The new study, published in the current issue of the Proceedings of the National Academy of Sciences, is based on mammal remains from Saint-Césaire, a Stone Age site in southwestern France.
Analysis of bones found there suggests a decrease in the variety of large mammals that prehistoric hunters would have targeted, the study found.
Prey species found at the site include bison, horses, red deer, and reindeer.
Unlike the other species, however, reindeer became increasingly abundant about 40,000 years ago, the evidence suggests.
The ratio of reindeer bones at the French site rose from 35 to 87 percent during this period, Morin said, indicating a rapidly cooling climate.
Remains of a tundra-dwelling rodent species, the narrow-skulled vole, likewise suggest suddenly colder conditions.
"About 40,000 years ago, the diversity of animals that could be hunted shrank severely, and that would have impacted human populations," Morin said.
This shift coincides with a period called the Middle/Upper Paleolithic transition, a period marked by significant changes in early-human culture, including the use of more sophisticated stone tools, he added.
But "there were fewer people around after the transition than before," he said.
Neandertals' reliance on reindeer would have been especially risky, since the mammals experienced extreme fluctuations in population, Morin said.
"This was also likely to have impacted human populations that depended on them," the anthropologist said.
Furthermore, if large mammals such as bison and horses were frozen out of the region, there wouldn't have been enough prey to support incoming modern-human migrants from Africa, he added.
Were Modern Humans More Prepared?
Paul Mellars, professor of human evolution at the University of Cambridge, U.K., said the new study provides a good analysis of the mammal fauna and climate during the transition period.
The new findings are supported by evidence from deep seabed sediments that indicate a dramatic drop in temperature of at least 14.4 degrees Fahrenheit (8 degrees Celsius), Mellars said.
"This occurs pretty well at precisely the time that you get the transition," he added.
But the Saint-Césaire site also shows an abrupt transition in artifacts that only an invasion by modern humans can explain, Mellars said.
Entirely different stone-tool technologies suddenly appear in the region, he said.
"Just on the basis of the technology alone, it looks like a sudden population replacement of Neandertals by modern humans [took place]," Mellars commented.
Evidence from similarly dated human bones also indicates the presence of anatomically modern humans that spread from the east across Europe, he noted.
Mellars said other researchers argue that the dramatic climatic cooling highlighted by the study would have given incoming modern humans, with their more advanced tools, a competitive advantage over the Neandertals.
(Read related story: "Did Neandertals Lack Smarts to Survive?" [March 3, 2006].)
These tools included new kinds of projectile points made from bone and antler, and small blades that attached to either spears or arrows, he added.
"With this new technology, modern humans may simply have been better able to cope," Mellars said.
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