Was the split up of
Rodinia the driver behind the Cambrian Explosion?
The rapid evolution and diversification of animals from single
celled organisms to complex multicellular, hard shelled organisms in the early
Cambrian (~530Mya) has been debated and
discussed amongst scientists for decades. It marks the most rapid period of
biological evolution recorded in earth’s history. Most of the organisms we see
today such as Worms, Molluscs, Echinoderms, Arthropods and Chordates are
considered to have originated during this time period. Though there is still no
universally accepted answer to explain the cause or causes behind this rapid
diversification event. Is this solely a biological phenomenon or are tectonic
processes partly to blame?
Evolution in theory requires genetic variation. This variation
can arise from natural selection and adaptive radiation, whereby organisms are
passing on more favorable alleles to better adapt to a variety of different
ecological niches. Or by genetic drift or mutation in which there is a random
change in allele frequency in a population. Natural selection is the main
process in which organisms are positively adapting to changes in the
environment and will therefore be more successful than random changes within
the gene pool. Evolutionary triggers can be split into 2 categories extrinsic
factors (e.g. tectonics, climate, ocean salinity, oxygen and nutrient
availability) or intrinsic factors (e.g. genetic and development innovations).
It
is believed by some that the explosion of life was due to only biotic factors
but in an earth system as diverse as this it’s probable that complex abiotic
and biotic interactions between the hydrosphere, biosphere and the geosphere
caused the Cambrian Explosion.
Map of landmasses during the
Cambrian Period
(Green=Land above water; Light Blue= Continental Shelf; Dark blue:
Deep ocean basins)
During
the Cambrian, the supercontinent Rodinia comprising of a significant proportion
of earths landmass, was beginning to break up and uplift more complex
topography. The uplifted topography induced higher weathering rates on land and
the rifting increased the area of the continental shelf forming widespread
shallow eperic seas. Coincidentally we also began to enter an interglacial
period following the Late Proterozoic ice age. The melt of continental ice
sheets generated a rise in sea level and brought down ions in melt water as it
made its way to sea. The interplay of these 2 abiotic processes increased the
area of diverse of ecological niches for organisms to specialize into.
One
hypothesis of the cause of the explosion is represented by the great
unconformity. This is an unconformity between ancient crystalline base rock and
younger undeformed marine sediments.
The Great Unconformity in the
Grand Canyon
Tectonics
uplifted land and instigated a period of continental denudation allowing the
weathering of newly exposed base rock, followed by the deposition of shallow
marine sediments deposited during marine transgression. The idea is that the
large area of continental margins exposed due to the rifting apart of Rodinia,
was inundated by ice melts from the late Proterozoic ice age causing sea level
rise. This process weathered newly exposed base rock releasing a significant
amount of ions such as Ca, Fe, K and SiO2, which changed the ocean water
chemistry. When organisms are subject to an excess of something (in this case
ions) they have to remove it from their system. The super saturation of ions in
solution could have facilitated the origin of biomineralisation to remove ions
from their system to create skeletons. This is an advantageous evolutionary
adaption because skeletons provide protection from predators, structural
support for movement onto land, muscle attachment for more efficient movement
and allow the growth of weapons and appendages.
Schematic showing the links between the earth system spheres
(Biosphere – Geosphere – Hydrosphere – Atmosphere)
There
is also evidence of increased carbonate mineral formation, which supports this
idea. It is possible that the excess of phosphorus brought down with glacial
melt water triggered an increase in the rate of primary productivity therefore
caused an increase in oxygen. This is thought because phosphorous plays a
critical role in the Krebs cycle (second step in respiration) as it’s a
compound for components of the Krebs cycle such as ADP and ATP.
Photosynthesis equation
A
lot of evidence points towards tectonics instigating rapid diversification of
animals during the Cambrian. The rifting apart of Rodinia allowed the spreading
of shallow seas across large areas of continental shelf, ultimately increasing
the habitable volume for all organisms. This would have temporarily ceased
pressures associated with food, space and predator-prey relationships.
Therefore
the generation of new ecological niches and the changing of the selective
pressures on existing ecosystems would have promoted organisms to evolve into new
niches. Without new habitats to live in, the number of individuals that can be
sustained in an area becomes limited by the carrying capacity of a certain
location. So without the rifting apart of Rodinia there would have been a
smaller surface area or less habitable area for organisms to specialise into.
So the selective pressures would have been greater. Also the super saturation
of ions in ocean water may not have occurred without such a vast exposure of
un-weathered base rock.
There
is evidence to suggest that tectonics played a massive role in the Cambrian
explosion but the event will not be solely based on processes within only the
geosphere. We know that processes within out earth systems are complexly interlinked
with other spheres. This is represented by the interplay between the
hydrosphere and the geosphere on the base rock and water interface. It could be
safe to say however that without the action of tectonics, biological processes
alone may not have produced such a rapid explosion of life. The real question
is, would this explosion of life ever have occurred without tectonic processes?
Josephine Turnbull
Josephine Turnbull
References:
Hsu, K., Kang, M., Lavarias,
A., & Prabaker, K. (2000). Tectonics
of the Cambrian. Retrieved from.
Astrobiology magazine. (2014). Massive geographic change may have triggered explosion of animal life. Retrieved from.
The Virtual Fossil Museum.
(n.d). The Cambrian Explosion.
Retrieved from.
ZHANG XingLiang SHU DeGan. (2014). Causes and
consequences of the cambrian explosion. 中国科学：地球科学英文版, 57(5),
930-942. doi:10.1007/s11430-013-4751-x
Smith, M. P., & David A. T. Harper. (2013).
Causes of the cambrian explosion. Science, 341(6152), 1355.
doi:10.1126/science.1239450
Zhang, X., Shu, D., Han, J., Zhang, Z., Liu,
J., & Fu, D. (2014). Triggers for the cambrian explosion: Hypotheses and
problems. Gondwana Research, 25(3), 896. doi:10.1016/j.gr.2013.06.001
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