherers studied in 19 sixties 19 seventies had historically been pushed out of their traditional territory into more marginal habitats by the encroachment of agriculturalists. So there's no surprise that their hunting success rates were so low. So if we were able to study these groups 200 or 500 years ago, they would have had much better access to game animals. Meat would have been much more accessible. Hunting success rates would have been much higher. And, undoubtedly, meat would have played a much greater role in their diets. In fact, among traditional hunting and gathering groups, meat is generally considered real food. There's also now a lot more archeological evidence that early hominins were eating a lot of meat over the last 3000000 years and even more during the last 2000000 years. This includes butchered animal remains. Archaeologists have excavated many thousands of butchered bones from hundreds of early sites across Africa. Bone broken open by hominins to get the marrow out, and many with cut marks on them from hominins using stone tools to remove the meat and with the stone tools found lying right next to them. More recently, bone chemistry analysis has become a common method for understanding past diets. An analysis of the analysis of the bone chemistry of early hominins indicates that they were eating a lot of meat. The evidence demonstrates conclusively that meat eating has been an important component of our diet for at least the last 2000000 years, and this continues to be the case well into historic times. It is also worth taking a brief look at human digestive physiology. While we humans are definitely omnivores, and we can eat a wide range of plant and animal foods, our digestive system is in many ways much more similar to a carnivores than a herbivores. Herbivores like the sheep illustrated here have much longer intestinal tracts, especially the small intestine, because the extraction of usable nutrients and energy from the plants they eat is a longer process. A sheep's digestive system is 27 times the length of its body. This is 20 to 30 meters in length, and its stomach is never empty because the food passes through so slowly. In contrast, carnivores like wolves have much shorter digestive tracts, only 5 or 6 times the length of their body. This is mainly because extracting calories and nutrients from fats and proteins is faster. This mean means food passes through more rapidly, and the stomach typically empties 3 to 4 hours after a meal, which means they can safely ingest potentially dangerous things like old scavenged meat without accumulating dangerous levels of microbes. The human digest digestive system is about 5 times the length of her body, and in most respects, is much more similar to a typical carnivores. There's also much more recent information on primate hunting and meat eating behavior. Most nonhuman primates do not eat meat. However, some do, like baboons and gorillas, and for some, like chimps, meat is their favorite food. They love meat, and they actively hunt small mammals, including monkeys and antelope. While meat likely makes up a relatively small proportion of the overall diet of most chimpanzee groups, they try to obtain it as much as possible and obviously prefer it over plant foods. Researcher Jill Preetz, who we saw in the film Ape Genius, has observed them making crude spears and using these to hunt bush babies. Recent research also indicates that our gentler relative, the bonobo, also hunts and eats monkeys. And chimta chimpsamonobos have a very similar digestive system to us as well. Ominous evolved from earlier primates that were almost entirely herbivorous. Why did meat eating develop in our evolutionary line at all? Currently, there's one hypothesis with significant potential for explaining this. It's called the expensive tissue hypothesis, and it argues that it was necessary in early hominins diets to allow the evolution of our very large brains. It was developed by Peter Wheeler and Les Aiello in the 19 nineties, and their argument goes like this. An animal's body can only produce a finite amount of energy to run its various systems and organs, the muscular system, the digestive system, respiratory system, etcetera. Mammals have 11 major systems that require some portion of the available energy in the form of calories that are provided by the digestive system when it breaks down food. The amount of available energy depends on 4 major factors. The first is the animal's body size. Larger bodies require more energy, of course. The second is the environment that it lives in. Is it cold and an animal needs to use more energy to heat itself, or does the animal need to climb up and down hills to get around? The third is the organism's metabolism. How efficiently is food turned into energy by the animal's digestive system? And the 4th is the source of nutrition. What is the animal eating? Some foods are more easily turned into energy. Because the available energy is limited, a species cannot evolve characters characteristics that are extraneous. That is systems or organs that require energy to function, but do not provide some adaptive advantage in return. Components of an animal's physiology must pass a cost benefit test by natural selection. Maintaining any one organ requires energy from the body's reserves, and so it must constantly provide returns in terms of some adaptive advantage. If some organ or system is not providing major adaptive advantages, it will eventually disappear due due to a lack of natural selection for or even active natural selection against it. For example, eyesight. For most animals, eyesight provides a huge adaptive advantage and represents a very good return on the energy it takes for the eyes to function effectively. However, among organisms adapted to environments with no light, like this cave salamander, eyes become useless. They