What would humans need to sacrifice for a better sense of smell?

Question

Humans have some genes that allow for a precise sense of smell, but they were turned off millions of years back, just like in many other primates. To name an exeption, lemurs are primates with a developed sense of smell, complete with a VNO. I would like for the humans in my story to have developed VNOs and a sense of smell similar in application to that of a lemur. Of course, there are structural limitations on the humanoid nose, so the sense of smell may not be as good as a lemur's, but much like it.

In particular, the humans in my story are nocturnal and eat fruit. They are tetrachromats and have sharp vision, slightly better than that of real humans to see fruit at night. Would that evolutionarily detract form their olfactory sense? Most importantly, would a stronger sense of smell hurt intelligence?

Answer 1

Nothing, humans already have an excellent (if underused) sense of smell.

https://www.theguardian.com/science/2017/may/11/not-to-be-sniffed-at-human-sense-of-smell-rivals-that-of-dogs-says-study

In the latest paper, published in Science, McGann points out that in absolute terms the human olfactory bulb is bigger than in many mammals and a literature search revealed that the absolute number olfactory neurons is remarkably consistent across mammals. “We went to the medical school and looked at a human brain,” he said. “We put the human bulb next to the mouse bulb and gasped. It was gigantic.”

https://www.scientificamerican.com/article/the-human-nose-knows-more-than-we-think/

The lack of a standard metric for scent is the main challenge, McGann says, in comparing absolute olfactory abilities across species. “It’s tempting to say humans are way more sensitive than mice at smelling human blood, and that sounds like a good ecological story,” he says. “But then you look at a whole range of other odors and realize that actually it just seems like there’s quite a lot of odors that humans are better at detecting than mice, dogs or rats, and other odors that we’re less good at detecting.” It’s impossible, therefore, to make sweeping generalizations about which species has the winning nose.

The issue for us in modern life is that we don't need to use smell much and so don't develop a great deal of use for it. Trained professionals can use their noses much more effectively, and can follow scent trails through fields, distinguish between perfumes and food types and such. The VNO genes turned on wouldn't provide much of a buff. Ours already works fairly well.

https://academic.oup.com/chemse/article/26/4/433/266106

Opinion: The EVG constitutes evidence for a selective and sensitive response to human-derived chemicals located in the region of the VNO. Systemic autonomic responses and emotional changes elicited by stimulation in this region suggest some chemosensitivity, even though the anatomical substrate is difficult to demonstrate and seems unlikely to be conventional VSNs.

https://biology.stackexchange.com/questions/62624/why-do-humans-not-have-a-powerful-sense-of-smell

Related.

Answer 2

The short answer to whether or not intelligence is affected by a better sense of smell is no, but the one thing you can say about evolution is that it trends towards efficiency and the question must be asked whether you need great vision AND great olfactory senses AND intelligence.

The brain (depending on the scientific paper you read) uses around 20% - 25% of the body's energy every day. That means that it's a massive energy sink that has to provide a substantial benefit to us to make up for that energy cost; and it does. It originally made us better hunters, more efficient with the resources around us, then better at building and refining tools that we use to achieve our ends.

Our five senses are in essence a part of our nervous system and provide key inputs to the brain. That sensory input also costs energy to sustain. I don't have exact figures on how much that cost is exactly, but there is a cost just the same. So the question becomes whether or not the development of an advanced brain is necessary if the combination of heightened visual and olfactory senses already make us better gatherers and more adept at avoiding danger.

Of course, there is another consideration here. Most evolutionary biologists believe now that the brain development in humans was only possible after the introduction of some meat into their diet. There are certain nutrients that proto-humans could not get from fruit and vegetables alone, and that the introduction of small amounts of meat, that transition into an omnivorous organism, was a key component of the development of modern human intelligence.

If that is the case, then it's arguable that heightened olfactory and visual senses has made your proto-humans adept in their environment; being able to forage and avoid danger sufficiently that the further investment in neural advancement (especially without the key nutrients provided by meat) may never happen.

Of course, the other consideration here is that most anthropologists believe that while we had the capacity for intelligence for many millennia before we actually started using it, the manifestation was brought on by our control of fire. Before that, we spent all our time surviving. After we harnessed fire we could sit around one, relatively safe from predators, and spend time actually thinking.

In other words, being capable of intelligence is not the same as having it.

What all this means for your proto-human stock is that by giving them a better sense of smell, you don't necessarily deprive them of intelligence, but you do very much change the priorities which evolution would apply to humans as improvements, and you also change their preferred environment to such an extent that two key contributors to the development of human intelligence (meat and fire) may not occur.