"All the world's a stage we pass through." - R. Ayana

Friday, October 8, 2010

Where Are the Neanderthals?

Where Are the Neanderthals?
Our Neandertal Brethren:

Why They Were Not a Separate Species

Genome sequencing has revealed our common humanity
According to the late Harvard University biologist Ernst W. Mayr, the greatest evolutionary theorist since Charles Darwin, “species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.”
Reproductive isolation is the key to understanding how new species form, and many types of barriers can divide a population and split it into two different groups: geographic (such as a mountain range, desert, ocean or river), morphological (a change in coloration, body type or reproductive organs), behavioral (a change in breeding season, mating calls or courtship actions), and others.
After isolation, if members of the split populations encounter one another and cannot produce viable offspring that can themselves later successfully interbreed and produce viable offspring (hybrids such as mules are infertile), then these two populations constitute two different species.
Let’s say that a species migrates out of Africa into Europe around 400,000 years ago and becomes reproductively isolated from its ancestral population for the next 320,000 years. It evolves distinctive anatomical features and adaptations for the colder climes. Moreover, even after other descendants of the original ancestral population move into Europe around 80,000 years ago, the skeletons from both groups show no obvious signs of blended characteristics. Modern scientists classify the creatures as two different species.
Then, however, genetic analysis reveals that members of these two species interbred and produced viable offspring that populated Europe and spread eastward as far as China and Papua New Guinea. By Mayr’s definition, these two interbreeding populations are not two species after all, but two sibling subspecies of the original African species.
A subspecies has a characteristic appearance and geographic range, Mayr explains, yet he adds this significant qualifier: “It is a unit of convenience for the taxonomist, but not a unit of evolution.”
Thus it is — revealing the identity of my example — that we must reclassify Homo neanderthalensis as Homo sapiens neanderthalensis, a subspecies of Homo sapiens. A comprehensive and technically sophisticated study published in the May 7 issue of Science, “A Draft Sequence of the Neandertal Genome,” by Max Planck Institute evolutionary anthropologists Richard E. Green, Svante Pääbo and 54 of their colleagues, demonstrates that “between 1 and 4% of the ge­nomes of people in Eurasia are derived from Neandertals” and that “Neandertals are on average closer to individuals in Eurasia than to individuals in Africa.”
In fact, the authors note, “a striking observation is that Neandertals are as closely related to a Chinese and Papuan individual as to a French individual.... Thus, the gene flow between Neandertals and modern humans that we detect most likely occurred before the divergence of Europeans, East Asians, and Papuans.” In other words, our anatomically hirsute cousins are actually our genetic brothers.
This modified Out of Africa theory holds that around 400,000 years ago a population of hominids migrated northward through the Middle East and into Europe and parts of western Asia. Between 80,000 and 50,000 years ago another population from the ancestral continent journeyed a similar route into the Eurasian landmass, and there the two populations met and mated. We are their descendants.
The Neandertal species did not go extinct, because it was never a separate species; instead population pockets of Neandertals died out around 30,000 years ago, whereas other Neandertal populations survived through interbreeding with their modern human brothers and sisters, who live on to this day.
I always suspected that Neandertals and anatomically modern humans interbred, based on a simple observation: humans are the most sexual of all the primates, willing and able to do it just about anywhere, anytime, with anyone (and even with other species if the Kinsey report is to be believed in its findings about farmhands and their animal charges). Given the viable hybrid offspring that the most diverse members of our species have produced as a result of cultural conjoinings through both ancient migrations and modern travel, one has to suspect that close encounters of the corporeal kind occurred not infrequently in those dark and lonely cave nights over the course of those long-gone millennia.
From http://www.scientificamerican.com/article.cfm?id=our-neandertal-brethren

