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PANSPERMIA
Perhaps life did not begin on Earth at all, but was brought
here from elsewhere in space, a notion known as panspermia.
For instance, rocks regularly get blasted off Mars by cosmic impacts, and a
number of Martian meteorites have been found on Earth that some researchers
have controversially suggested brought microbes over here, potentially making
us all Martians originally. Other scientists have even suggested that life
might have hitchhiked on comets from other star systems. However, even if this
concept were true, the question of how life began on Earth would then only
change to how life began elsewhere in space.
SIMPLE BEGINNINGS
Instead of developing from complex molecules
such as RNA, life might have begun with smaller molecules interacting
with each other in cycles of reactions. These might have been contained
in simple capsules akin to cell membranes, and over time more complex
molecules that performed these reactions better than the smaller ones
could have evolved, scenarios dubbed "metabolism-first" models, as
opposed to the "gene-first" model of the "RNA world" hypothesis.
RNA WORLD
Nowadays DNA needs proteins in order to form, and proteins
require DNA to form, so how could these have formed without each other? The
answer may be RNA,
which can store information like DNA, serve as an enzyme like proteins, and
help create both DNA and proteins. Later DNA and proteins succeeded this
"RNA world," because they are more efficient. RNA still exists and
performs several functions in organisms, including acting as an on-off switch
for some genes. The question still remains how RNA got here in the first place.
And while some scientists think the molecule could have spontaneously arisen on
Earth, others say that was very unlikely to have happened.
Other nucleic acids other than RNA have been suggested as
well, such as the more esoteric PNA or TNA.
CHILLY START
Ice might have covered the oceans 3 billion
years ago, as the sun was about a third less luminous than it is now.
This layer of ice, possibly hundreds of feet thick, might have protected
fragile organic compounds in the water below from ultraviolet light and
destruction from cosmic impacts. The cold might have also helped these
molecules to survive longer, allowing key reactions to happen.
DEEP-SEA VENTS
The deep-sea vent theory suggests that life may have begun
at submarine hydrothermal vents, spewing key hydrogen-rich molecules. Their rocky nooks
could then have concentrated these molecules together and provided mineral
catalysts for critical reactions. Even now, these vents, rich in chemical and
thermal energy, sustain vibrant ecosystems.
COMMUNITY CLAY
The first molecules of life might have met on clay, according
to an idea elaborated by organic chemist Alexander Graham Cairns-Smith at the
University of Glasgow in Scotland. These surfaces might not only have
concentrated these organic compounds together, but also helped organize them
into patterns much like our genes do now.
The main role of DNA is to store information on how other
molecules should be arranged. Genetic sequences in DNA are essentially instructions
on how amino acids should be arranged in proteins. Cairns-Smith suggests that
mineral crystals in clay could have arranged organic molecules into organized
patterns. After a while, organic molecules took over this job and organized
themselves.
ELECTRIC SPARK
Electric sparks can generate amino acids and sugars from an
atmosphere loaded with water, methane, ammonia and hydrogen, as was shown in
the famous Miller & Urey experiment reported in 1953, suggesting that lightning might
have helped create the key building blocks of life on Earth in its early days.
Over millions of years, larger and more complex molecules could form. Although research since then has
revealed the early atmosphere of Earth was actually hydrogen-poor, scientists
have suggested that volcanic clouds in the early atmosphere might have held methane, ammonia and
hydrogen and been filled with lightning as well.
There are quite a lot theories which are also really important in the science world suchs we are able to see in the last pages.
Anyway nobody knows certainly how life in Earth appeared so the only thing we can do is to choose a theory and try to defend it with scientific researches.
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