Anthropology help

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It is tempting to think that human evolution stopped once modern Homo sapiens evolved and migrated all over the world. But, there is both anatomical and genetic evidence that this is not the case at all. Consider, for instance, our teeth. So many humans today have crooked teeth that require braces. Of course, early hominids would not have worn braces, but a close look at their teeth reveals that there is plenty of space in their mouths for a full set of straight teeth. Tooth crowding, and malocclusion, like that shown in the bottom image, is a recent and common problem today. Why? In the last 10,000 years, humans have shifted from a diet of wild plants and animals to a diet of domesticated plants and animals. Most humans have shifted from a hunting-gathering lifestyle, to one that relies on agriculture. Foods are processed more than ever before, making them softer and more easy to chew and digest. As a result, our chewing muscles and the underlying bone does not grow as large or as robust as it used to. However, our teeth continue to grow as though there was plenty of space for them. Our biology has been strongly impacted by the dietary and behavioral trends of the last 10,000 years.

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In this chapter, we will look closely at the last 10,000 years, and how the adoption of agriculture and the expansion of populations has shaped our biology. We will look first at the origins of agriculture and consider when, where, and why humans first domesticated plants and animals. We will look at the archaeological and skeletal evidence for the impacts that agriculture had on our living circumstances and on our biology. We will then consider some of the important forces still shaping human biology today, and will consider the question of whether humans are still evolving. This last lecture hopefully will convince you that human evolution is not just a thing of the past.

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Up until about 10,000 years ago, all humans around the world were hunters and gatherers. They collected a wide variety of plants, hunted game (both large and small) and began fishing and collecting shellfish. But, at the transition of the Pleistocene to the Holocene (a transition period called the Neolithic), humans began to domesticate some of the local plants and animals. This, of course, did not happen overnight, and shown here is an illustration of the gradual shift from a hunter-gatherer lifestyle to one that relies on agriculture. This shift from gathering plants to growing them has had a profound impact on our biology. The process of domestication is actually quite simple. Humans deliberately chose certain variants to grow based on favorable characteristics, such as plants with soft outer coats more easily digested, or with larger seeds that were more flavorful or had more calories. Eventually, many of these domesticated plants and animals grew to rely on human intervention to survive and reproduce, resulting in a completely dependent and mutualistic relationship between humans and our domesticated crops and our animals. But, why did we start growing our own food after hunting and gathering it for the last several millions of years?

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In a few words, we don’t know why we started doing this. However, as with other major evolutionary events, the climate most certainly played a role. Shown here is the global temperature over the course of the last 100,000 years. Notice that during most of the last 100,000 years, the climate was cooler, drier, and highly variable. In such conditions, it would be difficult for crops to grow consistently, given the rapid fluctuations in local environments. However, starting around 10,000 years ago, the earth emerged out of the last glacial cycle (ice age) and became warmer. But, perhaps more importantly, it became stable. This stability meant that local climates were predictable and humans could more easily rely on what they grew from year to year for survival. Around this time, humans began to selectively choose teosinte (to the bottom left) with larger and larger seeds—the beginning of maize domestication. The tool shown to the bottom right is an antler with blades inserted into it, which was used by Neolithic humans to harvest wild plants. But, domestication does not happen without people, and there is evidence from all over the world that population growth may have triggered domestication. As populations grew, humans could no longer support themselves with hunting and gathering techniques. They needed more food, and deliberately growing it, storing it, and controlling its distribution not only allowed populations to grow but encouraged these populations to become more anchored to a piece of land. Village life, with permanent settlements, began to appear in the archaeological record around this time.

