Skeleton in Mo. Came From Ancient Mammoth (AP) AP - Everything about the bones that turned up in a Grain Valley back yard last summer suggested to scientists that they came from a 50,000-year-old mastodon. But evidence yielded by additional digging has led the experts to conclude they were off-base. The remains are actually those of a mammoth that lived 500,000 to 1 million years ago, according to the scientists.