This article is timely as I was
about to tackle the same set of issues.
Thus I will use this as a back drop to my own ruminations.
First, we have shown that the ice
age itself plausibly ended as a consequence of the crust been shifted thirty
degrees south with the original North Pole ending up in Hudson
Bay . This conjecture is
written up extensively here under the heading Pleistocene Nonconformity.
Thus we have a powerful argument
for almost all of the mega fauna been destroyed then and there with survivors
succumbing to the environmental disruption that arises with the removal of the
main players.
The removal of the buffalo
assisted the restructuring of the Great Plains
with a sharp change in active participants that today we are now learning to
remedy.
Without the herds of mammoths,
the forests regrew and choked out open grasslands that would have supported a
large suite of herbivores.
The problem I have with that is
that remnant populations will survive and certainly did. The elimination of those populations most
likely was due to human predation, or that is the only reasonable explanation available
to us. Except we then ask the question
of why the African and Indian elephants survive. Certainly forest cover made the mammoth more
vulnerable to human hunting by establishing game trails that could be made over
into traps.
Quite simply, the elephant is neither
easy to kill and it produces so much meat that a large group is necessary to
exploit it. Yet it was done occasionally
in Africa .
I have to conclude that the
population was both decimated and that humanity bounced back fast enough to overwhelm
the remaining population.
That still leaves us with
populations in South America that I assume were strong enough to hold this off
if the extinction event were merely local to the northern Hemisphere. A crustal shift is a global event able to
impact global populations and in the nonce, Africa
was the least effected been on the axis of rotation. That is not true for the Indian Elephant, but
tropical conditions may have served to protect survivors.
Survivors from the remainder of
the menageries likely died off as the loss of the mammoth hugely altered the
environment.
Let us understand what that all
implies. The Boreal forest is solid
woodland. Mammoths would tear down most
of these woodlands to produce open country with grasslands leaving country attractive
for a full range of herbivores, rather that the present subdued populations.
The impact conjecture is still
been kicked back and forth in academe and no one has yet connected the dots to
understand just how deliberate it all was.
Yet it is able to explain the collapse and the ending of the Ice Age
itself. Without the shock removal of the
mega fauna, the Boreal forest would be well populated with them and plausibly
uninhabitable by humanity.
Mammoth mystery: Why giants no longer rule the north
31 March 2011 by Henry
Nicholls
Magazine issue 2805.
Woolly behemoths ruled the frozen steppe for hundreds of thousands of
years. Were they wiped out by climate change, a killer asteroid – or our
ancestors?
IN 1643, workers unearthed some huge bones in a field outside Bruges in Belgium
In 1728, British anatomist Hans Sloane identified similar remains from Siberia as belonging to elephants. But what were animals
that lived in hot climes doing in Siberia ? It
wasn't until the end of the 18th century that French zoologist Georges Cuvier
concluded that giant bones like these were from a relative of elephants that
died out long ago - the mammoth.
So where did these mysterious giants come from? What were they like?
And what drove them to extinction? Biologists have been arguing over these
questions ever since Cuvier's time. In the past few years, however, a wealth of
new information has emerged, thanks in part to DNA studies.
As far as fossil records go, the mammoth has one of the best, offering
an incredible insight into the evolution of this lineage. "You can trace
how the anatomy has changed from a general elephant-like animal to this very
specialised creature that is the woolly mammoth," says Adrian Lister of
the Natural History
Museum in London
By themselves, though, bones can only tell us so much. Luckily, the
freezer-like conditions in which woolly mammoths lived and died have preserved
not only bones but also flesh and hair. Sometimes entire animals have been
found frozen, such as Lubya, a 1-month-old mammoth discovered in 2007. Not only
do these give us a greater idea of what mammoths were like, they also preserve
their DNA blueprint. Thanks to hairs from two frozen specimens, around half
the woolly mammoth genome has now been sequenced.
Ancient DNA is helping to fill in many of the gaps in our knowledge.
"In the space of just a few years, with a relatively small amount of work,
we've gone from not really knowing anything at all about the movements of
mammoths to being able to say roughly when a migration happened, where the
animals came from and where they went to," says Ian Barnes, a molecular
palaeobiologist at Royal Holloway, University of London.
