Would specie (humans included) actually surely go extinct if reduced to certain numbers, or would it just stack disabilities? These guys are product of single pair and were at one point reduced to one pair of siblings, but looks like they still function.

Would specie (humans included) actually surely go extinct if reduced to certain numbers, or would it just stack disabilities? These guys are product of single pair and were at one point reduced to one pair of siblings, but looks like they still function.