Big Freeze Didn't Kill Off Neandertals, Study Says

James Owen
for National Geographic News

September 12, 2007

Climate change might have had a hand in the demise of the Neandertals, but it wasn't the main culprit, a new study says.

The research is the latest contribution to a longstanding debate as to what finally did in the ancient human species some 30,000 years ago.

Some scientists blame the sudden onset of ice age conditions in Europe and western Asia, while others link the extinction to the arrival of modern humans from Africa.

Now a team led by Polychronis Tzedakis of the University of Leeds in England says it has detected no extreme climate shifts during the time the Neandertals (often spelled "Neanderthals") disappeared.

"We can eliminate catastrophic climate change as a factor in the Neandertal extinction," Tzedakis said.

Carbon Clues

The research, reported this week in the journal Nature, focused on finds from Iberia (modern-day Spain and Portugal)—the region identified as the Neandertals' last European stronghold.

Recently analyzed remains from Gorham's Cave in the British territory of Gibraltar suggest that the Neandertals died out there sometime between 30,000 years ago—in line with previous estimates—and 24,000 years ago. (See a Gibraltar map.)

So Tzedakis' team compared the radiocarbon dates of pinecone scales and charcoal recovered from the Neandertal cave to prehistoric climate records.

The scientists used a new method in the comparison, because radiocarbon years don't necessarily match up with calendar years.

Radiocarbon dates are calculated by measuring the radioactivity of the carbon found in all living things. Radiocarbon, or radioactive carbon, is continually being added in small amounts during life but decays at a fixed rate after death.

This carbon ultimately comes from the air, and varying levels of radiocarbon in the atmosphere "can cause the clock either to race forward or stand still," Tzedakis said.

Tzedakis added that "21,000 carbon years is equivalent to about 26,000 calendar years, so it can be quite a large discrepancy."

The mismatch is especially marked around the time the Neandertals went extinct, making it difficult to work out the chronology of their last days.

To get around this, the researchers compared the radiocarbon dates for the Gorham's Cave finds with those for fossil plankton from a deep-sea core drilled in Venezuela's Cariaco Basin.

The plankton remains were found in seabed sediments that provide a record of past climate change.

"At present, it's the only core you can do this with, because you have hundreds of radiocarbon dates from marine plankton," Tzedakis said.

Balmy Mediterranean

Using the sea-core record, the team worked out the climate 32,000, 28,000, and 24,000 years ago.

The earlier two periods coincided with centuries of climatic fluctuation but no abrupt, catastrophic changes, the study found.

The latest, more speculative date did mark the onset of a transition to much colder, arid conditions. But these took hold only gradually, so this wouldn't explain a sudden extinction, the team reports.

"What we have is more of a gradual climate shift where you have the ice sheets building up and a significant drop in temperatures in the high latitudes," Tzedakis said.

Furthermore, the Mediterranean region escaped these chilling events, the team found.

"Since the conditions remained relatively warm in southern Iberia and Gibraltar, we can't envisage that climate had a direct impact on the Neandertal population," Tzedakis said.

Ongoing Controversy

The new findings contradict research published earlier this year in the journal Quaternary Science Reviews.

That study, headed by Miguel Ortega Huertas of the University of Granada, Spain, suggested that southern Iberia suffered a major freeze some 24,000 years ago that finished off the Neandertals.

But the University of Leeds's Tzedakis noted that the Neandertals survived various episodes of severe climate change after first appearing in the fossil record 350,000 years ago.

They only "disappeared once modern humans appeared on the scene," he said.

So if climate did play a role in the Neandertal extinction, the impact was indirect, the study team writes. Ice age conditions could have increased competition as Neandertals or modern humans fled south to escape increasingly hostile conditions, for example.

Tjeerd van Andel is an earth sciences professor at the University of Cambridge.

He described the study team's arguments as "sensible."

"However, as far as I can see, these are speculations or hypotheses that in the context of the data offered are free-floating," he said.

But van Andel agreed that the final extinction of the Neandertals likely had more than one cause.

Researchers have previously argued that the Neandertals weren't able to adapt their hunting techniques to the changing fauna and environment or that they were outcompeted or slaughtered by their modern human rivals. (Related: "Did Neandertals Lack Smarts to Survive?" [March 6, 2004].)

Other experts suggest that rather than being replaced, the Neandertals became absorbed into the human race through interbreeding.

Local climate change, different animals and plants adapted to more arid conditions, and "a slow decline of a smaller and smaller population due to reduced fertility would do the job," van Andel said.

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