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Chapter 25: Brief History of Life
Transcript of Chapter 25: Brief History of Life
(Brief) History of Life
Photosynthesis leads to the accumulation of O2 gas in the Earth.
First: O2 reacts with Iron in the ocean. This produces "
Next: O2 concentration in the atmosphere increases (fast!)
Before we begin:
How do we know what we know about things that we weren't around for?
Should we just not bother to try?
Evidence! Extraordinary Claims = Extraordinary Evidence
Why do we think this happened?
This kind of thinking is called
What was Early Earth like?*
Hot: Due to the formation of the Earth
Violent: Due to the formation of the Solar System
Toxic: Lots of nasty gasses in the amosphere (CO2, NH3, SO2, H2O). NO OXYGEN GAS!
The "Heavy Bombardment": Comes from being bombarded by comets & asteroids...a lot.
Something really big blasted off the moon
The atmosphere was either reducing, or neutral (the modern atmosphere is oxidizing)...favoring increasing chemical complexity.
* We Think
Make Sure You Can
How old is the Earth? Where did it come from?
How did life begin?
How have the history of life and the history of Earth influenced each other?
Explain how scientists are able to date the ages of all events discussed in this presentation.
Describe the hypothetical steps that had to occur for life to arise in the Universe.
Explain the significance of all time periods discussed in this presentation.
Describe the effects of oxygenation of the atmosphere, extinctions and adaptive radiation on the history of life.
Explain how the field of evo-devo has informed our thinking about the relationship between changes in genes and changes in forms.
The Origin of Life in 4 Steps
1. Formation of Biological Molecules
2. Accretion of proto-cells
3. Development of an information molecule
Biological molecules are complex.
Could they be created on Early Earth?
Research suggests a resounding "yes"!
Stanley Miller (1953)- simulated "Early Earth" conditions in the lab.
Created amino acids, other organic molecules
Other experiments have since demonstrated biological molecule formation in:
Asteroids (in space)
Comets (in space)
Collections of biological molecules ("
") has been investigated extensively in labs.
Have a primitive
Absorb & excrete biological molecules
a hypothesis that proposes that RNA was the first information molecule.
Based on the fact that RNA has both information storage & catalytic functions.
: RNA enzymes.
Artificial evolution of RNA molecules in vitro has been demonstrated in laboratory investigations.
A few things to keep in mind:
1. It could have taken hundreds of millions of years to happen (we've got plenty of time).
2. Once it starts, evolution creates a positive feedback loop that leads to increasing efficiency & complexity of life.
3. Maybe it happened somewhere else, first ("
How do we know when things happened?
A Note about Ages
of radioisotopes is a universal constant
The amount of particular radioisotope left = age (within a few million years)
~15 billion years old
(based on light years of background radiation from the Big Bang)
Solar System & Earth:
~4.5 billion years old
(dating of asteroids)
Photosynthesis evolves ~500 million years after life evolves.
The Heterotroph Hypothesis
prior to photosynthesis, life ate other life (was
Evolution of Eukaryotes
Cyanobacteria: photosynthetic bacterial cells.
Stromatolites: Large colonies of cyanobacteria
Oxygenation of Earth
Any life more complex than bacteria is
Eukaryotes have many compartmentalized
Where did these organelles come from?
The availability of Oxygen in the atmosphere allows for the evolution of
aerobic cellular respiration
, a process that produces more than 10X the amount of ATP from breaking down biological molecules than oxygen-free ("
") respiration does.
...how should this effect life's evolution?
And Away We Go!
The theory that particular organelles (
) evolved from free-living bacteria which were engulfed by larger bacteria.
Chloroplasts & Mitochondria:
Reproduce independently from cell division
Have their own DNA (bacterial in structure)
DNA sequencing demonstrates bacterial origins
Have their own ribosomes (bacterial in size)
Have a double membrane
Endosymbiosis was a win-win situation.
Clues about the evolution of multicellularity can be seen in modern organisms.
It involves advances in cellular messaging and communication, present in unicellular life.
a species of social amoeba that has unicellular and multicellular phases of its life cycle
Patterns in the Evolution of Life
After a slow start (~3.5 billion years). The
of life increases exponentially from ~550 million years ago until today.
This kind of pattern is indicative of a positive feedback relationship between evolution of complexity and time. Why would such a relationship exist?
The Evolution of Homo
Even though the diversity of life has increased, most species (99.9%) that have ever evolved have gone
Examination of fossil evidence has show that the history of life has been punctuated by 5 particularly large extinction episodes (the "
"). There is much debate about the causes.
Contrasted with extinctions, the pattern of evolution also includes many examples of the relatively fast appearance of many species.
: When one species rapidly evolves to fill many diverse niches. Typically happens on islands (finches) or after a big extinction (mammals)
The evolution of humans is noteworthy for the effect that humans have had on the rest of the planet.
- The current geological period (from about 11,ooo years ago), wherein the actions of the human population are able to have a global impact.
Evolution Up Close
150 years on, we have a much better understanding of how evolution can work to drive changes in a population. Much of this understanding is driven by our current understanding of genetics and development.
- The field of evolutionary biology that deals with these questions
A little change can go a long way
Very few genes are in control of development.
Hox genes are responsible for the development of specific sections of animals.
Changes in the patterns of expression of these genes can lead to big changes in an animal's body plan.
The Action is staged, the age is not
Time sequence model of early solar system
An example of a ribozyme that manipulates other RNA molecules
The Revolution of Photosynthesis
Complexity of life increases in exponentially shorter time periods
Don't see too many Dimetrodons anymore
Extinction rate vs. Number of taxonomic families over time (can you spot the 5 great extinctions?)
Artists rendering of the Chixulub impact event which is thought to have precipitated the KT extinction.
The radiation of all animal phyla
3 Adaptive radiations:
Mammals: at the end of the cretaceous
Cichlid fish: in lake Victoria
Dubautia plants: on the Hawaian Islands.
Why were these radiations possible??
The progression of the hominid lineage from divergence with Chimpanzees to Homo sapiens.
Three (of many) species that were driven to extinction by the actions of the human population:
The Passenger Pigeon
The Tasmanian Tiger
The Great Auk
Human population growth vs. number of recorded extinctions since 1400 ad
Hox gene expression correlated to body segments in a fruit fly
Differences in 1 Hox gene expression in insects and crustaceans of the arthropod phylum
The expression of a specific Hox gene (purple) is responsible for limbless segments in chordates
In suitably operatic tones...
Putting this all a slightly different way...
RNA WORLD Hypothesis
Metabolism First Hypothesis
Two possible means (A and B) by which lipid-enclosed cells
could have detached from their iron-sulfur catalyst
Unit 1: The Origin of Life