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Archaeological evidence shows that domestication did not start in one place and spread. Instead, it took hold independently in 11 regions around the world. Certainly from these central areas, the idea spread quickly through a process called diffusion. The only region that did not adopt agriculture was Australia. The 11 epicenters of plant domestication are shown in this top map. Notice that in each region, different plants (locally found) were domesticated, like maize in Central America, sunflowers in North America, wheat in the Middle East, and bananas in New Guinea. Domestication appears to have begun first in a region called the Fertile Crescent in the Middle East, followed soon after by evidence of plant and animal domestication in Central America and South America, and in China. Again, this did not happen overnight. The archaeological and genetic evidence finds that Neolithic humans in the Middle East, for instance, were harvesting wheat and barley for part of the year, and then hunting and gathering for the rest of it. At some point, these people noticed that the seeds that fell off the wheat and barley made new wheat and barley plants. Their selection of particular seeds to deliberately plant started a long (and continuing) practice of controlling the life cycle of plants.

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As humans grew more and more food, these resources could support more and more people. But, these resources were not necessarily mobile, and humans began to settle into more permanent communities called villages. Soon after, these villages grew in size and developed into cities. One of the earliest of these is illustrated here: Ҫatalhöyük in Turkey. The archaeological record reveals that Neolithic humans were farming in Turkey by about 10,000 years ago. Soon after, villages emerged, with closely spaced (bottom left) living spaces, reconstructed as seen in the top image. To the upper right are skeletal remains of a pregnant female and her unborn fetus from Ҫatalhöyük. These skeletons can reveal what life was like for these early farmers.

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Agricultural origins are often focused on the Middle East (the so-called Fertile Crescent). But, it is important to recognize that Neolithic humans all over the world began to plant seeds and grow their own food. In China and Southwest Asia, people domesticated rice by 8,000 years ago. In Mexico, corn was domesticated around that time. This idea spread throughout the Americas, quickly reaching the American Southwest, and eventually making it all the way to the Atlantic coast by about 1,000 years ago. There is little evidence that the corn itself was moved from Mexico to these other places. Instead, the knowledge of how to grow corn spread from community to community. The same thing happened in Europe, as the domestication of wheat and barley, which began in the Middle East, spread to Greece by 8,000 years ago and into western Europe.

We have neglected to mention the domestication of animals in this whole process. In fact, the very first thing domesticated was probably not a plant, but an animal: dogs. There is evidence that humans domesticated the local wolf by about 15,000 years ago, leading to dogs that were helpful in hunting and guarding. Eventually, wild cattle, sheep, and pigs were domesticated for their meat and their coats.

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One cannot overstate the importance of domestication. Domestication allowed humans to grow enough food to sustain a population growth at the beginning of the Holocene. This population boom continues today. Two-thirds of all calories and protein intake comes from cereal grains, like wheat, barley, corn, and rice that were domesticated in the early Holocene. Rice alone accounts for 50% of the caloric intake of nearly 2 billion humans. Domestication has led to the establishment of permanent human settlements: villages, cities, and whole societies. Many of the great civilizations of the past—in China, Egypt, South America, and Mexico—were fueled by agriculture. And agriculture has driven many technological innovations that allow us to grow, store, and transport more and more food to a growing population. It is therefore tempting to think that agriculture has been nothing but a positive innovation. However, there are always trade-offs, and domestication is no exception. Notice in the top graph how the population has skyrocketed since the start of agriculture 10,000 years ago. This is known as the Neolithic Demographic Transition. The greater availability of grains cooked into soft mushes allowed mothers to wean their offspring earlier, allowing them to have more offspring, and have a higher fertility over the course of their lives. Prior to the advent of agriculture, it is estimated that there were about 2–3 million humans. But, only 8,000 years later, there were 100 times as many humans: close to 300 million. And today, there are over 200 times as many humans as 2,000 years ago: 7 billion. There are consequences for this many people crammed into a limited amount of habitable space. There is more competition for resources, and thus more violence and organized warfare for access to land useful for growing food. And the utilization of land to grow food to sustain our growing population has an impact on the environment. This is not new: Landscapes were being altered around the Mediterranean 6,000 years ago, and this modification of our world continues today.