Neandertal Gene Study Reveals Early Split With Humans
Genetic study bolsters theories of an early human-Neandertal split
by Elizabeth Svoboda
for National Geographic News
Controversy has long swirled in the scientific community over how closely the Eurasian hunters resembled modern humans, with some researchers even claiming Neandertals (often spelled Neanderthals) were actually members of our own species, Homo sapiens. (Related: "Neandertals' Last Stand Was in Gibraltar, Study Suggests" [September 13, 2006].)
A study by geneticist James Noonan at Lawrence Berkeley National Laboratory, however, reveals that modern humans and Neandertals' most recent common ancestor probably perished about 400,000 years ago.
The research was presented earlier this month at the American Society of Human Genetics conference in New Orleans, Louisiana (get a genetics overview).
Richard Potts, director of the human origins program at the National Museum of Natural History in Washington, D.C., called Noonan's work "highly significant."
"Each part of the Neandertal genome is an archive of the similarity and distinction [between Neandertals and] all people living today," he said. "Comparison to a lineage in our own family tree helps us understand which elements of the genetic code make us human."
Going Nuclear
To obtain the raw material for his study, Noonan extracted DNA from fossilized Neandertal bones.
Combing the samples for Neandertal-specific genetic sequences was a painstaking process bogged down by large amounts of contamination.
"Most of the DNA we got was bacterial DNA from organisms that had colonized the specimens," Noonan said. "We can pick out the ancient DNA sequences because they're shorter and more degraded."
After analyzing the genetic content of the sequences, Noonan and his colleagues began cataloging them in a library similar to that used to help organize the human genome.
Initial results indicate Neandertals have contributed surprisingly little to modern humans' genetic makeup.  Noonan's work represents a significant advance over earlier studies of Neandertal genetics, such as those conducted by William Goodwin of the University of Glasgow in Scotland. (Related: "Neandertals Not Our Ancestors, DNA Study Suggests" [May 14, 2003].)
That early work involved analysis of mitochondrial DNA, which tends to stay preserved longer than DNA found inside the nuclei of cells. But Noonan analyzed nuclear DNA, which holds a much greater wealth of information.
"Nuclear DNA is where all the biology is," Noonan said. "We want to understand how traits like language and cognition are encoded, and none of those traits can be found in mitochondrial DNA."
Race to the Finish
Like the multiple groups who worked simultaneously to sequence the human genome, Noonan faces competition from other inspired teams.
Genetic anthropologist Svante Paabo of the Max Planck Institute in Leipzig, Germany, [worked] on a similar sequencing project using DNA from bone specimens belonging to a Neandertal who lived in Croatia about 45,000 years ago…
While Noonan's focus is on studying the sequences of Neandertal DNA he considers most significant — those he can compare to modern human DNA sequences — Paabo's goal [was] to sequence the entire Neandertal genome. …
"Neandertal DNA is degraded in specific ways that we had not anticipated, and in some ways Neandertals actually look closer to humans than we had expected," Paabo said…
"The genetic analysis of Neandertals complements the study of fossils and the archaeological record of Neandertal behavior," he said.
"All this evidence allows us to understand exactly how Neandertals lived and adapted to a changing world that eventually included our species."
From http://news.nationalgeographic.com/news/2006/10/061026-neanderthals.html

Volcanoes Killed Off Neanderthals, Study Suggests

Modern humans escaped extinction due to their farther-flung populations?

