One of the problems with studying the origin and evolution
of life is that our mother Earth has a shady memory. The farther back in time we go, the rarer and
more fragmented the rock record becomes. What this basically means is that for
most of the first third of Earth’s history, we run out of rock record before we
get back far enough it time to discover the origin of various fundamental early
steps in our own evolution. Even for more recent developments, like the
oxygenation of the atmosphere and the recent great extinction events, the rock
record is frustratingly incomplete. This has several effects.
On the observational side, it
requires scientists to draw bigger and bigger conclusions from slimmer and
slimmer data. Was there life in the Hadean? If all you have is a pinhead pile
of ground up zircons, there is only so much evidence you can put forth.
On the theoretical side, there is
of course even more speculation and unconstrained hypothesizing. With older
rocks more common on smaller, deader worlds, and hypotheses like
Panspermia
positing that space is no barrier to the spread of life, there is literally a
universe of possibilities. As a result, many theorists have lapsed into
quasimystical approaches to the framework for how life has evolved from very
early primitive micro-organisms to space age simians who none-the-less waste
their time reading this blog. The approaches generally fall into two broad
categories.
The first is the “Manifest
Destiny” approach. This school of though believes that life is an unstoppable,
inexorable force that will climb every mountain, contaminate every spacecraft,
and spread in an inexorable evolving wave throughout the universe. Most
astrobiologists subscribe to this belief, as it is easier to justify your life’s
work if you think that there is actually something out there to find.
The second is the “There but for
the Grace of God” approach, which envisages life as a blind, reactive
encrustation to grand events and processes far beyond its control. Proponents
tend to be hard rock geologists and extinction researchers.
It is important to note that
these are hypothetical endmembers- most researchers lie on a solid solution
between them, albeit generally closer to one end than the other. It is also
important to note that although I have deliberately used non-scientific labels,
as these leanings are often manifestations of inclination rather than
deduction, an inclination towards one camp or another is in no way an
indication that a particular research is not a great scientist. Rather, it is
an attempt to colorfully illustrate two diametric approaches taken to thinking
about the early history of life.
Tonight, however, I’d like to
draw attention to a paper that combines these approaches in a fascinating way.
Konhauser et al. 2007 posit an Archean Earth where Nickel-dependent methanogens
had evolved to become the dominant life form on the planet. The oxygenation of
the Earth’s atmosphere was not a result of oxidative photosynthesis evolving
and outcompeting the methanogens. Rather, the decrease in high-temperature,
nickel-rich komatiitic volcanism at the end of the Archean weakened the
methanogens by creating a shortage of the nickel they needed to survive,
reducing methane production and allowing oxygen producers to take over.
Scientifically,
this idea is appealing because increasing lines of evidence, such as that
summarized in
Geosonnet 21, indicate that oxygen production was going on long
before the great oxygenation at a limited local level. But for hundreds of millions
of years, it was never more than a transient, small scale local phenomenon.
This hypothesis is also nice in that it ties
the large scale tectonic and igneous changes between the Archean and the
Proterozoic with the change in atmosphere. Linking those two fundamental shifts
in the Earth’s history is always nice, as having them coincidentally
synchronous seems somewhat implausible.
On a purely
personal level, however, the proposed narrative reminds me of the H. P.
Lovecraft novel, “
At the Mountains of Madness” The difference is that the
Archean overlords who ruled the hostile ancient Earth were not 3 meters tall.
They were 3 microns tall instead. And it was mantle convection, not decadence
in intergalactic civilizations, that allowed our distant aerobic forbearers to liberate
their planet.
