Tag Archives: Australopithecus

Grassroots and fisticuffs

A couple of recent perspectives on Australopithecus and early Homo:

Tropical grasses and sedges commonly use the C4 pathway for photosynthesis, which evolved from the C3 pathway used by most plants, in response to lower atmospheric carbon dioxide levels. We can trace an isotopic signature in the teeth of animals that eat C4 plants (and animals that eat those animals). From about 3.5 Mya we see evidence of hominins eating more C4 foods: either grasses (maybe including underground parts) or animals that ate grasses. Paranthropus boisei seems to be a particular champion with the C4s. Chimpanzees, even when they live in open woodlands, mostly stick to eating fruits and leaves. By contrast, exploiting grassroots and sedges to varying degrees may be an important component of australopithecine adaptation to the savannah.

Not only do australopithecines have enlarged back teeth relative to great apes, but they also have smaller canines. Canines are mostly good for fighting, so it’s nice to think that canine reduction is a sign of our ancestors becoming more peaceful. Could be. But it’s possible they just switched fighting styles. David Carrier argues that australopithecine faces are built to withstand punches, starting from the time that australopithecine hands were capable of forming fists. This might go together with high levels of sexual dimorphism – males, the more violent sex, being bigger than females (but early hominin sexual dimorphism is a contentious subject.)

Two Million BC

Two million years ago, there were multiple hominin species in Africa. There were two robust species that we know of, boisei in east Africa and robustus in southern Africa. The robusts had huge premolars and molars, and enormous jaw muscles running through a large zygomatic arch to attach all the way up to the sagittal crest in the middle of the skull. Gorillas have sagittal crests too, but with them it’s about powerful bites from incisors and canines. You, on the other hand, don’t have a sagittal crest. Your temporalis muscle only runs up to your temple.

The robusts are sometimes given their own genus Paranthropus, other times included in Australopithecus. This reflects uncertainty about their relationships. Did robust morphology evolve once, with both boisei and robustus evolving from aethiopicus? Or did it evolve independently several times, with robustus evolving from Australopithecus africanus in southern Africa?

P. boisei. Robust ancestor?

There was at least one genus of Homo around two million years ago, Homo habilis, with a larger brain than any australopithecine, but similar body size and long arms. There was also the recently discovered Australopithecus sediba, with brain not especially large, but maybe reorganized in a human direction, and teeth trending human-wards as well. . We don’t know if either of these guys was a human ancestor. Another candidate, Homo rudolfensis will show up soon.

A. sediba. Composite based on 3 individuals

Turnover pulse

Elizabeth Vrba, a South African paleontologist, coined the term “turnover pulse.” The idea is that most species most of the time are in an equilibrium with their physical and biotic environments, and not changing much. But every once in a while an environmental disturbance comes along, resulting in large scale extinctions, and a pulse of speciation. Most evolutionary change, on this theory, occurs during these pulses. Vrba saw evidence of a pulse 2.5 million years ago among South African antelopes (affecting specialized more than generalized feeders). The pulse was associated with a shift toward cooler weather, and a shrinking of forests and expansion of grasslands. Part of the turnover pulse hypothesis is that the same pulses should affect many different species at the same time: there is arguably a turnover among hominins as well, giving rise to a later generation of savannah-adapted australopithecines.

Vrba was one of a circle of paleontologists pushing the idea of punctuated equilibrium ¬– that during most of their existence species don’t change much (stasis), and evolutionary change is concentrated in the times and places when a new species branches off from another. Steven Jay Gould and Niles Eldredge were others pushing the idea. There are various possible explanations for why we might see a combination of stasis and punctuated equlibrium (assuming the theory is true in the first place). Environmental controls, as suggested by Vrba are one possibility. Or maybe species’ developmental systems are tightly integrated and resistant to change. Or (for sexually reproducing species) the need to find a similar-enough mate might foster stasis most of the time.

