Sea otters may have been using stone tools millions of years ago, perhaps even before the first pre-human thought of cracking open a nut with a lump of rock.
The marine creatures knew about tools long before dolphins, which have only learned to use them in the past 200 years, research suggests.
Scientists based their conclusions on a genetic study of more than 100 wild marine otters living along the Californian coast.
Sea otters use rocks or other hard objects to break open clams and marine snails.
The genetic analysis showed that, unlike dolphins, sea otters appear to have an innate tool-using ability.
Not all the otters used tools but those that did were no more related to one another than to the general population.
In contrast, tool-using dolphins within a particular area were likely to be closely related, with Australian dolphins observed using conical sponges to protect their noses while probing the sea floor for small fish.
The US team, led by Dr Roderick Gagne, from the University of Wyoming, said in the Royal Society journal Biology Letters the lack of genetic association among tool-using sea otters compared with dolphins may result from the length of time each species has been using tools.
"Tool use in dolphins appears to be a relatively recent innovation (less than 200 years) but sea otters have probably been using tools for many thousands or even millions of years."
Unlike dolphins, all young sea otters appeared to be predisposed to use tools, the authors said.
"Orphaned otter pups raised in captivity exhibit rudimentary pounding behaviour without training or previous experience, and wild pups develop tool-use behaviour before weaning, regardless of their mother's diet type."
Ancient pre-human ancestors of people living today are now thought to have been using stone tools more than three million years ago.
