Abiogenesis, Lithopanspermia and Translocation
The 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.
Abiogenesis 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. In most cases the life that is found on the worlds of the Terragen Sphere has emerged there, from processes of abiogenesis; there are many different routes to abiogenesis, and it seems to be the case that every case of abiogenesis is unique; the number of possible biochemical signatures is so high that there could be a different biochemistry on every planet in the visible universe, purely by chance. 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.
Lithopanspermia 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. The cluster M67 is an old cluster which has never fully dispersed, and several events of panspermia have occurred there.
On rare occasions life may pass through space unprotected by rock ejecta, but only where the originating world and the destination world are very close together. This occurrence is most frequent within the moon systems of large planets, where the moons are fairly close together; an example of this is Macrystis, where life has been transferred as spores into the parent gas giant.
Translocation 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. These events are known as translocation events, or directed panspermia if deliberate. The first examples of directed panspermia 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.