We’ve seen some enormous episodes of mass extinction before on Logarithmic History. The turnover pulse hypothesis implies that the same phenomenon on a smaller scale is behind most evolutionary change. Not everybody buys this though. Punctuated equilibrium is a hugely contentious topic. And plenty of evolutionary biologists think that arms races among (and within) species keep evolution running along pretty constantly even without environmental changes. We’ll see some of the same issues – punctuation versus gradualism, ecological forcing versus internal dynamics driven by arms races – when we take up human history.

Stonecraft as soulcraft

2.71-2.56 million years ago

Until recently the earliest known stone tools dated back to the Oldowan, 2.6 million years ago, although just last year stone tools going back 700,000 years earlier were reported.

We now know that tool making is not uniquely human. But Oldowan tools – including choppers (below), pounders, and scrapers — go beyond anything chimpanzees, or other animals, do. Kanzi, a bonobo (pygmy chimpanzee, who was also taught to communicate with an artificial set of symbols) learned to use sharp stone flakes for cutting, but never mastered the art of striking a stone core at the proper angle to produce useful sharp flakes. Apparently australopithecines (or maybe early Homo or Kenyanthropus) had taken a step further by 2.6 million years ago (or earlier).

oldowan

Early evolutionary theory developed in tandem with the Industrial Revolution and included an appreciation for the importance of manual labor. Darwin, in The Descent of Man, argued for the central role of toolmaking in human evolution, and, not surprisingly, the same point was echoed by Friedrich Engels in 1876, in his unfinished essay “The Part Played by Labor in the Transition from Ape to Man.” Engels was pushing back against the attitude in most traditional stratified societies that manual labor is low class, while symbolic labor (and/or wielding weapons) is high class. For example the fingernails on this Chinese scholar advertised that he didn’t work with his hands.

fingernail

Nowadays, a common complaint about the post-industrial economy is that so much education and employment revolves around pushing symbols around that manual labor is relatively devalued. The recent book Shopcraft as Soulcraft: An Inquiry into the Value of Work is a statement of this lament. Maybe today is a good time to celebrate the part played by labor in the transition from ape to man — by making something, or mending something. I’ll be doing my part by neandering a big branch that broke off the apricot tree in my back yard. (But if food is more your thing, the next post will suggest a recipe.)

Lucy

LSD

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.’

Lewis Carroll Alice in Wonderland

Southern apes

Two years ago to the day saw the announcement of a new hominin species, Australopithecus deyiremida, discovered in Ethiopia, based on a jaw and some teeth. There’s good news coverage from Carl Zimmer. The new species overlaps in time with Australopithecus afarensis (the long lasting species to which Lucy belonged) and Australopithecus bahrelghazali, as well as Kenyanthropus platyops. This might be more evidence that human evolution for a long time was extremely “bushy,” involving a proliferation of species with variable combinations of traits. But it could also be telling us that what constitutes a species is less clear cut than you might have been taught in high school biology – an issue we’ll keep coming back to.

People want to know who their relatives are, so an awful lot of press coverage is about whether A. deyiremida is a human ancestor or not. It’s already been claimed that Kenyanthropus platyopslooks closer to genus Homo than other hominins from around the time period. But right now we’re at a point where new discoveries seem to make it harder, not easier, to draw lines connecting ancestors and descendants.

Maybe it’s best to step back and notice some general traits of australopithecines (“southern apes”). We’ve talked a lot about bipedalism. Australopithecines are clearly bipedal, but there’s an ongoing debate over whether some or all of them might also have spent some time climbing in trees. Certainly this sounds like a good idea for getting away from predators but just how important it was is in dispute. Australopithecus teeth are telling us something too. Mostly the front teeth are reduced compared to chimpanzees and gorillas, but the back teeth are large and thick enameled, implying that australopithecines, more than African great apes, were supplementing their fruit diet with tough hard-to-chew “fallback” foods. There’s a lot of variation between species though: A. deyiremida apparently looks closer to later species than A afarensis.

Four legs good, two legs better

4.75-4.49 million years ago

There’s been a lot of hullabaloo in the last few days 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.

Here I cover some of what we know about the evolution of bipedalism. This is mostly in the context of Ardipithecus ramidus, but I have some suggestions at the end of the post about how the Graecopithecus find might be relevant.

ardipithecusWith Ardipithecus ramidus (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.

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.