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.

Bunches of monkeys

Our descent, then, is the origin of our evil passions!! The devil under form of Baboon is our grandfather.

Charles Darwin, Noteboook M

Maimoun angushti shaitan ast.

(A monkey is the devil’s fingers.)

Tajik proverb

Monkeys and apes are not only exceptionally brainy, but also distinctively social. Most mammals are solitary (apart from mothers and their juvenile offspring of course). Among a minority of mammals, adult males and females set up pairbonds. And some mammals form larger groups. In most cases, however, these are relatively unstructured aggregations: a herd of buffalo is more like a crowd of people than a human community. A handful of mammals – elephants, cetaceans, and the majority of monkeys and apes – form more structured groups, enduring and internally differentiated.

Social evolution is path dependent: primate social organization is affected by ecology, but also has a strong phylogenetic component.  This makes it possible to offer a tentative reconstruction of the stepwise evolution of stable sociality in primates. Here’s an evolutionary tree, showing inferred transitions between solitary living, and multimale/multifemale, unimale/multifemale, and pairbonded groups:

A diagram of the possible evolutionary dynamics looks like this:

And the accompanying story goes like this: about 52 million years ago, the solitary nocturnal ancestor of monkeys and apes switched to being diurnal. This allowed for the exploitation of a whole range of new foods, but it also exposed the ancestor to new forms of predation. The first step in the evolution of monkey and ape sociality, then, was aggregation in multimale/multifemale groups to cut predation risk. At first these groups would have been loosely structured and unstable, but eventually they would have evolved into something like what we see today among most Old World monkeys: stable (sometimes lifelong) networks of relatives and friends, dominants and subordinates, nested within enduring communities.

A later development, going back to 20 million years ago or less, was a shift, among some of these social primates, to unimale/multifemale groups, or pair-living family groups.

The inferred development of structured social groups in primates bears a remote similarity to the evolution of eusociality among social insects. According to current theories, the starting point for eusociality is the development of a defended nest site where females lay eggs and raise offspring. Sometimes an established nest site is so valuable that it’s adaptive for the next generation of offspring to stay on when they mature rather trying to found new nests. The eventual result may be the evolution of a highly structured society, with strong reproductive skew: some nest members specialize in reproduction, others in foraging or defending the nest. The latter may evolve into an obligately sterile caste.

Primates too have developed an intensified, structured sociality as a response to obligate group living. But the parallels with eusocial insects go only so far. The great majority of primates give birth to one offspring at a time. There are no queen bee baboons whelping one vast litter after another and pushing subaltern kin into caring for them. This goes for humans as well. Our species matches the social insects in the scale of cooperation, but we manage this through a complicated dance of coalition-building and reputation management. For a primate, building a honey-bee style superorganism has to be an aspiration rather than a reality.

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).

Blood and brains

Humans are brainy animals. One way to show that is by looking at brain size: our species has the biggest brains, in relation to body size, of any animal. But there’s more to it than that. An earlier post covered the work of Susan Herculano-Houzel. She developed a technique for counting the number of neurons in a brain, or part of a brain. Among most mammals, big animals have a lower density of brain neurons, so they aren’t actually as brainy (measured by neuron number) as you’d think just based on their brain size. Primates however break the usual mammalian rule. Big primates have the same neuron density as little guys, so they really are quite brainy. And humans, with really big brains and (following primate rules) a high density of neurons, stand out even among primates as exceptionally brainy.

This work isn’t much help if we are looking at extinct hominins, when all we’ve got is their fossil skulls. But now there’s some interesting recent research with a new take on the subject. Brains need to be supplied with blood. The more energy they use, the more blood flow is needed. We can now figure out fairly accurately how much blood flow a brain is getting by looking at the size of the hole that lets the carotid artery in through the base of the skull. And then we can apply this technique to look at humans, and at extinct hominins. It turns out that humans are even more exceptional when we look at blood flow to the brain: we’re getting double the flow that you’d expect based on brain size alone.

Early hominins however, Australopithecus and early Homo, aren’t very impressive upstairs, many with less blood flow to the brain than modern apes. Looking at the graph it looks like there are really two grades of brain evolution. In the lower grade, which includes early hominins and modern apes, there is a gradual increase over millions of years. (I’m just guessing here that the ancestors of chimps and gorillas millions of years ago were about as brainy as contemporary hominins, but we’d still like to find more fossils.) And then there is a big leap up to a higher grade with early Homo erectus, and a rapid increase after that. It looks like something major changed with the appearance of Homo erectus, either on the supply side – improvements in food supply making brains more affordable – or on the demand side – a greater fitness payoff to a high energy brain – or both.

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. …”

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.

Even more exotic are octopuses (octopi, octopuses) – easily the smartest invertebrates, solitary creatures with little social life, but very handy with their tentacles. Peter Godfrey-Smith, philosopher and scuba diver gives his take here.

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 and octopus cases suggest that hands (or their near-equivalent) are merely unusual, not absolutely unique to humans and near relations.

Ardi

4.4 Mya

We’ve got earlier hominins – Sahelanthropus, Orrorin tugensis, even an earlier species of Ardipithecus – but Ardi (her nickname, her species is Ardipithecus ramidus) stands out because she left us an exceptionally complete skeleton. 

And she helped to upend a lot of theories based on drawing a straight line between modern chimpanzees and later australopithecines. She was a biped. Her legs tucked under her pelvis, with a short lower spine, and a broad pelvis (all closer to modern humans than to chimps). Her outer foot was adapted for walking on the ground. But her toes were long-ish. And her big toe stood way off to the side: well-adapted for grasping branches. So when she was walking on the ground, she had to use her second toe to push off, instead of – like you and me – her big toe.

Ardi really shifted people to realizing that gorillas and chimps are specialized beasts, specialized in being big apes adept at climbing trees, grasping branches, swinging around, and pulling themselves up by their arms, but paying a price in inefficient knuckle walking when down on the ground. Ardi was taking up a different specialization, still a tree climber (her habitat was woodland, more than savannah), but ambling around (not quite striding yet) two-legged on the ground. 

So the common ancestor of chimps and humans, back before Ardi, apparently didn’t look all that much like either. In some respects she might have been somewhat closer to humans. She might even have been a sometime biped on the ground.

For a great recent summary, including lots of information about Ardi, and also about the politics, academic and otherwise, of digging fossils, check out Fossil Men: The Quest for the Oldest Skeleton and the Origins of Humankind.