2. When did these events occur?
1. Find the length of masking tape at your table.
2. Write Earth forms on one end of the tape, and
today on the other end.
3. As a group, place the following six events in
the timeline:
a. Mammals first appear
b. Amphibians first appear
c. Fish first appear
d. Birds first appear
e. Reptiles first appear
f. Dinosaurs first appear
14. MASS
EXTINCTION 1
445 Ma
Nov 27
MASS
EXTINCTION 1
445 Ma
Nov 27
MASS
EXTINCTION 3
252 Ma
Dec 12
MASS
EXTINCTION 3
252 Ma
Dec 12
MASS
EXTINCTION 5
66 Ma
Dec 27
MASS
EXTINCTION 5
66 Ma
Dec 27
MASS
EXTINCTION 2
360 Ma
Dec 4
MASS
EXTINCTION 2
360 Ma
Dec 4
MASS
EXTINCTION 4
198 Ma
Dec 17
MASS
EXTINCTION 4
198 Ma
Dec 17
Mass Extinctions
542Ma
today
FISH
AMPHIBIANS
REPTILES
DINOSAURS
MAMMALS
BIRDS
CO2
T°CO2
T°
CO2
T°
CO2
T°
CO2
T°
We just explore 2 snapshots in the history of life on Earth – how do those snapshots fit in the bigger picture?
When geologists look at rocks, they examine a variety of characteristics (composition, structure, deformation features, etc) including biological/fossil assemblages. These assemblages are snapshots of some of the organisms that existed at a given point in the past. By figuring out how old the rocks are (radiometric dating, relative dating), they can then determine the (minimum) age of the fossils found in them. For instance: Neanderthal fossils have been found in rocks as old as 400,000 years in other words, Neanderthals first appeared 400,000 years ago.
We are going to look at the Earth’s history and together use evidence to figure out where these animals first evolved. First, I want to see where your thinking is, so at your table groups place the 6 events in the timeline.
Share out, come up with whole class starting point.
walk around to see if all groups have the same order debrief accordingly
What do these organisms have in common?
All breathe oxygen
All made of prokaryotic cells
All but 1 need land
Take CARD 1
Oldest mineral 4.4 Ga, Zircon
Scientists estimate the age to be 4.6 Ga
Oldest Moon rocks 4.4-4.5 Ga
Most meteorites 4.4-4.6 Ga
Radiometric dating of lead ores (Galena) – 4.56 Ga
Why is it so hard to find older rocks?
Talk about how 4.6 billion is a very big number. It is hard to comprehend how big. If we counted to 4.6 billion, one number at a time, and said one number per second, it would take us 146 years to get there….for this reason, we will use some tools to help us scale time to something we can comprehend.
A map is a way of looking at very large areas on a small surface – for geographical navigation, display of resources or topography, or other geographical based distribution of information. Maps are a scaled down version of the real world
We need a scale to tell us how the map relates to the real world.
This map is at a 1:4,000,000 scale
That means that 1 unit on the map equals 4 million units on the ground 1 cm on the map equals 4,000,000 cm in the real world
A MAP IS A WAY TO SCALE SPACE
BUT YOU CAN ALSO SCALE TIME
This is scaling (one of the CCC) a concept that we can use with other very large distributions of information, like time.
In fact, you just did that with the masking tape and the different classes of animals.
Your whole 100cm-strip is meant to represent 4.6 billion years (4,600,000,000), so each cm represents 46 million years.
This may seem pretty abstract to a middle schooler, so we’re going to use something familiar to help make sense of the vast time that we’re working with.
We’re going to use a calendar as our scale model for geologic time.
Carl Sagan did this in his book The Dragon’s Eden and in his Cosmos tv show – he used a calendar to visualize the vast history of the universe, which is 13.8 billion years old. In his model, January 1st is the Big Bang, and December 31st is today.
We’ll take this model, but modify it to only look at the history of the Earth – only 4.6 billion years!
So January 1st will represent the Earth’s formation, and December 31st (the very last milliseconds of it) will represent today.
(write on the board)
1 day = 12.6 million years
1 hour = 525,000 years
1 minute = 8,750 years
1 second = 145.8 years
Hadean Eon = 4.6 – 4 Ga = Jan 1-17 Color calendar
Name means of the underworld – this was a very inhospitable world
Earth forms – 4.6 Ga = January 1 – What did the Earth look like then?
