Humans and Pre-Industrial Climate

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

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

Climate and Human Evolution

Radiometeric dating

Humans evolved in Africa

Evidence of Human Evolution5 Distinctive Developments

Onset of bipedalism

Use of stone tools

Initial branching off from primate ape

Development of large brainsBranching of the pre-human

line into genus Homo

Human ancestors & chimpanzees branched off

Humans evolved from a small shrew-like mammal

Primitive apes evolved

Appearance of Human Ancestors

Molecular Biology

Walking Upright

The development to walk is still argued today

Hominins

Hominidae

4.3 myr ago

3.6 myr ago

Use of Stone Tools

A natural evolutionary development

Used to butcher dead animals

“killer ape” hypothesis

Butchering = Diet change

Cutting = Use of all the animal

Stone = Digging

Appearance of Homo

Ancestral australopithecines

Paranthropus

Homo erectus

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

Brain Size

Hominid Braincases Growth by Volume

Type 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

Braincase – part of the skull that houses the brain

Braincase volume has 3x

Unusually rapid compared to many evolutionary changes

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 Deglaciationand Agriculture

Hypothesis about cause and effect link

=

Proposed Cause and Effect Links

• The changing conditions made agriculture more favorable

• The Younger Dryas– Climatic reversal

Early Civilizations and Climate Change

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

10,000 years ago there were fewer than 50 left

50,000 years ago more than 150 genera of mammals larger than ~100 pounds existed

Megafauna comes from the ancient Greek word mega

meaning “Large” and the new Latin word

fauna meaning “animal”

Did Humans Cause Megafaunal Extinction?

AustraliaDuring the most recent glaciation (est. 50,000

year ago) many marsupials and non-marsupials 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 summersScientists have been critical of climate hypothesis because many of the mammals went extinct over

12,500 years ago

Overkill HypothesisThe 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 HypothesisWere there enough people to cause extinction of the megafauna?

o Large mammals have slow reproductiono 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?

Black Sea Flood Hypothesis

tries to explain the worldwide flood and the widespread

deposit

Diluvial Hypothesis

Early Anthropogenic Hypothesis

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.

Proposed byWilliam Ruddiman

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 HighlightsWhat 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?

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.

What is the main difference between the Savanna Hypothesis and the Variability Selection Hypothesis?

Chapter Highlights