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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, 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

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.

The bottomlands

There’s a book from back in 1954, now out of print, called Engineer’s Dreams by Willy Ley (who was most notable as a spaceflight advocate). The book lays out various grandiose engineering projects that people have proposed over the years. Some of these dreams have actually been realized: after centuries of people talking about it, there is now a tunnel under the English Channel.

Others … well …

One project the book discusses is damming the Congo River, creating a huge lake in the Congo basin, then sending the water north to create another huge lake in Chad. (There’s a small lake there now, almost dried out, which was a lot bigger 10,000 years ago when Africa was wetter.) From Lake Chad, the water would be sent further north to create a great river – a second Nile — running through Libya into the Mediterranean. All that fresh water is just running uselessly into the Atlantic now. Why not send it someplace where it’s needed?

Another engineer’s dream is to refurbish the Mediterranean Sea by building a dam across the Strait of Gibraltar. This actually isn’t an impossible project. The strait is less than nine miles across at its narrowest, and about 3000 feet deep at its deepest. A dam across the strait would have some dramatic consequences. The Mediterranean loses more water from evaporation than it gains from the rivers running into it. The difference is made up by a flow of water from the Atlantic. Cut this off, and the sea will start shrinking. You could let the Mediterranean drop 330 feet before stabilizing it, run a huge hydroelectric plant at Gibraltar, and open up a whole lot of prime Mediterranean real estate.

Sadly, whenever people have dreamed great dreams, there have always been small-minded carpers and critics to raise objections. Okay, so maybe the mayors of every port on the Mediterranean would complain about their cities becoming landlocked. And maybe massively lowering the sea level in an earthquake-prone region would lead to a certain amount of tectonic readjustment before things settled down.

So probably the Gibraltar dam will never be built (although Spain and Morocco are considering a tunnel). But we’ve seen already that Mother Nature sometimes plays rough with her children, and it turns out (although Ley couldn’t have known this back in the 50s) that damming the Mediterranean has already been done. The story begins back in the Mesozoic (late March), when the Tethys Sea ran between the northern continent of Laurasia and the southern continent of Gondwanaland. The sea was still around 50 million years ago (April 11) when whales were learning to swim. But it has been gradually disappearing over time. When India crashed into Asia and raised the Himalayas, the eastern part of the sea closed off. And as Africa-Arabia moved north toward Eurasia, a whole chain of mountains was raised up, running from the Caucasus to the Balkans to the Alps. The Tethys Sea was scrunched between these: what’s left of it forms the Caspian, Black, and Mediterranean seas.

Starting about 6 million years ago, the story takes a really dramatic turn. The continents were in roughly there present positions, but the northern movement of the African tectonic plate, plus a decline in sea levels due to growing ice caps, shut off the Strait of Gibraltar, sporadically at first. With water from the Atlantic cut off, the Mediterranean began drying out. By 5.6 million years ago, it had dried out almost completely – the Messinian Salinity Crisis. (The Messinian Age is the last part of the Miocene Period). There were just some hyper-saline lakes, similar to the Great Salt Lake in Utah or the Dead Sea in the Near East, at the bottom of an immense desert more than a mile below today’s sea level. The Nile and the Rhone cut deep channels, far below their current levels, to reach these lakes. This lasted until 5.3 million years ago, when the strait reopened and a dramatic flood from the Atlantic restored the Mediterranean.

All this was happening just around that time that hominins were committing to bipedalism. Did the cataclysmic events in the Mediterranean basin have some influence on hominin evolution in Africa? At this point we can’t say.

Harry Turtledove, prolific writer of alternative history, has a novella, Down in the Bottomlands, set on an alternative Earth in which the Mediterranean closed off, dried out, and never reflooded. In the novella, terrorists are plotting to use a nuclear weapon to reopen the Mediterranean desert to the Atlantic – sort of Engineer’s Dreams in reverse.

Grasses and gases for a cooling world

The standard sort of photosynthesis uses a so-called C-3 chemical pathway. But  maybe from 8-7 million years ago there’s an increasing proliferation of so-called C-4 plants. They use an alternative, more-efficient pathway to incorporate carbon from C02. C-4 plants evolved independently 45-60 times.

Tropical grasslands are mostly C-4. The profusion of grasses, herbivores, and carnivores on tropical savannahs will owe a lot to C-4 plants. This evolutionary transition is probably a sign that C02 levels are declining, and have reached a threshold where C-4 plants are favored.

Going back about 7 million years, C02 levels stood at maybe 1500 parts per million (ppm). (They were higher earlier in the Miocene.) Levels decline pretty steadily, leading to global cooling and eventual Ice Ages. But things never again reach the extremes of Snowball Earth 750 million years ago.

At the beginning of the Industrial Age C02 levels stood below 300 ppm. They went above 400 ppm last year.

Toumaï

7.04-6.65 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) is 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.

Apeman

Was our splitting off from the chimpanzee/bonobo line really such a good idea? If you’re nostalgic for life before the split … or just for 1970 … here are The Kinks. (Of course we’ve learned since 1970 that there’s sort of a biker/hippy split between chimpanzees and bonobos. But that’s a subject for another post.)