Unit 5 this week covering chapters 16. Notes below are the things I highlighted in my book.

Chapter 16 Humans and Preindustrial Climate

16-1 Evidence of Human Evolution

• Anthropologists focus either on distinctive events that break the continuous process of evolution into separate stages or on quantitative traits that can be measured as they gradually change.
• Human evolution is marked by five distinctive developments:
  • the initial branching off from primitive apes between 6 and 4 million years ago, the onset of bipedalism near 4 million years ago, the use of stone tools beginning near 2.5 million years ago, the branching of the prehuman line into the genus Homo and other forms by 2 million years ago, and the development of large brains in the Homo genus since 2 million years ago.

Appearance of Human Ancestors

• Human evolution can be traced back to primitive mammals that evolved during the millions of years after the massive extinctions caused by the asteroid impact. 
• By 10 million years ago, one such line had further evolved to primitive apes. Subsequently, a group of apes that included both our human ancestors and chimpanzees branched off from the primitive apes, with modern apes evolving along the other branch.
• Molecular biologists measure the composition of DNA molecules in the protein of living organisms.
  • They make use of DNA as an evolutionary clock: the longer the time that has elapsed since two organisms branched off from a common ancestor, the more dissimilar their DNA will have become. Assuming that this DNA dissimilarity increases through time at a constant rate, the amount of dissimilarity can be used as a clock to measure elapsed time.
• Molecular biologists have concluded that the line that led to humans diverged from the line that led to the modern great apes between 6 and 4 million years ago.

Walking Upright

• The evolutionary line that led to modern humans, called hominins, appeared by 4 million years ago.
• Fossil remains of ankle bones with a distinctive structure suggest that walking had become the primary means of movement by 4.3 million years ago.
• A remarkable deposit dated to 3.6 million years ago in Tanzania holds footprints of individuals walking across a freshly fallen layer of volcanic ash that had cooled.
• Scientists argue whether these creatures developed the ability to walk in order to exploit food resources on the grassy savanna lying between stands of trees, or whether they developed upright postures in order to stand on the lower tree limbs and reach up for fruits and nuts.

Use of Stone Tools

• The first firm evidence that hominins used stone tools dates to about 2.5 million years ago.
• One early hypothesis suggested that humans evolved mainly as “killer apes” because of the aggressive use of tools to kill their prey.
  • But many anthropologists today believe that hominins simply made opportunistic use of the remains of animals previously killed by lions and leopards, and that they were constantly forced to contend with hyenas and other scavengers for this food.
• Use of tools for butchering implies an important change in diet. The available evidence suggests that toolmaking followed more than 1 million years after the ability to walk upright.
• Some scientists infer that toolmaking was a natural evolutionary development for creatures whose hands were freed for other uses when they began to walk on two legs.

Appearance of Homo

• Sometime after 2.5 million years ago, the ancestral australopithecines evolved into several new forms. One line led to the genus Paranthropus which became extinct by 1 million years ago.
• The other major group carries the name of our own genus, Homo.
  • These were more graceful creatures with larger heads and braincases. The patterns of wear on their teeth indicate a broader-based diet of meat, fruits, and vegetables.
• The first appearance of these humanlike creatures appears to follow closely after the earliest use of stone tools, but the precise timing is difficult to determine because the fossil record is so fragmentary.
• The use of tools is likely to have favored those prehumans who were able to move easily across the landscape in pursuit of a large variety of seasonally changing food sources. Frequent movement may also have called on a greater use of intellect and imagination.

Brain Size

• Over time, our human ancestors developed larger brains, shown by the increasing size of preserved skulls that encased and protected the brain.
• In broad outline, the volume of the braincase tripled over the last 3 or 4 million years.
  • unusually rapid compared to most evolutionary changes.

