Geoscience Reference
In-Depth Information
associated with the Shuram-Wonoka anomaly, the oxygen demand for
Dickensonia
seems to fit with our geochemical arguments suggesting that
around 580 million years ago oxygen rose to at least 15% of present
levels. Sounds pretty good. In this scenario, the evolution of large and
possibly motile animals like
Dickensonia
was enabled by a rise in oxygen
to levels conducive for their livelihood. In the words of Andy Knoll,
motile animals evolved into a “permissive environment.”
This type of logic makes some paleontologists cringe, and particu-
larly Nick Butterfield from Cambridge University. I can almost see him
shudder every time he reads about a possible relationship between oxy-
gen concentrations and animal evolution, especially if the argument
comes from a geochemist. Nick's view is that oxygen should be left out
of it. He argues that animals themselves have engineered the conditions
of environmental change, and any evolutionary developments that we
may attribute to a “permissive environment,” were made permissive by
animals themselves. Nick wrote the following in a 2011 article:
By facilitating and forcing the diversification of, for example, eu-
karyotic phytoplankton, large body size, bioturbation and bio-
mineralization, early animals reinvented the chemical interchange
between the biosphere and the planet. In this light, the biogeo-
chemical perturbations of the Ediacaran-Cambrian interval are
more likely to be top-down consequences of animal evolution than
its bottom-up cause.
Nick may be surprised to hear that I like this idea very much. There
can be no question that animals themselves (including the obvious
recent impact of humans) have fundamentally influenced the biogeo-
chemical cycling of elements and the chemistry of the oceans. In this
view, the oxygenation we see at about 580 million years ago, or perhaps
a bit earlier, might be better interpreted as a redistribution of oxygen
in the oceans due to animal activity, rather than an increase in the levels
of atmospheric oxygen. Indeed, this idea dates back to Graham Logan,
John Hayes, Glenn Hieshima, and Roger Summons; they argued in 1995
that the evolution of tiny animal plankton, so-called zooplankton, com-
pletely changed the carbon cycle and the distribution of oxygen in the
oceans. The idea is that zooplankton produce fast-sinking fecal pellets.
These would decompose less in the upper layers of the ocean as they sink
