استماع TOEFL C1: Sauropods

In the past, North America supported a wide variety of enormous creatures.

Between 11,000 and 10,000 B.C.E,

beasts like Mastodons, Giant Sloths and the Mammoth.

I could go on, the list is massive.

Rather suddenly, around 10,000 B.C.E,

all of these enormous beasts had vanished.

They were North American Megafauna, and they were gone.

No one is exactly sure what happened to them, but several theories have been discussed.

Human hunting, is one theory.

The beasts could have been hunted to extinction.

At the time that those animals roamed the continent,

the humans were called the Clovis People.

There is a site in southern california, featuring those people.

Within the site is a number of tools for processing meat,

as well as spear points, fireplaces, and the skeletons of 13 mammoths.

Mammoth bones have also been found at other Clovis People excavation sites.

However, just because they hunted the mammals didn't mean they did so to extinction.

Further evidence to the contrary includes strong hints

that the Clovis people tended to favor plant gathering and small game hunting.

Likewise, similar gatherings of mammoth skeletons have been found at...

what is obviously the site of an ancient natural disaster.

Avalanche, flood, any one of these could kill a large group of them.

Perhaps at the Clovis site mentioned,

the humans only took advantage of the aftermath of a natural disaster before they, too, were claimed.

Another theory is that of severe climate change.

Around 11,000 B.C.E,

humans started settling into areas, instead of being Nomadic hunter-gatherers.

The world was moving into one of its interglacial periods, or the warm times between Ice Ages.

Due to carbon dioxide and methane buildups in the atmosphere,

the entire world was insulated and warmed up significantly.

With warmer weathers, animals built to live in extreme colds would suffer stress in the heat,

and die from heat-stroke or dehydration.

Their physiology simply couldn't cope.

We take for granted that there are tropical zones near the equator,

grasslands, woodlands, and finally tundra as you move northward.

During Ice Age periods, the plants would have all been fairly mixed together, with high plant diversity in any one area.

With warmer climates, plants expanded outward,

survived as best they could, and resulted in more plant volume,

but less diversity in each area.

Another theory in addition to the heat-stroke one

is that large animals like our Megafauna could not get the plant diversity needed for their large diets,

and died of malnutrition.

While that may seem like a sound theory,

it does have one fatal flaw:

The world goes through cycles like this regularly,

and the last Ice-Age followed by a interglacial period had no mass extinction.

An article was recently published about an archeologist using the latest technology to try and solve this puzzle.

He scripted a computer program that would simulate the effects of certain environmental changes on a mammoth.

For example, if a drought persisted for a dozen years,

or hunters culled ten percent of the total mammoth population,

what would happen?

While no solid conclusion was made, he did have this to say:

"Hunters killing off every mammoth was not needed to push them toward extinction".

Huge creatures like elephants and, by extension, mammoths, reproduce very slowly.

The man created a simulation where humans killed off a few young females from the herd,

and gave them some added environmental stress.

The simulated mammoth herd all died within a few decades.

For now, scientists suspect a mixture of hunting and changing climate would have done the trick to smother these mighty beasts.

Unfortunately, we still do not have a concrete answer,

and no amount of computer simulations will replace finding solid evidence.