I have been trying on and off for years to find a short story I would have read in an anthology during the 80s, but was probably originally a bit older. It was about time travelers from a distant future in which humanity was [mysteriously] dying due to having exhausted its genetic potential. That civilization had determined that there was a population bottleneck in the last ice age when humanity had been reduced to a really small number- one band of survivors. They decided to save just one extra person from dying during a perilous trek through an icy wilderness, thereby adding millennia to humanity's downstream genetic viability.
Time travel complications probably ensued.
I cannot remember title, author, or exact plot. And random googling has proved most ineffective.