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.
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.
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.)
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 can track down sources if you want.
But be careful! There have been some big recalibrations of DNA dates lately that are especially important for human evolution. Human beings and chimpanzees differ at about 1.2% of their DNA sites. Humans and gorillas (and chimps and gorillas) at about 1.6%. Humans and orangutans (and chimps and orangutans, and gorillas and orangutans) at about 3.1%. DNA divergence accumulates a fairly steady rate, so this level of divergence has been used to estimate a chimp/human split around 7-6 million years ago.
But lately we’ve got direct estimates of DNA divergence rates, based on measuring mutation rates in living populations, instead of estimates based on calibrating genes with the fossil record. These suggest that the DNA “clock” is running more slowly than we thought. A report from just three years back based on the new estimates puts the chimp/human split around 13 million years. But there’s disagreement how reliable the new methods really are.
In a sense it may not be that big a deal knowing when the human lineage (the hominins) split from our closest living relatives, the chimpanzees (including bonobos). It’s possible that even after the split the two species looked similar for a long time before hominins started on a distinctive adaptive pathway – in particular, before they started being bipedal. Still, it would be interesting to know. The fossils put some kind of lower bound on the time of the split. – probably at least 6 million years ago. But the upper bound is now in dispute.
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.)
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.
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.
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.
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. It’s easier to tell 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:
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.