| As I
have previously noted, many astronomers and geologists have pointed out that the early
earth almost certainly never possessed a strongly reducing atmosphere as assumed in the
Miller experiments. The early earth would have had an atmosphere similar to that of Mars
and Venus - our two nearest neighbor planets - in that it would have been near neutral.
The Miller method of producing amino acids and other organics simply does not work with
near neutral gas mixtures. Yet variations on the Miller experiments continue to be
published and presented unquestioningly in textbooks and journals. In fact, the best
evidence Miller himself can offer for an early reducing atmosphere is the presence of life
itself - that the organics for pre-biotic synthesis must have come from somewhere. As I
have noted, others suggest meteorites may have been the source of the organics, citing a
single meteorite that fell in Australia in 1969, which contained 100 PPM of organics which
were similar in make up to those produced in the Miller-type experiments. What is rarely
mentioned is that these carbonaceous meteorites are very rare, with the vast majority of
meteorites having either and Iron or silicate based make up (this is why the 1969
Murchison meteorite is nearly always used as the only example). Even considering that
meteorites fell at a far more frequent rate in the past, the rarity of carbonaceous types,
their very low (<100 PPM) organics content as well as the fact that then, as now, only
larger meteors make it through our atmosphere without burning up, have led many
scientists, including Stanley Miller, to conclude that meteorites simply could not have
been large contributors of organics to the early earth. The same sort of criticism can be
offered for materials such as Hydrogen Cyanide thought to have come into our atmosphere
from comets. So if we cannot identify how large quantities of organics might have come to
be present, why should we then just assume their existence and move on to the issue of
formation of more complex pre-biotic organics? Another difficult issue that is often
glossed over without significant consideration is the issue of random D and L amino
acid mixtures (often referred to as left and right oriented forms). All natural processes
(including the Miller-type) produce random mixtures of both types. Yet all
life forms use proteins which are made almost exclusively of the Left-handed variety
- how these could be sorted out under pre-biotic conditions remains to be answered.
|
Another little considered problem is the scarcity of soluble phosphorous, since phosphates
tend to precipitate out as Calcium Phosphate, a common inorganic mineral. Phosphates are
needed for both RNA and DNA synthesis in concentrations far greater than those commonly
seen in any natural bodies of water.
A possible solution to a number of concerns which is
often tossed out by many scientists, has been the thought that various mineral surfaces,
such as clays, could provide significant abilities to collect, sort out and catalyze
various desirable organic reactions. Unfortunately, under actual test conditions, minerals
provide only the most meager assistance, and then only in a few types of reactions. Along
similar lines, hot vents on the ocean bottom (including some known as "black
smokers") are suggested by some as the focal points for the origin of life, providing
heat to create reactions and sulfides that could be used as a "food"
source. But researchers like Stanley Miller have pointed out that the vents do not
make organic compounds, they destroy them. If these vents were as common on the early
earth as some suggest, essentially all ocean waters would by cycled through them in
geologically "short" periods of time, greatly limiting the amount of complex
organic "soup" that could possibly build up in pre-biotic oceans. Although most
researchers hold that life must have evolved through an "RNA World" stage during
its initial development (because there is no explanation for sub-life complexity without
some form of self-reproduction) they do so without any direct evidence. Many of the
optimistic students of this science assume it must have happened - in spite of the fact
that all the considerable research conducted to date fails to indicate any circumstance
under which the RNA World might be possible. On top of this, without a self-replicating
system, the RNA World theories are a moot proposal - they could not have accomplished
anything. Yet something is required to explain the complexity of pre-biotic building
blocks - this seemingly purposeful and organized complexity cannot be simply glossed over
or ignored. |
