Tag Archives: apes

Four legs good, two legs better

ardipithecusWith Ardipithecus radius (about 4.5 million years ago) we have the strongest evidence so far that hominins have adopted bipedalism. Earlier fossils, including the earlier Ardipithecus kadabba, are too fragmentary to be very sure. Even “Ardi” was not bipedal quite the way we are. She had a somewhat diverging big toe, and arms and hands well-adapted for suspension, suggesting she was bipedal on the ground, but still spent a lot of time in trees.

We’ve seen bipedalism before on Logarithmic History. Bipedalism allowed ancestral dinosaurs to overcome the tight coupling of locomotion and respiration that prevents sprawling lizards from breathing while they run. But human bipedalism, with no counterbalancing tail, is different. As far as we know it evolved only once in the history of life (or maybe twice if Oreopithecus was bipedal).

In part human bipedalism is related to the general primate phenomenon of having grasping hands. Both humans and macaques, for example, devote separate areas of the brain (within the somato-sensory cortex, specifically) to each finger on each hand. Brain areas for the toes, by contrast, are more smooshed together.
monkeyhands

Human bipedalism is more specifically related to tradeoffs in locomotion in  great apes. Other great apes pay a big price for being the largest animals well-adapted for moving around under and among branches: great ape locomotion on the ground is particularly inefficient. Chimpanzees spend several times as much energy knuckle-walking on all fours as you would expect based on comparisons to similar sized quadrupedal mammals. Remarkably, chimpanzees don’t take any more energy walking on two legs than they do walking on all fours, even though they aren’t at all well-adapted to bipedalism. Humans by contrast take a little less energy to walk around than a same-size four-legged mammal, and way less than a chimp. (No one has yet done the research to figure out how (in)efficient humans are when getting around all fours. More research needed!) A recent study shows that the same was true of Ardipithecus: she was an efficient bipedal walker on the ground. In other words, being good at climbing trees (although not as good as a chimp) didn’t hurt her when it came to getting around bipedally on the ground.

That said, efficiency isn’t everything. Human beings are lousy at sprinting – try outsprinting your dog, or a squirrel for that matter. Our top speed is less than half that of a chimpanzee.

So there’s a tradeoff between the efficiency advantages of bipedalism (at least compared to knuckle walking), and the loss of speed. It may be that bipedalism evolved initially in an environment where predation pressure wasn’t very intense, and the need for speed was not as great. This argument has been made for Oreopithecus, living on an island in the Mediterranean. Perhaps Graecopithecus initially enjoyed a similar isolation, and freedom from predation, associated in some way with the drying and flooding of the Mediterranean.

Bikers and hippies and apes

We’ll have more to say about the evolution of our very distinctive social organization as the blog carries on. But it may be informative to consider our closest relatives, chimpanzees and bonobos. The two species are closely related, having diverged only about 2 million years ago. They remain physically quite similar, and people didn’t even figure out that bonobos are a separate species until the twentieth century. There are some broad similarities in their social organization. Both species have fission-fusion societies, in which subgroups form and reform within a larger, more stable community. Both species have male philopatry: males spend their lives in the community they were born in, while females transfer out of their natal community to a new community when they reach sexual maturity. In both species, females commonly mate with many males over the course of an estrus cycle. But there are some important differences.

Jane Goodall began studying chimpanzees in the wild at Gombe National Park, Tanzania, in the 1960s. The early reports from Gombe captivated the world with stories of chimpanzee social life, tool use, and interactions with human observers. It was the 1960s, and chimpanzees – hairy, sexually promiscuous, grooving in the jungle ­– looked familiar: they were hippies.

The picture darkened a lot in the 1970s, when the community at Gombe split in two. Between 1974 and 1978, the two daughter communities were effectively in a state of war. Males from the larger of the two communities carried out a series of raids against the smaller, with raiding parties opportunistically picking off and killing isolated individuals, eventually eliminating all the males and some of the females. Subsequent studies of other chimpanzee populations have made it clear that this was not an isolated incident: intergroup warfare and group extinction are general features of chimpanzee life. Chimpanzees are still hairy, still sexually promiscuous, but they now look less like hippies and more like bikers. Really scary bikers.

Bonobos look like the real hippies. They are more peaceable. They show less violence between groups, with members of neighboring groups sometimes even feeding peacefully in proximity to one another, something unthinkable for chimps. There is also less within-community male-male violence among bonobos. Bonobo females play a major role in regulating and intervening in male-male competition, and may even be dominant to males. There are tensions within bonobo communities but these are often resolved by (non-reproductive) sexual activity. For example, females, who are generally not related to one another because they were born elsewhere, might be expected to find themselves fighting over food. Instead they settle potential feeding conflicts peaceably by “g-g (genital-genital) rubbing,” rubbing their sexual swellings together until they reach orgasm. Or do a pretty convincing job of faking it: “I’ll have what she’s having.”

