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!