Mammoth DNA is also helping to settle questions about the origins of
mammoths. It has long been clear that mammoths first arose in Africa ,
says Lister, as fossils of ancestral mammoths dating back as far as 5 million
years ago have been found there. However, from anatomy alone it was not clear whether
these ancient mammoths were more closely related to African or Asian elephants.
In 2006, three groups sequenced the woolly mammoth's mitochondrial DNA,
revealing the structure of the elephant family tree. The studies show that the
lineage leading to African elephants split off from the common ancestor first,
around 6 million years ago. This was followed soon after by the mammoths
forking away from what would become the Asian elephant (PLoS
Biology, vol 8, e1000564).
These early mammoths had the spirally curved tusks characteristic of
their kind, but otherwise probably looked much like elephants. They remained
restricted to Africa until around 3 million years ago, when they began spread
across Europe and Asia .
The migrants really were mammoths, as they were larger than modern
elephants. Plant remains found with fossils suggest they lived in partly open
habitats, feeding on trees and bushes. There is little evidence of adaptation
towards cold in these individuals, which makes sense because the climate was
still relatively mild. But times were changing.
Around 2.5 million years ago, an epoch of ice ages began, with
temperatures plummeting ever lower with each successive ice age. Many forests
were replaced by open grassland.
These dramatic changes led to the evolution of a new kind of mammoth,
known as the steppe mammoth, with clear adaptations towards life in a colder
world and to the changing vegetation. "The steppe mammoth's teeth had more
enamel ridges to deal with a more grassy diet and a higher crown to tolerate
greater wear," says Lister.
Until recently, it seemed as though this stage in the mammoth story was
a case of gradual evolution, with the first steppe mammoths appearing around
750,000 years ago. But this picture was based on fossils found in Europe .
Fossils recently unearthed in China
It was around this time that some mammoths crossed a land bridge
joining Siberia to North America . There
mammoths evolved into distinctive North American forms and some eventually
spread as far south as central America.
Meanwhile, some steppe mammoths were becoming ever more specialised for
cold climates and open grassland, giving rise to the woolly mammoth, the most
famous of its kind. Again, while fossils in Europe suggest it appeared
relatively recently, around 150,000 years ago, we now know the woolly mammoth
began evolving around 700,000 years ago in northern Siberia ,
says Lister.
Its most distinctive feature was its shaggy coat, which was up to a
metre long. Preserved mammoths have a wide range of hair colour, with specimens
sporting blonde, red, brown and even black hair. Recent analysis of a gene
known to determine hair pigmentation, however, suggests most mammoths actually
had a dark-brown coat. The blonde, red and black tinges seen are the result of
differences in preservation, says Barnes, who was a member of the team. The
work will be published in Quaternary
Science Reviews.
Besides its long fur the woolly mammoth had a thick layer of fat
beneath its skin to insulate against the cold. It also had smaller ears and a
shorter tail than its forebears to minimise heat loss. Its huge tusks were
probably used to warn off predators and to settle disputes, but they may also
have been used like a snowplough to expose vegetation to eat or to break up
ice.
The woolly mammoth's DNA is now revealing more ways in which this
creature was adapted to life in the cold. For instance, its version of the
blood protein haemoglobin was quite different to that of modern elephants, says
Michael Hofreiter of the University of
York , UK
So woolly mammoths were built for the cold, and they thrived during a
series of ever deeper ice ages. The species spread west and east to occupy much
of the northern hemisphere, including North America ,
while other mammoth species died out. Studies of mitochondrial DNA from 40
woolly mammoth specimens by Barnes and colleagues show its population and range
expanded as the world entered the last ice age around 100,000 years ago and
remained stable during the ice age itself (Current
Biology, vol 17, p 1072). And then, as the ice age ended, it went extinct.
What happened? Some biologists think the extinction took place very
rapidly, triggered by a sudden, dramatic event around 12,000 years ago. One
suggestion is that some kind of "megadisease" wiped out the species.
Another is that a meteorite impact in North America
triggered catastrophic change. And then there's the "blitzkrieg
hypothesis", which blames the mammoth's demise on the spread of
spear-wielding human hunters.