However recent DNA tests have revealed an unexpected twist to the chimpanzee/bonobo comparison. In spite of the more peaceable nature of male bonobos compared to male chimps, it turns out that there is actually greater reproductive inequality among male bonobos and a stronger relationship between dominance rank and reproductive success! Dominant male bonobos are more successful than dominant male chimps in monopolizing reproduction. If bonobos still look like hippies, then they are the kind of hippies where a lot of free loving is going on, but the whole happening is run by and for the leader (backed up by his mom) and his groupies.

Toumaï

6.80-6.44 million years ago

Sahelanthropus is a 7-6 million year old species whose remains have been found in Chad. “Toumaï” (“hope of life” in the Daza language) is the nickname for one individual, represented by a fairly complete skull. Otherwise Sahelanthropus is known from some jaws and teeth.

toumai

One of the things that distinguishes hominins (the human line) from great apes is that the front teeth – canines and incisors – are reduced. (Back teeth are another story. They stay big, or even get bigger, for a long time.) By this standard, Sahelanthropus looks like an early hominin. It’s got reduced incisors and canines and a short mid-face. And depending on who you talk to, it might or might not have been bipedal, although the foramen magnum (where the spine enters the skull) was maybe not positioned to balance the skull on top of the spine. Not that there was much brain inside the skull: the cranial capacity (maybe 360 cc) would be at the low end for a chimp.

So Sahelanthropus could be one of the very first species related to us after the chimp/human split. Chad, where Sahelanthropus was found, is a long way from East Africa, where most other hominins have been found, which suggests there may have been a profusion of hominins across Africa, waiting to be discovered.

The face in the Logarithmic History banner for the month of May is a Sahelanthropus.

Two roads diverged

7.61-7.20 million years ago

There was a lot of hullabaloo a few years back over claims that a jaw assigned to the 7.2 million year old Graecopithecus freygbergi represents the earliest known human relative after the hominin/chimp split. The jaw was found in Greece, which suggests that the split happened around the Mediterranean, rather than in Africa. (This doesn’t take anything way from the claim that Africa is the main center of later human evolution, up to 2 million years ago, which would have taken place when Graecopithecus’ descendants migrated to Africa).

All this needs to be taken fairly skeptically: a mandible with one tooth isn’t overwhelming evidence.

The date for Graeccopithecus is at the upper end of dates given for the chimp-human split. The TimeTree site, sponsored by Penn State, Arizona State, the National Science Foundation, and NASA, lets you enter any pair of species you want (common names or Latin) and find out the time since they split from a common ancestor. You get a range of estimates from the scientific literature, along with means and medians. The site lets you track down sources if you want. Entering Homo sapiens and Pan troglodytes gives you a median estimate of  the time of the split of 6.4 million years, a mean estimate of 6.7, and a whopping confidence interval of 5.1 to 11.8 million years, based on 79 studies.

Stories of O

9.00 – 8.52 million years ago

There were several interesting apes around 9 million years ago.

Ouranopithecus (sometimes called Graecopithecus) could fit almost anywhere on the great ape tree. Some people think it looks like an Asian great ape. Others think it looks more like the African great apes, maybe gorillas especially. This would be consistent with African great apes evolving outside Africa, then moving back. But maybe it only looks gorilla-like because it’s pretty big. In any case, we should expect that at this point different lineages of great ape will be hard to tell apart; they have only recently split.

But the award for weird goes to Oreopithecus. (If you think that sounds like a good species name for the Cookie Monster – you’re not the first person to have that thought.) From 9 to 6.5 million years ago, Tuscany and Sardinia were part of an island chain. Oreopithecus evolved there in relative isolation. It may be important that big predators weren’t abundant. Oreopithecus spent significant time arm-hanging. It’s when it was on the ground that things get strange. O’s big toe stuck out sideways at an extreme angle, so its foot was tripod-like, with a triangle formed by heel, little toes, and big toe. It’s possible that O was a biped, walking around on its two tripod feet when it was down on the ground. (Although measurements on the lower spine published in 2013 cast doubt on the biped theory.)

oreopithecus foot

Oreopithecus is just one find showing that apes early in the Late Miocene, well before our ancestors parted ways with chimpanzees, were experimenting with a lot of different types of locomotion, possibly including versions of bipedalism. Many of these experiments were taking place in Europe. (A few more examples: Danuvius guggnemosi and  Rudpithecus hungaricus.)

Biped or not, Oreopithecus was probably pretty awkward on the ground. When a land bridge reconnected O’s island chain with the mainland, predators arrived and, perhaps in consequence, Oreopithecus went extinct.

