How can we know that scientific knowledge is true?
We can’t 100% know. But, scientific knowledge is evidence-based, testable, and based on probability. More evidence = better supported knowledge.
What does probability do in science?
It explains the most predictable outcomes based on large scale sample sizes. It demonstrates the most likely pattern.
What is important to remember about scientific probability?
There are exceptions to most rules in science, but that does not mean we discard the rule completely. Living systems don’t behave identically, which creates variation in probability.
What was the atmosphere of early earth made up of?
Hydrogen sulphide, methane, and carbon dioxide. We know this based off of scientific testing.
What was the role of oxygen in the atmosphere of early earth?
There was little to no oxygen in the atmosphere, it was in a reduced state meaning that if it was present, it was taken away.
Why was oxidation prevented on early earth?
Because oxygen was not present in the atmosphere, no redox reactions could occur.
What is primordial soup? How was it created?
Primordial soup refers to the ocean on early earth filled with organic molecules (like amino acids) being turned into organic compounds by lighting. Theorized that this is how the first life forms evolved.
What was the Urey-Miller Experiment?
- Simulated early earth conditions with atmospheric elements they believed were present at the time.
- They also simulated the water cycle to mimic the ocean.
- Used an energy source to mimic the lighting reacting with organic molecules.
- Amino acids were found to be present, which contain carbon (believed this is how early life forms evolved).
What were reducing environments?
Still pockets in the deep ocean that released chemicals that reduced oxygen. A habitat for sulphur bacteria and thermophiles that don’t “eat” oxygen.
What did reducing environments on early earth suggest?
That the vent communities (sulphur bacteria and thermophiles) might provide an early model for the early evolution of life where there was no oxygen present.
What are chemolithotrophs?
Prokaryotic species that don’t feed on sugar, carbon, or oxygen. Allowed prokaryotes to use a different food source (like hydrogen sulphide gas) to maximize their ATP production in the electron transport chain. This occurs in highly reduced states, like on early earth.
What are the first documented prokaryotes?
The oldest specimens are billions of years old, they are called cyanobacteria and have been found in stromatolites.
What are stromatolites?
They are fossilized proof of early life forms. Cyanobacteria form a biofilm as their cells die. This biofilm trapped dust and debris over time, layering until it eventually became too heavy and sunk to the bottom of the ocean.
Why were cyanobacteria so important?
They obtained their energy through sunlight (photosynthetic) which made them better suited for survival. This photosynthesis created oxygen as a product in the atmosphere (oxidized environment). This oxygen dramatically stimulated biodiversity, and lead to the near-extinction of organisms intolerant to oxygen.
What was the great oxygenation event?
Occurred over a few billion years ago. Because of cyanobacteria, oxygen was being released into the atmosphere.
What is the proof for the great oxygenation event?
Banded iron formations were abudent between 2.5 and 1.8 billion years ago, with a decline after. This proved that iron was interacting with oxygen at high levels, creating rust layers. Hydrated and oxidized minerals emerged during this time in general.
Why was there a long gap of time before the atmosphere was largely oxygen?
- It is possible that there was a long period of an-oxygenic photosynthesis, where oxygen was not being released.
- Free oxygen was immediately reacting with ocean chemistry.
When did oxygen begin to become very abundant in the atmosphere?
- Oxygen in the ocean eventually reached a threshold, so it became released more often into the atmosphere instead.
- During the carboniferous era, land plants emerged and a huge spike of atmospheric oxygen appeared (as high as 35%).
How do eukaryotes differ from prokaryotes?
- Larger
- Nucleated (which contains genetic information)
- Membrane bound organelles in cells
How did eukaryotes first develop?
Through endosymbiosis (symbiosis between separate single-celled organisms), key eukaryotic organelles originated. Hypothesized that archaea and bacteria became eukaryotes, as bacteria could produce more energy for archaea if they stayed in their system.
What evidence proves endosymbiosis?
- Organelles are bound by membranes with similar composition to prokaryotes (ex. phospholipids).
- Organelles have circular DNA separate from DNA in the nucleus (which prokaryotes have).
- Mitochondrial DNA sequences similar to some bacteria, chloroplast DNA sequences similar to some cyanobacteria.
- Mitochondria replicate by pinching, like prokaryotes.
What was the Cambrian explosion?
The rapid appearance of many groups of organisms (around 500 million years ago). Some of these organisms are still around today. These organisms created small shells (armour), evidence of crabs and lobsters. The emergence of heads, mouths, eyes, and legs.
Why did the Cambrian explosion occur?
- There was more genetic diversity.
- Oxygen level increase allowing for higher metabolism and larger body size to capture and create energy.
- Evolution of grazing (eating), and reduction of algae mats which opened up new niches.
- Shift in ocean chemistry favouring the production of calcium carbonate (to create shells).
What is mass extinction?
When more than 75% of a species or ceases to exist. The rate of extinction usually exceeds the rate of speciation. Mass extinctions are periodic.
