This would have created selection pressure for organisms in this intermediate zone to acquire the ability to tolerate and then to use oxygen, possibly via endosymbiosis, where one organism lives inside another and both of them benefit from their association.
Isto crearía unha presión de selección para os organismos nesta zona intermedia para adquirir a capacidade de tolerar e despois de usar o oxíxeno, posiblemente por endosimbiose, na que un organismo vive dentro doutro e ambos os dous benefícianse desa asociación.