Bushbaby growth hormone is much more similar to nonprimate growth hormones than to rhesus monkey and human growth hormones.

Unlike other mammals, Old World primates have five growth hormone-like genes that are highly divergent at the amino acid level from the single growth hormone genes found in nonprimates. Additionally, there is a change in the interaction of growth hormone with its receptor in humans such that human growth hormone functions in nonprimates, whereas nonprimate growth hormone is ineffective in humans. A Southern blotting analysis of the genome of a prosimian, Galago senegalensis, revealed a single growth hormone locus. This single gene was PCR-amplified from genomic DNA and sequenced. It has a rate of nonsynonymous nucleotide substitution less than one fourth that of the human growth hormone gene, while the rates of synonymous substitution in the two species are less different. Human and rhesus monkey growth hormones exhibit variation at a number of amino acid residues that can affect receptor binding. The galago growth hormone is conservative at each of these sites, indicating that this growth hormone is functionally like nonprimate growth hormones. These observations indicate that the amplification and rapid divergence of primate growth hormones occurred after the separation of the higher primate lineage from the galago lineage.