16-2 Did climate change drive human evaluation

• Several hypotheses have been put forward to explain the rapid evolution of our ancestors during the last 3 or 4 million years.
  • Some ideas focus on the impact of new technologies, such as tools to facilitate gathering of food and to serve as weapons for defense and hunting.
  • Others emphasize changes in social factors, such as the development of communication skills and the need for early humans to gather food for infants who spent several years in a defenseless state.
  • Other hypotheses focus on climate change as the key control, and several components of the climate system are potentially relevant to environmental changes in Africa that could have had an impact on humans.
• For example, over the last 4 million years, orbital-scale variations in monsoonal summer rainfall have occurred in the Nile River headwaters. Northern hemisphere ice sheets are another potential factor. Pollen counts in a lake deposit in the mountains of East Africa suggest that mountain vegetation descended to lower elevations during some glaciations. Limited amounts of long-term evidence are available from Africa and nearby oceans, including material blown off the continent.
• Based on these various kinds of evidence, different hypotheses involving climatic factors have been put forward to explain human evolution.
  • The oldest, the savanna hypothesis, ties human evolution to long-term drying in Africa.
    • In this view, tropical rain forests became interspersed with small patches of semiarid grasslands, which then gradually spread between groves of trees and coalesced into grassland savannas.
    • This slow fragmentation of once continuous forest caused our ancestors to move on the ground for ever-longer distances, forcing more rapid movement to cover longer distances and also greater resourcefulness to survive in a more exposed environment. 
• Human evolution has also been linked to the onset and intensification of northern ice sheets, perhaps transmitted by the intermediary of the North Atlantic Ocean. Model simulations have shown that the climatic effects of even the very large ice sheets of the last million years on North African climate are relatively weak at orbital time scales.
• The anthropologist Richard Potts proposed the variability selection hypothesis.
  • Its basic premise is that evolution occurred at times when rapid changes in climate (temperature and/or precipitation) put new demands on our ancestors, favoring those individuals or groups who evolved traits that proved useful for adapting to an increasingly challenging environment. 
  • This hypothesis seems broadly consistent with the increased amplitude of climatic variability evident in many regions during the last 3 million years.

16-3 Testing climatic hypotheses with fragmentary records

• A major problem is the fragmentary nature of the record of hominin remains because of prevailing aridity.
• For hominin bones made of easily dissolved calcium phosphate (Ca3PO4), preservation is even worse in the acid-rich soils of the rain forests.
  • the total record of human evolution over 5 million years is based on just a few dozen fragments of skeletons, enough to reveal some of the broad outline of human evolution but few of the details.
• We have seen that sampling records of orbital-scale climate change can lead to gross misrepresentations of the shape of the actual climate signals.
• Aliasing can produce erroneous indications of the time of first acquisition of new physical or technical skills or of the first or last appearance of a new hominin species.
  • With only a few samples, the actual first or last appearances are likely to be missed, and instead we will see a much-reduced range.
  • The true ranges of most hominins are probably longer than those shown and complicates attempts to define the relative timing between climate changes and the first use of new skills
• A second undersampling problem has to do with quantitative measurements of the evolution of physical traits. In this case, the basic problem is that a broad range of natural variation occurs within all human populations.
• The actual range of braincase sizes present at any one time in the past is fairly large in comparison with the evolution of the mean value.
  • If the fossil record provides only one or two well-preserved specimens every hundred thousand years or so, a good chance exists that some of these specimens will not be representative of the brain size of the entire population living at that time, but will fall above or below the mean.
• Depending on the specific samples collected and analyzed, an inaccurate picture of the long-term trend could emerge.
• Even if the available fossil record is sparse, it should be possible to obtain a general sense of the direction in which the trend is moving, especially if the net amount of evolution far exceeds the natural range of variation at any one time.
  • But the limitations of sparse data make it impossible to define the true rates of change.

The Impact of Climate on Early Farming

• By the time of the most recent glaciation, our ancestors painted amazingly lifelike portrayals of animals on the walls of caves and rock shelters. They also made small statues of human and animal figures and created jewelry by stringing together shells, and they buried their dead with food and possessions for use in a future life.
• More sophisticated stone tools designed for specific functions also appeared, and, for the first time, people began to use bone, a much more “workable” substance than stone, yet hard enough for many uses. Learned to make tools and clothes.
• With greater protection from the elements, people pushed north into higher and colder latitudes of Asia. There they built dome-shaped houses with large mammoth bones supporting the superstructure and animal hides. draping the roof for protection from rain and snow.
• People also learned how to make rope to make snares and lines and nets to catch small animals, birds, and fish. People shaped bone or wood into spears that held stone spear points and used rope to help bind the spear points to the long shafts.
• This new technology produced a lethal and revolutionary new way of hunting.  Hunters could now bring down larger game from a safer distance
• By the start of the present interglaciation, an explosive alteration of basic human existence was underway.

