History of Life on Earth
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Formation of the Earth
The Earth was formed over a period of about 20 million years concluding in 4.54 ±
0.04 billion years ago.
Earth gets a friend
About 10 million years after the
formation of the Earth, a Marssized body struck and knocked off
enough material to form the Moon.
→ The Moon is by far the largest
satellite in the solar system relative
to its planet!
Late Heavy Bombardment
From approximately 4.1 to 3.8 billion years ago, the clearing of planetesimals
from the solar system caused a barrage of large impacts on the Earth and Moon.
When did life begin?
When life emerged on Earth will never be precisely known. This is because, the
farther back you go, the more challenging it is to date things.
Reasons for difficulty:
Finding an old sample sets the lower limit for the age of life.
When did life begin?
When life emerged on Earth will never be precisely known. This is because, the
farther back you go, the more challenging it is to date things.
Reasons for difficulty:
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Fewer samples survive
Extant samples more likely to have been altered
Older things are more difficult to date
Finding an old sample sets the lower limit for the age of life.
Stromatolites
Stromatolites are accumulations of sedimentary material formed by floating
microbial mats.
Fossilized stromatolites date back about 3.5 billion years.
Microfossils
Microfossils are the fossilized
remains of single-and multicellular organisms too small to be
seen by the unaided aid.
Microfossils have been found
preserved in rocks more than
3.5 billion years old.
Isotope ratios
An isotope is an atom of an element with a
different number of neutrons. The ratio
between these types is called the
fractionation.
→ The processes of life fractionate carbon by
creating abnormal concentrations of carbon-12.
There are unusual concentrations of
carbon-12 in materials dating back at least
3.8 and maybe 4.1 billion years.
One cell? No problem!
For around 500 millions years, all life
on Earth consisted of unicellular
organisms. Fully self-contained, they
are the most compact, and perhaps
most versatile, form of life.
→ Bacteria are among the most
common forms of unicellular
organisms that survive today.
Living at the edge
An important class of unicellular
organisms are extremophiles, creatures
that exist in the harshest environments on
Earth.
→ Many scientists believe that
extremophiles are some of the closest
representations of life at its inception.
To make food or find food
Autotrophs
Heterotrophs
Organisms that produce their own energy
Organisms that consume other organisms
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Earliest life survived off surrounding
chemical compounds
Later organisms developed
photosynthesis, the process of
converting sunlight and CO2 into
oxygen and energy
The photosynthesis equation:
6CO2 + 6H2O + sunlight → C6H12O6 + 6O2
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Depend on the existence of
autotrophs
Comprise ~95% of all life on Earth
today
Energy transfer is inefficient
Poisoning the air with oxygen
About 2.5 billion years ago, cyanobacteria began using photosynthesis to convert
sunlight into energy. This process produces oxygen. After 200 million years, this
oxygen begins to build up in the atmosphere.
Oxygen is toxic to most life
on Earth at the time and the
so-called Great
Oxygenation Event
causes what may be the
first mass extinction in
history.
Rusty rocks
How can we know how much oxygen was
produced billions of years ago? We think
the early ocean was full of dissolved iron
and when oxygen began to proliferate, it
rusted the iron, which sank to form
banded iron formations on the seafloor.
The geologic record is our key to
understanding the physical conditions
on ancient Earth.
The eukaryote revolution
An organism whose cells contain a
nucleus is known as a eukaryote. All
complex life, including plants and
animals, are eukaryotes.
→ Eukaryotes developed around 2
billion years ago
→ Eukaryotes can be single- or multicellular
From one cell to many
Less than 1 billion years ago, organisms consisting
of multiple cells began to proliferate. The origin of
these multicellular organisms is not well
understood.
→ One hypothesis suggests that different species of
unicellular life became so dependant on one another
that they eventually just merged.
Multiple cells with different functionality allow life
to become dramatically more complex.
The invention of sex
An important innovation for multicellular organisms was the development of
sexual reproduction. Prior to sexual reproduction, unicellular organisms would
simply produce copies of themselves. Now, multiple sets of DNA would be
combined.
Benefits of sex
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Introduce minor genetic
differences
Defense against parasites
Drawbacks of sex
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More likely to eliminate a useful
variation
Less efficient for population growth
Competition for mates can
promote anti-survival traits
The timeline of life
Multicellular life
Eukaryotes
Photosynthesis
Unicellular life
Earth forms
4500
4000
3500
3000
2500
2000
Millions of years ago
1500
1000
500
0
Small group discussion
WIth those around you, discuss the following questions:
1. What evidence of possible ancient life on Mars do you think would be
easiest to find?
2. If we visit a random planet around a random star, what soft of life
would you expect to find?
Be prepared to share your answers with the class!
An explosion of life
Beginning 542 million years ago, the fossil record grows dramatically. One
interpretation of this is a rapid increase in the complexity and diversity of life,
called the Cambrian explosion.
From sea to land
In the aftermath of the Cambrian explosion,
life began to venture out of the oceans and
onto land. This necessitated major evolution
to solve a number of problems.
Some major evolutionary steps:
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●
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Switch from extracting oxygen from water
to extracting it from the air
Reshaping of the eyes to account for
different density of air
Development of internal fertilization for
reproduction
About that land...
The crust of the Earth is broken into a series of
tectonic plates, which move independently from
another another. Through history, landmasses have
separated and recombined numerous times.
→ Plates move a few centimeters per year. That’s
about the speed your fingernails grow
→ Collisions between plates result in earthquakes
and the formation of mountains.
Once life left the ocean, it was strongly
influenced by the motion of the continents.
The worst time on Earth
In a brief period beginning around 252 million years ago, 96% of species went
extinct during the Permian extinction.
→ Increased in atmospheric CO2 caused global warming and disrupted
photosynthesis
→ Ocean acidification killed most sea creatures
→ Only extinction to dramatically affect insects
Possible causes:
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Massive volcanic eruptions in what is today Siberia
Giant impact similar to what killed the dinosaurs
Death begets life
In the aftermath of the Permian extinction, three new kinds of life arise:
That part we all know… but how?
66 million years ago, a giant impact from space killed
the dinosaurs and launched mammals on their path as
the dominant complex species. But how do we know
this?
The thinking creatures
Australopithecus
4.5 - 2 Mya
Homo habilis
2 - 1.5 Mya
Homo erectus
2 - 0.07 Mya
Creativity
The timeline of life
Dinosaurs
Humans
Multicellular life
Eukaryotes
Photosynthesis
Unicellular life
Earth forms
4500
4000
3500
3000
2500
2000
Millions of years ago
1500
1000
500
0
Small group discussion
By one estimate, the Milky Way is home to 11 billion Earth-sized planets orbiting
Sun-sized stars at or near that star’s life-supporting region, called the habitable
zone.
Discuss with those around you:
1.
Based on the history of life on Earth, can we narrow down which planets to
search for complex life?
2. What might be indicators from afar that a planet has life on it?
Be prepared to share you answers with the class!