Humans and Preindustrial Climate

Humans and Pre-Industrial Climate

The black curve shows the northern hemisphere temperature in a climate model.

Michelle Letendre, Evan Wise, Chance Snow, Mary James, Sara Smith, Cristy Echaves

Climate and Human Evolution

Humans evolved in Africa

Radiometeric dating

Evidence of Human Evolution 5 Distinctive Developments Initial branching off from primate ape

Onset of bipedalism

Use of stone tools

Branching of the pre-human Development of large brains line into genus Homo

Appearance of Human Ancestors Humans evolved from a small shrew-like mammal Primitive apes evolved Human ancestors & chimpanzees branched off

Molecular Biology

Walking Upright Hominins

Hominidae

4.3 myr ago

3.6 myr ago The development to walk is still argued today

Use of Stone Tools Used to butcher dead animals “killer ape” hypothesis

Cutting = Use of all the animal Butchering = Diet change A natural evolutionary development

Stone = Digging

Appearance of Homo

Ancestral australopithecines

Paranthropus

Homo erectus

Stone tools and use of intellect & imagination = Frequent & vast movement

Brain Size Braincase – part of the skull that houses the brain Hominid Braincases Growth by Volume Age (Myr ago)

Braincase (cm3)

Homo sapiens

0.2 – 0

1100 – 1500

Homo erectus

2.4 - 1.8

800 – 1000

Australopithecus

4.1 - 3.1

400 - 500

Type

Unusually rapid compared to many evolutionary changes

Braincase volume has 3x

Did Climate Change Drive Human Evolution? • Savanna Hypothesis • Sediments from Indian and Atlantic Ocean Support this Hypothesis

Atmospheric CO2 • A long term decrease in atmospheric CO2 could have been a factor in the change in vegetation • There was a change from C3 to C4 carbon on all continents

Volcanic Plateaus • Vegetation shifted from forest to grassland

• Cooling of Western Indian Ocean

This may be more complicated than we think… Hominins lived in many different environments (woodlands, grasslands, river margins) This leads to a different Hypothesis… The The Variability Selection Hypothesis: Rapid evolution occurred because rapidly changing climate put new demands on our ancestors, which favored those who were more adaptable.

• Links have been found between glaciation and the appearance of the genus Homo

• Pollen records from East Africa support this Hypothesis • Records from the Eastern and Southern Plateaus of Africa support the hypothesis as well.

Problems with the Variability Selection Hypothesis… • It is believed that the ice sheets would’ve had little effect on the African climate.

• Summer monsoons continued for millions of years. No trend toward larger or smaller oscillations.

Aliasing • Different signals become indistinguishable when sampled, or aliases • Also refers to distortions or artifacts that appear in signal reconstructions

Sparse Fossil Records Composed of easily dissolved calcium phosphate

Bad preservation of remains in acid rich soils of rain forests Brief snapshots of human evolutionary history

Aliasing = Misrepresentation Specimens may not be representative of the population

Depiction of inaccurate long-term trends

Aliasing = Misrepresentation

Aliasing is a Problem A formidable concern with just one sample every 100,000 years or so Erroneous indications of significant developments (e.g. first evidence of walking and use of tools)

Complicates correlation of relative timing between climate changes and first use of new traits

Volcanic Plateaus Vegetation shifted from forest to grassland

Cooling of Western Indian Ocean

First Evidence of Agriculture •Fertile Crescent oAbandonment hunting/gathering

oDomestication of: cattle, livestock, barley, etc.

•

Association of Deglaciation and Agriculture

= Hypothesis about cause and effect link

Proposed Cause and Effect Links • The changing conditions made agriculture more favorable

• The Younger Dryas – Climatic reversal

Climate change is hypothesized to be a major contributing factor to the deterioration of early civilizations

Changing Saharan Monsoons coincided with the movement of peoples throughout the Sahara and the development of the early Egyptian dynasties.

The Mayan Classic Period 300 AD-1,000 AD

Data Collected from lake beds and stalactites indicate severe drought events during the declining years of the Maya.

Anasazi

Climate Change and the Anasazi

Resource Depletion Coupled with Changing Climate

Did Humans Cause Megafaunal Extinction? Megafauna comes from the ancient Greek word mega meaning “Large” and the new Latin word fauna meaning “animal” 50,000 years ago more than 150 genera of mammals larger than ~100 pounds existed 10,000 years ago there were fewer than 50 left

Australia During the most recent glaciation (est. 50,000 year ago) many marsupials and nonmarsupials became extinct

Just before the glaciation humans had first entered Australia through southeast asia because of the low seas The humans used fire to help hunt these animals and leads scientist to believe the humans were the primary cause of extinction

Climate

One of the major explanation of the megafaunal extinction in North America is climate change The deglaciation caused strong summers

Scientists have been critical of climate hypothesis because many of the mammals went extinct over 12,500 years ago

Overkill Hypothesis The idea that human hunting caused extinction of the megafauna

The hunting could have been from the either from the first human arrival in America or by the first appearance of new hunting technology or strategy The first humans inhabited America 12,500 years ago. They crossed into Alaska by using land bring from Asia

Around the same time, 12,500 years ago, a new technology appeared which allowed for humans to hunt bigger game

Criticism of the Overkill Hypothesis Were there enough people to cause extinction of the megafauna? o Large mammals have slow reproduction o Humans worked in team to drive animals to their death over step cliffs. So many animals died but only a fraction were used for food and clothing.

Many carnivores went extinct but how? o With the prey that carnivores eat going extinct the carnivores would decrease their population. How did the megafauna we see today survive?

Diluvial Hypothesis tries to explain the worldwide flood and the widespread deposit

Black Sea Flood Hypothesis

Early Anthropogenic Hypothesis Proposed by William Ruddiman

 States that cyclic variations in CO2 and CH4 driven by Earth’s orbital changes during the 350,000 yrs predict a decrease in CO2 and CH4 but anomalies in the data show an increase in CO2 occurring 8,000 yrs ago as well as a rise in CH4 occurring 5,000 years ago.  Deforestation began 8,000 yrs ago leading to an increase of CO2.  Large scale rice cultivation began in India and China 5,000 yrs ago producing higher levels of CH4.

CO2 & CH4 Influences

Further Evidence for the Early Anthropogenic Hypothesis 5, 000 years ago the Monsoon weekend across the Sahara reducing CH4 emissions from wetlands. Arctic summers became cooler England in 1086 was 85% arable land was pasture or crops, 15% remaining forest.

Rice irrigation began in SE Asia 7,000-6,000 years ago increasing considerably 5,000 years ago. 2,000 years ago 50 million people were living in China Circulation Models support this idea indicating that a new glaciation would already be underway.

Critiques of the Early Anthropogenic Hypothesis

Argue that there were not enough humans to effect greenhouse gases in a substantial way. Anomaly can not be accounted for even if all the forests in Southern Eurasia were cut down centuries before.

Chapter Highlights What are the 5 distinctive developments of humans? What issue does the effect of aliasing raise?

Where was the first evidence of agriculture found? What was the main contributing factor to the abnormal high precipitation during the height of the classic Mayan period? What is the general idea of Early Anthropogenic Hypothesis?

Chapter Highlights What is the main difference between the Savanna Hypothesis and the Variability Selection Hypothesis? What were the two hypothesis for the extinction of the megafauna? Evidence points more toward the support of the Savanna hypothesis rather than the Variability selection hypothesis.