I recall a rather grim short story/novelette read during my teens (early to mid 1970's) from either a magazine or anthology, so could be written prior to 1970. The setting was either future Earth or on an Earth-like planet. The protagonist lived in a somewhat primitive warlike society (spears, clubs, knives) on an island who was 'drafted' into a night-time or dawn attack against another village on a different island.
The attack was by a small war party using kayaks/canoes, but an added danger was the presence of dangerous aquatic creatures - which were descended from humans. An alarm is raised and the attack ends in disaster with the aquatic creatures involved in the mayhem, and the protagonist barely survives, fleeing to another land.
He encounters flying creatures there which attack him and one of which he kills by his bare hands scaring off the others - again creatures descended from humans. He then encounters another individual who welcomes him and who is described as being of the 'original stock' of humanity.