The more I study paleontology and evolution, the more I find myself taking language for granted. I have begun to casually and habitually use words that have specific meanings for paleontologists and evolutionary biologists, but that mean something quite different in colloquial speech. Understandably, this sows a bit of confusion now and then.
Take the words "ancestral" and "primitive," for instance. In several recent off-line contexts -- and one online one: my debate with Permavegan -- I have used one or the other of these words to describe some trait of humans that my interlocutor takes to be important in our evolution. On this blog, the relevant trait has been meat-eating or omnivory. I have pointed out that some degree of meat-eating behavior is "ancestral" to primates.
Some people take that statement to mean, "humans evolved to eat meat," but that's not actually what it says at all.
In paleontology, the words "ancestral" and (more commonly) "primitive" describe traits that are so evolutionarily deep that they are no longer distinctive enough to define a species. The words are contrasted with "derived" traits, which can be used to define a species (or other taxon).
The distinction is subtle, but important, and perhaps best illustrated by an example.
Let's say we are comparing humans and chimps, trying to determine how related they are to each other, and which of their respective traits should be used to define them as separate species.
Because they are both primates, there will unavoidably be traits that both chimps and humans share with other primates (say, opposable thumbs). Such traits -- shared by all primates, not just chimps and humans -- would be called "primitive" or "ancestral" for chimps and humans, because they are shared by all other primates, as well. Thus, these traits do not distinguish chimps and humans from other primates; they are, as it were, nothing special.
On the other hand, both chimps and humans are also going to have traits unique to themselves and to no other species (humans, for instance, are obligate bipeds, unique among primates). Such traits -- possessed by only one species (or taxon) but no others -- are called "derived" traits, because though they may be unique to that species or taxon, they were developed from a history that nonetheless relates them to other taxa.
Finally, there are going to be traits that chimps and humans share with each other, but with no other primate species. Such traits, shared by two species within the same taxon but not with others in that taxon, are "shared derived" traits, which they each inherited from their most recent common ancestor.
This distinction between ancestral/primitive, derived and shared derived traits is one that almost never turns up in colloquial discussions of evolution. In the context of the debate between vegans and carnists, for instance, each side will produce laundry lists of traits designed to prove that modern humans are "meant" to eat specific foods. Yet, neither camp makes any effort to distinguish whether their pet traits are ancestral, derived or shared derived, which muddies the whole discussion.
For example, I recently encountered a hunter who insisted that humans are meant to eat meat because our pancreas produces elastase, an enzyme that breaks down the proteins used to build and maintain connective tissues. This, as far as he was concerned, closed the case.
When I pointed out to him that elastase production is a primitive trait for all jawed vertebrates (because we are all ultimately descended from carnivorous fish), he didn't get the point, which was that because it is nearly universal in the vertebrate world, elastase cannot be used to define H. sapiens as an obligate carnivore. Natural selection is conservative, and nearly all jawed vertebrates -- even cows! -- retain the elastase-producing trait because it hasn't proved to be a constraint on their evolution.
Pick any trait you desire, be you a vegan or a carnist. Before you can plausibly argue that your pet trait proves something distinctive -- and thus definitive -- about modern humans, you're going to have to establish that the trait is derived (or at best, shared derived) for us. This is done by reference to the fossil record and (increasingly) genome analysis.
In the case of most traits cited by either camp, this will prove impossible, since the majority of traits possessed by H. sapiens are either ancestral/primitive or shared derived.
But, it doesn't stop there! In addition to establishing the primitive or derived nature of trait, you're also going to have to avoid a common fallacy called adaptationism, which argues that every single trait of an organism exists because it played an adaptive role in that organism's evolution; or, relatedly, that the modern use of a trait necessarily points to its adaptive history.
Not all traits are adaptations for a given organism. Many, if not most of them, were simply retained by all descendants of a common ancestor because the trait did not pose a hindrance to their evolutionary fitness. And what's more, such traits may get co-opted into new uses, or prove to be beneficial in new contexts separate from the selective pressures under which they originally arose.
A classic example is feathers, currently used by birds to achieve flight. It's tempting to think that flight is the "purpose" of feathers, but the fossil record shows us that the species of dinosaurs from whom birds descended developed feathers millions of years before they developed flight. So, why feathers? The prevailing hypothesis is that feathers developed as a method of thermoregulation, and only later got co-opted in the development of flight. Whatever the case, the fact that feathers are far more ancient than flight among dinosaurs indicates that flight was not the "purpose" of feathers.
A similar situation applies to an array of human traits often cited as evidence that we are "meant" to eat specific foods, such as canine teeth and binocular color vision (perennial faves among carnists), or small intestine length and stomach pH of 4 to 5 (popular among vegans). The fact that we possess these traits does not, in itself, mean that those traits were adaptive for our species in particular; nor does the fact that we currently use these traits in particular ways mean that those uses were what the traits were "meant" for. The likeliest explanation for a given trait is that it represents an adaptation to the environment of some distant ancestor, not to our own, and that we retained it because it posed no obstacle to our own evolution.
Modern humans are a patchwork of primitive and shared derived traits, most of which didn't arise in response to selective pressures faced by H. sapiens in particular. When defining what makes us uniquely human, it's only going to be our derived traits that count. Most carnist and vegan pundits miss this point, and are probably unaware that the distinction exists at all.
We should all keep this in mind when making appeals to evolution to defend our veganism, and be circumspect when making claims about what nature "intends" us to eat. There's nothing to fear in admitting that meat-eating is ancestral for humans; at best, it concedes a capacity for digesting animals, not an obligation to do so.
On the other hand, I have found using this distinction to be a lot of fun when confronting antagonistic armchair anthropologists, proclaiming from their self-presumed expertise that humans are "designed" to eat meat. Even knowing about this primitive-vs.-derived distinction in a cursory way can be enough for you to confound their presumption, and pull a Will Hunting on them. Just challenge them to demonstrate that their pet human trait is derived rather than primitive, and watch the apoplectics ensue.