Tag Archives: bipedalism

It’s a small world after all

595-563 thousand years ago

The story of human origins is partly a story of Big Things like The Taming of Fire and the The Dawn of Speech. But it’s also the story of some odd byways and quiddities. A nice introduction to some of these is Chip Walter’s book Thumbs, Toes, and Tears: And Other Traits That Make Us Human. (His more recent Last Ape Standing is good too.) Walters considers funny bits of anatomy like our unique big toes and thumbs, and funny bits of behavior like our habits of laughing, weeping, and kissing. Toes and thumbs fossilize, but behaviors can be hard to date, evolutionarily. Presumably these behaviors appeared sometime before modern humans evolved and spread, so let’s pick today’s date. It’s also hard to figure out the exact evolutionary rationale for some of these behaviors. Humor, for example, is not a simple phenomenon: intellectually appreciating a joke, actually finding it funny and enjoying it, and finally laughing, each involve separate areas of the brain.

Laughter, specifically, is a minor human oddity that sheds an interesting light on some big events in human evolution. Robert Provine, a leading laughter researcher, spells out the argument in “Curious Behavior: Yawning, Laughing, Hiccuping, and Beyond.” Chimpanzees have a kind of laugh, a modified vocalized panting synchronized with inhalation and exhalation. Presumably laughter first resulted when panting-during-play evolved into a play signal. But the short bursts of human laughter go further, having freed themselves from synchrony with the inhalation/exhalation cycle. Laughter, in other words, is just one instance of the more general phenomenon of humans having separate controls for vocalization and for respiration. Interestingly, the most prominent examples of complex vocalization – songbirds and some other birds, whales, bats, and humans – are all found in non-quadrupeds. In quadrupeds, breathing is tightly coupled with locomotion: lungs need to be full to stiffen the thorax when the forelimbs hit the ground. Giving up quadrupedalism seems to have allowed for an “adaptive release” in the evolution of vocal abilities in a number of unrelated lineages. So the study of laughter (and other vocalizations) suggests that two key human adaptations – bipedalism and spoken language – are more closely linked than one might have expected.

Another and overlapping set of human particularities involves facial expressions of the emotions. Darwin got a whole book out of this. He concluded (admittedly based on somewhat anecdotal evidence) that different emotional expressions are largely innate. It’s an interesting illustration of his ability to reason from small facts to large conclusions that he also drew a big conclusion about human evolution from this. In Darwin’s day, there were scientists who believed that different human races had evolved from very different prehuman progenitors: one prehuman species giving rise to Europeans, another to Africans, and so on. But Darwin reasoned that the very close similarity in facial expressions (and he had traveled a lot, and witnessed a lot of expressions in a lot of places) and the very similar emotional makeup of humans around the world was evidence that human populations shared a fairly recent common ancestry. Here as in several other cases, a mixture of close reasoning and sheer luck led Darwin to the correct conclusion about evolution long before there was much solid evidence.

Darwin’s work on emotions was neglected for most of the twentieth century by anthropologists favoring a blank slate view of human behavior, but was eventually largely vindicated by a number of researchers, notably Paul Ekman. There is now good evidence for six basic facially expressed emotions: Fear, Disgust, Joy, Anger, Sadness, and Surprise.

If you’re a movie watcher, this list may seem familiar. These emotions (all except for Surprise) are all depicted as little homunculi living inside the head of an 11 year old girl in the animated feature “Inside Out.” Somebody at Pixar Studios knows their Ekman.

insideout

So the sappy song is right: There is just one moon and one golden sun, and a smile means friendship to everyone.

Homo erectus

Evolutionary theory implies that the transition from one species to another takes many generations. There’s never going to be a point at which a non-human animal gives birth to a human offspring. But on the scale we use to measure things at Logarithmic History, the time 1.8 million years ago has a good claim to be the time when human beings began. Genus Homo has been around for a while, but there are major evolutionary changes around now in the human direction. We can start with geography. It’s now that we find the first hominins outside of Africa, at least as far as Georgia in the Caucasus. The Dmanisi fossils from Georgia can probably be assigned to the new species Homo erectus, albeit somewhat shorter and smaller-brained than later erectus. Homo erectus also appears around this time in Africa.

It’s still possible that hominins got out of Africa even before erectus.  A recent article came out suggests that Homo floresiensis, the famous diminutive “hobbit” from Flores island, Indonesia, has its closest affinities with African Homo habilis, raising the possibility of an earlier exit. It’s even possible that H. erectus evolved outside Africa from this earlier emigrant (so far undiscovered), and then some migrated back to Africa.

H. erectus has a bigger brain than earlier forms, and reduced jaws and teeth. And there are dramatic changes below the neck. Erectus has a body shape and size quite similar to ours. Strikingly, the changes in body form seem to be systematically related to distance running. Tendons in the feet and calves turn into springs that put a bounce in our running stride (but also rule out serious tree-climbing). Our neck gets longer and shoulders and head get more independent so we can swing arms for balance without twisting our heads from side to side. And the gluteus maximus becomes the largest muscle in the body, to prevent our bodies from toppling forward with each step. Homo erectus is the first hominin with a serious butt.

