More On Miocene "Apes"

It's not that I don't trust you. I know I included a link in my last post to an article about a time when Earth really was the Planet of the Apes. And I hope you read it. But I still can't stop thinking about it. For about the third or fourth time in my life, I've started going ape for the Miocene apes.

Of course, "apes" don't really exist. That's an old, pre-cladistic term that we paleo students and professionals call a paraphyletic taxon, because it usually excludes the direct human lineage, who sprouted from the same common ancestor. A more proper term would be "Miocene hominoids," but let's face it, when's the last time you called that boorish meat-head who's always trying to muscle you out of the way and steal your mate a "big dumb hominoid"?

It'll never catch on.

So, apes it is. You might think you know what "ape" means, so I'll get back to that in a sec. First, let's talk about this word "Miocene."

As mentioned in my post about giraffes, the Miocene Epoch was a time of great tectonic, climatic and ecological change that proved a crucible of evolution for many lineages. At its start, about 23.8  Ma, the non-avian dinosaurs had already been extinct for 42 million years. Avian dinosaurs (aka, birds) and mammals flourished in their absence, expanding into all terrestrial niches. Mammals had returned to the sea, and were starting to develop their special brand of echolocation (like "apes," cetaceans reached the period of their greatest diversity during the Miocene). Grasses and kelps were radiating into new regions, while the planet's tectonic plates were coming close to their current positions (although South America and North America were not yet connected, and India was only beginning to collide with Eurasia). The uplift of the Andes and Himalayas mountains and the closing off of the Tethys Ocean slowly but radically altered Earth's climate, so that by the end of the Miocene, many animals had been forced to migrate to mid-latitudes as the world got colder.

At the start of the Miocene, monkeys were the biggest game in town among the primates. At the end, they had been supplanted by the apes, who had already begun to evolve into our direct ancestors.

When I was a kid, I used to call the Miocene "the Age of Apes," to make it sound equal to the "Age of Dinosaurs." Mostly, that was human bias talking, but there's still some justification for the name (provided you pay no never-mind to those cetaceans and grasses staking their claims): apes reached the height of their diversity in the Miocene, radiating across Eurasia and Africa into nearly 100 different species. Sadly, most of them went extinct as the Miocene climate got steadily colder, and the survivors fled to Africa and southern Asia, where they became gorillas-chimps-humans and gibbons, siamangs and orangutans, respectively.

When most people think of an ape, they imagine either a knuckle-walker like the gorilla and chimp, or a tree-swinger like the gibbon and orangutan; but the apes of the Miocene were more diverse than that. They had mastered just about every form of terrestrial locomotion except flight and bipedalism... and we're not sure about bipedalism.

But I'm putting the monkey ahead of the banana here.

What's In An Ape?

It's common to use the words "monkey" and "ape" interchangeably, but this isn't quite accurate. Apes diverged from a monkey-like common ancestor a long time ago, most likely before the Miocene began, and there are numerous anatomical differences between them. Notably, apes have no tails and larger brains in relation to body size than monkeys do. Their arms are longer and, along with their hands, more flexible than those of monkeys. Some people get confused here, though, because when you look a little more closely, you find that apes have many things in common with some monkeys, but not others. Usually, the monkeys who share traits with apes are called "Old World" monkeys (because they're found in Africa and Eurasia), and the ones who don't are called "New World" monkeys (guess where they come from?).

Apes share a common ancestor with Old World monkeys, and together with them make up a group of primates called the Catarrhines, who are characterized by their downward facing nostrils and tails used for balancing rather than grabbing. New World monkeys form a group called the Platyrrhines, or flat-nosed monkeys.

The lines of the Catarrhines and Platyrrhines diverged from each other about 40 million years ago, when a group of Platyrrhine ancestors somehow made it across the Atlantic Ocean to South America, in one of paleontology's biggest mysteries. We're still nowhere close to figuring out how that happened.

So, apes are monkeys, sort of, because they are Catarrhines and display most of the Catarrhine characters. Except that "monkeys" aren't really monkeys, since there are two independent lineages who diverged from some common ancestor that wasn't quite a monkey,  either.

Got that? Good, because there will be a quiz.

The main point I'm working up to here is that most people today, if pressed to describe how an ape moves, would say either, "walking on their hands and feet" or "swinging from trees." Those of more precise observatory skills would point out that ground-walking apes specifically walk on their knuckles rather than their palms, and tree-dwelling apes get around by hanging and swinging below branches rather than on top of them. These methods are distinct ways of movement among living apes, so there's a temptation to define the group "apes" by them.

But just as, paleontologically, giraffes are not defined by their long necks, "apes" aren't defined by these ways of moving. And for the same reasons: most of their ancestors didn't do it that way.

The earliest true "ape" (whom you met in my hokey screenplay spoof) was Proconsul africanus, a hominoid who lived between 25 and 15 Ma in the forests of east Africa, but had cousins all over Eurasia, as well. If you saw a Proconsul today, you'd probably think it was a giant tailless monkey, because like monkeys, it walked with its body parallel to the branches (rather than swinging perpendicularly below them like gibbons and orangs) and walked on its palms (instead of its knuckles like chimps and gorillas). The technical term for this is "pronograde arboreal quadrupedality."

P. africanus retained several monkey-like features, such as thin tooth enamel, a narrow torso and relatively short arms. But its brain was larger than those of monkeys relative to body size, and it lacked a tail. Since Catarrhines used their tails for balance, their hominoid descendants had to find a way to compensate for this ability upon losing their tails. That's where their hands and feet come in.

Proconsul's hands and feet are distinct from those of monkeys in that their first rays are well-developed and their phalanges are elongated. In layman's terms, Proconsul had more flexible wrists, ankles, fingers and toes than other Catarrhines, and used them rather than a tail to balance themselves.

P. africanus and the other members of its genus represent the basal-most "ape" body plan, the foundation from which subsequent species evolved increasingly diversified and specialized forms of locomotion, including the knuckle-walking of chimps and gorillas, the tree-swinging of gibbons and orangs, and the obligate bipedality of humans.

Understanding this point about Miocene hominoid locomotion is important, because a lot of people tend to think that evolution stood still for apes while it marched onward for humans. They imagine that our common ancestor with, say, chimps was basically a chimp. But chimps have been diverging from the common ancestor for 6 to 7 million years, just like we have. Unfortunately, the fossil record of chimp evolution is currently poor, but it's likely to contain as many extinct intermediaries as the human one does, possibly even with as much diversity.

The same is true for all other apes, extant and extinct. Various traits of the still-living hominoids -- including precision grasping and a possible form of bipedalism -- all have their precursors among the Miocene apes, who added in many other locomotive and dietary specialties no longer found among the living "great apes."

Since this post is getting a bit long for my preferences, I'll end it here. But this won't be the last you hear of the Miocene apes. I'll do future posts on many of them individually.

Meanwhile, please do not neglect the ape protection and rescue links I provided in my previous post. If no other animals deserve "rights," the apes most certainly do.