provide no adaptive returns for the energy they require, and eventually, they will evolve away. The brain is one of the most metabolically expensive organs. For example, it uses 22 times more energy per gram than muscle tissue. So it uses a lot of energy from the body's reserves. We hominins dramatically increased our brain size over the last several 1000000 years. Therefore, we had to evolve some physiological response to deal with this. We needed to balance out our increased energy requirements with our limited available energy reserves. But we cannot simply evolve a larger digestive system in order to process more food to increase energy reserves because the gut is just as metabolically expensive as the brain. By looking at the average size of different organs across mammals of different sizes, we can compute the expected size of organs for a mammal of our size. If we were typical mammals of our size, we would have a heart that weighs around 320 grams, kidneys of 240 grams, liver, 1600 grams, and a gut of 1900 grams, and a brain of about 450 grams. And we can compare this to their actual size in us living humans. Our heart, kidney, and liver are close to what we'd expect, but our digestive system, our gut, is much smaller than the expected size, and our brain, as we already knew, is about 3 times larger. In fact, it turns out that the hominins evolved a smaller digestive system, which would actually free up a bit more energy. But this little extra would be nowhere near enough to support such a huge brain, and now the gut can't process as much food. For a smaller gut to support a larger brain, we're left with one solution. Hominins had to adopt a higher quality diet. They had to switch from an entirely plant based diet to one heavily based on meat or more specifically fat. Fat represents a much richer source of energy than carbohydrates. In general, fat has far more potential energy per gram than carbohydrates, and most fat and protein molecules are easier to break down than many carbohydrate molecules, which means they provide higher net energy returns. With fatty meat, the digest digestive system does not have to use as much energy to break the molecules down so there's more energy in the form of kilocalories left over to provide to other systems. We can see what sort of energy returns in kilocalories we can actually get from different types of food available to hunter gatherers. Leaves have very low net energy returns partly because they start out with low caloric values, and then it takes a lot of energy to break them down to release those calories. The average edible wild plant is better, but 1.7 kilocalories per gram is still very low. Seeds and nuts are actually quite high quality foods in terms of their energy energy returns, especially very fatty ones like macadamia nuts. But seeds and nuts also cost much more in terms of the energy expended to require them. They occur in very small individual packages, and a person often has to often has to pick each one. Large packages of meat, especially from very fatty animals, represent the best return on collection efforts. In hot, dry environments like Bunch of Africa, most animals are actually quite lean. Their muscles have quite low fat content, but significant quantities of fat can be found in bone marrow and bone grease and in animals' organs, especially the brain. The expensive tissue hypothesis currently appears to a significant support, but we'll have to wait and see if it holds up in the face of continued research. The current thinking is though that we have long underestimated how important fat was to hominin diets. This would especially be the case among hominin populations that eventually left Africa for colder climates in Asia and Europe. Finally, let's examine one other food and brain related idea, the cooking hypothesis. Richard Brangam, a primatologist, has suggested an alternative explanation for how we were able to evolve big brains. He argues that it was not meat that allowed our huge brains to evolve. It was our ancestors' discovery of fire and starting to cook their food. Cooking food, whether meat or plants, starts breaking down large molecules, so cooked food digests more efficiently. Cooking is like starting the digestive process before you put the food in your mouth, which means your digestive system does not have to work as hard to get the available calories out. This significantly increases net energy returns. While we clearly do not need to cook all of our food or even most of it, Rangam has shown that modern humans need to cook at least some of our food to maintain healthy energy levels and long term re reproductive success. Among people who don't cook any of their food, members of the raw foodist movement, women eventually stop menstruating, and men have low sperm counts. He suggests that the development of hominin use of fire and cooking better explains the apparent jump in encephalization around 2,000,000 years ago with the appearance of Homer Gaster. However, archaeological evidence doesn't support this hypothesis. If the use of fire and regular cooking began 2,000,000 years ago, even if the vast majority of such archaeological sites were lost, there still should be dozens and dozens of early sites with evidence of fire. As it is, the number of archaeological sites in Africa older than 500000 years with potential evidence for hominin use of fire is incredibly low, maybe 3. In fact, the evidence for fire use prior to even just 250,000 years ago is thin. We'll talk more about this in the lecture on the genitals. The need to cook some food may be a much more recent physiological development. Also, as we'll see in the next unit, there might not have actually been a big jump in brain size with Homer and Gaster. The rate of brain size increase over the last 3000000 years was certainly rapid, but it was probably also fairly smooth.