There has been a lot of argument about how to classify primates, and on what basis. Molecular data have shown that the “haplorhines” (tarsiers, New World monkeys, Old World monkeys and hominoids) are monophyletic – we all share a common ancestor which did not give rise to the remaining primates, the “strepsirhines” (lemurs and lorises). Those of you with a classical training will have guessed that the “rhine” business suggests this classification is to do with the shape of the nose in these animals. Furthermore, haplorhines generally have acute colour vision (they are mostly diurnal), while strepsirhines, which are generally nocturnal, do not.
This has led to the suggestion that, during primate evolution, vision and the sense of smell have been traded, in particular in the human lineage. As we grew to be increasingly visual animals, we came to rely less on our sense of smell. This can be seen, it is claimed, in the number of “dead” olfactory receptor genes (“pseudogenes”) to be found in the human genome compared to that in New World Monkeys, a direct result of our acquisition of full trichromatic vision. But is this actually true? Was there a “trade-off” of one sense against another?
A study by a group of Japanese researchers (Atsushi Matsui, Yasuhiro Ho and Yoshihito Niimura), about to appear in Molecular Biology and Evolution [subscription needed], looks at this widely-accepted suggestion and finds little evidence to support it.
They looked at the olfactory receptor (OR) genes in five primate species (human, chimpanzee, orangutan, marmoset and rhesus macaque) together with two “strepsirhines” – the bushbaby and the mouse lemur, with the tree shrew as a comparison (“outgroup”). Surprisingly, they found no significant differences in the number of functional OR genes between the marmoset (New World Monkey) and the macaque (Old World Monkey) and the hominoids. In fact, humans had the second largest number of intact ORs (396), just behind the chimpanzee (399), and as against only 296 in the orangutan. This suggests that – whatever you might think – you can probably detect a similar number of odours as any of the other primates.
They then looked at how these genes evolved over time. They looked at which genes seemed to be the same in each of the different species (“orthologous” genes). This is particularly difficult in the case of OR genes, which evolve extremely rapidly, for reasons we do not understand. They estimated that the most recent common ancestor of all primates would have had around 550 OR genes. 62 of those genes are still shared by all the species they studied, and 34 of them are present in a single copy in each primate genome. Indeed, when you take into account gene duplication, nearly 77% of the human and chimpanzee OR genomes are common to the two species.
This figure shows how many of those 550-odd genes each species has lost over the last 48 MY or so (in the white bar on the right). Probable gene duplications are shown with a plus sign on the right of the bars, followed by the total number of functional ORs for each species. The number of genes that were lost at each branching event in our evolutionary tree are shown on the left. Strikingly, the same number OR genes were lost at the point we split with the strepsirhines (51 – arrowhead) and when we and the chimps split from the orangutans. This is not what was predicted by the “gain colour/lose smell hypothesis” – we would have expected to see a larger number of OR genes lost when we first parted company with our less-visual primate cousins.
This is not the end of the story, however. We already know that in humans, our OR genes have become “pseudogenized” at a greater rate after we split from the chimps, suggesting a relaxation of selection pressure on (some part of?) our olfactory genome. People have speculated that this may be due to our apparent non-reliance on pheromones for sexual and territorial communication. However, there is no evidence for this, any more than we have any idea as to what the relatively small OR repertoire in orangutans might indicate. For the moment, we cannot take the sequence of an OR gene and say what the corresponding receptor protein detects.
Although this study has not found evidence to support the trichromatic vision hypothesis, the authors point out that more comparisons from more species are needed, in particular because New World Monkeys and strepsirhines show highly variable colour vision systems. We are still far from understanding the effect of evolution on sensory systems, even in the species that are closest to us, including our own.
Matsui A, Go Y, Niimura Y. ( 2010) Degeneration of Olfactory Receptor Gene Repertories in Primates: No Direct Link to Full Trichromatic Vision. Mol Biol Evol. Jan 8. [Epub ahead of print]