Planet had just formed and was cooling
Very high volcanism, partially molten surface
Frequent collisions with other solar system bodies (Late Heavy Bombardment)
100 million years into Earth’s history: solid crust had formed, oceans began to form (from comets and volcanic outgassing), and an atmosphere began to accumulate. Solid Crust, atmosphere, and oceans January 7
Key aspects of the Earth that allow this to happen:
Earth is the right distance from the Sun – the atmosphere and oceans regulate the temperature, and keep it within the range where liquid water is stable
Earth is big enough for its gravity to keep the atmosphere in place
The atmosphere and the magnetic field protect the earth’s surface from harmful UV rays
It is possible that some microbial life may have also existed, but there is no evidence left.
How do we know? Oldest zircon (Jack Hills, western Australia) – 4.4 billion years old. Dated using Ur Pb January 16
Does this look like the kind of place where the animals we sorted could live?
Is there oxygen? No, not free oxygen in the atmosphere
Are there oceans? Yes
Is there land? Yes
NO – these organisms could not live in this environment (move organisms in timeline, if needed)
Archean Eon = 4 – 2.5 Ga = Feb 18 – June 16
Name means ancient beginning
Earth’s heat flow was nearly 3X as high as it is today – remnant heat from planetary accretion, from core formation, and radioactive decay.
High volcanic activity
Sun had 70-75 % present luminosity
Reducing atmosphere consisting primarily of methane, ammonia and other gases that would be toxic to modern life forms – NO FREE OXYGEN
Beginning of formation of continents, but liquid water was prevalent – shallow and deep water deposits exist.
CARD 2 - stromatolites
First traces of life – simple, unicellular organisms (microbial communities found in Australia) - appear 3.5 Ga = March 28
Fossils are stromatolites: layered mounds of mud produced by the growth of microbial mats. These organisms photosynthesize
But there is still no free oxygen throughout this eon
Does this look like the kind of place where the animals we sorted could live?
Is there oxygen? No, not free oxygen in the atmosphere
Are there oceans? Yes
Is there land? Yes
NO – these organisms could not live in this environment (move organisms in timeline, if needed)
BUT WHY IS THERE NO OXYGEN, IF IT’S BEING MADE?
You may wonder, or students might ask questions like “How do we know? How do we know that stromatolites photosynthesized?”
Turn to shoulder partner and share ideas – share out w/ whole class
We have fossil evidence of organisms that has been dated using radioactive dating techniques
Some organisms, like stromatolites, are still alive today – we can see how they live, grow, and undergo life processes today, and work with the concept that the processes that created the fossil stromatolites are the same as the ones that are creating modern stromatolites today (the present is the key to the past)
We will use this idea of using information from rocks to tell us about the past to decipher the second half of the year, and to narrow down where the classes of animals belong in the timeline.
BUT WHY IS THERE NO OXYGEN, IF IT’S BEING MADE?
During the Archean, oceans were filled with dissolved iron (iron easily dissolves in water as long as there is no oxygen present)
Where did the iron come from? Volcanism, weathering and erosion of fe-bearing rocks
When cyanobacteria began producing oxygen, the oxygen reacted right away with the iron in the water and precipitated out as BIF’s (the iron in the water was an oxygen sink)
Iron eventually ran out what then? Free oxygen could now start to accumulate. (next slide)
So we are in the next Eon
Proterozoic Eon = 2.5 Ga – 542 Ma = June 17 – November 18
Name means early life
First stable continents first appeared and began to accrete
First abundant fossils of living organisms, mostly bacteria and archea (microbial prokaryotes – no cell nucleus, nor any membrane-bound organelle in cells)
CARD 3 – ATMOSPHERIC OXYGENATION
Atmospheric Oxygenation 2.3 Ga = July 9
First evidence of oxygen buildup in the atmosphere = global catastrophe – was doom for most living organisms of the time, but made possible the explosion of eukaryotic forms (multicellular algae, and eventually the first animals)
considered Earth’s first large-scale pollution event
evidence points to a fairly rapid in crease in O2 levels (in Archean = 1%, Proterozoic = 15%, now = 21%)
Oxygen is a powerful degrader of organic compounds, and so lethal to many life forms
Organisms had to evolve biochemical methods for rendering oxygen harmless – one of these methods, oxidative respiration, had the advantage if producing large amounts of energy for the cell, and is now found in most eukaryotes.
