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Luck of the draw?
Dec 02, 2011

 

What happens when another global catastrophe strikes the Earth? Will mankind survive the next “Big One?” 
In today’s world, where climate change is a topic of intense interest in the media, especially as a result of the recent United Nations conference in Durban, South Africa, there are both proponents and skeptics. Some of the debate centres around whether industrialized nations, such as Canada, need to be overly concerned that they are spewing too much greenhouse gases into the atmosphere. And if such emissions are dramatically curtailed, whether it will have dire consequences for a nation’s economy. Science and economics seem to continually be at loggerheads in the climate change debate, with political decisions more apt to lean toward the latter, especially when the powers-that-be in Canada react to the issue.
Past events show that mass extinctions periodically occur and survival of any species involves having the “right stuff” at the “right time.” The creatures that contributed to the evolution of humanity survived the great cataclysmic events that destroyed other supposedly well-adapted species, and luck certainly did play a part.
When the Cambrian Explosion, the period when complex organism first arose and arthropods such as trilobites ruled the shallow seas, ended with a mass extinction event around 480 million years ago, the descendants of a tiny creature called Pikala glacilens survived. They were chordates, animals which swam fish-like through the water by contracting the muscles of their body wall. Pikala had a stiff internal rod (notochord) running along the entire length of its body — the beginning of a backbone. If this little creature’s descendants hadn’t survived and spread, the progression toward higher species leading to man would have abruptly stopped in its tracks. 
The Great Dying, the most massive extinction event of all time, when 90 per cent of the Earth’s living creatures met their demise, occurred 252 million years ago. In a November 23 National Geographical News article, two scientists placed a 200,000-year-long time frame for the event at the end of the Permian, which nearly wiped out all life on Earth. In fact, 70 per cent of all land species died out, including 75 per cent of the reptiles and 67 per cent of the amphibians. A staggering 95 per cent of all marine species expired, including the formerly prolific trilobites, and most of the plants covering the land disappeared (fungi was a survivor and dominated until other plants began to recolonize the land). Unique to all major extinction events, 33 per cent of the usually quite hardy insect orders perished. 
There are two causes for the Great Dying proposed: typically one involves a massive asteroid impact, but most scientists believe the cause was run-away greenhouses gases flung into the atmosphere by volcanic eruptions in the Siberian Traps. It would have been “Hell on Earth” on an unimaginable scale, since 200,000 square kilometres of land were covered by torrents of lava which emerged continually from beneath the Earth’s crust for thousands of years, releasing  carbon dioxide and sulphur into the atmosphere. Once the carbon dioxide began to warm the Earth, frozen methane hydrate deposits in the oceans melted and methane gas rose from the depths, The sulphur contributed to acid rain, while carbon dioxide and methane acted as greenhouse gases that formed a blanket over the Earth that trapped in heat, rising the temperature to a point more than sufficient to kill off all life.
Before the Great Dying, synapsids, ranging up to the size of a present-day mouse to an hippopotamus had evolved. From these synapsid reptiles emerged the “mammal-like reptiles,” or therapsids. Most would imagine a reptile as having splayed out legs similar to a lizard, but some therapsids had legs directly beneath their bodies and skulls resembling that of a wolf or dog. The jaws contained canines and molars, which is a characteristic of mammals, not reptiles. This branch of therapsids referred to as cynodonts (dog-teeth) were among the few groups of synapsids that survived the Great Dying. 
It is likely that cynodonts were at least partially, if not completely, warm-blooded and covered with hair, which would have insulated them and helped to maintain a high body temperature.  
In the end, mammals became the only surviving synapsids, as all other groups went extinct. But the path toward true mammals was fraught with uncertainty, as there was another group of animals that would soon dominate the landscape. In the Triassic period, a previously obscure group of small reptiles, the archosaurs, proliferated, and from this group, the dinosaurs arose to dominate the rest of the Mesozoic era. The Dinosaur Age forced the proto-mammals into nocturnal niches, and may have contributed greatly to the development of mammalian traits such as a large brain. Later in the Mesozoic, when the proto-mammals evolved into true mammals, the new species on the block spread into other ecological niches, with some becoming aquatic (similar to today’s muskrats) or gliders (similar to today’s flying squirrels). But most mammals of this era were either  land-based insectivores or omnivores, and they were decidedly miniscule in size when compared to the large meat-eating and plant-consuming dinosaurs.
Mammals bided their time. When another extinction event occurred some 65 million years ago — most likely caused by an asteroid impact —  the dinosaurs were wiped out as were the large marine reptiles, which were not dinosaurs, but arose from a separate evolutionary path. The only reptiles to survive were crocodilians, turtles and some lizard species. As well birds, a descendent of small, bipedal carnivorous dinosaurs, made it through the extinction event, as did fish, another vetebrate species. 
Dinosaurs had dominated the land for 160 million years, but tiny shrew-sized and cat-sized mammals survived the extinction event that wiped out the large reptiles. In this case, smaller was definitely better. Adaptations needed to hide in the shadows in order to survive the Dinosaur Age had saved the mammals.
With the end of large reptiles dominating the Earth, mammals branched out to fill the vacated ecological niches, and eventually, some four million years ago, arose the bipedal hominid species that led to the evolution of Homo sapiens sapiens — modern man.
So far, mankind has been the biggest winner in the extinction sweepstakes, but will our luck continue to hold out?