Would you recognize your earliest primate ancestor if you met it face to face?

What if it didn’t look like a monkey, or an ape, or even a lemur?

Meet Purgatorius, a kind of mammal called a plesiadapiform that might’ve been one of your earliest ancestors.

Around 65 million years ago, these little shrew-like creatures were climbing around in the trees of western North America, munching on insects and fruit.

They’re known from bits of jaws and teeth that look more like those of later, true primates than any other group, and also from bones that show that their ankles were mobile -- perfect for an arboreal lifestyle.

But Purgatorius is also kind of mysterious, because it didn’t have what we think of as the classic primate traits, like forward facing eyes, or nails instead of claws.

So how did we get from this thing -- a mouse-sized creature that looked more like a squirrel than a monkey -- to you, a member of Homo sapiens?

To answer that question, it helps to understand the primate family tree, the whole history of evolutionary relationships that ties together the Order of Primates.

Then, you can trace your way back, all the way from Purgatorius up to your very own perch on the primate family tree.

To reconstruct the evolutionary history of any organism -- whether that’s you or a tree frog or a chicken -- scientists rely on two things: traits and dates.

Think for a second about your own family tree: You look more like the people you’re closely related to than your more distant relatives.

But you can still pick out some features that you share with, say, your second cousins or great-grandparents.

And this also holds true when you’re looking at the primate family tree.

Researchers start by finding synapomorphies, traits shared by two or more groups that are inherited from a common ancestor.

Groups that are more closely related have more of these synapomorphies in common than more distantly groups do, and this can be used to organize certain groups together.

Thinking about evolution this way emphasizes your unique features as a species, as well as your similarities with other primates.

But of course, a lot of your evolutionary history shows up only in what you can’t see.

Specifically, your genome.

In addition to revealing genetic similarities between you and other primates, your genome can provide a sense of how far apart in time you are from your ancestors, with the help of what’s known as the molecular clock.

This is based on the idea that DNA accumulates mutations at a fairly constant rate over time in different organisms, But the clock only works if you’re looking at parts of the genome that aren’t under selective pressure, like non-coding DNA - parts of the genome that don’t code for particular proteins.

That’s because mutations can only happen at a constant rate if they're not being selected for or against, by nature or anything else.

And scientists can then “set” the clock for a certain group of organisms, with the help of well-dated fossils.

So, say you take two groups of organisms, and quantify the amount of genetic difference between them.

Then you divide that amount by the age of a relevant fossil that has a known, radiometric age.

You can then use that rate to calculate the timing of the split between those two groups.

And the relative that existed just before that split is known as their last common ancestor, or LCA.

LCAs appear at each branching point on the primate family tree.

And all of the connections between the branches show their evolutionary relationships.

So we can use all of this information to create a taxonomy - a method for classifying and naming organisms.

Including us!

Now, we’re members of Homo sapiens.

Homo is our genus, and sapiens is our species.

Genus and species are taxonomic ranks, probably the ones you’re most familiar with.

They fit within a nested hierarchy of taxonomic ranks, with each higher rank being more inclusive than the one below it.

And these ranks reflect evolutionary relationships.

So, being part of the genus Homo puts you in a pretty exclusive group.

It includes only our immediate ancestors and our very closest fossil relatives.

The earliest fossil from our genus, a partial mandible, or lower jaw, from Ledi-Geraru in Ethiopia, is dated to 2.8 to 2.75 million years ago, in the Pliocene Epoch.

We don’t have enough of this jaw to know which species it belonged to, but it’s thought to belong to a member of our genus because of its teeth.

Now, the next rank up from genus isn’t one that’s mentioned very often.

It’s the level of tribe, and our tribe is Hominini, the hominins.

The hominins include us plus all of our extinct relatives that lived since our Last Common Ancestor with chimps and bonobos.

That ancestor lived between 4 and 8 million years ago, in either the Pliocene or Miocene Epoch.

The date varies based on what part of the genome is analyzed and what fossils are used to calibrate the molecular clock.

Traditionally, the key traits of hominins are that they’re bipedal, and the size of the canine teeth are closer to the same size in both males and females.

Sounds obscure, I know, but in chimps and our more distant ancestors, males always had noticeably larger canines.

And there are three main contenders for the earliest known hominin: Sahelanthropus tchadensis at around 7 million years old, Orrorin tugenensis at about 6 million years old, and Ardipithecus kadabba between 5.8 and 5.2 million years old.

Around this point in our family tree, you’ll note that our ancestors are still looking more like you than like Purgatorius.

Now, above the rank of tribe, there’s family.

And our family is the Hominidae, the hominids or great apes.

They include us and chimps and bonobos, but also orangutans and gorillas.

Hominids are usually large-bodied, with males that tend to be larger than females.

And we all lack what’s known as ischial callosities.

These are the specialized fatty pads that gibbons, siamangs, and many Old World monkeys have on their butts.

