Seafood on Spanish Coast Attests to Climate Cataclysm 8,200 Years Ago

She sells shrinking sea shells by the seashore in Mesolithic Spain, and from doting on pretty top shells, the dwellers of climate-stricken El Mazo cave were reduced to dining on limpets

Ruth Schuster
Ruth Schuster
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Dinner is served: Patella lusitanica, Mytilus edulis, Monodonta lineata and Balanus balanoides.
Dinner is served: Patella lusitanica, Mytilus edulis, Monodonta lineata and Balanus balanoides.Credit: Luis Miguel Bugallo Sánchez
Ruth Schuster
Ruth Schuster

Climate change is barreling at us like a freight train and its consequences are unpredictable. Our terror of the unknown might be mitigated by studying past events of climate change, which can help us plan. In any case, the more we know about past environmental upheavals, the better we can prepare for the potential future.

Now a new study published in Nature sheds light on human adaptation in a cave called El Mazo, on the northern coast of Spain, during the most abrupt and violent climatic event of the Holocene. It is known as the “8.2 kiloyear event,” which just means it happened 8,200 years ago.

The time was one of transition from the Mesolithic to the Neolithic periods. People were gradually, at different times in different places, stutteringly shifting from foraging, hunting and gathering to settlement and subsistence agriculture (augmented by foraging, hunting and gathering), over generations.

In the Levant and Anatolia, and parts of Asia, farming and animal domestication had begun; and then came the 8.2ka event.

Let it be clear that average global temperatures have fluctuated and do fluctuate. But 8,200 years ago, very suddenly, global temperatures plunged, in a dramatic reversal from the warming trend following the Last Glacial Maximum (which was 26,000 years ago).

A group of common limpets (Patella vulgata) in Pembrokeshire, Wales.Credit: Tango22

Evidence for the 8.2ka event has been found worldwide, though the greatest impact seems to have been in the Northern Hemisphere. Why it happened in the first place is still argued about, but a main theory is that as the Ice Age waned and the glaciers receded, North American lakes filled with freezing cold meltwater that suddenly burst into the North Atlantic Ocean. The massive influx of freshwater disrupted the ocean circulation systems.

The result was that sea surface temperatures around the world dropped and the weather worldwide became drier and colder, to different degrees at different places.

Until now, little proof of how the 8.2ka event affected specific locales, let alone people, has been found, though theories abound. For instance, some associate drought and chill with increased social stratification and urbanism in what is today Syria and changing occupation patterns in Egypt. 

The new paper gets specific about the effect on the northern Spanish coast, in the province of Asturias: in the coldest winter during the 8.2ka, the temperature dropped to 9.4 degrees Celsius (about 49 degrees Fahrenheit). The most intense seasonality – the difference between winter and summer average temperatures – was 13.8 degrees Celsius. The summers were chilly, at just over 22 degrees Celsius, though note: just as today, global warming is accompanied by some stunning snowstorms, and winter has become characterized by blazing sun. Then too, the 8.2ka brought some days of warmth in winter.

And what did this cause? The researchers believe they have found a proxy that reflects the degree of climatic disruption: a stratified, vast pile of discarded seashells from meals. Altogether they analyzed over 80,000 individual shells belonging to 23 mollusk species, but there were some clearly dominant types.

The depressed limpet, Patella depressa. The name refers to its shape, not its mental well-being.Credit: H. Zell

That shell midden had built up over 1,500 years at exactly the critical point, from the perspective of studying startling change in the environment and in human habits.

The analysis detected change in the species of shells the foragers had been eating and in the size of the animals they harvested, report Asier García-Escárzaga of the Autonomous University of Barcelona with an international group of colleagues in Scientific Reports. They grew smaller.

Before the 8.2ka the favorite was the sea snail Phorcus lineatus, better known as the lined top shell. An eye-catching beastie, the top shell is a mollusk that loves warm water. And as the 8.2ka event howled and the weather and water turned cold, it vanished.

It was supplanted in the cave persons’ diet by Patella vulgata, aka the limpet, which does better in cold. Anecdotally, limpets are very chewy and it’s easy to overcook them. As they are essentially sea snails, they are reportedly tasty with garlic butter.

The researchers also note an increase in another limpet, Patella depressa, which likes warm water but is more resistant to cold temperatures than other warm-water species, the team explains. So it soldiered on.

Phorcus lineatusCredit: H. Zell

In short, the mollusk populations available on the sea shore of Mesolithic Spain changed as the weather turned cold, judging by the animals’ exploitation by cave-dwelling humans.

Apropos of whom, the evidence at El Mazo indicates if anything it intensified consumption of the seafood – but the people were harvesting smaller animals, on average. If one can catch a big animal, one does, so the increase in smaller-sized shells indicates increased harvesting.

Intensifying exploitation

There is a disturbing parallel in today’s collapsing fisheries: it’s getting harder and harder to catch big fish, because they are being caught before they can grow up properly.

As for the intensified exploitation of the mollusks, the researchers suspect this was because the population in the cave was growing. Possibly people headed to Atlantic coastal refugia from the icy inland.

At least the shells were not exploited to the point of unsustainability. The average animal size did not fall below 2 centimeters (almost an inch), which is the minimum size specified by fishery regulators today to guarantee long-term species survival. 

Ultimately, the team proved that the climatic burp had a profound effect on the mollusk populations of El Mazo 8,200 years ago, and that if anything, the people in the cave made merry and multiplied, simply shifting their culinary attentions to whatever sea snail they could catch, as one does.

It begs adding that the researchers asked themselves whether it was only temperature that was killing off the top shells and reducing shell sizes, or perhaps change in nutrients because of the changes in the ocean. It was temperature.

“Climatic change had an important impact, not only on the mollusk population numbers of each species, but also on human foraging behavior, by conditioning the food supplies available for consumption by Mesolithic societies,” the researchers write.

Can we glean lessons for our future from this past?

Frankly, it’s not news that the starving eat anything and when one’s top shells vanish, one eats whatever edibles one can find.

We can glean the lesson that water temperatures definitely affect marine life, which we had begun to notice in this modern era of accelerated, and accelerating, climate change.

However, it bears adding that today’s climate change is anthropogenic, not due to malice of the deity or the vagaries of the cosmos. Also, global warming isn’t happening in a vacuum: changing temperature is one parameter, invasive species are another, pollution and contamination are yet another, overfishing, and so, so much more.

The bottom line is apparently that people do what they need to do to survive, and if that involves eating ever-smaller limpets, so it will be.

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