Our planet
had only single celled life for nearly three billion years after its
birth. It was around ~543 million years ago (Ma) that was at the
beginning of the Cambrian chapter of the earth's history there was a 'big
bang' of evolution. All of a sudden life started to prosper and
proliferate from single celled organisms to multicellular and from soft
boneless to forms with external skeleton. The ocean was the cradle of
this biodiversity.
It is quite
perplexing to imagine what were the causes that triggered a sudden
explosion of life? Well life can not survive without food. Thus the
answer to the problem perhaps lies in finding out what was the source of
food for the life evolving rapidly in the depths of the sea?
The
vehicle of evolution never turned back once it was set in motion
during the Cambrian Period. But yes, at around 65 Ma there was a
mass extinction of a variety of fauna, including the then rulers of
the earth, the dinosaurs.
Even in present day the total living organisms are much more in the
oceans than on the ground. From where do they get their food? A
corollary would help in understanding the events of the past that
provided food for life. The marine fauna thrives on the dissolved
mineral ions in the sea water. Similarly most of the marine
organisms have skeletons (internal or external) made of calcium
carbonate and some are of phosphatic material which are again
present in a dissolved form in the sea water. The source for these
and many other useful elements for life in the water is the detritus
or the eroded material painstakingly transported by the rivers.
Bay of Bengal which receives detritus from the Ganges-Brahmputra
river systems is known as the world's largest sediment fan. It is
estimated that since it came in to being some 20 Ma over 12 million
cubic kilometer of Himalayan derived sediments has been discharged
in to the Bay of Bengal. The detritus is rich in minerals many of
which are food for the life.
The reasons of sudden spurt of life could be many. Geologists have
been deeply pondering over the issue and in the decade of 1990s
several hypotheses emerged. It was coincidental that several
geological events coincided with the 'big bang' between ~640-542Ma
that is during Neo-proterozoic to Early Paleozoic part of the
earth's history. It was thus thought that alternating spells of cold
glacial and warm climates, plate reorganization induced by true
polar wander, fluctuations in ocean and atmospheric chemistry,
decrease in the earth's tilt causing rapid shifts in earth's climate
zones, influence of a giant meteorite impact, gene doubling and
sudden thermal recycling events due to sudden release of methane
from oceans were the cause for spurt in biological activity on the
planet.
Amongst these hypotheses, the 'true polar wander' hypothesis of Dr.
Joseph L. Kirschvink of California Institute of Technology
speculated that a major reorganization of tectonic plates during
Precambrian time changed the balance of mass within the Earth. This
acted as a trigger for reorientation. Thus the regions that were
previously at the north and south poles were relocated to the
equator, and two antipodal points near the equator became the new
poles. Kirschvink felt that the dance of life that began about 530
Ma lasted for about 15 million years that gave birth to practically
all the groups of animals present on the planet. Citing geophysical
evidences he says all of the major continents experienced a burst of
motion during the same interval of time. The sudden motion of the
continents caused a global imbalance leading to spurts of carbon
released from the oceanic abyss which ultimately led to
proliferation of life.
Meanwhile researchers in various parts of the world continued to
collect evidences in favour of their hypothesis. It was in 2006 Rick
Squire a researcher in Geology with Monash University, Clayton,
Victoria, Australia created quite a stir in the geological community
with his paper published in September. Rick has ascribed the cause
of proliferation and also answered the queries regarding the source
of food etc to an entirely different process. Taking a cue from
Rick's paper published in Earth and Planetary Science Letters, A.V.
Sankaran of from Bangalore published an analysis in the January
issue of the Current Science.
Rick postulated that as a result of a collision between giant
continental blocks corresponding roughly with Arabia, India and
Antarctica. These continents or the continental plates began to
collide with the eastern edge of Africa, some 650 to 515 Ma. He had
taken the idea of this collision from the published research of Ian
W.D. Dalziel of Department of Geological Sciences, University of
Michigan, who had actually given the concept of colliding continents
and the rise of a large mountain. The colliding continents gave rise
to the planet's longest mountain chain termed as Transgondwanan
Supermountain approximately 100 km wide and more than 8000 km long
along the equator.
The rising mountains caused a change in the geography and climate of
the region. Being in equatorial region the newly formed mountainous
terrain experienced more rain and hence became prone to more
erosion. The process of rapid erosion as per the evidences available
continued for nearly 296 million years. The material carried by the
rivers during this period was so enormous that if accumulated today,
it would have formed a 10 km high pile over all the 50 states of
United States. The influx of material in to the oceans of the years
of the yore was three times more than what takes places in the
present day Bay of Bengal, says Sankaran. The high rate of erosion
had been noticed by earlier workers. M.D. Brasier of Department of
earth Sciences, Oxford University, U.K. and J.F. Lindsay of
Australian Geological Survey Organisation, Australia had published
their findings in 2001. Based on Strontium and Nubidium isotope
ratios they had concluded that such ratios in the sea water were
only possible because of much higher influx of sediments in the
geological past.
This gave reasons to Ricky to conclude that such heavy influx can
take place only if a mountain of appropriate size is present.
The rapid increase in the influx of dissolved phosphorous, iron,
strontium, cadmium and bicarbonate ions in the oceans during early
Cambrian, that is ~640-543 Ma provided nutrients for the rapid rise
of primitive life, such as the green algae that became the food for
more complex life forms during the Cambrian period. The ocean at the
plate margins was more enriched with phosphorous and calcium
carbonate. Many organisms like trilobites and mollusks made use of
these nutrients for developing an external skeleton or shells.
The diversification of life at that time from soft bodied single
celled animals to multicellular, organisms with hard parts must not
have been that simple as narrated here. But yes the sudden influx of
nutrient rich mineral in the ocean and also the release of oxygen
due to complex chemical reactions of some the minerals were
certainly behind the explosion of life.
But for the great collision this story would not have appeared
because then neither would you have been there to read it nor me to
write about it!
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