First there was the mushroom.
Then the mushroom said 'let there be life throughout the galaxy'
Seriously though, the idea of DNA being transferred by spores between planets is now a totally viable scientific theory for the origin and explanation
of interstellar seeding of life. Not just mushroom spores but bacterial spores. They are light enough to enter through the atmosphere without gaining
enough velocity to burn up, and resistant enough to cold and radiation to survive in space for very long periods of time.
Can spores survive in interstellar space?
There is good evidence that life appeared on earth just 200-400 million years after the crust had cooled (assuming conventional methods of measuring
age). Two hundred million years seems a bit on t
he short side for the spontaneous generation of life, although no one really knows just how long this process should take (forever?). The apparent
rapidity of the onset of terrestrial life has led to a reexamination of the old panspermia hypothesis, in which spores, bacteria, or even nonliving
"templates" of life descended on the lifeless but fertile earth from interstellar space.
P. Weber and J.M. Greenberg have now tested spores (actually Bacillus subtilis) under temperature and ultraviolet radiation levels expected in
interstellar space. They found that 90% of the spores under test would be killed in times on the order of hundreds of years -- far too short for
panspermia to work at interstellar distances. However, if the spores are transported in dark, molecular clouds, which are not uncommon between the
stars, survival times of tens or hundreds of million years are indicated by the experiments. Under such conditions, the interstellar transportation of
life is possible.
But perhaps the injection and capture phases of panspermia might be lethal to spores. Weber and Greenberg think not -- under certain conditions. The
collision of a large comet or meteorite could inject spores from a life-endowed planet into space safely, particularly if the impacting object glanced
off into space pulling ejecta after it. The terminal phase, the capture of spores from a passing molecular cloud by the solar system and then the
earth, would be nonlethal if the spores were somehow coated with a thin veneer of ultraviolet absorbing material. In sum, the experiments place limits
on panspermia, but do not rule it out by any means.
(Weber, Peter, and Greenberg, J. Mayo; "Can Spores Survive in Interstellar Space?" Nature, 316:403, 1985.)
I heard a talk a while ago from a mycologist where they said the best way to store mushroom spores stably for long term use is to keep them as near to
a perfect vacuum and absolute zero as you can get. Exactly the conditions of space, but with a little extra radiation.
are the most likely candidate out of many.
"Endospores can survive without nutrients. They are resistant to ultraviolet radiation, desiccation, high temperature, extreme freezing and
chemical disinfectants. According to scientist Dr. Steinn Sigurdsson, "There are viable bacterial spores that have been found that are 40 million
years old on Earth - and we know they're very hardened to radiation"
One of the problems with abiogenesis research is that we can't define "life". We can describe MODERN life, but when we try to write a cohesive
definition of life as such it breaks down. Non-Earth-like life (Peter Ward's term, in "Life As We Do Not Know It") almost certainly will be able to
survive in situations that would be instantly lethal to Earth-like life, and of course vice versa. There are examples (which Ward illustrates in that
book) of biologists finding life on Earth and simply not realizing it.
Life as we know it can't survive, sure. No one is going to argue that. But what about OTHER types of life? It takes supreme arrogance to think that
DNA-based cells composed of bilipid layers and a handful of amino acids is the only possibility, and frankly until we find other possibilities it's
really useless to speculate about them. Asking "Can non-Earth-like life survive in space?" is akin to asking "Why did God create parasites?" in
that it can only, at this point, be speculation heaped upon speculation with no actual data to serve as a foundation.
It doesn't even have to be DNA. They could easily have another genetic molecule--RNA, for example, or protein (proteins were long thought to be the
molecule of heredity in Earth-like life). At the very least, the actual code itself--how many nucleotides are used for each amino acid, and which ones
code for which acids--would differ wildly.
complex molecules are rather ubiquitous in space. We're not talking a handful of sugar mollecules or some barely-detectable amino acids in a small
number of weird sampels--we're talking siginificant quantities of these materials pretty much everywhere we look. (I used "significant" rather than
"large" because we're still talking nebula that are thinner than the stratosphere and dead rocks. These aren't large quantities, but theyr'e
detectable and at MUCH higher rates than we expected when we started looking.)
Second, the way the Earth formed lends itself to such seeding. The Sun formed in a nebular cloud that included...well, frankly all the elements that
aren't man-made. The Earth formed via the accretion of material over a few hundred million years. So we already know that raw elements seeded the
planet--the planet can be thought of as one giant ball of seeded material. The material is there, and the mechanism is there.
Third, we know it happens. I don't mean life coming from outer space; I mean that the way we know that bolides can carry amino acids is that we've
found bolides with amino acids in them (and under conditions that prevented contamination). You can actually hold an example of what I'm talking
about in your hands if you want to (and can convince the owner to let you). This won't have much impact on modern life, but since it's happening now
it's reasonable to conclude it happened in the past (unless someone can demonstrate a mechanism preventing it--this is basic uniformitarianism, and
the non-controversial bits at that).
So we know it happens today. We know that the Earth was more or less constantly bombarded with extraterrestrial material for the first few hundred
million years. And given how common complex organic mollecules are in space, it'd be much more surprising if there weren't such molecules in the
early solar system than if there were. So it's reasonable to conclude that complex molecules came to Earth from extraterrestrial sources at all
times, including during its early formation.
The question is, how much of a role did they play? The answer is "Somewhere between none at all and they're the reason we're alive today", and
there's not enough data to put it any more firmly than that, far as I know.
I expect that planets are like islands to a certain extent, simple building blocks for life occasionally swim in from elsewhere on occasions.