But, by and large, the sense of smell reaches its apogee with the mammals. It's not just dogs that have a far better sense of smell than you do. It's also mice, deer, cats, and many others besides. The mammalian olfactory system is superbly evolved, and this dates right back to the origin of the class. In short, it's usually far better than anything we've got.
Except... um... we are mammals. So what gives? Why are we humans so useless at something that our fellow milk-giving relatives have perfected? Something that, presumably, has a lot to do with the success of mammals as a class?
Well, it's not just us. For one, there's the cetaceans, who just don't need a good sense of smell when they're underwater all the time, and, having to actually breathe through their nostrils, can't do what fish do. (Fish typically do have nostrils, but they're solely there as sense organs, and aren't even remotely attached to their respiratory systems). But obviously, that doesn't apply to us.
Yet primates in general seem to have a poor sense of smell by mammalian standards. It's likely that this is due to us having shifted the primary focus of our sensory systems to vision. High quality vision is important to anything that's going to be swinging through the trees, and our unusually good colour vision may have evolved to help our fruit-eating ancestors tell what was ripe. Some support for the idea that it's our superb vision that sidelined our sense of smell comes from the fact that the other group of land-dwelling mammals with a weak sense of smell are the bats... who have done with hearing what we did with sight.
The upshot of this is that primates have a worse sense of smell than other mammals, and that, within the primates, monkeys are less adept at smelling than the other primates. So that's two points in our evolution when our sense of smell declined, then: once when primates evolved, and a second time when monkeys did.
But is it really that simple? Evolution doesn't always follow a straight line, after all. And how would we tell how good various kinds of primate are at smelling, anyway? A recent analysis tried to answer the question indirectly, by examining the anatomy of a wide range of primates, including some fossil ones, and comparing them to each other, and to our closest non-primate relatives. Inevitably, such indirect inference isn't as conclusive as physically testing the animals would be, but it's the best way of looking at a large number of species in a sensible time, and, obviously, the only way that's going to work on fossils at all.
To understand what they chose to look at, though, we do need to briefly discuss what the anatomical structures associated with smell are. Perhaps one of the first things to mention is that there are, in fact, two different senses of smell in mammals. There's the one you're likely thinking of, which is the one where you breathe in through your nostrils, and detect volatile chemicals wafting about in the air. Then there's the other one, the "accessory olfactory system", that involves a pair of organs at the base of the nasal cavity with ducts running down to the roof of the mouth, just behind the teeth. This detects non-volatile chemicals, and is therefore, in some respects, closer to the sense of taste.
If you've no idea what I'm talking about, it's because, being human, you haven't had those organs since you were an embryo. In both we apes and in other Old World monkeys, they're just not there in adults. We don't even have the genes to make them function; evolution has long since wiped them out as unnecessary. But other animals still have such a thing - it's called a vomeronasal or Jacobsen's organ. This is why cats and horses, among others, sometimes make that strange lip-curling grimace (the "flehmen response") - they're wafting chemicals up to the vomeronasal organ to identify ones that their regular sense of smell can't detect. It's also how snakes smell, which is why I said they cheated; they're not doing it the same way we are.
So, setting that aside, since we clearly haven't got one, let's turn to the regular sense of smell that all primates do have. This is due to sensory cells located on the upper surface of the nasal cavity, which are connected to paired structures called the olfactory bulbs lying just above it. In fact, the olfactory bulbs are the front part of the brain, where it stretches forward into the rear part of the snout, and lie just in front of the main cerebral hemispheres.
Or, at least, that's where they are in most animals. In humans, the cerebral hemispheres are so huge, and the olfactory bulbs so small, that they're squeezed underneath the great bulge of the brain, instead of sitting in front of it. And it's that sort of change that gives us our proxy for how well animals, especially fossil ones, are able to smell: we know from anatomical studies on living species that the bigger the olfactory bulbs, the better the sense of smell is likely to be.
Even then, there's the question of whether it's the simple size of the organs that we should be looking at, or it makes more sense to consider their size relative to the rest of the brain, or even to the whole animal. Generally speaking, of course, as animals get larger, so does their brain, but, especially in primates, the brain may still be larger than you'd expect given the size of the animal. While you'd tend to expect that relative size is therefore the important factor, it's at least possible that the olfactory bulbs might be an exception to this general rule - that the more sensory cells you have in the nose, the better your sense of smell regardless of the size of the rest of you.
The study therefore, attempted to look at both the absolute size of the olfactory bulbs of animals, and at how it scaled with the rest of their brains. They compared over 80 species of primates and insectivores, using estimates from fossils to help confirm how the size and ratio of the organs had changed over the course of evolution. The answer for primates as a whole was, by their criteria, ambiguous: it seems that the bulbs of the first primates were not that different in absolute size from those of their closest relatives, but, because their brains as a whole were larger, the ratio was noticeably smaller. Whether that would have made a difference depends on which you think is more important. (Although it seems to me that, at the very least, it's likely to show a decreased reliance on the sense, relative to others).
The picture was, however, clearer when it comparing primates with one another, and it doesn't show the simple pattern that might have been predicted. Rather than showing that animals like lemurs and lorises have retained a better sense of smell than monkeys, it seems to show that they have actually improved their sense of smell over the ancestral condition. That is, while early primates may have been less effective at smelling than their own ancestors, the trend reversed again during the evolutionary line that led to the so-called "lower primates". If so, it's a significant reminder that, just because an animal looks more "primitive" than another, doesn't mean that it hasn't been evolving just as long, and that nothing has happened to it over the millions of years since the two parted company.
On the other hand, obviously there has to be a significant decrease somewhere along the evolutionary line leading to us, because we can see the effects of it today. The study placed that most clearly at the root of the branch that led to monkeys and tarsiers, but there seemed to be two, separate, losses of olfactory sensation in the lines that led to the New World monkeys on the one hand, and the Old World ones on the other. As to the apes, while the olfactory bulbs of related monkeys (such as baboons) continued to shrink, those in apes did not - they just look smaller because the rest of the brain is so much larger.
If all of this is right, it demonstrates that just looking at what we have now can sometimes obscure the pattern of exactly how we got there.
[Photo by "Mary03101983", from Wikimedia Commons]
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