Complex life could certainly exist as long as the world isn't also cold like the Everest death zone. Cold blooded reptiles have metabolic rates that average 7.2 times less than that of placental mammals. This means that reptiles would still have plenty of oxygen to spare in the thinner atmosphere.
Plants would also tend to grow slower if they cannot get as much CO2 but without mammals around eating obscene amounts of plant matter, this should not be a particularly limiting factor. In short, life could still be pretty complex, just... slower.
Anaerobic respiration works for simple organisms, but it can not replace aerobic respiration in complex life forms because it creates a toxic buildup of lactic acid over time. Simple life forms can compensate for this by pushing the acid out of themselves and using non-reactive cell membranes to keep it out, but flushing acid out of a single cell and flushing it completely outside of a complex organism are two different feats. While many complex organisms are designed to be able to do anaerobic respiration on the short term, it creates an oxygen debt. This means we need to eventually breath in more oxygen to be able to break the lactic acid down when we are done exorcising making this an unviable mechanism.