The skull of a Neandertal woman.
Female Neanderthal skull
Photograph by Kenneth Garrett, National Geographic
by Ker Than
Catastrophic volcanic eruptions in Europe may have culled Neanderthals to the point where they couldn't bounce back, according to a controversial new theory.
Modern humans, though, squeaked by, thanks to fallback populations in Africa and Asia, researchers say.
About 40,000 years ago in what we now call Italy and the Caucasus Mountains, which straddle Europe and Asia, several volcanoes erupted in quick succession, according to a new study to be published in the October issue of the journal Current Anthropology.
It's likely the eruptions reduced or wiped out local bands of Neanderthals and indirectly affected farther-flung populations, the team concluded after analyzing pollen and ash from the affected area. (See volcano pictures.)
The researchers examined sediments layer from around 40,000 years ago in Russia's Mezmaiskaya Cave and found that the more volcanic ash a layer had, the less plant pollen it contained.
"We tested all the layers for this volcanic ash signature. The most volcanic-ash-rich layer" — likely corresponding to the so-called Campanian Ignimbrite eruption, which occurred near Naples (map) — "had no [tree] pollen and very little pollen from other types of plants," said study team member Naomi Cleghorn. "It's just a sterile layer."
The loss of plants would have led to a decline in plant-eating mammals, which in turn would have affected the Neanderthals, who hunted large mammals for food.
"This idea of an environmental cause for the Neanderthals' demise has been out in the literature. What we're trying to do is point out a specific mechanism," said Cleghorn, an anthropologist at the University of Texas, Arlington.
Other theories propose that modern humans played a vital role in the fall of the Neanderthals, either through competition, warfare, or interbreeding. (See "Neanderthals, Humans Interbred—First Solid DNA Evidence.")
If the volcanoes theory is correct, the Neanderthals' end was much more tragic: dying slowly in a cold and desolate landscape bereft of food sources.
"It's hard to say what it would have been like to be the last few groups out there, seeing other groups less and less over the years," Cleghorn said.
Uniquely Powerful Eruption
The Neanderthals were a hardy species that lived through multiple ice ages and would have been familiar with volcanoes and other natural calamities. But the eruptions 40,000 years ago were unlike anything Neanderthals had faced before, Cleghorn and company say.
For one thing, all the volcanoes apparently erupted around the same time. And one of those blasts, the Campanian Ignimbrite, is thought to have been the most powerful eruption in Europe in the last 200,000 years.
"It's much easier to adapt to something that's happening over a couple of generations," Cleghorn said. "You can move around, you can find other places to live, and your population can rebound.
"This is not that kind of event," she said. "This is unique."
Neanderthals Had Short Bench?
There may also have been small bands of Homo sapiens living in Europe at the time, Cleghorn said. They too would have been affected by the eruptions.
But modern humans likely avoided extinction because they had larger populations in Africa and Asia, she said, while most Neanderthals were in Europe around this time. (Related: "Neanderthals Ranged Much Farther East Than Thought.")
"With their small population groups, Neanderthals did not really have a great source population," Cleghorn said.
"They didn't really have the numbers and the density" to rebuild their populations after the eruptions.
"Not Convinced" by Volcanoes Theory
The researchers acknowledge that there are gaps in the volcanoes theory. For instance, the time line needs to be better defined—did the volcanic eruptions occur in a period of months, years, or decades?
"At this point, it's impossible to pin down a reliable date" for the eruptions, Cleghorn said. "We can't say, This eruption happened 50 years before the next eruption. We just don't have that kind of resolution."
It's also unknown exactly how long it took the Neanderthals to die out—or how long after the eruptions modern humans began settling Europe in force, she said.
Anthropologist John Hoffecker, though, suggests that modern humans had already begun crowding out Neanderthals in Europe long before the eruptions in question.
Judging from discoveries of modern-human artifacts in former Neanderthal strongholds, Hoffecker said, "Neanderthals were clearly in trouble well before 40,000 years ago, because modern humans were occupying certain places, such as Italy, where Neanderthals had been present. So something clearly had gone wrong there."
Perhaps, he added, the volcanic eruptions just dealt the final blow.
"I'm not entirely convinced that's the case either," said Hoffecker, of the University of Colorado. "But at least that's a plausible scenario that's consistent with the chronology."
Study co-author Cleghorn counters that the modern human populations living in Europe 40,000 years ago were small and isolated, and only after the Neanderthals were gone did Homo sapiens populations explode.
"If modern humans were making any forays into European Neanderthal territory prior to this, they were doing it only on the very margins," Cleghorn said.
"What was keeping them from moving very quickly into the heart of Europe? We think Neanderthals were still holding their own and might have held out for much longer, if it hadn't been for the devastating impact of these eruptions."
From http://news.nationalgeographic.com/news/2010/01/100922-volcanoes-eruptions-neanderthals-science-volcanic-humans/
Images - http://www.powerlineblog.com/archives/media/Neanderthal662.jpg

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