Hunting clearly did happen, as cave paintings and the occasional
spearhead lodged in bone testify. But there's growing evidence that woolly
mammoths didn't die out as suddenly as such cataclysmic visions would have us
believe. Dating of mammoth remains by Lister and others suggest the woolly
mammoth's range had been in decline for several thousand years before they
finally disappeared. And genetic studies show a loss of genetic diversity, a
sign of a shrinking population. This was probably a result of trees replacing
grassland as the world began to warm up again. By 12,000 years ago, woolly
mammoths were restricted to the steppes of Siberia .
"The original, huge unbroken range of the species in its heyday shrank and
became fragmented in ways that map onto the way the climate was changing and
the way the vegetation was changing," Lister says.
Urinating DNA
However, dating remains can only give a very rough idea of when
mammoths died out, says Eske Willerslev, an evolutionary biologist at the University of Copenhagen
in Denmark
"Modern elephants urinate about 50 litres a day, which is
basically DNA all over the landscape," Willerslev says. Working on the
assumption that mammoths would have done the same, Willerslev and colleagues
extracted sediment that could be accurately dated from the Alaskan tundra and
looked for mammoth DNA. Although the most recent mammoth remains from this
region date to over 13,000 years ago, the team's findings suggest that mammoths
were still living there 10,500 years ago (Proceedings
of the National Academy of Sciences, vol 106, p 22352).
That's 3500 years after the first recorded human settlement and almost
2500 years after the mooted meteorite blamed for wiping out mammoths in North America . "Our data show that neither the
blitzkrieg hypothesis nor the idea of an extraterrestrial impact can be the
whole story," Willersley says.
In fact, a
paper due to appear in Earth-Science Reviews concludes there was no
impact after all. But climate change seems unlikely to be the whole story
either, as mammoths had survived previous warm periods, or interglacials.
"It's possible that you only get extinction if you get a combination of
factors coming together," says Lister.
The last stand of the mammoths took place on Wrangel Island in the Arctic Ocean . Here a population of mammoths was cut off
from the Siberian mainland 9000 years ago as ice sheets melted and sea level
rose. However, the climate and vegetation remained suitable for them, and they
survived here for 5000 years before dying out around 4000 years ago - around
the time humans arrived.
Nobody knows for sure what caused the extinction of the Wrangel
mammoths. There is no direct evidence of humans killing mammoths, so it is
possible the island was simply too small to support a mammoth population.
However, ancient DNA collected from the remains of some of these last mammoths
paints a picture of a stable population that thrived for 5000 years and then
suddenly died out, rather than a population in terminal decline (Proceedings of
the Royal Society B, vol 277, p 2331). So the smart money is on our
ancestors having a hand in it.
The emerging consensus, then, is that as mammoths gradually became more
specialised for cold conditions and grassland, they became ever more vulnerable
to climate change. Their range and population shrank dramatically as the world
warmed, but they might well have clung on as they did in previous interglacials
had human hunters not put further pressure on them. It seems we delivered
the final blow
What killed the megafauna?
Not long ago, giants ruled the land. The woolly mammoth was just one of
dozens of gargantuan beasts, including cave bears, giant ground sloths, Irish
elk, mastodons and woolly rhinoceros. Between 40,000 years ago and the end of
the ice age around 10,000 years ago, they vanished. Most continents lost
around 80 per cent of their large mammals forever.
Recent studies show megafauna weren't the only animals affected.
"All of the surviving species for which we have ancient DNA have reduced
genetic diversity at the end of the Pleistocene," says Michael Hofreiter
of the University of York , UK
The cause of these extinctions has long been fiercely debated. Studies
of the mammoth suggest they were wiped out by a combination of habitat loss as
the world warmed and hunting (see main story). But that may not be true
for other animals' extinctions. "It may well be that there were different
causes or combinations of causes for the extinction of each species," says
Hofreiter.
In the case of cave bears, for example, Hofreiter and his colleagues
have found that the population began to decline around 50,000 years ago before
finally going extinct 25,000 years ago, right at the coldest point of the last
ice age. Brown bears, by contrast, remained relatively stable over the same
period (Molecular
Biology and Evolution, vol 28, p 879). Hofreiter believes it's possible
that humans competed directly with cave bears for suitable caves to live,
pushing them out into the cold.
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