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Let’s briefly review before we discuss the biological changes we see in human populations as a result of agriculture. There are always trade-offs in biology and in evolution. By domesticating plants and animals and controlling their growth and production, we were able to feed more of us, leading to an increase in our population. In places, this led to a food surplus, which can be used to feed those who are in need. Long-term food storage allows us to feed ourselves during the unstable years where excess rain or drought reduces the crop yield. However, the astounding growth in our population has led to a strain on the environment. Humans have exploited their landscape and altered it so that they can either live or grow food on it. Competition for valuable, fertile, land has increased, leading to conflict. Humans have also altered the landscape enough to render many other plants and animals extinct, leading to an overall decrease in biodiversity. Let’s look more closely now at how the transition to agriculture has impacted our biology.

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There is a common misconception that human evolution ground to a halt with the Upper Paleolithic modern Homo sapiens. This is entirely false. Human evolution has continued, and our biology has changed since the late Pleistocene. Much of that biological change has been affected by the advent of agriculture and other changes related to our diet. For example, let’s look at some of the changes that have happened to our skulls and faces. A general trend that we can trace from about 2.5 million years ago to the Holocene is a general reduction in robusticity of the chewing muscles and the teeth. The early transition from Australopithecus to Homo is related to an increased use of stone tools and an incorporation of more meat into the diet. The latest part of this trend (Neolithic) may be related to the dietary adoption of domesticated plants. This is known as the masticatory-functional hypothesis. As humans began to eat soft-textured agricultural foods like millet, the chewing musculature has been reduced. The softness of grains was further increased by cooking them in pottery containers until they were soft mushes that need not be mechanically broken down by a strong masticatory system. By reducing the activity of the temporalis and masseter chewing muscles, the bones of the face and skull undergo less strain, grow less, and produce a skull that is more globular, with a less projecting face. The less projecting face results in less room for teeth, leading to an increased rate of malocclusion. In this way, our culture (domestication of plants, cooking vessels), has directly impacted our biology. Because the size and position of our teeth is determined by our genetics, and less influenced by the food we eat, there is a mismatch between our large teeth and our small, gracile faces. This mismatch is so severe that it can lead to molar impaction, infection, and even death in the absence of dental care. There is thus selection for individuals with fewer teeth, and as predicted, more individuals today have congenitally absent wisdom teeth than ever before.

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Before we move onto changes in the postcranial skeleton related to agriculture, let’s review the skull. Notice the differences between hunter-gatherers and agriculturalists. These are not universal differences, but are general tendencies that characterize these two populations of modern humans. Keep in mind that these differences are entirely related to diet, and do not reflect any phylogenetic connection of one group or another to earlier humans. One thing to consider is whether you would have developed a larger, more robust mandible and a longer cranial vault had you been raised in a hunter-gatherer group. The experimental evidence on primates fed different diets shows that the answer to this question is “Yes.” You would look quite different had you been raised in another way, eating a different diet.

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It is quite easy to reconstruct the activity patterns and workloads of modern human hunter-gatherers. All one has to do is go study them. In fact, in the 1960s, researchers Richard Lee and Irven DeVore organized a conference in which anthropologists gathered to discuss the life of a hunter-gatherer. What they determined was that hunter-gatherer life depends very much on the ecology and the kinds of foods being sought. Hunter-gatherers unquestionably work hard to survive, but there is leisure time as well, like that shown here for the !Kung. But, how does one reconstruct what life was like for Neolithic people and the early agriculturalists? How can we reconstruct activity patterns from skeletal remains? One tool physical anthropologists use is called biomechanics. Because bones are plastic during development, their growth can reflect the activities being engaged in by a population. Larger, thicker bones tend to reflect higher levels of activity. More specifically, biomechanists use the principle of an “I beam” to reconstruct ancient activity patterns. Material that is farther away from the center of the bone is stronger (resists bending) than material closer in much the same way that a ruler can very easily be snapped in half by bending it in one direction (along the flat edge) than by bending it along the other axis (perpendicular to the flat edge). So, in this image of the femur, it is stronger in the up and down direction (more bone farther from the center) than in the side-to-side direction. This suggests that this individual is doing more strenuous activity that bends the bone in one direction compared to the other.

