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Abiogenesis, Lithopanspermia, and TranslocationThe origin of life in planetary environments |
While
Humankind was confined to the Earth's
surface the origin of life remained a subject for speculation and
controversy.
One of the many aims of early space exploration was the collection of
data
about possible life outside the Earth, and it was hoped that any such
information gathered would help to uncover the mystery of the origin of
life on
Earth and in the universe as a whole. Unfortunately very little
information on
this subject could be obtained from the exploration of the Solar
System. Apart
from a few prokaryote fossils on Mars,
and some disputed non-organic structures
on Titan, which were eventually determined to be ancient relics of the
Muuh
race, there was no sign of life in the solar system outside of Earth. A
false
alarm occurred after the Dark
Ages when life was found in some abundance on
Europa;
but this turned out to be a forgotten bioscience experiment from the
fifth
century a.t..
Most encouraging was the discovery of Martian fossils, the latest of
which were
dated with some confidence to the end of the Noachian period. However
there was
no surviving DNA
to be found (assuming those organisms ever used DNA) and it
was impossible to determine whether the organisms concerned had been
transferred from Earth to Mars by some method. Some astrobiologists at
that
time even suggested that the organisms originated on Mars, and some
were
transferred to Earth during the Hadean period of Earth's history
(roughly
contemporary with the Noachian of Mars). Others still speculated on a
back-and
forth trade of microorganisms, facilitated by the frequency of large
meteoric
impacts on both worlds at that time.
So even the discovery of Martian fossils was not sufficient to confirm
or
disprove any one of the two main theories of life's origin on Earth
current at
that time; namely Abiogenesis, the spontaneous generation of
self-replicating
lifeforms on the surface of a planet due to the formation of organic
compounds
by chance, or lithopanspermia, the transfer of life from one world to
another
within rocky ejecta. Another early contender, cometary panspermia, was
eliminated quite early on when a selection of cometary bodies were
examined and
found to be as sterile and free from fossils as the rest of the Solar
System
(excepting of course Earth and Mars).
During this period, a number of extrasolar planets had been discovered
which
almost certainly held biospheres; for instance at 83 light years
distance the
world HD
3823 d showed an absorption spectrum indicating the presence
of
chlorophyll in some abundance. When probes and eventually manned
missions
reached these nearby worlds, a number of alien xenoecologies of many
varied
types were found, and the science of astrobiology finally had an
abundance of
data to examine; from this data the modern science of xenobiology
emerged, and
with it a system of classification for the many pathways by which life
has
emerged or arrived on the life-bearing worlds of the Orion Arm volume
and
environs.
As well as planets that have fully developed biospheres, many more
worlds were
found outside the Solar System that were in various early stages of the
development of life. The most common form of life-bearing world has
been found
to be one where simple unicellular organisms exist in a fluid medium.
Water
based, protein/lipid life, which uses nucleic acid as an information
transfer
mechanism is one of the most abundant forms of simple life; this form
of life
is generally anaerobic and not associated with oxygen-rich atmospheres.
In fact
representative worlds can be found for most of the early stages of the
Earth's
early history; particularly interesting are worlds in the Banded Iron
stage of
development, where periods of relatively high oxygen content alternate
with
periods of low oxygen content as populations of photosynthetic
organisms bloom
and die.
But perhaps even more interesting are those worlds that hold the
precursors to
cellular life. On many worlds these protobiotic stages are short,
transient
periods, and some few of the precursor biospheres that have been
discovered are
already in a state of transition towards cellular life. But other
worlds of
this kind are seemingly locked at the stage of self-replicating organic
molecules,
a condition which allows xenobiologists to study the various stages of
abiogenesis in detail.
The most primitive stage in the development of living organisms from a
non-living environment is the organic monomer stage. Various simple
organic
chemicals such as amino acids are created in various ways generally
during the
late stages of the formation of a world; in many cases some of these
organic
monomers are synthesised in space, in the molecular clouds associated
with
starbirth and in the protoplanetary cloud. Some cooler worlds retain
populations of pre-biotic monomers for billions of years, showing that
these
chemicals are widespread in planetary environments. While on a few
worlds the
conditions are right for the progression from the monomer stage to the
organic
polymer stage to be observed. Small pools or muddy puddles of organic
soup
containing polymers of 40 sub-units or more in length are quite often
found on
young terrestrial worlds.
The next stage, sometimes known as the Lipid World stage, is known from
a
number of examples; bundles of fatty molecules, often forming
bubble-like
structures known as prebionts, enclose a concentrated soup of
autocatalysing
molecules. The most primitive of these soups consist of enzymes which
catalyse
the formation of new proteins in a so-called hypercycle. Polycyclic
aromatic
hydrocarbons (PAHs) can also form self-replicating cycles in these
conditions. Not all prebionts take the form of bubbles; some manifest
as gels, or meshes of organic polymers, or single strings with organics
adhering to the outside (an example of this being the
so-called Angel
Hair from the gas giant Big Bob). One or two prebiopheres of
great antiquity have been found; but generally these
prebionts progress to the next stage in a few million years. This next
stage
includes self-replicating information-rich molecules such as nucleic
acids.
