History of Life
Once the first living cells arose, 3.9 BY ago,
Natural Selection easily explains the appearance of all other species
Chemical evolution became biological evolution
3.8 BY
indirect evidence that life may have existed
3.5 BY
1 st true fossils: very simple cells = procaryotes
11 different species have been found
In 1800’s, Challenger Expedition – deep sea collections
 thought they had found the first life forms:
Bathybius haekelii – “primordial slime”
 turned out to be precipitate formed when sediment was
mixed with alcohol
How do We Know How Old Rocks and Fossils Are?
Relative Ages
There are several kinds of rock found on earth
the easiest rocks to date are Sedimentary Rocks
 rocks formed in the ocean of thousands or millions
of years from the slow deposition of successive layers of sediment and
dead shells and organisms
we have 350,000 ft (75 miles) of sedimentary rock
deposits preserving an extensive record of past life
but still many gaps; not a continuous series
at first was only a relative dating method
later became more accurate as we got a better
understanding of how sedimentary rock is produced
Absolute Dating
much more accurate dating technique
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based on the very precise rate of radioactive decay for
various isotopes
some systems currently used:
Decay System
Rubidium Strontium
Lutetium  Hafnium
Uranium Lead
Potassium  Argon
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C
Half Life
1.42X10-11 yr-1
1.94x10-11 yr-1
1.55 X10-10 yr-1
0.581 X10-10 yr-1
5568±30 yr3
Useful Timespan
48.8 BY
35.7±1.2BY
4.47BY
1.93BY
Once life arose, what were some of the major events
that occurred from then until now?
Major events in History of Life (fossils):
1. Photosynthesis (3.5 by)
2. Aerobic Respiration (1.5 by)
3. Eucaryotes (1.8 by)
4. Multicellular Forms (~0.8 by)
first animals
first fungi
first plants
Photosynthesis
3.5 BY ago
Once life appeared the organic molecules that had
been accumulating for a billion years were quickly consumed
as food supply dwindled any adaptation that would have enabled an organism
to manufacture its own food would be favored
some of the earliest fossils we have are blue green bacteria (= autotrophs)
use sunlight as energy to make large organic molecules
 so photosynthesis arose fairly quickly
photosynthesis uses carbon dioxide and some source
of hydrogen as building blocks to make sugars
these molecules could then be broken down
when energy was needed
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eg. plant/algae photosynthesis:
CO2 + H2O
sunlight
sugar + O2
first autotrophs probably did not produce oxygen
hydrogen source was H2 or H2S (produced S2)
CO2 + H2S
sunlight
sugar + S2
only later ~2.8BY [BG bacteria] was water used as
H source (far more abundant as other sources dwindled)
 probably the most important innovation in the history of life on earth:
life was freed from energy scarcity forever
from then on life was limited primarily by the
scarcity of just a few specific nutrients, eg P or N,
but not an energy source; had limitless energy
when the new form of photosynthesis originated the
waste product of the process was O2
this increased autotroph efficiency and global
productivity by 100’s or 1000’s of times
but:
O2 is lethal to most cells
O2 was probably fatal to most of the earth’s
early life forms (an unrecorded mass extinction?)
cells exposed to oxygen had to develop
protections against it
eventually oxygen began to accumulate in the atmosphere
oxygen gas didn’t show up in the atmosphere for ~500MY after
it was first produced [2.8BY BG bacteria  2.3 BY atm oxygen]
 abundant metals and volcanic gasses of early earth
reacted with free O2
indications of oxygen accumulation are found in rocks up to 2.3 BY old
produced banded iron formations (oldest evidence for bacterial
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oxygen production
greatest “pollution” crisis the world has ever known
 it took over 1.5 BY for O2 to accumulate to near current levels
 ALL the free O2 in our atm was produced by living cells
ozone began to accumulate in upper atmosphere blocking UV rays
UV
O2---- O + O
O2 + O  O3
UV was main energy source in origin of life
Life could probably never again arise spontaneously on earth
Aerobic Respiration
~2BY ago (2.2-1.9BY)
new forms of generating energy probably arose that were more efficient
 aerobic respiration
original heterotrophs probably used fermentation-like processes which
produce very little energy
Only partially breaks down organic molecules:
CCCCCC  CCC + CCC
aerobic respiration extracts much more energy from organic food
breaks sugars down into inorganic molecules; carbon dioxide and water
eg
fermentation of sugar  2 units of energy
aerobic respiration of sugar  36-38 units of energy
(19x’s more)
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The Age of Bacteria
most of life’s history (~2 BY of it) consisted of prokaryotes (bacteria) only
by 1.5 BY most biochemical evolution had been accomplished
all the unique and unusual metabolic pathways found
in all other kingdoms today were already present in bacteria
along with many other more exotic pathways
many of the bacteria alive today seem to have
changed very little from those earliest ones:
eg. stromatolites are still formed today
eg. bg bacteria seem to have changed little
even today bacteria are the most tenaceous beings known
Bacterial Accomplishments:
1.