They’re basically built-in seat cushions.

Within the hominids, the orangutan lineage branched off between 12 and 15 million years ago.

Some of their earliest members were part of the genus Sivapithecus, whose fossils have been found throughout Asia.

And around the same time, another tribe of hominids, known as the Dryopithecines, were living in Europe.

And this tribe may have given rise to the African apes, including gorillas, chimps, and bonobos.

Now, let’s talk more about apes.

Because you are one.

Taxonomically, you’re part of the Superfamily known as Hominoidea.

This superfamily includes two families: the “great apes” or Hominidae and the “lesser apes” or Hylobatidae, which includes gibbons and siamangs.

These two families probably split sometime between 16 and 24 million years ago, during in the Miocene.

And all living hominoids - including you - share features that are related to having an upright posture and living in the trees.

We have stiffer lower backs with fewer lumbar vertebrae, as well as the ability to rotate our arms above our heads at the shoulder, and we lack tails.

Also we mature more slowly than other primates, we live longer lives and have relatively larger brains.

And, just to be thorough here, we’ve also got a characteristic pattern of cusps on our lower molar teeth; this is the kind of thing that primatologists look for!

There are 5 of these cusps, and the lines between form a Y shape.

So we call them Y-5 molars.

Check them out the next time you’re in front of a mirror.

Now, if we check in with Purgatorius, we can at least point to a couple of things that it has in common with hominoids, like living in trees.

But it still looks nothing like an ape.

But members of the next rank will probably look familiar.

That rank is Infraorder, and the one we belong to is Simiiformes, which includes both monkeys and apes.

Among the monkeys, you have your New World monkeys and your Old World monkeys.

Both have eye sockets that are completely enclosed by bone.

And they also have dry noses, meaning they lack a rhinarium, the wet part of the snout that some animals, like dogs, have that gives them increased smelling abilities.

And since we share a more recent common ancestor with Old World monkeys, our nostrils open downward, like theirs do.

But in the New World monkeys, they open sideways.

The hominoid lineage split from the one leading to Old World monkeys between 24 and 38 million years ago, probably around 29 million years ago, in the Oligocene.

But the lineage leading to New World monkeys branched off a bit earlier, sometime between 33 and 44 million years ago, in the Eocene.

Now, one rank up from Infraorder is Suborder.

And ours is Haplorhini.

This includes us, the other apes, the monkeys, and the tarsier, which is the only primate that exclusively eats other animals -- generally insects and small vertebrates.

Haplorrhines, like you, have dry noses, broad, flat incisors, and eye orbits that are at least partially enclosed with bone at the back.

Tarsiers and Simiiformes might have parted ways as far back as 65 million years ago, in the early Paleocene, shortly after the extinction of the non avian dinosaurs.

The ancestor of all haplorrhines was probably small, arboreal, and active during the day.

And it might’ve eaten both insects and fruits.

So, it may have been more like Purgatorius than anything else we’ve encountered so far.

And finally, we’ve made it to the level of Order.

And ours is Primates, which is all of the haplorrhines plus the lemurs and lorises, which are considered strepsirrhines.

All primates have forward-facing eyes, which gives us binocular vision and good depth perception, important for life in the trees.

And we also have eye orbits that are partially or completely enclosed by bone.

We also have opposable, grasping thumbs; fingernails instead of claws; and relatively large brains and slower life histories than other mammals, meaning we mature more slowly and live longer.

Not all primates have all of these features, but they’re the traits that define us as an order -- ones that evolved in our early ancestors and were passed on, contributing to our success.

And this brings us, at long last, to Purgatorius, the earliest known potential primate whose fossils date from around 65 million years ago.

But molecular clock studies suggest that the origin of our order may be 10 million years before that, during the Cretaceous period.

Plus, like I said at the very beginning, Purgatorius is a plesiadapiform.

And there’s still debate about whether plesiadapiforms are primates, because they don’t have enclosed bony orbits, or nails, or even forward-facing eyes.

But some researchers support their primate status on the basis of the anatomy of their teeth and ankles.

The earliest uncontroversial primates -- the ones that have all the primate features, like the adapoids and omomyoids -- show up about 55.8 million years ago, at the start of the Eocene.

So that’s where you are on the tree of life!

Your species is sapiens, in the genus Homo.

You’re also a hominin and, beyond that, a hominid, or great ape.

You’re a member of the hominoids, one of all the apes that has ever existed, and you’re a simiiform, placing you on the branch that includes monkeys, too.

You’re also a haplorrhine, like the tarsier, on the opposite branch of the family tree from the lemurs and lorises.

But they’re your distant cousins, too - because they’re fellow members of the primate order.

That’s your evolutionary heritage in a nutshell.

Your immediate ancestors are upright walkers and tool users.

But your distant ancestors were small, tree-dwelling creatures, like Purgatorius, that would go on to diversify into the incredible array of lemurs, lorises, monkeys, and apes alive today.