News: Ancient rocks in India give clues to early life.
Source: The Hindu
The “Cambrian Explosion” refers to the sudden appearance in the fossil record of complex animals with mineralized skeletal remains 541 million years ago.
The rapid appearance of a wide variety of animals - particularly bilaterians - led to the development of radical new ecological interactions such as predation. Consequently, ecosystems became much more complex than those of the Ediacaran. As the number and variety of organisms increased, they occupied a variety of new marine environments and habitats. Cambrian seas teemed with animals of various sizes, shapes, and ecologies; some lived on or in the sea floor (a benthic lifestyle), while others actively swam in the water column (nektonic).
The fundamental ecological structure of modern marine communities was firmly established during the Cambrian. By the end of the Period, some animals had also made the first temporary forays onto land, soon to be followed by plants.
Triggers of the Cambrian Explosion
Before complex animals could evolve on Earth, there had to be an environment favourable for their survival. Researchers have examined a number of environmental factors that might have been instrumental in the evolution of new body plans, but the two strongest contenders are a rise in oxygen levels and the end of extreme glacial conditions.
Multicellular animals use oxygen to fuel their metabolism. At low oxygen levels, they don't function well … without it, they cannot survive. Photosynthesis could have caused a rise in the amount of oxygen in the seas and atmosphere near the beginning of the Cambrian, allowing the evolution of larger, more complex animals with respiratory and circulatory systems.
However, there does not seem to be much variation in oxygen levels across the Ediacaran-Cambrian boundary. Earlier increases might have triggered the evolution of large Ediacaran metazoans prior to the explosion, and a subsequent post-explosion rise in oxygen levels may have allowed animals to adopt more active, energy-intensive lifestyles such as swimming and hunting.
Another possible environmental explanation for why the explosion occurred when it did involves glaciers. Some researchers have suggested the entire Earth was covered with ice before the Cambrian explosion. (This is known as the "Snowball Earth" hypothesis.)
The ice would have limited the number of evolutionary niches for life in the sea, and blocked most of the sunlight on which cyanobacterial mats and algae depend. But once the glaciers receded, huge expanses would suddenly be opened for life: an ideal situation for experiments on different body plans. Unfortunately for the hypothesis, the last worldwide glaciation seems to have ended around 635 million years ago - nearly 90 million years before the first signs of the Cambrian explosion in the fossil record (which was followed by another major regional glaciation around 580 million years ago).
Even if there is no direct triggering link between Precambrian glaciations and the Cambrian explosion, the post-glacial period was a crucial time in evolution. The appearance of the first large and complex multicellular organisms shortly after the return to a warmer global climate suggests that environmental conditions had become ripe for them to evolve.