Tuesday, April 12, 2011

Mammoth Mystery

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. The naturalists who studied them were convinced they had come from a human-like giant. Their length, after all, tallied with a biblical reference to Og, a giant king supposedly slain by Moses.

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 and the author of Mammoths: Giants of the ice age.

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 paint a very different picture. They show that the steppe mammoth evolved there about 1.7 million years ago and gradually spread out across the northern hemisphere, replacing earlier forms. "East Asia was the key area of mammoth evolution after the initial radiation of early forms out of Africa," says Guangbiao Wei, director of the Chongqing Three Gorges Institute of Paleoanthropology 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. But did these differences matter, or were they just a result of random mutations? To find out, he and his colleagues made some mammoth haemoglobin and put it through its paces in the lab. "The mammoth haemoglobin releases oxygen at much lower temperatures than elephant haemoglobin," says Hofreiter. This means their blood could still deliver enough oxygen to cells even when their extremities became cold. Similar adaptations are seen in modern-day Arctic mammals like reindeer and musk-ox (Nature Genetics, vol 42, p 536).

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. That's because only a tiny fraction of the population will have been preserved and we have found only a few of these. So he has taken another, more unusual, approach.
"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. "So something was reducing genetic diversity massively and across the board." The larger species went extinct most readily because their smaller populations, and the longer time it takes for juveniles to reach maturity, made it harder for them to recover.

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.

Henry Nicholls is a freelance writer based in London. Follow him ontwitter.com/WayOfThePanda

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