A Brief History of Life:
Origins
In the beginning, God created the heavens and the earth. (Genesis 1:1)
The question
many people have been asking for the past several thousand years, is HOW?
We now have good evidence
that the universe is about 15,000,000,000 years old!
We can still see some of the "light" (microwave radiation) leftover from the creation of the universe.
Link
to COBE project to detect microwave radiation in the universe, evidence
for the "big bang"
The Earth
was likely formed roughly 5,000,000,000 years ago.
Link to more information
about the Formation of the Earth:
Origins
Extra-terrestrial:
In 1969 a meteorite (left-over bits from the origin of the solar system)
landed near Allende, Mexico. The Allende Meteorite (and others of its sort)
have been analyzed and found to contain amino
acids, the building blocks of
proteins, and thus of life. It has been hypothesized
that life originated out in space and came to earth inside a meteorite.
The amino acids recovered from meteorites are in a group known as exotics.
They do not occur in the chemical systems of living things. The ET theory
is now not considered to be correct, although the August 1996 discovery
of the Martian meteorite and its purported fossils has revived thought
of life elsewhere in the Solar System than the Earth.
Supernatural:
Since Science is an attempt
to measure and study the natural world, this theory is outside science
(at least our current understanding of science). Science classes deal with
science, and this idea is in the category of not-science. My
own (David Ussery's) view on this subject is that the natural world is
certainly within the bounds of religion, but that we are merely looking
at different aspects of the same Truth. Thus, one could say "God
did it", but a scientist would want to know HOW God did it - what mechanism
did He use? Understanding the mechanism does not exclude (nor necessarily
include) the existence of God.
Organic
Chemical Evolution: Until the middle 1800's
it was thought that organic chemicals (those with a C-C skeleton) could
only form by the actions of living things. A French scientist heated crystals
of a mineral, and discovered that they formed urea (an organic chemical)
when they cooled. In 1922, Russian scientist, A.I. Oparin hypothesized
that cellular life was presaged by a period of chemical evolution.
The above image is modified from http://www.whfreeman.com/life/update/.
In 1950, a graduate student (Stanley Miller) designed an experimental test for Oparin's hypothesis. Oparin's original hypothesis called for : 1) little or no free oxygen (oxygen not bonded to other elements); and 2) C H O and N in abundance. Studies of modern volcanic eruptions support inference of the existence of such an atmosphere. Miller discharged an electric spark into a mixture thought to resemble the primordial composition of the atmosphere. In a water receptacle, designed to model an ancient ocean, Miller recovered amino acids. Subsequent modifications of the atmosphere have produced representatives or precursors of all four organic macromolecular classes. Miller's first report of his results was in: Miller, S. L., "A production of amino acids under possible primitive Earth conditions", Science 1953; 117: 528-529.
The above image is modified from http://www.whfreeman.com/life/update/.
The primordial
Earth was a very different place than today, with greater amounts of energy,
stronger storms, etc. The oceans were a "soup" of organic compounds that
formed by inorganic processes. Miller's (and subsequent) experiments have
not proven life originated in this way, only that conditions thought
to have existed on the ancient (over 3 billion years ago) planet were such
that the spontaneous (inorganic) formation of organic macromolecules could
have taken place. The simple inorganic molecules that Miller placed into
his apparatus, produced a variety of complex molecules:
The above images are
modified from http://www.whfreeman.com/life/update/.
The
interactions of molecules thus generated would have increased as concentrations
increased. Reactions would have led to the building of larger, more complex
molecules. A pre-cellular life would have began with the formation of nucleic
acids. But (if you're observant!) you
might notice that there are NO NUCLEIC ACIDS produced in Stanley
Miller's experiment. Where would these come from? In subsequent
experiments, nucleic acids WERE DETECTED; a likely sheme for their formation
is from polymerisation of cyanide (!), (which can be readily formed in
a primitive atmosphere), shown below:
Now, you're probably saying "so what? This still doesn't look much like a nucleic acid to me!" However, the tetramer can be rearranged as follows:
Adenine
is a purine, very similar to caffeine (remember the DNA
is like Coca-cola lecture??) Chemicals made by these nucleic
acids would have remained in proximity to the nucleic
acids. Remember,
nucleic acids
(like lipids) hate water, and would tend to stack on top of each other
and form a helix.
This
process is known as SPONTANTOUS self-assembly, and works for membranes
as well as nucleic acids.
Eventually the pre-cells would have been enclosed in a lipid-protein membrane,
which would have resulted in the first cells.
Biochemically,
living systems are separated from other chemical systems by three things:
1.) The capacity for replication
from one generation to another. Most organisms today use DNA
as the hereditary material, although recent evidence (ribozymes) suggests
that RNA
may have been the first nucleic acid system to have formed. Nobel laureate
Walter Gilbert refers to this as the RNA world. 2.)
The presence of enzymes and other complex
molecules essential to the processes needed by living systems. Miller's
experiment showed how these could possibly form. 3.
A membrane that separates the internal
chemicals from the external chemical environment. This also delimits the
cell from not-cell areas. The work of Sidney W. Fox has produced proteinoid
spheres, which while not cells, suggest a possible route from chemical
to cellular life.
Fossil
evidence supports the origins of life on earth earlier than 3.5 billion
years ago. The North Pole fossils from Australia are such that even more
primitive cells must have existed earlier. From rocks of the Ishua Super
Group in Greenland come possibly the earliest cells, 3.8 (?) billion years
ago. J. William Schopf of UCLA recently discovered possibly
photosynthetic prokaryotes from North America in 3.5 billion year
old rocks, suggesting the existence of even older heterotrophic forms.
The oldest known rocks on earth are 3.96 billion years old, and are from
Arctic Canada. Thus, life appears to have begun soon after the cooling
of the earth and formation of the atmosphere and oceans.
These
ancient fossils occur in marine rocks, such as limestones and sandstones,
that formed in ancient oceans. The organisms living today that are most
similar to ancient life forms are the archaebacteria. This group is today
restricted to marginal environments. Recent discoveries of bacteria at
mid-ocean ridges add yet another possible origin for life: at these mid-ocean
ridges where heat and molten rock rise to the earth's surface.
Click here for more information about Stanley
Miller's experiments.
The
First Life (was probably RNA-based)
A "slightly less brief" History of Life:
above here will be Wednesday's lecture
BELOW here will be covered on Friday, 13 March, 1998.
Recently (in February,
1998) researchers in China have found specimens of eukaryotic cells dating
from BEFORE the "Cambrian explosion". Actually the date for exactly
WHEN the Cambrian period began is a bit uncertain, with dates ranging
from as early as 1,200,000,000
years ago, to as recent as 500,000,000
years ago! It is hoped that
more careful studies of these types of fossils can help to identify when
the first eukaryotic-like organisms appeared.
Multicellularity
The
Invasion of the Land
Human
Evolution
Australopithecines,
the first true hominids, could stand and walk upright.
 |
Evolutionary
Connections:
Why
Do Humans Walk Upright?
Back to the Biology 101 Syllabus 
