Tag Archives: Miocene

Toumaï

6.65-6.30 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) would be 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.

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Two roads diverged

7.93-6.66 million years ago

There was a lot of hullabaloo last year 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 6.65, and a confidence interval of 7.07 to 6.23 million years, based on 72 studies.

Stories of O

9.8-9.3 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 foot

There’s no reason to think Oreopithecus was close to the human line. If it’s true that it was a biped, this suggests that several versions of bipedalism evolved independently as solutions to the problem of how does an arm-hanger get around on the ground – knuckle-walking being another solution.

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 Oreopithecus went extinct.

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 surviving species 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

  • Snackwells cookies
  • Jelly beans prominently labelled “No fat.”
  • A fellow grad student laughing about an older relative who said bread and pasta can 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 Mya) 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.

Land of the thoats

17.2-16.3 million years ago

There’s a great expansion in the diversity of horses in the mid-Miocene, especially horses adapted to grazing rather than browsing. The shift to grazing is goes 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.)

thoat

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.