Beyond beginning

Beyond beginning

THE origin of life has intrigued and fascinated researchers over the years. Every time a new finding is made, a link in the chain of events is added. A similar finding now seems to put another block of the puzzle in place. Based on genetic evidence, recent molecular research suggests that progenitors of creatures small and big, from beetles to humans, appeared on earth one billion years ago. This is twice as early as once widely thought.

For billions of years life on our planet was simple in the form of bacteria, plankton and algae. Suddenly animal life exploded all over the world. The time was the beginning of the Cambrian period, around 565 million years ago, when the progenitors of the animal kingdom took shape. In a span of not more than 20 million years, the ancestors of virtually all the present day creatures were born in this unique phenomenon called Cambrian explosion.

Charles Darwin had set the ball rolling for an in-depth research when he argued in the 1850s that the appearance of multicellular animals during the Cambrian period merely seemed sudden and, in fact, had been preceded by a lengthy period of biological evolution for which the geological record was missing. However, scientists discovered in layers of rocks the fossils of the ancestors of almost all the species that exist on earth now, dating back to the Cambrian age. This led to the hypothesis that it was no other period than the early Cambrian age that nature chose to invent the animal body plans that define the broad biological groupings called phyla, which encompass everything from classes and orders to families, genera and species (Down To Earth, Vol 4, No 17).

Evolutionary zealots since Darwin's time have tried to get an insight into the years that preceded the Cambrian explosion. And it is finally molecular biologists who carried the day.
Life was elsewhere The major breakthrough came recently at the State University of New York at Stony Brook, us. Based on the chemical clues in living creatures, scientists led by Gregory A Wray reported in a recent issue of Science, that long before animal organisms grew bones, shells and spines that left a clear mark in the fossil record, minute creatures crawled or slithered between grains of mud or sand in the primordial waters. Since these simple animals did not have skeletons, they were not fossilised. And as these species were slight in their make, with not enough substance, there were no detectable burrows and tracks as proof of their existence. Apparently, for about half a billion years these cryptic organisms were slowly evolving, diverging into different lineages and finally becoming big and complex to leave a profusion of fossils.

Researchers made this discovery using patterns of genetic changes as molecular clocks. This technique is being increasingly used in unveiling evolutionary history so deep in time when fossils were either absent or were of uncertain value. In this study, scientists examined detailed records of seven different genes in different species, revealing their rates of divergence over time from their common ancestors. This was to estimate the length of the time span over which the structure of these genes had been changing. They found this to be 1 billion to 1.2 billion years. This shifted the origin of animal life back by around half a billion years, causing a major shift in the evolutionary time line.

In 1982, based on the results of his study of blood proteins, Bruce Runneger, a palaeontologist at the University of California at Los Angeles argued that Cambrian explosion did not erupt out of the blue but was "the continuation of a process that began long before". Due to lack of substantial evidence however his preposition did not gather much support. Welcoming the new findings that seem to confirm the result of his more limited study, Runneger says that, "there is no doubt any more about a longer history of multicellular animals. It is just a question of how longer".

Confirming his notion, Wray and his colleagues cast a doubt on the prevailing theory that all the major animal groups diverged explosively during the Cambrian or late Vendian, the geological period preceding Cambrian. They suggest that "there was an extended period of divergence beginning about a billion years ago". Meaning thereby that precursors of the animal species that seemingly exploded in Cambrian period emerged and evolved over a much longer period.

The finding is provocative and is certain to stir a lot of debate and research. Many palaeontologists who look at the past through the lens of fossils may question the reliability of the molecular clock methods for probing deep time. In a commentary accompanying the report, Geerat J Vermeij, a palaeontologist at the University of California at Davis, us, cautions that "acceptance of the proposed ancient origin of animals hinges on the accuracy of the calibration of genetic changes in living organisms. Also on the assumption that the rate of divergence was constant through time". Scientists may not be sure whether such an assumption is valid.

However, the takers of the new theory may raise important questions about the pre-Cambrian animal species and the factors that led to the Cambrian explosion. Delving on the factors that led to huge expansion in the size, complexity and body architecture of pre-Cambrian animals at the beginning of the Cambrian period, Andrew H Knoll, a palaeontologist at the Harvard University, feels that the causes were environmental--specifically, a significant increase in atmospheric oxygen. Increased oxygen and other factors could have improved the efficiency of cells and production of collagen, a protein that holds animal cells together, leading to complex structures. Shifting continents and drastic climate change may have been other factors, too. Interestingly, some scientists suspect that the introduction of predation set off the revolution.

Evidence and explanation
Geochemical evidence suggests that oxygen levels in the oceans and atmosphere rose enormously at the beginning of the Cambrian period. In the pre-Cambrian period the oceans were smothered by vast populations of algae. They got energy by using the process of photosynthesis which uses carbon dioxide, water and sunlight. However, tonnes of vegetative debris resulted from these algal weeds which were decomposed by bacteria, by a process that is exactly photosynthesis in reverse. That is, large quantities of oxygen were consumed by these scavenging bacteria, releasing equal quantities of carbon dioxide into the atmosphere. Thus carbon dioxide mopping by the algae was balanced by oxygen consumed by the bacteria to clear the algal debris. Thus, for oxygen to rise, the planet's decaying organic matter had to decline. Knoll suggests that amidst a tumultuous climate at the beginning of Cambrian period this happened. And how did it happen? Knoll speculates that a chain of geological and geochemical changes took place that infused the primordial waters with oxygen.

The eve of the Cambrian explosion was marked by violent earthquakes due to turbulent tectonic forces. The climate was in great turmoil as ice ages came and went. The stage was all set to unleash events that could change the chemistry of the atmosphere and oceans. One mechanism, says Knoll, could have been the erosion from steep mountain slopes. Tonnes of sediment and rocks poured into the sea that could have buried the algal remains to the ocean floor. Rifting continents changed the geometry of ocean basins that did not let the water to circulate as vigorously as before. The organic carbon that fell to the sea floor thus stayed there never cycling back to the water surface and eventually into the atmosphere as carbon dioxide. The earth would have cooled, as levels of atmospheric carbon dioxide dropped due to the unavailability of decomposable algae. Thus, the raised oxygen levels in the oceans and atmosphere towards the beginning of the Cambrian age is felt to have led the pre-Cambrian creatures modify suddenly into complex forms that left a profuse of fossils behind.

Another explanation offered by some scientists like David Jablonski, a University of Chicago palaeontologist, says that the introduction of predation may have also played a role in the Cambrian revolution. It is suspected that with time some tiny pre-Cambrian species could have developed teeth or grasping claws. "And once one lineage of animal life figured how to eat another, armour became a matter of survival," explains Jablonski. Evolution by natural selection would then favour animals with self defence innovations leading to larger body size, protective shells and spine, and the ability to burrow and swim to flee the predators. Hard defence mechanisms developed all of a sudden round these soft bodied pre-Cambrian creatures, allowing them to make a mark finally in the fossil record.

The new molecular research gives a different perspective to the Cambrian explosion. The pre-Cambrian world, its environmental and biological dynamics may get more probing after this pathbreaking discovery.

Summing up the implications of this discovery,Vermeij says, "the new work in no way diminishes the significance of the Vendian-Cambrian revolution, but it does separate the ecological innovations of that episode from the earlier evolution of basic animal body plan."