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.