Friday, May 9, 2008

Once Lush Sahara Dried Up Over Millennia, Study Says


The grassy prehistoric Sahara turned into Earth's largest hot desert more slowly than previously thought, a new report says—and some say global warming may turn the desert green once again.

The new research is based on deposits from a unique desert lake in remote northern Chad.

Lake Yoa, sustained by prehistoric groundwater, has survived for millennia despite constant drought and searing heat.

The body of water contains an unbroken climate record going back at least 6,000 years, said study lead author Stefan Kröpelin of the Institute of Prehistoric Archaeology at the University of Cologne in Germany.

Ancient pollen, insects, algae, and other fossil clues preserved in the lake's sediments point to a gradual transformation to a desert environment.

Sahara Myth

The study contradicts past research that suggested the region dried up within a few hundred years. That research was based on windblown Saharan dust found in Atlantic Ocean sediments.

"This was a hypothesis used by most of the modelers and many of the scientific community who were not working themselves in the Sahara," Kröpelin said.

"To a large degree we can now show that such an abrupt drying out of the Sahara was a myth," he said.

The new study, which appears tomorrow in the journal Science, instead found evidence for a slow decline in tropical plants, followed by the gradual loss of savanna-type grasslands, and then the eventual spread of desert species.

Pollen samples revealed, for example, that the decrease in tropical trees accelerated after 4,800 years ago, while desert plants took root between 3,900 and 3,100 years ago.

Sand particles in the lake show that fierce desert winds didn't start picking up until about 3,700 years ago, the study found.

The only rapid change noted was in the lake itself, which switched from a freshwater to a salt lake between 4,200 and 3,900 years ago.

The transformation happened exactly in the time period when monsoon rains began moving away to the south, Kröpelin said.

This meant there was no longer surface water flowing in to counter salinity caused by evaporating water.

The study supports previous archaeological findings that human populations in the Sahara moved south over several millennia, following the monsoon rains, Kröpelin said.

First Reliable Record

About 20 feet (6 meters) of water evaporate from the lake every year, which is equivalent to the annual water consumption of about a million people, Kropelin noted.

"No team had ever succeeded in getting geological and paleoclimate information for the past 4,000 years since practically all the lakes had dried up, so there were no more geological archives available," he said.

The Lake Yoa data represent the first "reliable and high-resolution record" in the Sahara for verifying climate models, he added.

Such checks are important, he argues, "because if climate computer models don't work for the past, they probably won't work for the future."

Understanding climatic effects in the Sahara are especially important, since the region covers an area larger than the United States, Kröpelin said.

"Climate evolution in the Sahara reflects to a very large extent climate evolution on the African continent and beyond," he added.

Jonathan Holmes, of the Environmental Change Centre at University College London, was not involved in the study.

He wrote an accompanying commentary on Kröpelin's research in the same issue of Science.

The latest findings fill "an important gap in our understanding of the past 6,000 years of North African climate," he wrote in the article.

The study provides a more accurate picture of climate change in the region since the last ice age, because the "record comes from one of the few Saharan lakes in which sediments have accumulated without a break."

Similar lakes "probably do not exist," according to Holmes.

"However, improving existing geological records and using these to refine climate models would go a long way toward furthering our understanding," he wrote.

Modern Climate Change

Future research at Lake Yoa should provide clues to a potential regreening of the Sahara, triggered by the current trend of global warming, according to Kröpelin.

"I'm expecting reliable information on this possible trend," he said.

The last green phase, which started some 12,000 years ago, may be due to increased water evaporation from oceans. This led to monsoon rains that penetrated the interiors of tropical continents, he said.

"Now, today, man is probably causing the same thing," he said.

Kröpelin, who has studied the region for almost 30 years, said that since 1988 "there [has been] a strong indication [of] a return of increasing rains" in the eastern Sahara.

Already in some areas "you can see slight changes in the vegetation," he said.

Study confirms ancient Chile settlement is 14,000 years old

Scientists have confirmed that the famed Monte Verde archaeological site in southern Chile is about 14,000 years old, making it the earliest known human settlement in the Americas, the journal Science reported Thursday.

The age of Monte Verde has been the subject of controversy over the years, since estimates appeared to conflict with other archaeological evidence related to the settlement of North America.

The new findings support not only the age of the Monte Verde site, but also the coastal migration theory currently ascribed to by most scholars, which hypothesizes that people first entered the New World through the Bering land bridge more than 16,000 years ago.

The study, based on the first data compiled about the Monte Verde site in about a decade, identified nine species of seaweed and marine algae used as food by the settlement's inhabitants.