Something else happened in this Eon…
First Eukaryotic Cells 2.1 Ga = July 17.
Prokaryotic cells appeared first (starting with the stromatolites). Prokaryotic cells don’t have much – a cell wall, a cell membrane, and a few ribosomes are visible here. The cell has no nucleus or any of the other organelles
Eukaryotic cells are much more complex. Eukaryotic cells have a nucleus and many different organelles
Since all of these organisms are made of eukaryotic cells, these organisms could not have existed before these cells evolved, i.e. before 2.1 Ga
Move organisms in timeline as needed
Phanerozoic Eon = 542 Ma – present = November 19 – December 31
Name means visible life
Current geologic eon
Only eon during which abundant animal and plant life has existed. This time includes the rapid emergence of a number of animal phyla, the evolution of these phyla into diverse and complex life forms.
541 million years to the present
CARD 4 – Oldest Land Fossil
At the beginning of Phanerozoic all land was bound to the ocean
earliest/oldest land fossils (plants) show up in the rock record 440 Ma = November 27
What does the age of the oldest land fossil tell us about when our animal classes could have evolved?
The fish could have evolved before 443 Ma
But since amphibians, reptiles, dinos, mammals, and birds live on and depend on land, these organisms could not have existed before life existed on land, i.e. before 443 Ma
Move organisms in timeline as needed
Life in the Phanerozoic
Because the Phanerozoic is characterized by greater diversity and greater amount of biomass, and also because the rocks record is younger and less likely to have been destroyed by Earth’s dynamic processes (weathering & erosion, plate tectonics), we have a much more detailed fossil record of life and how it changed over time.
The story of this eon is one of life, but also one of mass extinctions. Let’s explore. The 5 major mass extinctions of multicellular life that we know of in Earth’s history happened in the Phanerozoic
With that last piece of information, we can now place (at least relatively) where our organisms would fall on the timeline – fish are only organisms that can go to the left, rest have to be to the right
The emergence of life from the oceans and the conquering of land is a key event of the Phanerozoic.
There is another important story here
An important aspect of this Eon is that it also holds the 5 major mass extinctions of multicellular life that we know of in Earth’s history we will explore this eon through that lens.
If we look at the fossil record of the Phanerozoic, we see the story of life becoming more complex and diversified, and we also see the story of mass extinctions
The 5 major extinctions of multicellular life happened in the Phanerozoic
Go through (briefly or extensively – choose short or regular version of pwp)
Phanerozoic begins with life entirely confined to the oceans; great explosion of life forms
Extinction 1 – caused by drop of CO2 in atmosphere (weathering of silicate rocks) and tectonic movements placing Gondwanaland in polar regions; 82-88% of species died off
After extinction, diversity increases again, fish dominate marine and freshwater, plants conquer land
Extinction 2 – caused by drop of CO2 because of increased plant photosynthesis on land
After extinction, sharks fill ecological gap left by placoderm extinction, reptiles evolve on land
Extinction 3 – The Great Dying, over 90% of all species died off – life almost eliminated. Massive volcanism.
After extinction, it took a long time for life to recover, mammal-like reptiles flourished, dinos and mammals evolve
Extinction 4 – MAR rift begins, increased volcanism increase CO2 76-84% species
After extinction, with most large amphibians and mammal-like reptiles gone, there was an opening for a new dominant land animal = age of the dinos.
Extinction 5 – ongoing massive volcanism followed by the famous asteroid impact
After extinction, the age of mammals
A NOTE ON EXTINCTION
How do you think extinctions may be important in the context of evolution?
Mass extinctions have played many evolutionary roles:
differential survivorship or selectivity of taxa and traits
the disruption or preservation of evolutionary trends and ecosystem organization
the promotion of taxonomic and morphological diversifications—often along unexpected trajectories—after the destruction or marginalization of once-dominant clades.
A system can be stable on a short/small time scale, but on a longer/wider scale it can be changing
Change can be difficult for humans to observe and understand, so we will use bridging examples –
When looking at a living organism over the course of an hour or a day, it may seem unchanging.
Over months or years, the organism may grow, age, and eventually die.
That organism is part of species that can remain seemingly unchanged for even longer periods, but it actually being changed through natural selection.
The collection of all of the changes in species over the history of the Earth, the evolution of life, is a process that has occurred over billions of years, and has brought forth all life forms that exist and have ever existed on Earth.
