A primaeval origin for plant and animal haemoglobins?

Abstract

The discovery and study of haemoglobin and its genes in nitrogen-fixing nonleguminous plant root nodules and in the roots of non-nodulated and non-nodulating plants has shown that these haemoglobins, the leghaemoglobins of legume nodules, and animal haemoglobins, all have a common evolutionary origin. It would be simple to assume that they are related by vertical descent from a haemoglobin-containing proto-organism that evolved in the early biosphere some 1500 million years ago, but this might not be so. Recently, haemoglobin has been recognised in Vitreoscilla, a bacterium, and homology between this protein and its gene and animal and plant haemoglobins and their genes is demonstrated. This could mean that bacterial, plant and animal haemoglobins are related by vertical descent from haemoglobin present in a common proto-organism, possibly a primitive archaebacterium, that developed in the early Archean era more than 3500 million years ago. The known properties of contemporary plant and animal haemoglobins and their genes suggest that plant haemoglobin, with its ability to function at nanomolar concentration of free oxygen and its three-intron gene structure, could most resemble this putative primaeval haemoglobin.