Rama’s ape

12.6 – 12.0 million years ago

Ramapithecus (Rama’s ape) is no more. Another Hindu god has taken over the franchise; Ramapithecus is now subsumed under Sivapithecus, an earlier discovery, and is no longer a valid taxon name.

The story is interesting from a history-of-science point of view. Ramapithecus used to be presented as the very first ape on the human line, postdating the split between humans and great apes, maybe even a biped. This was given in textbooks not so long ago as established fact. Then geneticists (Sarich and Wilson) came along, and declared that the genetic divergence between chimps and humans is so low that the split had to be way later than Ramapithecus. There was a lot of fuss over this. Paleoanthropologists didn’t like geneticists telling them their job. Eventually, though, the paleoanthropologists found some new fossils. These showed in particular that the line of Ramapithecus‘s jaw was not arch-shaped, like a human’s, but more U-shaped, like a non-human ape’s. So after thinking it over a while, paleoanthropologists decided that Ramapithecus (now part of Sivapithecus) looked more like an orangutan relative: likely ancestor of a great radiation of orangutan kin that left just one genus, Pongo, in the present.

rama jaw

There are plenty of examples of experts in different fields coming up with different answers. For example, paleontologists didn’t like physicists telling them why dinosaurs went extinct. And we’ll see other examples in days to come: geneticists, physical anthropologists, and archeologists arguing over modern human origins. And very recently geneticists coming in on the side of old-fashioned historical linguists, and against recent generations of archeologists, in the matter of Indo-European origins.

It would be nice if there were a simple rule of thumb to decide who’s right in these cases. Maybe experts know what they’re talking about (except that experts were telling us recently that low fat diets were the key to losing weight and eggs would kill us with cholesterol*). Or maybe harder science experts know better than softer science experts (except that physicists like Kelvin were telling geologists that the Sun couldn’t possibly have produced enough energy to support life on Earth for hundreds of millions of years – then along came Einstein and E=mc2). So the best we can do maybe is realize people, scientists included, are prone to overconfidence and group think – and not just those other people, either, but you and me.

* Memories of the fallout from 1990s nutrition expert wisdom

  • SnackWell’s cookies, low fat, loads of sugar
  • Jelly beans prominently labelled “No fat.”
  • A fellow grad student laughing about an older relative who said bread and pasta make you fat, and confidently declaring “Fat makes you fat.”
  • Me in Brazil talking with my landlord, who was planning to lose weight by sticking with sausage, and cutting starch. I told him modern science had shown this was precisely the wrong approach. He politely (Brazilians can be pretty good-natured) disagreed, stuck with his plans, and lost weight.

Oak ape

13.3-12.7 million years ago

We’ve known about Dryopithecus (“Oak ape”) for a while. The first specimen was found in France in 1856. They’ve since been found all over Europe, from Spain to Hungary. There are about 4 species of Dryopithecus, roughly chimp-sized.

The various Dryopithecuses are interesting because they look like good candidates for being somewhere in the ancestry of the great apes, Asian and/or African. (They could just as easily be on a side branch though. As any good cladist will tell you, it’s easier to say whether something is a close or distant relative than to figure out whether it’s an ancestor or a collateral.) Dryopithecus had made the move to suspensory brachiation – hanging from branches – and had the freely-rotating shoulders, long arms, and strong hands you need for that. But it wasn’t specialized for knuckle walking like a gorilla or a chimpanzee. This could mean it spent almost all its time in trees. Later on (10 million years ago) at Rudabanya, Hungary, we find Dryopithecus living in a moist subtropical forest, among fauna including Miocene versions of pigs, horses, rhinos, and elephants. The fauna also included predators: the lynx-like Sansanosmilus, weighing about 170 lbs, and “bear-dogs” up to five feet long. So maybe up in the trees all day was the safest place to be.

The evolutionary position of Dryopithecus matters for one of the big unsettled questions in human evolution: did bipedal human ancestors evolved directly from a tree-dweller like Dryopithecus, or were human ancestors chimp-like semi-terrestrial knuckle walkers before they started standing upright? Many scenarios for human evolution start with something that looked like a chimp, and maybe lived in chimp-style social groups (dominated by gangs of males ready to rumble with neighboring gangs) consistent with reconstructions of ancestral multi-male/multi-female groups among monkeys and apes. But there’s a lot of guesswork in this.

David Begum has recently written a book, The Real Planet of the Apes, covering this period in the evolution of human ancestors and collaterals. Begum argues that Dryopithecus was not just a great ape (now generally accepted) but close to the ancestry of present-day African great apes (i.e. gorillas, chimps (genus Pan), and humans, as opposed to Asian great apes – orangutans (genus Pongo)). This implies that African great apes may have originally evolved in Eurasia, and migrated back to Africa. Here’s one possible evolutionary tree, from Begum’s book:

dryopithecus tree