Cro-Magnon guide went by across the snow-covered yard, a tall handsome fellow dressed rather like an Eskimo (why had romance never credited paleolithic man with enough sense to wear jacket, pants, and footgear in a glacial period?), his face painted, one of the steel knives he had earned at his belt. The Patrol could act quite freely, this far back in time; there was no danger of upsetting the past, for the metal would rust away and the strangers be forgotten in a few centuries."
-Poul Anderson, Time Patrol (New York, 2006), p. 174.
A Few Points To Note
(i) A few centuries do not matter that far back.
(ii) If there is time travel, then maybe our remote descendants and ancestors know about it although we don't.
(iii) Those ancestors would have had enough sense to dress properly in cold weather.
In another fictional timeline, SM Stirling's characters, in Island In The Ocean Of Time, learn that ancient astrologers and mariners realized that the world was round but did not tell anyone else. The knowledge was either esoteric or a trade secret. Mariners in particular would see masts disappearing below the horizon. It was philosophers who first wanted to share and publish knowledge: Patet veritas omnibus, "Truth lies open to all."