terça-feira, 15 de junho de 2010

A short history of Life on Earth

The Earth had already been revolving around the Sun for nearly four billion years when Life entered a major new stage. For more than two billion years, the only life forms had been isolated cells floating in the worldwide ocean. But now these cells began to associate with one another, becoming the first multicellular organisms.

This was some 700 million years ago. It would take only another 100 Ma for certain organisms to develop a skeleton: hard parts that could be preserved in rock long after the organisms died. What we know of the past forms of Life on Earth is largely based on these fossils: they have given us a far more accurate picture of the past 600 Ma than we have of the billions of years that went before.

Another 100 Ma, and the seas are now populated with fish. Yet another 100, and their descendants can lay sturdy eggs; now equipped with lungs, they grow bolder, abandon the water, and conquer the continents, as yet uninhabited. Then, 260 Ma ago comes the “invention” of warm blood, and the first proto-mammals begin to prosper. Here, at the end of the Paleozoic Era, the abundant and varied fauna and flora bear every mark of success, both in the ocean depths and on the emergent land. Yet almost all at once, 250 Ma ago, a catastrophe causes 90% of all species to vanish forever. For an entire species to disappear, every individual it comprises must die without descendants. When 90% of all species die out, the populations of the remaining 10% will certainly be hard hit as well: in fact, perhaps 99% of all animals living at the end of the Paleozoic perished. This is the most extensive of all mass extinctions known today.

But not all died, and the survivors set out to reconquer the space so unexpectedly swept clear for them. This start of the Mesozoic Era is dominated by pig-sized plant-eaters called Lystrosaurus. They have large amphibians for company, along with other reptiles who will soon give rise to the first true mammals and the first dinosaurs. A new catastrophe, less violent than the first, arrives to decimate the last proto-mammals, the great amphibians, and (in the oceans) almost all species of ammonoids.

Small, hiding in the trees and living on insects, our mammal ancestors were anything but conspicuous. You might almost say they encouraged the world to forget they were there. For this, in fact, was the real beginning of the age of dinosaurs. Recent paleontologic research has given us a whole new perspective on these beasts. Some may have been warm-blooded. The great long-necked, plant-eating sauropods, like the celebrated Diplodocus, gradually gave way to animals sporting horns and duckbills, grazing no longer on the treetops but on grass and bushes. Their predators were those great carnivores, colorful and agile.

Then, 65 Ma ago, a huge catastrophe once again ravaged this world, which had seemed so perfectly adapted and balanced. This was the end of the dinosaurs and many mammals, but also of a great many other terrestrial and marine species, including the well known ammonites and a considerable number of smaller and less familiar organisms that constituted the marine plankton. In all, twothirds of the species then living (and possibly 80% of all individuals) were wiped out. This is the second great mass extinction. Yet again the momentum resumes, and in less than 15 Ma we find the ancestors of most animals that still live on our Earth today.

As the climate turns colder, modern fauna comes into place some 30 Ma ago. The age of dinosaurs has yielded to the age of mammals, delivered at last from their chief rivals. And the Mesozoic is succeeded by the Cenozoic Era.

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