Examples of this Nucleic Acid world stage are widespread, and molecules
such as
RNA, PNA, TNA and even DNA are found in simple cells known as
protobionts,
often associated with the previously mentioned PAH compounds. These
nucleic
acids become associated with the production of ever-longer and more
complex
protein strings, which form into simple cooperative organisms of
various kinds.
Many diverse worlds have biospheres consisting entirely of protobionts
with the
general characteristics of mitochondria, chloroplasts or other
subcellular
components, which have never yet combined to create more complex cell
communities of the prokaryotic type.
So the many stages of the origin of life have been observed on
different worlds
in the Terragen Sphere, leading to a number of useful systems for
classification of the various paths which lead from non-biological
matter to
self-replicating organisms. But not every biosphere is the result of
abiogenesis on that particular world; in fact in a number of cases life
has
arrived by one of two means; lithopanspermia or deliberate
translocation by an
intelligent agent.
Organic lifeforms are generally ill-suited for survival in space, so if
they
are to be transported from world to world they require protection from
radiation and the ability to survive cold temperatures and desiccation.
Many
simple organisms can suspend all biological activity for an indefinite
period
then revive when conditions are right; these organisms would need extra
protection from radiation during a space journey, but this can be found
if they
are surrounded by a thickness of rock. So in theory a rocky meteorite
ejected
from a planet due to a large impact could carry with it a cargo of
dormant organisms.
There are several constraints on this process, however; the ejection
process requires the acceleration of the rock to escape velocity in a
matter of
instants, and most ejected objects are heated during the process so
much that
they are sterilised. For worlds with relatively low escape velocities
the
initial impact energy requirement for an ejection event is less, so the
chance
of an ejected rock carrying viable lifeforms is correspondingly greater.
Examples of lithopanspermia within solar systems are quite common;
there is
some debate about whether Mars infected Earth or vice versa in the old
Solsystem, but definite examples of this process have been demonstrated
elsewhere many times. One of the first examples was Fortuna (Gamma
Cephei b,
latterly known as Silence),
a gas giant infected by ejecta from the terrestrial
moon Hope (latterly Anomie).
But another, more long-range form of lithopanspermia has also been
demonstrated. In the energetic conditions of a young stellar nursery,
stars of
a similar age are found in close proximity to one another; calculations
show
that lithopanspermia events occur on average once per cluster between
the
planets of separate stars. If a world develops life in the early
history of a
cluster, it is not unlikely that it will infect a planet orbiting a
nearby
star. Later the cluster will almost certainly disassociate into a
co-moving
stream of stars, which will then scatter completely. In some cases the
only
indications that two stars formed in the same cluster are the facts
that they
are similar ages, and have biospheres which share several distinct
characteristics.
A spacefaring
civilisation
such as the Terragen Expansion inevitably carries with it lifeforms
from the
original home planet; sometimes that civilisation
will infect a
sterile world by accident, simply by dumping biological waste on or
near its
surface. However in many cases that
civilisation
will deliberately establish, or try to
establish, an artificial biosphere on another world. The first examples
of deliberate
bioengineering by an alien race were discovered in the so-called Garden
of
Paradise cluster, not in fact a stellar cluster at all but a collection
of
separate worlds in the
Puppis
region terraformed long ago by the otherwise
unknown Mysterians.
In the same time period several cold worlds were discovered
with similar ammonia or methane based xenoecologies; it appears that
most of
these were infected by accident millions of years ago due to an
abandoned
expansion of the Muuh Empire or by their client race the Soft Ones.
Other translocation events have been identified; the many chlorine rich
worlds
of the Halogenic
subtype were created 780 million years ago then apparently
abandoned, and the artificial terraforming swarm known as the Cybyota
has been
active within the last thirteen million years. Another incident
of translocation occurred perhaps 2 gigayears ago, details here.
In fact confirmed
translocation
events predating the Terragen Expansion number several hundred
instances within
the Terragen sphere alone, and if the worlds terraformed or infected by
humanity and
the human-derived Terragen
civilisation are
included, translocation events count for the vast majority of life
bearing worlds in the local volume. It is notable that very few of the
ancient civilisations responsible for these translocation events
are still extant; apart from slow-developing species such as the Muuh
and the Jacks. One
would expect that a single very successful "seeder" civilization would
have blanketed the entire galaxy with its own form of life at some
point in the distant past, especially given the number of times that
this has happened on a smaller scale, but this has not happened. It
would appear that some factor has prevented this from happening in the
Milky Way galaxy (if not elsewhere), since life forms in this region
are quite diverse and many have independent origins.
