The History of Life
History of Life
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It is now believed by scientists that the earth
is about 4.6 billion years old.
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However, in its first 750 million years the
earth was extremely hot…
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So hot that the oceans did not form until 3.9 to
3.5 billion years ago…when the earth finally cooled enough for there to be
liquid water.
History in rocks
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Scientists cannot
be sure that the earth formed this way… they can only hypothesize based on clues
left behind in rock.
Fossils
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There are many different types of fossils that
hold clues to the past.
-Trace
Fossil: Footprints, trails
-Petrified: Mineral Copies of once living
things.
-Cast/Mold:
Empty space left in a rock by an organism’s body
-Imprints: Small indentations
-Frozen: Quickly trapped in ice
Age of Fossils and Rocks
Relative Dating
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Uses the idea that newer rocks sit on top of
older rocks
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If a fossil is found in a
higher layer of rock, then it also younger than those below.
-Relative dating cannot give exact dates though…
Radiometric Dating (carbon dating)
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By examining the amount of organic (once living)
material still in a fossil, it’s age can be
determined.
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Also works for rocks…rocks will decay from one
form to another.
Geologic Time Scale
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The entire
history of the earth is measured on the geologic time scale.
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Just like a
regular time, it is broken down into different intervals.
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The largest is ERA
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An ERA is several
hundred million years long.
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Then each ERA can
be broken down into PERIODS.
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Each Period is
only in the tens of millions of years.
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And each period
can be broken down and so on.
There are four major eras of the earth
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Precambrian
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Paleozoic
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Mesozoic
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Cenozoic
Cenozoic
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The current era is the Cenozoic which goes back
about 66 million years ago.
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Your book claims that humans first began around
200,000 years ago, however, humans as we know them did not appear until 10,000
years ago.
Spontaneous Generation
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The idea that nonliving
material can produce life.
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This concept was, at the time, a reasonable
explanation of events.
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Later discoveries however would prove it false.
Francesco Redi
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Francesco Redi
was the first scientist to disprove the spontaneous generation theory.
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His experiments
were based on raw meat and flies. (page 388)
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However, Redi’s
experiments did not deal with bacteria at all…
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This is important
because most scientists believed that bacteria were created through spontaneous
generation.
Louis Pasteur
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Finally Louis Pasteur was able to disprove s.g. entirely with his experiments involving broth filled
flasks.
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Eventually the theory of Biogenesis (life only
from life) was accepted and integrated into the cell theory.
The first cells
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Scientists currently theorize that the first
forms of life were prokaryotes, around 3.5 billion years ago.
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Over the course of the billion years,
prokaryotes grew in numbers and diversity.
endosymbiont theory
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Eventually, according to the endosymbiont
theory, new types of cells, eukaryotes evolved.
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The endosymbiont theory was first developed in
the 1960’s and is still the current theory that scientists use to explain the
formation of life.
However, it is still just a theory.