Bottom-up apes

According to the latest research, chimpanzees recognize other chimps not just by their faces but by their butts. 

Which raises the further question: Why are chimps down on all fours, while we’re not? This and related matters are subjects of a major recent review, “Fossil apes and human evolution,” which contrasts “top-down” and “bottom-up” approaches to understanding comparative ape and human ancestry. The authors write “top down approaches have relied on living apes (especially chimpanzees) to reconstruct [human] origins.” By contrast, “bottom-up” approaches pay more attention to the fossil record. The review brings to the fore something that’s been brewing for a while: fossil apes from the mid to late Miocene, leading up to the time that gorillas, chimps, and humans go their separate ways, are a varied bunch, and the last common ancestors of gorillas, chimpanzees, and humans, and of chimpanzees and humans, may be creatures that didn’t look all that much like any of the three. More specifically, the common ancestors may have been “orthograde,” standing upright and using both feet and hands to clamber vertically through trees. From ancestors like this, gorillas and chimps may have evolved similar innovations in parallel, getting bigger, evolving longer arms, larger palms, and shorter backs – all of which helped them as big animals to get around in trees, but also led them to take a more crouching posture, and to adopt the expedient of knuckle-walking on the ground (and hence to spend a lot of time looking at one another’s butts). The ancestors of humans, on this account, followed a different path, adopting bipedalism – not such a big step, already a big part of their postural repertoire – when walking on the ground. 

In some ways, gorillas and chimps seem to be caught in a “specialization trap.” This seems to show up in the energetics of locomotion. For a human being, it takes about 50 kilocalories to walk a mile (or about 30 kcal to walk a kilometer). This is about the same as you’d expect for a standard mammalian quadruped of our size. But for chimpanzees, committed to knuckle-walking, the energy cost is about double.

Stories of O

9.00 – 8.51 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 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  Rudapithecus 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.

Oak ape

12.6 – 12.0 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; probably we were never chimps.

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:

Apes, on the road to great

14.1 – 13.4 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, but there have been interesting discoveries from Europe that we’ll cover in days to come. Maybe genetics will have something to tell us about whether chimp ancestors took to knuckle walking before or after they spit from human ancestors.

Land of thoats

There’s a great expansion in the diversity of horses in the mid-Miocene, especially horses that are adapted to grazing rather than browsing. The shift to grazing is going on world wide among many different groups. In South America the big grazers are the liptoterns, ungulates not closely related to horses that evolve to look a lot like them, with high-crowned grazing teeth, single-toed hoofed feet and legs built for speed. (Edgar Rice Burroughs took the name thoat – what his characters rode around on on Barsoom/Mars — from one genus of liptotern, Thoatherium.)

Thoatherium reconstruction

We often think of evolution as a matter of organisms adapting to their environments, but when the environment is other organisms, each side may be chasing a moving target. Or sometimes the sides may reach an equilibrium. In the case of grazing animals, there’s a process of coevolution that goes on between grazers and grasses. Where grazers are active, the plants that survive are grasses, which keep leaves above the ground but grow from underground. And this works in the other direction: in moderately dry climates, grasses are more productive than taller brushy plants, so it’s when grasses take over that there’s enough food around for grazers – a mutually reinforcing cycle. With drier climates from the mid-Miocene on, grasslands and grazers get to be more and more important.

So a lot of the story of life on Earth is not just the appearance of this or that cool animal, but also the evolution of ecosystems. At the same time grasslands were spreading on land, for example, kelp forests were spreading in coastal oceans. We’ll see how important grasslands are in human evolution and history. And kelp forests, with their rich fish populations, might have been important too, as the highway that the earliest Americans followed along the Pacific coast to the New World.

We were never chimps

It’s natural to turn to our closest living relatives, the great apes (chimpanzees and bonobos, gorillas, orangutans), for insights into what our remote ancestors were like. But the fossil evidence suggests that current great apes aren’t a good guide to our past. Below is a figure from a recent article reconstructing diet and habitat for Morotopithecus and some relatives, from just over 20 million years ago, and later. It looks like all these guys inhabited relatively open woodland – trees interspersed with grass – rather than the closed canopy tropical forest that is the modal habitat for all the living great apes. Also, they may have specialized in consuming more young leaves, and less fruit than, say, chimpanzees. On current evidence, then, our closest living relatives have all evolved away from our common ancestors, to become (not terribly successful) tropical forest specialists.

Also, more on this later: gorilla and chimpanzee knuckle walking may be a poor model for locomotion in our common ancestor. Asking how we evolved from a chimp-like ancestor is probably asking the wrong question. 

Pace Jared Diamond: a good book and a snappy title, but we are not The Third Chimpanzee.

Planet of the apes

17.6 – 16.7 million years ago

We are now covering history at the rate of one million years a day

The Miocene (23 – 5 million years ago) is a period of extraordinary success for our closest relatives, the apes. Overall there may have been as many as a hundred ape species during the epoch. Proconsul (actually several species) is one of the earliest. We will meet just a few of the others over the course of the Miocene, as some leave Africa for Asia, and some (we think) migrate back.

Sometimes evolution is a story of progress – not necessarily moral progress, but at least progress in the sense of more effective animals replacing less effective. For example, monkeys and apes largely replace other primates (prosimians, relatives of lemurs and lorises) over most of the world after the Eocene, with lemurs flourishing only on isolated Madagascar. This replacement is probably a story of more effective forms outcompeting less effective. And the expansion of brain size that we see among many mammalian lineages throughout the Cenozoic may be another example of progress resulting from evolutionary arms races.

But measured by the yardstick of evolutionary success, (non-human) apes — some of the brainiest animals on the planet — will turn out not to be all that effective after the Miocene. In our day, we’re down to just about four species of great ape (chimpanzees, bonobos, gorillas, and orangutans), none of them very successful. Monkeys, with smaller body sizes and more rapid reproductive rates, are doing better. For that matter, the closest living relatives of primates (apart from colugos and tree shrews) are rodents, who are doing better still, mostly by reproducing faster than predators can eat them.

So big brains aren’t quite the ticket to evolutionary success that, say, flight has been for birds. One issue for apes may be that with primate rules for brain growth – double the brain size means double the neurons means double the energy cost – a large-bodied, large brained primate (i.e. an ape) is going to face a serious challenge finding enough food to keep its brain running. It’s not until a later evolutionary period that one lineage of apes really overcomes this problem, with a combination of better physical technology (stone tools, fire) and better social technology (enlisting others to provision mothers and their dependent offspring).

The bottomlands

5.31 – 5.03 million years ago

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 the Sahara 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 (about 900 meters) 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 (about 100 meters) 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.

And here’s a completely unrelated post about bottoms.

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 are more tolerant of low carbon dioxide levels, using 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 a few years back.

Two roads diverged

7.88 – 7.46 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.