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So, what has been found? Researchers have discovered that the transition from hunter-gatherers to agriculturalists resulted in a significant decline in bone strength. What is illustrated here is a comparison of the cross-section of the humerus and femur (in two places) between hunter-gatherers and early agriculturalists from the southeastern United States. We met these early agriculturalists at the very beginning of the first lecture. Notice that these early prehistoric farmers from Georgia had considerably thinner bone than hunter-gatherers, reflecting reduced mechanical stress, and a lower overall activity pattern. In addition, it has been found that humans have reduced their bone mass as a result of their smaller size. Of course, this depends on the local environment, but, in general, humans have had a 10% decrease in body weight in the last 20,000 years. As a result of agriculture, we did not get bigger, we got smaller.

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But, the reduced activity patterns also has led to a reduction, in some populations, of degenerative joint diseases, like osteoarthritis. Excess stress on joints from carrying, lifting, and any physically demanding activity can break down the protective joint cartilage over time and lead to excess bony growth. This happens most often in the spine, hand, knee, and hips. Shown here is the osteophytic lipping, or “spurs,” that can form from arthritis of the lower back. These pathologies are often found in skeletons of individuals who were quite active.

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As we’ve already seen, agriculture and population increase are intimately connected. Linked to these changes in human life in the Neolithic is a shift from a migratory lifestyle to a sedentary one. These changes led to humans living in densely populated, unsanitary conditions. This is a recipe for the spread of infectious diseases, and human evolution has been strongly shaped in the last 10,000 years by our exposure to these diseases. One way we know that early Holocene agriculturalists were exposed to many infectious diseases is to study the bones. Infections, such as those caused by the staph bacteria, Staphylococcus aureas, can impact bone growth, causing a swollen region called a periosteal reaction. Though the bones indicate that there was an infection, it is difficult to determine precisely what caused it. Nevertheless, periosteal reactions are extremely rare in pre-agriculturalists, but more common in more recent skeletons.

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Other infections, like the one shown here, not only show up on bone (and are thus detectable in ancient populations) but signal the precise cause. These lesions on the skull are often accompanied by leg bones that are enlarged and bowed. What causes this? A group of diseases called treponematoses, which can include syphilis and yaws. Other diseases found in the skeletal remains of Neolithic humans include tuberculosis, measles, mumps, cholera, the flu, and small pox. There is a common tendency to think that these diseases were not present in the New World and were only introduced through the colonization of the New World by Europeans in the 1400s. However, there is skeletal evidence that many of these diseases were present in overcrowded regions of the New World.

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One of the negative results of eating domesticated plants is the increase in dental caries, or “cavities.” Many domesticated plants are rich in carbohydrates, which support the growth of bacteria like Streptococcus mutans and Lactobacillus acidophilus (shown on the far right image). These produce lactic acid as a by-product, which literally dissolves the enamel of the tooth. Of course, cavities are a mere nuisance to those today who have dental care. However, for Neolithic humans, dental caries could lead to gum infections that could ultimately be fatal. Certainly, some foods are richer in carbohydrates than others. For instance, rice tends to not cause cavities, but corn does. Look at the graph at the bottom left of the page. Archaic, Early and Middle Woodland North Americans lived prior to 1,000 years ago and had not yet domesticated corn. Notice the low levels of cavities. However, around 1,000 years ago, corn was domesticated here and the Late Woodland, Mississippian, and Contact periods preserve skeletons with a very large percentage of individuals with cavities.

