But the most persuasive of past human destructiveness comes from fossils coming to light around the world

By admin
Ballet Shoes Center.com

Published: July 25, 2010

commentComment        

But the most persuasive of past human destructiveness comes from fossils coming to light around the world. The direct fossil evidence for human butchering is patchy, but what else could have caused such a wipe-out? Climate has again been indicted, but if this were the cause we would expect small species to suffer more than large, for they are more sensitive. Prof Martin calls this time of devastation “the Pleistocene overkill”.Overall, the evidence for Pleistocene overkill is circumstantial. Ancestors of the present-day native Americans crossed from Siberia to Alaska around 13,000 years ago and spread south during the next few thousand years Species after species of large mammal vanished as they went. Now they too are down to 12: the llama, tapir, jaguar, spectacled bear, and a few others.What lay behind this second round of losses which occurred over the past few thousand years, long after the invasion of the northern species? The most likely cause, according to Professor Paul Martin of the University of Arizona, was the arrival of humans. This impressive catalogue is now reduced to 12; including the world’s largest deer, moose and wapiti, and bison, musk-ox and a few bears In South America the destruction has been even greater.

In the late Pleistocene, the native southern survivors plus the northern invaders between them spanned an astonishing 58 genera; almost as great a variety of large mammals as now exist over the entire Earth. These included animals that no longer exist like mammoths and mastodonts, plus others that survive in Eurasia and Africa, like camels, horses, the yak and the cheetah. Thus, until at least 100,000 years ago, late in the epoch known as the Pleistocene, North Amer- ica had no fewer than 45 genera of large mammals, each of which may include several or even dozens of related species. No signs of inferiority there.But most large mammals that survived the Pliocene upsets – “large” means more than 100lbs – have disappeared in both Americas. Some even managed to migrate north, including giant ground sloths and glyptodonts which flourished for a time in the present United States, plus armadilloes and marsupial opossums which still abound. As the climate changed, they could stay in the kind of environment they were used to just by changing latitude; southerners were mostly obliged to stay put and died out as conditions changed.Many southerners did survive the invasion, however, including some of the native hoofed animals: one shaped like a heavyweight camel and another like a hornless rhinoceros. Yet their native forest did not escape unscathed; it retreated and opened as the climate grew colder and drier.

Northern immigrants were adapted to such open country, as it was the same as that of the north in former times. As things turned out, they could press on to Brazil.The southerners, by contrast, were already at the equator when the climate began to grow harsher They had nowhere to run and nothing to gain by fleeing. The equator, the hottest place, is at the level of the Amazon, so that is where they went. If the northern continent had not met the southern, some northerners might have died out in Panama.

She believes that, at the time the two continents collided, the world was growing cooler and drier – a prelude to the ice ages So, maintains Dr Vrba, animals migrated towards heat. But Elisabeth Vrba, a zoologist at Yale University, has formulated a powerful argument which supposes instead that the first mass extinctions of South American mammals was brought about simply by a change of climate and hence of landscape. They also included a host of creatures that have become extinct in their turn: one-toed and three-toed horses, elephant- like mastodons and even true elephants, and the formidable Arctodus – a bear half as big again as a modern grizzly but with a short face for seizing and long legs for running; a kind of ursine Rottweiler.The replacement of southern creatures by northerners was dramatic, and most biologists assumed the northerners were more agile, more versatile, more streetwise. Those northern newcomers included the immediate ancestors of the largest South American animals that survive today, like the llama and vicuna, the tapir, the jaguar and puma, and the spectacled bear. The second occurred after about 13,000 years ago, with the arrival of human beings in the Americas.During the first wave of extinctions, most of the unique marsupials and hoofed animals were replaced by creatures that flooded in from the north, via the new-formed isthmus of Panama.

The great mammals of South America vanished, it seems, in two main waves. The first mass extinction followed soon after South America met up with North Amer-ica in the Pliocene epoch, around three- and-a-half million years ago. The discovery of a maritime sloth shows that the xen-arthrans were even more versatile than anyone had previously suspected.These weird and wonderful native species of South America are almost all gone, as extinct as the dinosaurs which disappeared more than 60 million years previously. Uniquely, too, it had the xenarthrans which, besides the sloths, in-clude the anteaters, armadilloes and the extinct tortoise-like glyptodonts with a ‘’shell” as round as a golf-ball and as big as a bread van.


Comments are closed.

Ballet Shoes Center Latest

>Next Articles

Top Brands