It seems Neanderthals enjoyed a wide range of foods - a much broader menu than had previously been supposed.
Excavations in caves in Gibraltar once occupied by the ancient humans show they ate seal and dolphin when they could get hold of the animals.
There are even indications that mussels were warmed to open their shells.
The findings, reported in the journal PNAS, give the lie to the popular view that Neanderthals ate a diet utterly dominated by meat from land animals.
It is one more example of the greater sophistication now being ascribed to Homo neanderthalensis; and further complicates the story of how modern humans (Homo sapiens) out-competed and out-lived their evolutionary cousins.
"Moderns still had a more efficient way of extracting the maximum out of the environment compared with the Neanderthals," said lead author Chris Stringer from London's Natural History Museum.
"So there still is an element of superiority, but it is a much more finely balanced one now. This is yet another difference that had been proposed between Neanderthals and moderns which now disappears," he told BBC News.
Professor Stringer and colleagues have been investigating the fossil material from a number of seaside excavation sites in Vanguard and Gorham's Caves in eastern Gibraltar.
The cave deposits are throwing up a rich array of Neanderthal artefacts, demonstrating that Homo sapiens were not the only ones to live off the sea.
It has been known from earlier work that Neanderthals would eat some shellfish when available, but the Gibraltar study is the first to show the exploitation of marine mammals.
"We've got a shoulder blade of a seal with cut marks on it and we've got parts of the bones from a flipper with cut marks," explained Professor Stringer.
"These Neanderthals were skinning and dismembering seals. What's interesting is that they didn't always cook them; they often ate them raw, it seems.
"They were also heating bones, not to cook the meat but to get at the marrow inside. By putting bones in fires, they were making them more brittle so they could get them open more easily."
On the menu also was dolphin (Tursiops truncatus), probably dead animals that had washed up on the beach. Monk seals (Monachus monachus), on the other hand, were most likely juveniles clubbed to death at breeding grounds and then taken back to the caves to be butchered.
By analysing the different types, or isotopes, of atoms incorporated into Neanderthal bones as a result of the foods they ate, it is possible to glimpse something of their lifestyle.
In northern Europe, particularly, it is clear that big game meat - mammoth, deer, horse - dominated the Neanderthal menu.
The isotopes from early modern humans, by comparison, show a much broader range of foods - they were eating small grain, they were fowling and fishing.
This has been used to help explain Neanderthal extinction: H. neanderthalensis may have struggled at times to get the most out of their environment and could be out-competed by moderns.
The latest research, by demonstrating the exploitation of seal and dolphin, shows the extinction story is a little more complicated - at least as far as Gibraltar is concerned, believes Professor Stringer.
"We can't generalise to all Neanderthal populations, because the further north you go, away from the coast, you won't have those resources," he told BBC News.
The Gibraltar caves also contain hearths and flint stone tools, as well as butchered land mammals such as ibex (Capra ibex), red deer (Cervus elaphus), wild boar (Sus scrofa), bear (Ursus arctos) and rabbit (Oryctolagus cuniculus).
The remains of mussels (Mytilus galloprovincialis) are also evident. These are found in ash and are scorched - clear evidence that they were cooked near a fire to open them.
The caves in Gibraltar may be among the very last places Neanderthals lived before they became extinct.
Analysis of charcoal remains from the hearths indicates the species was present 28,000 years ago, and perhaps as recently as 24,000 years ago.
The excavation of the caves is a collaborative project between several institutes, including London's NHM, Gibraltar Museum, and the Museo Nacional de Ciencias Naturales, Madrid.