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There is a common misconception that the shift to agriculture improved nutritional health. It did not. Domestication of plants and the adoption of farming leads to a narrowing of the types of plants and animals consumed by a population. There is evidence that agriculturalists ate a very specific food in a given region: rice in Asia, wheat in Europe, millet or sorghum in Africa. Of course, other foods were consumed, but the variety does not approach that eaten by hunter-gatherers. Without the variety, agriculturalists are at risk for nutritional deficiencies. For instance, corn is lacking in three amino acids, and a person eating almost exclusively cornmeal will have growth deficiencies. Also, corn contains phytate, which can prevent the body from absorbing iron, causing iron deficiencies. So, how do we know if these early agriculturalists had growth deficiencies? If an individual is in nutritional distress, his body either slows or ceases growth. This is most obvious in the teeth, where ameloblasts (cells that make enamel) will halt production in times of nutritional stress. This creates lines on the teeth, called enamel hypoplasias. These are often caused by nutritional deficiencies, though severe infections can cause these as well.

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Just a moment ago, we discussed how corn can inhibit proper iron absorption. Iron deficiencies can also be caused by a lack of citric acid (found in fruits), which helps absorb iron. The most common source of iron, however, is meat, and an iron deficiency may signal a lack of meat in the diet without being compensated with other iron-rich foods. Iron deficiencies are also caused by infection, such as the hookworm (upper right image), which anchors to the intestinal wall and literally sucks the blood of the host individual, causing iron loss and anemia. When iron is deficient, the body responds by making more red blood cells. This leaves a mark on the skeleton, since the marrow producing regions of the body (especially in the skull) become more porous to increase the surface area of blood cell production. This results in a porous texture to the top of the skull (porotic hyperostosis) shown in the middle image to the right, and a projection of spongy bone into the eye orbits (cribra orbitalia). These skeletal indicators of iron deficiency are quite rare before the Neolithic and appear in many skeletal populations after the advent of agriculture.

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Let’s finish our discussion of agriculture with a review of the costs associated with growing our own food, and with a basic question: Why do we do it? Compared to hunter-gatherers, agriculturalists have a higher rate of periosteal reaction (infection), more cavities, more nutritional deficiencies, resulting in reduced child growth and development, more enamel hypoplasias, more anemias, and lower adult height. This last statement is surprising to many, but the skeletal record shows a decline in height in Neolithic humans compared to hunter-gatherers. If agriculture was so bad for us, why in the world did we adopt this new form of food acquisition? Let’s recall that evolution is about survival and reproduction. Agriculture is bad, but it is not so bad that individuals cannot survive (for the most part). But, as we mentioned earlier, agriculture has allowed women to be more sedentary, have a stable food supply (even if it is nutritionally wanting), to wean earlier, and ultimately to have more offspring. This increase in female fertility is the reason why agriculture has been so successful. It has led to an increase in the human population from just a few million to over 7 billion.

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The shift to agriculture had an obvious affect on our biology. But, even more recent transitions to a high-calorie diet rich in saturated fats and simple sugars, combined with an increasingly sedentary lifestyle, has led to increases in obesity, cardiovascular disease, and diabetes. Humans continue to be impacted biologically by the way we interact with our environment. And this is an environment that we are continuing to alter. There is little scientific doubt that the planet is warming and that global climate change is being caused by an increase in greenhouse gases in our atmosphere. What impact might this have on human health and human biology? Some models predict that many of the food-producing regions of the world will dry and become deserts. If this happens, it would have a significant impact on our food supply. Higher temperatures will also increase the number of breeding mosquitoes, which spread pathogens like those that cause malaria. Selection will in turn favor humans with sickle-cell genes that can ward off malaria; in other words, humans will continue to evolve. It is difficult to know just how humans will evolve in the future. There is no question that humans face difficult challenges ahead. But, we are remarkably adaptable primates, and our adaptability to changing conditions is an evolved behavior that has served us well right up through the Pleistocene, the Holocene, and today. No matter, as Clark Larsen, the author of your textbook says, “The evolutionary record makes clear that evolution, whatever its forms, will continue.”

Image of Earth courtesy of NASA and Wikicommons creative license.

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