16-4 Did deglacial warming lead to early agriculture

• The first evidence of agriculture dates to just over 12,000 calendar years ago in a region of the Middle East called the Fertile Crescent, encompassing present Syria, Iraq, Jordan, and Turkey.
  • The people living in this region abandoned the hunter-gatherer way of life and began to cultivate wheat, rye, barley, peas, and lentils rather than simply harvesting the same foods in the wild. These people took up residence in permanent dwellings. Within 1,000 years, the dwellings began to cluster into permanent village settlements.
    • Evidence of cultivation is based on preserved remains of grains found in regions where they did not naturally grow
    • Evidence of permanent occupation comes from the dental remains of animals from the settlements.
• Because of the close association in time between the later stages of the deglaciation and the origin of agriculture, several cause-and-effect links have been proposed.
• One seemingly plausible link is the possibility that the change from the harsh glacial climate with large short-term oscillations to the more stable and accommodating climate provided conditions more favorable for humans to begin the grand experiment of growing crops.
• On the other hand, a totally different climatic hypothesis centers on the Younger Dryas climatic reversal between 13,000 and 11,700 years ago.
  • In this view, the Younger Dryas episode intensified the already dry conditions across the eastern Mediterranean region and forced people to retreat to dependable water sources.
  • In these more closely clustered conditions, people who harvested and ate wild grains may have accidentally scattered some grains near their threshing sites, with the discarded grains sprouting in succeeding years as a form of primitive farming.
• Neither of these directly opposing hypotheses is easy to test. One problem is that agriculture in some regions may have begun earlier than the still-incomplete records indicate. Another problem is that the beginnings of agriculture in each region were one-of-a-kind events.
• Many such events from many regions, each related to a similar kind of climate change, would be required to show a conclusive cause-and-effect relationship.
• The independent emergence of agriculture in so many places within a period of about 5,000 years seems to indicate that humans had reached a threshold of cultural sophistication that made such advances all but inevitable for many cultures.

16-5 Impacts of climate on early civilizations

• Climate change has been hypothesized as the cause of the deterioration or collapse of several early civilizations.
• One hypothesis focuses on the role of an early flood in the Black Sea, but drought is the climatic factor most commonly invoked as influencing humans. 
• The first advanced civilizations of the early Egyptian dynasties developed between 6,000 and 5,000 years ago, when the North African summer monsoon was still stronger than today. When the Nile ran strong, large seasonal floods provided fertile soils and moisture for farming along the floodplain
• Climate in sub-Saharan North Africa turned much drier after 5,000 years ago as the summer monsoon weakened.
  • This drying trend affected the civilizations that had come into existence and grown in size during the wetter monsoon climates in the preceding millennium.
  • Greatly reduced the extent of summer flooding of the Nile. This change must have put greater stress on populations that had expanded in response to the stable food supply from large crop yields in a wetter monsoonal climate.
• Other examples are the Akkadian empire and the Mayan civilization and the Anasazi people.
• Depletion of resources is also a plausible competing explanation for some of these cultural changes.
• The Anasazi did abandon their cliff dwellings in the American Southwest during a drought, previous dry intervals of comparable or nearly comparable intensity had not driven them from the region.
  • Abandonment also coincided with the near disappearance of tree pollen from climate records, indicating that the Anasazi had cut down the juniper and piñon pine trees previously used as fuel for cooking and for winter heat on the high, cold plateau. If so, depletion of this crucial resource may also have been a major factor in the abandonment.
• Similarly, the Maya of Central America may have altered their own local environment.
  • They may have contributed to regional drought by cutting trees. Their farming methods may have exhausted the limited supply of nutrient-rich soils in a region of nutrient-poor limestone bedrock, making agriculture difficult or impossible. Wars with neighboring cultures may also have been a factor, along with disease.
•  Isolating climate change as the sole cause of changes in early civilizations is often difficult.

16-6 Did humans cause megafaunal extinctions

• Populations of large mammals called megafauna decreased drastically during the most recent glaciation.
  • Prior to 50,000 years ago, more than 150 genera of mammals larger than 45 kg existed.
  • By 10,000 years ago, 50 or fewer genera were left. This interval of mammal extinctions was unprecedented in millions of years of prior geologic history.
• In the Americas, the megafaunal population was highly diverse until the late stages of the most recent deglaciation, with a particularly rich array in North America.
  • Then, within an interval of a few thousand years centered on 12,500 years ago, over half of the large mammal species living in both North and South America became extinct.
• One long-standing explanation for this rapid pulse of extinction is that major climate changes at the end of the glacial maximum created new environmental combinations to which many mammals were unable to adapt.
  • • These conditions included strong summer warming and drying of the land south of the ice sheets by high summer insolation, reduction of habitat in the cooler north by the slowly retreating ice, and unusual mixtures of vegetation that developed as forests and grasslands shifted from their glacial positions to their modern locations. 
•  Critics of this climatic hypothesis note that no comparable pulse of extinction occurred in any of the fifty or so preceding deglaciations.
  