prokaryotes could assemble and disassemble all molecules of modern life
virtually all metabolic pathways:
photosynthesis
fermentation
aerobic respiration
with the exception of a few exotic chemicals
eg, essential oils and hallucinogens of flowering plants
eg. snake venoms
2. the earth’s modern atmosphere and surface were
largely established by bacteria
3. bacteria invented asexual and a simple form of sexual reproduction
they grow and divide quickly
 life cycle in minutes not days or years
bacteria are essentially immortal
 they don’t age as complex cells do.
bacteria are able to exchange genes with each
other and from the environment
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we can only exchange genes between males and females
 essentially makes all bacteria a “superorganism”
= one worldwide species
Many people contend that even today the world is run mainly by bacteria
the “age of mammals”; “age of dinosaurs”; “age of human
civilization” are myths
eg. more bacteria inhabit your mouth than there are people in NYC
eg. 9x’s more bacterial cells in your body than human cells
(90 trillion vs 10 trillion cells)
eg. eucaryotic food webs
“form a crown – intricate and unnecessary – atop ecosystems
fundamentally maintained by prokaryotic metabolism”
Knoll. 2003. Life on a Young Planet
 we cannot survive without bacteria
but they would do just fine without us!
Origin of Eukaryotes
1.8 BY ago
a stable aerobic environment was now established
no major changes for 1.5 BY
organisms had begun finding ways to protect themselves from free oxygen
and apparently promoted the development of the next
major step in the history of life
1.8 BY ago first eukaryotic cells appear (Protists)
molecular evidence indicates they may have existed a billion years
before that (2.7BY) but no true fossils
eukaryotes appeared suddenly, not due to gradual changes
this was a drastic change from over 2 BY of previous evolution
today all cells are either prokaryotes or eukaryotes
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all cells either have a nucleus or not
prokaryotes and eucaryotes are chemically similar;
all eukaryotic metabolic pathways are found in
procaryotes
major structural differences:
-generally larger cells
-genetic material enclosed within nucleus
-DNA is more complex, more genes, associated
with proteins called histones
-cell walls, when present are made of different chemicals
-contain many “membrane bound” organelles
-more complex kinds of cell division
must be some tremendous advantages:
1. more compartments,
more organization
enzymes for specific metabolic pathways
are grouped together in specific organelles
2. more efficient
almost all eucaryotes (including single celled) are aerobes
 most efficient metabolism
3. 1000x’s more DNA
eukaryotes have 1000’s times more DNA than prokaryotes
more genetic instructions
Eukaryotes evolved by symbioses of various prokaryotes (Mutualism)
=
endosymbiont theory
mutualism is common in prokaryotes:
1. amenable to rapid genetic changes
2. don’t reject intruders
3. chemically diverse, often complementary colonial associations
Evidence for endosymbiont origin
1. many modern organisms contain intracellular symbiotic bacteria
Bacteria are the best endosymbionts
have been doing it for Millions of years
invasive spirochaetes are one of most
successful lifeforms on earth
found in unusual niches:
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eg. crystalline style of oysters
eg. protists in termite gut
2. Some eukaryotic organelles still contain bacterial chromosomes
today in eukaryotes chloroplasts and mitochondria have given
most of their genes to the nucleus of the cell
but all have retained a functional genome of at least 5 genes
3. some eukaryotic organelles contain bacterial ribosomes
4. Some eukaryotic organelles divide asexually in
a process very similar to bacterial reproduction
eg. mitochondria:
 main energy producing organelle in all eukaryotes
-most protists, all plants, fungi & animals
-contain their own DNA resembling bacterial
DNA
-contain many enzymes present in bacteria
-contain bacterial type ribosomes
-produced only by other mitochondria;
host cell cannot produce them alone
-resemble purple nonsulfur bacteria
eg. chloroplasts:
 main organelle for photosynthesis in
eukaryotes
-contain DNA
-contain same pigment as BG bacteria
-photosynthetic procaryote ingested by
larger non-photosynthetic procaryote
-probably happened independently in several
lines
-some protists, fungi, plants and animals
contain symbiotic cyanobacteria
they lack cell walls
divide same time as host
are functionally chloroplasts
eg. cilia and flagella:
-perhaps from motile spirochaete-like
bacteria
-similar symbiotic associations are seen
today in termite symbionts
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eg. centrioles (mitosis):
-related to cilia and flagella structurally
-divide independently of cell
-some report finding DNA within them
Sexual Reproduction
one of the most significant shifts in going from
prokaryote to eukaryote is in how genes are exchanged
prokaryotes freely exchange genes with each
other or with environment
all bacteria can easily trade genes with other
bacteria
 don’t have to be same “species”
even though their main method of reproduction is
asexual fission their genes are “modular”
 freely traded
this creates a powerful tool for evolutionary
adaptation
eg. antibiotic resistance
eg. S. pneumonia pathogenicity in rats
once genes were trapped in a nucleus
an organism’s ability to freely exchange them
becomes very limited
 eukaryotes traded genetic flexibility for an
increase in size and complexity
more efficient use of energy
more complex cells and bodies
but now genetic change is only possible during
reproduction (with a few exceptions)
Eukaryotes and all their offspring (Fungi, Plants, Animals)
now mainly trade genes “vertically”
from generation to generation.