Carbon dating put the age of the seaweed samples at between 13,980 and 14,220 years old, confirming that the site was occupied some 1,000 years earlier than any other known human settlements in the Americas. The study appears in the May 9 issue of Science.

Discovered in 1976, Monte Verde is located in a peat bog about 500 miles (800 kilometers) south of Santiago, Chile.

Researchers say it could have supported between 20 to 30 people in a dozen huts along a small creek.

A wide variety of food has been found at the site, including extinct species of llama and an elephant-like animal called a gomphothere, shellfish, vegetables and nuts.

85,000-year-old finery recovered in Moroccan cave

Archaeologists have uncovered shells used for finery by prehistoric communities 85,000 years ago in a cave in eastern Morocco, the country's heritage institute said Tuesday.

A research team, led by archaeology and heritage institute (INSAP) member Abdel-Jalil Bouzouggar and Oxford University's Nick Barton, found the 20 perforated shells in a cave near Taforalt between March and April this year.

According to a statement from the Moroccan Ministry for Culture, the shells are the type prehistoric people would have worn. In 2007, Bouzouggar and Barton discovered 14 perforated shells in the same cave.

"This discovery shows that the making and use of objects of finery is very anchored in the traditions of Morocco's prehistoric people," said Bouzouggar, in whose opinion the country is the original center of artistic and symbolic creation.

Objects of finery discovered in Morocco are "now considered to be even more ancient than those discovered in Algeria, South Africa and in Palestine," said the Culture Ministry. Known as the "cave of pigeons," the 30-meter-deep, 10-meter high cave is situated 50 kilometers from Morocco's Mediterranean coast.

Neanderthals were separate species, new study finds

A new, simplified family tree of humanity has dealt a blow to those who contend that the enigmatic hominids known as Neanderthals intermingled with our forebears.

Neanderthals were a separate species to Homo sapiens, as anatomically modern humans are known, rather than offshoots of the same species, the new organigram published Sunday by the journal Nature declares.

The method, invented by evolutionary analysts in Argentina, marks a break with the conventional technique by which anthropologists chart the twists and turns of the human odyssey.

That technique typically divides the genus Homo into various classifications according to the shape of key facial features -- "flat-faced," "protruding-faced" and so on.

Reconciling these diverse classifications from a tiny number of specimens spanning millions of years has led to lots of claims and counter-claims, as well as much confusion in the general public, about how we came to be here.

Various species of Homo have been put up for the crown of being our direct ancestor, only to find themselves dismissed by critics as failed branches of the Homo tree.

The authors of the new study, led by Rolando Gonzalez-Jose at the Patagonian National Centre at Puerto Madryn, Argentina, say the problem with the conventional method is that, under evolution, facial traits do not appear out of the blue but result from continuous change.

So the arrival of a specimen that has some relatively minor change of feature as compared to others should not be automatically held up as representing a new species, they argue.

The team goes back over the same well-known set of specimens, but uses a different approach to analyse it, focussing in particular on a set of fundamental yet long-term changes in skull shape.

They took digital 3D images of the casts of 17 hominid specimens as well as from a gorilla, chimpanzee and H. sapiens.

The images were then crunched through a computer model to compare four fundamental variables -- the skull's roundness and base, the protrusion of the jaw, and facial retraction, which is the position of the face relative to the cranial base.

When other phylotogenic techniques are used, the outcome is a family tree whose main lines closely mirror existing ones but offers a clearer view as to how the evolutionary path unfolded.

The paper suggests that, after evolving from the hominid Australopithecus afarensis, the first member of Homo, H. habilis, arose between 1.5 and 2.1 million years ago.

We are direct linear descendants of H. habilis. H. sapiens started to show up around 200,000 years ago.

None of the species currently assigned to Homo are discarded, though.

On the other hand, the Neanderthals are declared "chronological variants inside a single biological heritage," in other words, evolutionary cousins but still a separate species from us.

The squat, low-browed Neanderthals lived in parts of Europe, Central Asia and the Middle East for around 170,000 but traces of them disappear some 28,000 years ago, their last known refuge being Gibraltar.

Why they died out is a matter of furious debate, because they co-existed alongside anatomically modern man.

Some opinions aver that the Neanderthals were slowly wiped out by the smarter H. sapiens in the competition for resources.

Other contend that we and the Neanderthals were more than just kissing cousins. Interbreeding took place, which explains why the Neanderthal line died out, but implies that we could have Neanderthal inheritage in our genome today, goes this theory.