  • The same basic combination of changing climatic conditions had occurred during all of the earlier deglaciations without causing pulses of extinction. 
•  Another vulnerability of the climatic hypothesis is the fact that so many kinds of mammals throughout the Americas suffered the same fate, even though they lived in environments ranging from semiarid grasslands to rain forests.
• A second explanation, called the overkill hypothesis is that human hunting caused this extinction pulse.
  • The immediate cause of the extinctions could have been either the first arrival of humans in the Americas, the buildup of their populations, or the first appearance of a new hunting technology or strategy among people already present.
• People from Asia first arrived by water, traveling rapidly down the food-rich Pacific coasts of the Americas, and later penetrating eastward across the continents
• A new hunting technology appeared at the same time that the extinctions occurred. Many archeological sites that date to near 12,500 calendar years ago contain spears fitted with a new and elegant kind of stone spear point fashioned by humans. 
• One criticism of the overkill hypothesis is that the early Americans were too few in number to have caused so many extinctions, but studies from population models have refuted this criticism.
  • Because reproduction times for large mammals are long, hunters only need to cull an extra 1–2% of a total species population per year beyond natural attrition rates to cause their extinction within a millennium.
• Another criticism of the overkill hypothesis is that some creatures that do not seem likely to have been hunted also went extinct, including large meat-eating mammals that may have preyed on humans rather than becoming their prey.
• A plausible response to this criticism is that carnivores that depended on the carcasses of large mammals for food may have gone extinct because much of their natural prey had gone extinct at the hands of humans.
• The overkill hypothesis does not explain why some large mammals that would seem to have been likely targets for hunters survived.

16-7 Did early farmers alter climate

• In the early anthropogenic hypothesis published in 2003, it claimed that early agriculture had a substantial impact on greenhouse gases and on global climate thousands of years ago, much earlier than previously thought.
  • This claim was based on the fact that the concentrations of CO2 and CH4 in the atmosphere increased during the last several thousand years of the current interglaciation, even though they had fallen during comparable intervals in previous interglaciations.
• By 10,000 years ago, agriculture had originated independently in several areas, including the Fertile Crescent, China, Mexico, and the South American Andes.
• The first appearance of cereal grains and other crop remains in hundreds of 14C-dated lake cores shows the spread of agriculture from the Fertile Crescent into and across Europe. Because growing crops in tree covered Europe required deforestation.
  • This regional increase in CO2 emissions from deforestation coincides with the start of the anomalous upward trajectory in atmospheric CO2 concentrations near 7,000 years ago.
• The anomalous rise of the methane concentration began near 5,000 years ago. Several human activities generate methane, including irrigating fields, burning biomass, tending livestock, and production of organic waste, including human waste.
• The archeobotanist Dorian Fuller and colleagues have traced the spread of rice irrigation across Asia based on archeological records.
  • Rice irrigation expanded rapidly into other countries during the interval of rising atmospheric methane concentrations. By 1,000 years ago, rice irrigation was practiced across all areas of Southeast Asia where it is in use today. Fuller estimated the total area of rice irrigated between 5,000 and 1,000 years ago. The rising trends in area irrigated and in methane emitted resembled the CH4 increase measured in ice-core records, with 70% of the observed methane rise accounted for by irrigation.
• Critics of the early anthropogenic hypothesis have questioned whether humans can explain the large anomalies in CO2 and methane that developed prior to the last two centuries.
  • This criticized that too few humans were present to have taken control of greenhouse-gas trends millennia ago. Some model simulations of forest clearance based on a direct link to population appear to confirm this view: by 1800, most of the world’s forests are shown as largely intact, even in China, India, and Europe.
• Historical evidence from both Europe and China has shown, however, that the average farmer 2,000 years ago used four to five times as much land as farmers in the 1800s.
  • This decrease during historical times resulted from the fact that population growth slowly reduced the size of land holdings, forcing farmers to obtain more produce per acre by adapting new innovations such as spreading animal manure for fertilizer and rotating crops.
• A model simulation based on historical evidence shows prevalent clearance in Europe, as well as other heavily populated areas by 1800, consistent with the early anthropogenic hypothesis.
• One corollary claim of the early anthropogenic hypothesis is that the anomalous rises in CO2 and CH4 kept the atmosphere warmer than it would have been if nature had remained in control.
• Throughout the last several millennia, the decrease in summer insolation across the Northern Hemisphere was driving a natural cooling at high northern latitudes. If the greenhouse-gas concentrations had fallen as they had done during the previous interglaciations, climate would have cooled even more, perhaps to the point of allowing new ice sheets to begin to form.
• Sensitivity tests with general circulation models support this idea. When greenhouse-gas concentrations are reduced to the “natural” level predicted by the hypothesis, some regions at high northern latitudes retain snow throughout the year.