Summing things Up! (Review)
Let's combine what we know about Earth's early history, conditions and such,
with what we've learned about requirements for life: mechanisms, properties,
energy sources, etc.
Properties of the `Pre-Biotic' Earth (circa >3.5 b.y.a.)
- Early on, numerous giant impacts. Decreasing in intensity and number with time.
In the first few 100 m.yrs, entirely sterilizing. Later, just evaporate most or parts of the
Oceans.
- Reducing atmosphere of NH3, CH4, CO2. No oxygen in atmosphere & NO OZONE: UV, some soft X-ray more easily penetrate atmosphere. Dangerous
to surface organisms.
- Oceans covering most if not all of the Earth.
- Organic compounds, naturally occurring from lightning, meteorites, UV/X-ray,
hydrothermal vents, etc.
- Global Temperature between freezing and boiling, liquid water always,
buffered by CO2 (greenhouse), maintained by C-cycle. But perhaps,
average Temperature quite warm (~150 F).
Implied Characteristics of Early Life
- Most unevolved organisms (genetically closest to Universal Ancestor) are
Archaea extremophiles (hyperthermophilic, halophilic).
- Energy probably NOT derived via photosynthesis (most complex), but likely
from organic (heterotrophic) or inorganic (chemo-autotrophic) chemicals.
- Probably lived deeply buried (dirt, if available! or water) to hide from
UV, X-rays, meteoritic impacts (consistent with non-photosynthetic).
The above pieces of evidence suggest, early life likely existed
exclusively at the bottom of the oceans.
Hyperthermophilic life would be protected from changes in the environment,
UV/X-rays, meteorites, and have ample energy at mid-oceanic ridges.
Life might have formed in a tidepool. However, it would have made its
way to the ocean floor with time, and the early environment would have
killed everything else that was not protected, leaving ocean organisms as
the sole ancestor to life today.
What is true: Life originated quickly! Once meteoritic
impacts were not absolutely sterilizing, life is found, utilizing ATP,
reproducing with DNA/RNA, and with complex cell walls and other structures.
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