Tag Archives: apes

Muscles, calories, and curves

1.46-1.39 million years ago

Chimpanzees and humans make different tradeoffs between strength, energy expenditure, and energy storage (i.e. fat). The differences are ordinarily not so evident on the surface, because chimps are also covered with hair. But this video of two hairless chimps, father and son, is (literally) revealing. It shows how buff chimpanzees are: muscular, and with little body fat obscuring muscle definition.

(And yes, it also shows off the chimps’ enormous testicles, adapted to produce lots of sperm and outcompete other males’ sperm in a promiscuous mating system.)

Chimpanzee muscle is 1.35 times stronger than human muscle (a smaller difference than is sometimes reported), with more of the fast twitch fibers the make for bursts of strength, and fewer of the slow twitch fibers that make for endurance.

As a further comparison, here’s figure is from a neat recent paper comparing energy expenditure (TEE or Total Energy Expended) and fat among humans and our closest relations: chimpanzees (genus Pan), gorillas (Gorilla), and orangutans (Pongo). (The numbers are adjusted for differences in overall body mass.)

 

1.46-1.39 million years ago

energyfat

This figure is from a neat recent paper comparing energy expenditure (TEE or Total Energy Expended) and fat among humans and our closest relations: chimpanzees (genus Pan), gorillas (Gorilla), and orangutans (Pongo). (The numbers are adjusted for differences in overall body mass.)

What stands out here is that humans are a high energy species. Also we carry a lot more body fat than the other great apes. This applies particularly to women, who need a lot of extra fat to meet the high energy demands of human infants. But it even applies to men. For both sexes, the high energy life style means you want to carry around an extra reserve of fat in case of emergencies.

We don’t know how long ago our ancestors decided to crank up their energy consumption. Maybe back with the rise of Homo erectus (just a few days ago on Logarithmic History). Or maybe later, when the typical modern human pattern of slow maturation was more firmly in place. At some point in the near future, we’ll actually nail down the specific genetic changes leading humans to accumulate more fat, and be able to put a date on the change. It may be that the distinctively human mating system also arose back then, with human females concealing ovulation (no chimp-style monthly sexual swellings) but advertising nubility (with conspicuous fat deposits appearing at puberty).

A high energy life-style also goes with extensive food sharing and changes in human kinship. (Here’s me, on beating Hamilton’s rule through socially enforced nepotism.)

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. And a study that came out just last year 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

Maimoun angushti shayton ast.

“A monkey is the Devil’s fingers.”

Tajik proverb

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.

Rama’s ape

12.3-11.7 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

13.8-13 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), 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

Apes, on the road to great

16.3-15.5 million years ago

Teeth are tough, and survive better than most bones. We can recognize apes by their teeth: ape and human molars have 5 cusps that form a distinctive Y pattern. Early Miocene apes like Proconsul already had this pattern. They had also already lost their tails. But in other respects they were more like monkeys than living great apes. They walked on their palms like monkeys, meaning they mostly walked on top of branches, instead of hanging underneath them.

How we get to modern great apes is somewhat mysterious. Apes may have left Africa for Europe and Asia as early as 16 million years ago, or maybe more like 14 Mya. A variety of great apes develop in Asia, although orangutans are now the only survivors. But we’re not sure whether the ancestors of African great apes are apes that stayed in Africa, or whether they’re apes that developed more modern features in Eurasia and then migrated back to Africa.

The various genera of great apes all make some kind of compromise between walking and hanging from branches. When orangutans are on the ground (which is not very often), they walk on the edges of their hands. Chimpanzees and gorillas walk on the knuckles of their hands. And of course humans walk on their hind legs. These are all pretty unusual ways to get around.

It would be nice to know whether human ancestors went through a knuckle walking phase. African fossils are skimpy for this period, and likely to remain so. Maybe genetics will have something to tell us about whether chimp ancestors took to knuckle walking before or after they spit from human ancestors.

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. And a study that came out just this year 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.