Biology 102 - General Biology
The Origin of Life
The current explanation of the origin of our one known universe is what
is called the BIG BANG, the cosmic explosion that occurred 13.7 billion
years ago. (There is evidence now that there are multi-verses but you
can read about this on your own. http://en.wikipedia.org/wiki/Multiverse)
Geochemical studies have provided overwhelming evidence that our solar
system including our planet, Earth, was formed about 4.5 billion years
Current ideas about the origin of the moon say it was formed 4.527 ±
0.010 billion years ago, about 30–50 million years after the origin
of the Solar System. The current most commonly accepted theory of the
formation of the moon is the "Big Whack" theory. It says the
moon was formed from the left overs from Earth's collision with another
planet-like body somewhat larger than Mars. On impact with the earth big
chunks of the terrestrial mantle were hurled into space to form the moon
along with residues of the impactor body. Much of that debris coalesced
into a rocky satellite, which was roughed up about half a billion years
later by a barrage of asteroids and other interplanetary material, leaving
scars in the form of craters.
The primitive earth was very inhospitable to life as we
know it. It was extremely hot with a lot of volcanic action and the sunlight's
UV light was not filtered out since the unprotected earth had no ozone
layer or cloud cover. The early atmosphere was a gaseous mixture of ammonia
(NH3), nitrogen (N2), hydrogen (H2),
carbon dioxide (CO2), carbon monoxide (CO), methane (CH4)
and liquid water (H2O). This highly reducing atmosphere was
devoid of free oxygen (O2).
Urey and Miller in 1952 (University of Chicago), and others
later, dramatically demonstrated that under conditions which mimicked
the primitive earth, using an appropriate energy input, an array of organic
molecules including nucleotides (including components of the "genetic"
molecule, RNA could be produced). It was believed that on the primitive
earth, these molecules accumulated to form an "organic soup"...an
ocean rich in organic molecules and that from this rich pre biotic "soup,"
However, this view of life beginning from a "Primordial
Soup" left a lot of unanswered questions. The next major advance
came in the 1980's with the idea of the "RNA World". (RNA you
will learn is closely related to DNA, the genetic material in all cells,
and the genetic material in some viruses). This RNA world idea originated
when some RNA molecules were shown to be able to act as enzyme-like catalysts.
While there is reason to believe that life went through an early RNA-dominated
phase, there are alternative theories that propose that metabolism came
first. The scientists who propose "Metabolism First" look to
the core metabolic pathways and networks known to occur in all organisms
today to find the primitive biochemical networks that were their progenitors.
Metabolism First sees life originating from networks of simple interactive
chemical reactions which became increasingly complex and diverse. Then
these systems could have taken on the features of replication and selection
distinctive of life. We eagerly await the development and progress of
the theory and experimental work behind this newer theory.
Microfossils of early organisms date back 3.5 BYA. The first
primitive organisms were thermophilic (heat loving) anaerobes, meaning
they lived in a very hot world with an atmosphere devoid of free oxygen.
In fact, oxygen would have been poisonous to them. Even today we take
"antioxidants" to protect us from the harmful effects of too
much oxygen. These first anaerobic cells flourished on the surface of
the planet for more than 500 million years before oxygen began to play
a role in the evolution of life as we know it. The earliest more primitive
cells are called prokaryotic cells, the simplest of cells. The more complex
eukaryotic cells, of which we are composed, evolved later. (We will discuss
these cells types in more detail in future lectures.)
Extreme environments such as the recently discovered submarine
hydrothermal vents where life abounds, may be where life on the planet
first arose. These hydrothermal vents are likely to be on other worlds
such as Jupiter's moon, Europa, or Saturn's moon, Titan, and Mars, where
minerals that on Earth are commonly formed at hydrothermal vents were
just discovered and life might very well have arisen there.
The free oxygen originated, as it does today, from photosynthesis
where H2O is broken down to form oxygen (O2) and
hydrogen ions (H+). And also, as today, the important part
was the formation of hydrogen ions which were used to reduce carbon dioxide
to form organic molecules (the building blocks of plants and animals).
The oxygen was toxic and the earliest cells had to learn to cope with
this "poison." Some of the early cells learned to "detoxify"
oxygen and then some eventually learned to use oxygen for their benefit.
We are now dependent on it to "burn" (oxidize) food molecules
to produce energy. (We will learn more about energy production in a future
Oxygen levels on Earth reached a critical threshold to enable
the evolution of complex life much earlier than previously thought. The
evidence is found in 1.2-billion-year-old rocks from Scotland. The evidence
came from a rock found on the coast near Lochinver.
THE PORPHYRIN RING ABOVE APPEARED EARLY IN CHEMICAL EVOLUTION
The word porphyrin comes from the Greek word for purple.
The porphyrin ring has a Mg (magnesium) ion at its center in the chlorophylls
and an iron ion when it appears later in the cytochromes of electron transport
enzymes that produce the body's energy. It also forms the "heme"portion
of hemoglobin, our oxygen carrying protein, where it also contains an
iron ion, Fe, in place of the Mg. It is the Fe ion in hemoglobin that
makes our blood red. Nature is a tinkerer and once it finds a useful biomolecule
it will use it in many different but related reactions. This molecule
appeared early on the earth and was critical in the capture of sunlight
in early photosynthesis. There are many porphyrin rings in nature. Since
they are in green plants and red blood cells, it has been proposed that
eating green plants (they have lots of chlorophyll and therefore porphyrins)
will help prevent anemia by providing the basic subunit of hemoglobin.
organisms were also heterotrophs
(other feeders). (We are
also heterotrophs.) These
early cells used the "organic
soup" to obtain molecules
to make more of themselves
and to break down for
energy. When the "organic
soup" became depleted
there was selection for
those organisms that could
manufacture their own
food. These organisms
are called autotrophs
(self feeders). Photosynthesis
is an example of a process
for making organic molecules
to be used as food, both
for the organism and for
those who eat it. One
of the early molecules
that was formed was chlorophyll
which plays an essential
role in photosynthesis
by capturing light energy
to break down water molecules.
The porphyrin ring it
contains has magnesium
(Mg) in the center. We
have a similar organic
structure in hemoglobin,
our blood protein that
carries oxygen. We have
iron (Fe) in the center
of our porphyrin ring
instead of magnesium (Mg).
It is interesting how
evolution is very conservative,
using similar molecules
in a variety of functions.
We see this over and over
in biology. (We will discuss
in a future lecture.)
The presence of abundant oxygen in the atmosphere stopped
any further production of organic molecules in the environment and also
any new "experiments" in the formation of living cells. Any
such de novo synthesis of organic molecules is no longer possible.
The molecules would be eaten by an existing organism or oxidized by the
oxygen-rich atmosphere. The ability to utilize oxygen to gain more energy,
however, resulted in a blossoming in the variety and complexity of organisms
on earth. By 500 million years ago there were multicellular organisms
and most major animal phyla had appeared. The emergence of animals in
the Late Proterozoic is believed to have been aided by the oxygenation
of Earth's atmosphere and oceans about 580 million years ago. Humans (and
their immediate ancestors) appeared only within the last one to two million
As you progress through the course you can refer back to
History of the Major Events in the Evolution of Life on