Rather than horizontally
 directly with their neighbors,
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and in the same generation
Result:
genetically fluid bacteria are immortal
but in eukaryotes, sex becomes linked with death
from ~2BY to ~1BY before today, even after the
origin of eukaryotes, bacteria reigned supreme
 eukaryotes existed for a billion years
without “going anywhere” in terms of evolution
why? if they were so much more efficient; such a
good design did they not “take off” and diversify
the answer might be due to low level of O2 in atmosphere and in ocean
O 2 1st appeared in atmosphere ~ 2.5BY ago
still, all but the uppermost layers of the ocean
remained anoxic
low levels of O2 were not enough to free up critical
nutrients such as Fe, Cu, Zn, etc from the minerals and
molecules that contained them
these minerals are required to make the enzymes
needed to extract Nitrogen from seawater
(N fixing requires Fe and other ions as cofactors)
 lack of excess O2 made nutrients
extremely hard to get for eucaryotes
Only after oxygen levels increased were
the micronutrients freed up for use by algae
Multicellular Life
4/5ths of all geologic time was dominated by primitive, slowly evolving
microbes
prokaryotes and eukaryotic single celled organisms
virtually the entire basic organization of the biological
world dates from this time:
aerobes/anaerobes
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fermenters
photosynthesis
aerobic respiration
producers/consumers
bacterial fission
sexual and asexual reproduction
mitosis and meiosis
shortly after eucaryotes appeared, multicellular
organisms appear and diversify in the fossil record
compared to the ~2BY that only bacteria were on the
earth, Eukaryotes appeared and diversified into all major kingdoms in a
relatively short time: a few 100 MY
Unlike the appearance of eukaryotes:
multicellularity was not a major step but
a natural progression toward increased
competitive interaction and specialization
while we tend to think of bacteria and protists as
single celled organisms they more typically form groupings, films,
filaments, etc
 in other words they were functionally
multicellular
poor fossil record during most of this time
mostly soft bodied forms  leave few fossils
there is molecular evidence that they may have
existed much earlier in fossil record
once multicellularity evolved the more complex
lifeforms evolved very quickly
all kingdoms, even bacteria, have “multicellular”
members
even bacteria don’t really function as single cells in
nature:
in any ecological niche teams of several kinds of
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bacteria live together responding to and creating each others
environment
 wastes of one becomes another’s food
 complementary enzymes
eg. stromatolites
 layers of different kinds of bacteria
in most multicellular forms the organism begins life as
a single cell and this cell is “cloned” to produce the adult organism
eg. human zygote  10 Trillion cells of adult
not all multicellular forms begin as a single cell:
eg. this is not the case in a few slime molds and fungi
whose multicellular bodies form from an aggregation of several
genetically distinct individuals
there are 3 main kingdoms of mostly multicellular forms:
animals
fungi
plants
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History of the Earth in a Single Year
days
(1)
Jan 1, 12:00am - earth formed
(63)
May 5:
(135)
May 16: - O2 producing photosynthesis
(2.9 BY)
(206)
July 26: - aerobic respiration
(~2.0 BY)
(222)
Aug 11:
- origin of eucaryotes
(1.8 BY)
(301)
Oct 28:
- multicellularity-1st animal fossils
(800 MY)
(320)
Nov 16:
- O2 levels near 20%; Cambrian Explosion
(570 MY)
(326)
Nov 21:
- 1st fossils of land animal
(490 MY)
(329)
Nov 24:
- 1st plant fossils
(460 MY)
(335)
Nov 30:
- 1st land vertebrate fossils
(380 MY)
(345)
Dec 10:
- Earth’s greatest mass extinction
(250 MY)
(353)
Dec 18:
- 1st fossil bird
(150 MY)
(355)
Dec 21:
- 1st fossils of flowering plants
(125 MY)
(359)
Dec 25:
- 1st Cells
(3.8 BY)
- end of dinosaurs; beginning of age
of mammals;1st primate fossils
(363)
Dec 28:
(364)
Dec 30, 9:00pm:
yrs b. p.
(4.6 BY)
(65 MY)
- spread of grasses
- 1st human fossils
Dec 30, 11:24pm: - 1st Homo sapiens fossils
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What is life?
“Life is the representation, the presencing’ of past chemistries, a past
environment of the early Earth that, because of life, remains on the modern
Earth.”
“It is the water, membrane-bound encapsulation of spacetime.”
“Life is a nexus of increasing sensitivity and complexity in a universe of parent
matter that seems stupid and unfeeling in comparison.”
“Preserving the past, making a difference between past and present, life binds
time, expanding complexity and creating new problems for itself.”
L. Margulis & D. Sagan. 1995. What is Life?
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