The simple, dictionary, definition is "a member of the order Primates". That's obviously a circular, and rather unhelpful, definition, but, then dictionaries aren't the same thing as encyclopaedias. But even it requires picking apart a bit. For a start, note the capital 'P' in the word "Primates". This indicates it's the name of a discrete thing, and not just the plural of the word "primate". It's a Latinate word, like "Rodentia" for the order of rodents or "Proboscidea" for the order of elephants and their extinct kin. Which means that it may not be pronounced the way you think; it has three syllables: Prime-ATE-eez. It literally means "of the first rank", because, you know, anything with us in it has to be of the highest rank. (Hence the title "primate" given to some high-ranking bishops).
Well, all right, but that doesn't really help us. What's an 'order'? Our current system of biological classification comes to us from 18th century Swedish naturalist Carl von Linné, better known by his Latin name, Carolus Linnaeus. His life's work was the creation of a scheme for classifying the whole of nature. He intended it to cover absolutely everything: minerals, cloud formations, you name it. However, it only survived for any length of time in biology, and, these days, we tend to forget that there were originally other bits, too.
By the time he'd finalised it, Linnaeus' scheme had living things classified into six increasingly narrow ranks, from kingdom at the top down to species at the bottom. Not long after his death, a seventh rank was added, making 'order' the fourth rank: a subdivision of a class that contains one or more families. The mammals as a whole are a class, so orders were intended as the highest division within the mammals, and one into which the various families could be grouped. The order Primates was first formally named, by Linnaeus himself, in 1758 in the tenth edition of Systema Naturae, the book from which all of modern taxonomy marks its beginning. It's therefore the oldest it's technically possible for a scientific name to be.
Linnaeus, of course, lived long before Darwin, and he didn't believe in evolution. He only intended his scheme as a means of identifying and naming things, not as reflecting anything actually true about the workings of life, at least when you got above the lowest two ranks. As we have come to understand evolution and gain a greater understanding of the variety of life, the scheme has been greatly expanded, and we now have such things as superfamilies, suborders, infraclasses, and so forth to try and make the scheme more meaningful. And that's before we get into unranked clades...
But, anyway, primates are an order, a large grouping of mammals that includes several different families. How many families? Well, that depends on your opinion. There are at least eleven, and a modern listing would likely include fifteen or sixteen. That is, as you might expect, quite a bit of variety. It includes all the various kinds of monkeys and apes, and also the so-called "lower primates", such as lemurs, bush babies, and the bizarre aye-aye. That's somewhere in the range of 450 named species, give or take.
So what binds all these together? What makes them primates?
Unfortunately, the fact is that there is no one defining feature that separates primates from all other mammals. We're actually quite generalised mammals, far less specialised than, for example, whales or hoofed animals. Nor do we have one unique thing that defines us, as rodents do, for example. On the other hand, we have the genetic evidence to prove that primates are a "real" group - that is, that they are all more closely related to one another than they are to anything else. Moreover, that primates could be identified as such must have been clear since at least 1758, when they were first named.
Linnaeus' original definition is no longer a particularly useful one. For the record, though, it went something like "has two hands, two mammary glands on the chest, and two canine teeth in each jaw, and usually has nails on the fingers and toes and four incisor teeth in each jaw". Which, when you realise that he was actually including bats in this, does rather stretch the definition of "hands", but there you go.
Johann Blumenbach, better known for his attempt at classifying human races, split off the bats in 1779, apparently deciding that the presence of wings was kind of significant. Skip forward another century or so, and we reach 1873 and the work of St George Mivart, a biologist with a particular interest in primates and their anatomy. By this point, Darwin is around, and Mivart initially believed in his theories. I say "initially" because, while he still believed in evolution, by 1873 he had rejected natural selection. Rather, he thought that new animals evolved from earlier species through the guiding hand of God, not by some entirely natural process. Evolution that is, towards some pre-ordained goal, which doubtless included primates, and, in particular, humans.
Anyway, Mivart came up with a new list of the defining features of primates. His description was "unguiculate, claviculate, placental mammals, with orbits encircled by bone; three kinds of teeth, at least at one time of life; brain always with a posterior lobe and calcarine fissure; the innermost digits of at least one pair of extremities opposable, hallux with a flat nail or none; a well-developed caecum; penis pendulous; testes scrotal; always two pectoral mammae."
