OK, WordPress ate my post on the wiring of human senses and I haven’t been able to recreate that post yet (although I have learned to make back-up copies). But, along those lines comes a new paper in PLoS about odor receptors. The authors report on the numbers of odor receptors in different animals, then talk about evolution.
In this study, we showed that the numbers of OR genes have changed extensively in mammalian evolution. Why did the number change so frequently in mammalian evolution? One obvious factor would be the requirement for a species to adapt to a particular environmental condition. For most mammalian species, detection of millions of different odorants is crucial for their survival. Yet, animals living in different environments require different numbers of ORs. For example, olfaction seems to be less important for the primate species that are endowed with trichromatic vision than for other dichromatic mammalian species, because trichromatic color vision is very powerful for perceiving environment signals. This could be the reason why humans or macaques have a smaller number of OR genes than rodents .
Apart from sounding like a just-so story, let’s delve into this a little. Humans and Macaques have trichromatic vision meaning that they don’t need as many ORs. My question is, why do humans have more than macaques? They report that macaques have 606 while humans have 802. Yet, isn’t our vision more evolved than that of macaques? So, what explains the discrepancy? Why did humans further evolve more ORs after supposedly splitting off from macaques?
Another question: why do the authors assume that these traits evolved to perfectly fit the environment? This is tautological thinking, which is typical for Darwinian “science.”