In humans and some other animals, bilirubin is excreted as the end product of haem originating from haemoglobin and other haemoproteins, and it has been the cause of jaundice and kernicterus.1 2 The conversion of haem to bilirubin is a two-step process.1 First, haem is converted to biliverdin by haem oxygenase. Then biliverdin is converted to bilirubin by biliverdin reductase. Interestingly, many animals take biliverdin as the end product of haem, without converting it further to bilirubin.2 Biliverdin is water soluble and readily excreted through bile or urine.

Why the energy-consuming conversion in some animals of the innocuous biliverdin to the water-insoluble, potentially toxic bilirubin, which needs more resources for transportation and excretion, takes place has been a great puzzle.1 2 Discovery of the antioxidant property and other properties of bilirubin has suggested that the change of biliverdin to bilirubin might be an advance in evolution.1 However, …