History of Life
spontaneous generation
spontaneous generation: since at least the time of Aristotle (4th Century BC),
people (including scientists) believed that simple living organisms like worms,
beetles, frogs, amd salamanders could come from dust, mud, etc., and food left out,
quickly “swarmed” with life by spontaneous generation. This was the idea that nonliving objects can give rise to living organisms. For example:
1- Observation: Every year in the spring, the Nile River flooded areas of Egypt
along the river, leaving behind nutrient-rich mud that enabled the people to grow
that year’s crop of food. However, along with the muddy soil, large numbers of frogs
appeared that weren’t around in drier times.
.
Conclusion: It was perfectly obvious to people back then that muddy soil gave rise
to the frogs.
.
2- Observation: In many parts of Europe, medieval farmers stored grain in barns
with thatched roofs (like Shakespeare’s house). As a roof aged, it starts leaking.
This could lead to spoiled or moldy grain, and there were lots of mice around.
Conclusion: It was obvious to them that the mice came from the moldy grain.
3- Observation: Since there were no refrigerators, the daily trip to the butcher
shop, especially in summer, meant battling the flies around the carcasses.
Conclusion: Obviously, the rotting meat that had been hanging in the sun all
day was the source of the flies.
In ancient times, people believed in spontaneous generation, that is, they
believed that life could be generated from nonliving matter. In 1668, Francesco
Redi disproved this theory. Back in the day, people believed that rooten meat
could turn into maggots because the maggots often appeared in decaying meat.
Redi did the experiment shown below, The conclusion from Redi's experiment
was that maggots appeared on the meat only if flies were able to enter the jars
and lay eggs.
http://www.kent.k12.wa.us/staff/TimLynch/sci_class/chap01/spontaneous.html
The First Cell Every cells is made up of many smaller parts. There is the nucleus
present in most cells, which contains DNA (deoxyribonucleic acid), which stores the
blueprint of the cell. There are mitochondria which break down compounds and
produce energy. There are many other parts in a cell, each of which has a specific
function. These parts of the cell are made up of proteins, which are made up of long
strands of amino acids, which are made up of different combinations of the base
elements of carbon (C), hydrogen (H), nitrogen (N), and oxygen (O). Of all the
possible types of combinations forming amino acids, only 20 are used in proteins.
However, these 20 amino acids can form almost infinite numbers of combinations to
create an almost infinite number of proteins.
the first proteins evolved from non-living matter.
The early atmosphere was too hot for oxygen (O), nitrogen (N), and other
elements to exist alone, as they do today. These elements combined to form
methane (CH4), ammonia (NH3), water vapor (H2O), hydrogen sulfide
(formula), hydrogen molecules (H2), etc. These where all, and thus the early
atmosphere was called a 'reducing atmosphere'. Further, as there was no free
oxygen to form an ozone (O3) layer to protect the earth from the harsh ultraviolet radiation (UV Rays) from the sun. Also, there where incredibly violent
electrical storms, which where more violent than any which occur today.
Oparin hypothesized that organic compounds where formed in the early earth
when the high energy provided by UV Rays, and storms, caused different
molecules to react, and create new organic compounds, such as Amino Acids
DNA, and RNA.
Urey and Miller began experimenting to prove Oparin's theory. Miller built an
apparatus which circulated gasses likely to be present in the early atmosphere
(Methane (CH4), Ammonia (NH3), Water (H2O), and hydrogen (H2) ) past an
electrical discharge, simulating the UV Rays and violent electrical storms
present in the early atmosphere.
Miller found many organic molecules present, the most important of created
compounds where amino acids. This, in effect, proved Oparin's theory that
organic compounds could have been created in the early atmosphere.
http://www.ucsd.tv/miller-urey/
-The amino acids would have then polymerized into proteins.
-The first cells were probably much like coacervates. As a group, these bacteria
are called heterotrophic anaerobes, means they were creatures which ate some
naturally occurring food and did not breathe oxygen. The fossils of some these
oldest known forms of life have been found in Australian rocks dating back 3.5
billion years.
-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. Chemicals
made by these nucleic acids would have remained in proximity to the 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 different from other chemical systems by
three things.
1- The capacity for replication from one generation to another( DNA)
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.
-Fossil evidence supports the origins of life on Earth earlier than 3.5 billion
years ago.
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookCELL1.html