In 2018, a team led by paleoanthropologist João Zilhão announced a discovery that fundamentally changed what we thought we knew about Neanderthals: They created cave art.

The finding was the first evidence to suggest Neanderthals were capable of symbolic thinking — an ability previously thought to be the preserve of our own species, Homo sapiens.

Now, Zilhão and an international team announce another Neanderthal discovery that flips the script on their cognitive abilities.

An excavation of Cueva de Figueira Brava, a rock shelter site on the western coast of Portugal, reveals Neanderthals lived there between 86,000 and 106,000 years ago. There, they took part in an activity that likely felt routine to them, but — seen through the lens of history — is revolutionary: They ate mollusks, crustaceans, and fish.

That diet of fresh seafood may have endowed Neanderthals with good brain health, boosting their cognitive skills, the researchers say.

The theory stems from the idea that a diet of seafood is crucial to human brain evolution. That's because the nutrients found in seafood, like omega fatty acids, iodine, iron, zinc, copper, and selenium, provide a necessary boost of brain growth. But scientists thought eating seafood only helped Homo sapiens develop big brains, not their Neanderthal cousins.

This study changes that, Zilhão explains.

"If this common consumption of marine resources played an important role in the development of cognitive skills, it did so on the entire humanity, including Neanderthals, and not only the African population that spread later."

A study detailing the revelatory finding was published Thursday in the journal Science.

The Figueria Brava caves, and their marine terrace front. Zilhao et al.

Until now, evidence of Neanderthals eating seafood was scant. There is some evidence that they used shells as tools, but it was unknown whether they actually ate creatures like shellfish. The lack of evidence, the team argues, likely stems from the fact that sea-level rise concealed any seaside habitats.

But the new site indicates that at least 50 percent of the Neanderthals who lived there's diet came from the ocean. And it was a relative cornucopia: Their food likely included clams, spider crabs, eels, and sharks — and that is just for starters. At the same time, the researchers also found evidence that these Neanderthals were not picky about where they got their produce. They also likely ate from terrestrial resources, like tortoises and goats.

Unlike the Neanderthals living in the cold tundras of Siberia, these ancient humans are perhaps better described as fisher-hunter-gatherers.

The fact that they gathered food from the sea is also indicative of other skills, the study suggests. The routine of harvesting shellfish implies that they must have understood how tides work, and, that if bivalves are harvested in the wrong season, it can put individuals at risk of biotoxin poisoning.

Zilhão and his colleagues posit that the "cognitive aspects" of the Figueria Brava data are in line with other evidence of Neanderthal intelligence, including the creation of cave art and jewelry.

The study adds to the mounting evidence closing the behavioral gap between Neanderthals and Homo sapiens. Ultimately, the "absence of evidence is not evidence of absence" when it comes to Neanderthal intelligence.

Abstract: Marine food–reliant subsistence systems such as those in the African Middle Stone Age (MSA) were not thought to exist in Europe until the much later Mesolithic. Whether this apparent lag reflects taphonomic biases or behavioral distinctions between archaic and modern humans remains much debated. Figueira Brava cave, in the Arrábida range (Portugal), provides an exceptionally well-preserved record of Neandertal coastal resource exploitation on a comparable scale to the MSA and dated to ~86 to 106 thousand years ago. The breadth of the subsistence base—pine nuts, marine invertebrates, fish, marine birds and mammals, tortoises, waterfowl, and hoofed game—exceeds that of regional early Holocene sites. Fisher-hunter-gatherer economies are not the preserve of anatomically modern people; by the Last Interglacial, they were in place across the Old World in the appropriate settings.