This is a rather general sort of description, but, when you consider that primates are fairly unspecialised mammals, that's basically what you're going to get. Crucially, nothing in that list is unique to primates, and you have to take all the features together to arrive at something distinctive. Still, having said that, it's not too bad, so let's try and unpack it, and see if we can come up with a general overview of what, exactly, a primate is.
"Placental mammals" is, of course, a given; we're not marsupials. "Unguiculate" means that the animal has either claws or nails. So, not really narrowing things down very much, but one has to start somewhere. Some of the other features relate to the fact that primates are adapted to living in trees.
Of course, not all primates do, in fact, live in trees. Apart from ourselves, there are, of course, the baboons, and many others are not wholly arboreal. Nonetheless, we do have features related to that lifestyle, such as the presence of those opposable thumbs. Humans may use them for manipulating tools, but they seem to have originally evolved for gripping onto branches. This is, as noted, not a unique feature of primates, as other tree-climbing mammals have them, too, but it is a common feature of primates. Most primates have nails, rather than claws, something that is probably also to do with gaining a firm grip. (Claws are, however, found on primates such as marmosets, which are small enough that they need to grip onto sheer bark rather than holding onto smaller branches).
By "claviculate", Mivart is referring to the fact that we have a collar bone, connecting the breastbone, or sternum, to the shoulders. This is an ancient feature of vertebrates, being found even in many fish, but it's by no means a universal feature among mammals. That it's so large in primates is to do with the increased shoulder strength and mobility that you need to swing through the trees.
Other features are related to climbing less directly, in that they reflect our reliance on vision rather than smell. Primates tend to have good binocular vision, presumably in order to judge distances while swinging through the trees. Our eyes tend to be large and forward-facing as a result, and most species have unusually good colour vision. Mivart's list of features includes the fact that the rim of bone around our eyes is complete, something that's hardly unique to primates, but which is far from common, either - in many, the bone doesn't reach all the way round, leaving a gap on the side away from the nose. Binocular vision, and the reduction in the sense of smell, may also explain our short snouts, and the fact that we have less teeth than some other mammals as a result.
The calcarine fissure, also mentioned in the list of features, is a groove on the brain that's associated with the primary visual cortex, and the processing of information from the eyes. While not unique to primates, its presence likely has to do with our greater reliance on the sense. A feature that Mivart doesn't mention is that primates tend to have large brains in general, although there is considerable variation in relative brain size between, say, lemurs, and humans. This, in turn, may help explain why primates tend to have complex social lives, and why our pregnancies last longer than might be expected from our size alone - it takes time to develop.
Mivart does mention the caecum, a blind-ending pouch at one end of the large bowel, to which the appendix is attached. It's larger in primates than in many other mammals, but smaller than in others (such as horses), and, since it's used to ferment vegetable matter in many animals, may be to do with the fact that primates are all either herbivores or omnivores, and never approach pure carnivory.
The remaining two features are reproductive in nature. The shape of our penis is not unique, although, again, it isn't standard, either. (The human penis is particularly odd. It hasn't even got spines on it!) And we have two breasts because we don't tend to have large litters of young, so there's no real need for any more than that. As noted in the description, they're located up on our chests, whereas many mammals have at least some of them further back. This is a feature we share with bats, and goes some way to explaining why, in the days when taxonomy was no more than a handy identification guide, Linnaeus put the two kinds of animal together.
So, there is no one feature that defines what it means to be a primate. Rather, there are many features that, taken together, separate us from the other orders of placental mammal. Modern definitions rely on genetic comparisons, or detailed analysis of dozens of points of similarity in fossil and skeletal remains. We could say that a modern definition is something like "anything descended from the last common ancestor of humans and lemurs", although in practice that might get difficult to determine when you're looking at the very base of the tree.
But, at the end of the day, you have to draw the line somewhere. The answer to the question "what is a primate?" may not be a simple one, but it's at least one we can take a stab at.
[Photo by Frank Vassen, from Wikimedia Commons]