Moving from what we know to what we guess, it looks likely that Homo erectus had shifted to a new diet and a new mode of acquiring food. David Carrier argues that H. erectus was a persistence hunter, running after prey until they were exhausted. Human beings, although pretty poor sprinters, have a big advantage in distance running, in that our breathing is uncoupled from our running. This lets us run efficiently at whatever speed we choose. Most mammals, by contrast, have to breathe and run in synch, and pay a heavy price – wasting energy and overheating – for running at non-optimal speed. Bipedal dinosaurs enjoyed a similar advantage.

Like anything else in paleoanthropology, there are arguments about this. For example, fire may or may not have played a significant role at this early stage. We’ll cover some of these arguments in posts to come.

Lucy

LSD

3.21-3.04 million years ago

AL (Afar locality)-288-1 is better known as Lucy: probably the most famous individual hominin fossil. Her skeleton is particularly complete, and demonstrated the existence of a previously unknown species, Australopithecus afarensis.

When she was discovered in 1974, she pushed back the hominin fossil record by over a million years. A. afarensis looked like a plausible ancestor to all the later hominins. Things have gotten complicated since then. “The first family” (AL 333) was discovered just a year later: bones of at least 13 individuals, a mixture of adults and juveniles, maybe all victims of a flash flood. This helped to fill in knowledge of individual variation among A. afarensis. And subsequent findings have documented the species over a million year span from 4-3 million years ago. At the same time, we have seen how later discoveries have also suggested that there were likely multiple hominid species around in Lucy’s day.

Also: Her pelvis is basin-shaped, and she stood with her legs under her body, not rocking from side to side. All this is like a human, not a chimpanzee. So she was certainly bipedal, but there are arguments about just how bipedal. The initial view was that she was an efficient walker (although not much of a runner: that comes later). But another school of thought points out that she’s got very long arms, and curved finger and toe bones, suggesting she spent a lot of time in trees. Her long toes might have made her an inefficient biped (a view derided by members of the Lucy-the-proficient-walker school as the clown shoe hypothesis).

Lucy got her name from this song, played on a camp loudspeaker the day she was discovered. John Lennon, who wrote the lyrics, always denied that Lucy in the Sky with Diamonds, in spite of the initials, had anything to do with LSD. He said that it was based on picture that his son Julian (then four years old) made in school, with some Lewis Carroll thrown in.

‘I could tell you my adventures—beginning from this morning,’ said Alice a little timidly; ‘but it’s no use going back to yesterday, because I was a different person then.’

Alice in Wonderland Lewis Carroll

Four legs good, three legs better

Having grasping hands (and having them coordinate with the eyes) is one of the important things that distinguishes primates from other mammals. And a special version of bipedalism, which allows hands to specialize for manipulation, and feet for locomotion, is one of the first things that distinguishes hominins from other primates, even before hominin brains get big.

You find the same arrangement — a pair of arms with hands and a pair of legs with feet – with most science fiction aliens. (For TV and movie science fiction this just reflects the fact that aliens, pre-CGI, were mostly played by actors made up with pointy ears or fur suits or whatever.) But there are wilder possibilities, with no Earth analog. One of the most imaginative is the Pierson’s Puppeteers invented by Larry Niven:

“…. I was fed up with humanoids. Chad Oliver in particular, an anthropologist, wrote story after story claiming that this is the only workable shape for an intelligent being. The puppeteers were my first attempt to show him a shape that could evolve to intelligence. …”

puppeteers

The Puppeteers’ brains are safely tucked away inside their bodies, but they have two “necks” ending in “heads” each including one eye, one mouth, and a set of “fingers” around the lips. And the body has three legs. Decapitation is bad news for a Puppeteer,  like having a limb amputated, but not a death sentence.

Even more exotic are Vernor Vinge’s “Tines.” These are dog-like aliens who have evolved a short-range ultrasonic communication system that transmits information at such a high baud rate that a pack of half a dozen separate organisms is integrated into an enduring single individual with a shared consciousness. Losing one member of the pack is more like losing a limb, or having a stroke, than like the death of an individual. The mouths of the pack act together, as coordinated as the fingers on a hand, allowing the Tines to build up a medieval level civilization. (Vinge is a computer scientist, not an evolutionary biologist, however, and he glosses over some potential problems in Tine sociobiology: “all for one and one for all” is all very well, but which member of the pack actually gets to pass on their genes when it’s time to mate?)

But we don’t have to travel to other planets to find alternatives to two hands / two feet. Elephant trunks, for example, let elephants browse while avoiding the need for a giraffe/diplodocus-style long neck. The trunks even have “fingers” (2 for African elephants, 1 for Asian elephants) that are sensitive enough to pick up a single piece of straw.

We’ll spend a lot of time on Logarithmic History asking how human beings got to be such an extraordinary species. Hands are an important part of the story, although the elephant case suggests that hands (or their near-equivalent) are merely unusual, not absolutely unique to humans and near relations.

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.

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.