Ancient European dogs doubled in size between 8,000 and 2,000 years ago to protect livestock from wolves and bears. Joshua Rapp, author of Science magazine, writes about this..

Dogs were domesticated between 15,000 and 30,000 years ago. Little is known about their size in those early times, although they were probably smaller than the gray wolves from which they are descended.. Scientists suggest that ancient dogs may have been used as hunting and driving dogs..

To see how the size and specialization of dogs have changed over time, Martin Welker, curator of zooarchaeology at the Arizona State Museum, and his colleagues examined the remains of 14 dogs found in ancient human settlements in Croatia.. They also included data from another 45 ancient dogs, some of which were from Croatia and some from neighboring countries.. The remains date from about 8,000 years ago from the Neolithic (Late Stone Age) to the Roman period, about 2,000 years ago..

Dogs have lived in this region before, but the first Neolithic farmers who arrived here from Anatolia and the Middle East brought a new breed with them.. These dogs weighed an average of about 15 kilograms - about the size of a border collie, scientists have found - although this was long before the appearance of most modern breeds..

By the Bronze Age, which began about 6,000 years ago in this region, the average weight of dogs had increased to about 17 kilograms.. And by Roman times, the weight of the average dog jumped to 24 kilograms..

Historical documents, including ancient Roman records advising farmers on herding and guard dogs, suggest that dogs were even larger at the time, reaching at least 32 kilograms, almost equal to the weight of today's large Pyrenean dogs.. The descendants of these dogs are still used to guard livestock..

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But not every ancient dog was a huge sheepdog.. According to frescoes and other evidence, the Romans may have been the first to breed lapdogs..