It’s alright, they haven’t developed complex society and a Communist agenda while we haven’t been watching.
What’s it all about then, I hear you ask. Click below!
The Chimpanzee Referential Doctrine.
These are the papers I’m basing this post on, from the 2012 volume of Annual Review of Anthropology:
-  Sayers, K., Raghanti, M. A., and Owen Lovejoy, C. (2012). Human evolution and the chimpanzee referential doctrine. Annual Review of Anthropology, Vol. 41: pp. 119 -138.
-  Stanford, C., B. (2012). Chimpanzees and the behavior of Ardipithecus ramidus. Annual Review of Anthropology, Vol. 41: pp. 139 -149.
Unfortunately, these are not freely available but I’ll do my best to do them justice in what follows.
This is going to be a 2-parter so keep an eye out tomorrow too!
The Clapped-Out Referential Doctrine
This is essentially the stance that Sayers, Raghanti and Lovejoy take in their paper.
Chimpanzees have been used as the ideal model for our early ancestors because although they are our closest living relatives, we are ‘clearly’ ‘more evolved’ than them. Can they drive cars? No. Can they even build a car? Still no. The old belief was that chimps have always been knuckle walkers, climbed vertically up trees and swung around below the branches. We know this is true now because we can see them do all of these things, even in a zoo.
More to the point, because gorillas diverged from the chimp/human lineage 7 or 8 million years ago (ref. ) and knuckle walk in the same way as chimps, it must have been a common characteristic of both lineages for at least 8 million years, and when we split from chimps, we grew out of it. All unique traits that we have evolved today in terms our skeleton and locomotion have been modified from a chimpanzee body plan. For example, as humans we hold our body upright because our chimp-like ancestors did so when they hung by their hands from branches.
In this view, gorillas are simply larger versions of chimps and have become too heavy to climb trees at all.
This Chimp Referential Doctrine allows us to make design very easy experimental comparisons of skeletal anatomy between chimps and humans because we can see exactly what has changed and what effect this had. That is why the CRD has been firm favourite for a long, long time.
Come to Realise the Difficulties
The problem comes when you realise that chimpanzees have been evolving for just as long as we have, only along a different path. There’s roughly 6 million years have passed since chimps and humans the Last Common ancestor of humans and chimps was around. So that’s 6 million years of evolution to produce humans, and 6 million years to produce the 2 chimpanzee species.
As you may have gathered, a lot can change in 6 million years. For both chimpanzees and humans.
The recent (year 2000 onwards) discovery of very early specimens such as Orrorin tugensis (Wikipedia) and Sahelantropus tchadensis (Wikipedia) have ruffled some feathers in the world of palaeoanthropology. Despite dating from right around the time of the gorilla-chimp/human and human-chimp split, they are unlike any apes alive today. All the available evidence points to knuckle walking evolving separately in chimps and gorillas, while the last common ancestor of chimps and humans was not especially like either a modern human or a modern chimpanzee.
Teeth from a slightly later specimen (Ardipithecus ramidus; Wikipedia) suggest a much more generalised and omnivorous diet than the largely ripe-fruit based diet of modern chimps.
Looking at limb proportions, Sayers et al. argue that the forelimb/hindlimb ratio in A. ramidus (89-91%) is closer to that of humans (70-76%) than of chimps (105-110%). That is, a person’s arm is about 73% the length of their leg. (See also Fig. 1)
I would contest that A. ramidus‘ limb ratio is actually closer to that of a chimpanzee but I’ll give Sayers and the others the benefit of the doubt. Let me explain. They go on to say that the last common ancestor was more likely to move about on all fours on top of branches in contrast to a chimp, and much more similar to an Old World monkey, whose limb proportions all seem to be 100% or lower (ref. ). That is, we are descended from an above-branch four-legged walker.
A suite of other traits are in this table.
Well, I’d love to show you this table but apparently the University of York don’t pay the people at Annual Reviews any money so I can’t see this paper any more either.
How do I know what I’m talking about then? I must have downloaded the pdf files a few weeks ago when my Durham login still worked. Durham clearly keep up with their bills.
Sorry about that, you’ll just have to trust me that it’s a very long table with lots of features which mostly point to a non chimp-like ancestor.
There’s another one of those tables too, this time listing a load of behavioural differences and differences in soft tissue arrangement between humans and chimps and how this would relate to A. ramidus, even though soft tissue doesn’t fossilise.
Consider the knuckle walking trait, which chimpanzees use to move about on the ground.
On the face of it, they are perfectly adapted for a life in the trees, swinging from branch to branch using their arms. Who in the world would think that chimps spent so much of their time on the ground and only really go up trees to feed, sleep and occasionally hunt monkeys, just by looking at their skeleton? They have long arms, an elongated hand perfect for hanging from, and grasping feet with opposable big toes. No way should a fossilised creature that looks like this spend any time on the ground. But chimps do.
It is curve balls like this that are almost impossible to see in the fossil record.
Others include displays by females to coincide with periods of fertility (chimps do this, humans don’t), and the structure of society ranging from gorilla harems, through chimp multi-male hierarchies and human multi-male (relative) equality. Comparisons are based on DNA analysis, not much of which is currently available for A. ramidus, so findings are pretty speculative at the moment.
The authors conclude that because it is near-impossible to determine which of these traits were present in 6 million year-old apes, it is no use trying to draw meaningful conclusions on behaviour or sexual appearance from modern apes. Maybe if we find a specimen with soft tissues preserved we’ll be able to answer these questions. How likely is that? Very, very not likely, I’m afraid.
Out Of Options?
Sayers and his colleagues advocate drawing together our knowledge of general evolutionary principles as we’ve worked them out in relation to other species and apply them in the context of what we do know about the environment at the time, and about the final outcome of evolution i.e. their extant descendants. This is likely to give a much more rounded theory.
As it brings together lots of different ideas, this is known as a conceptual model.
All in all, a pretty scathing attack on the Chimp Referential Doctrine.
(More to follow tomorrow!)
HERE IT IS:
Time for the Stanford paper to feel the heat of my sharp-edged analytical sword. Okay, so that’s a strained analogy and probably pretty inaccurate to boot. But what the hey – let’s have a look.
Clinging to the Referential Doctrine
Immediately, Stanford acknowledges that certain aspects of behaviour are difficult to infer from the fossil record, he goes on a bit about knuckle walking too. Here, though the similarities end.
Launching straight into an attack on conceptual models advocated in yesterday’s article, Stanford deems that pulling together ideas gained from studying diverse and unrelated taxa is likely to produce a model which contradicts itself a number of times, especially regarding features such as knuckle walking, or violence which has a variety of triggers and is not necessarily deeply rooted in an animals genes for all time.
One of the dangers, he says, is that these problems may be hidden by a lovely glossy narrative flowing over the top which causes us to accept it without even thinking.
You may notice that this is the same problem that the Sayers group find with the referential doctrine – it is very easy to just compare everything to a chimp because we know what it looks like.
Stanford even goes on to say that there is no real distinction between conceptual and referential models, since a conceptual model requires knowing a lot about each of the reference species included in the conceptual model.
He further talks down the utility of his own favoured referential doctrine by saying that up to now, most models of A. ramidus behaviour have been based on a suspect reconstruction of a badly crushed fossilised pelvis, and that just small changes in the way its reconstructed could lead to drastically different interpretations of behaviour.
So just what is Stanford saying that a referential doctrine is useful for?
Claiming a Reduced Doctrine
Acknowledging that certain aspects of behaviour are difficult to gather from fossils, and are more open to change over evolutionary time (it’s likely easier to change behaviour than it is to change large-scale [gross] anatomy), Stanford instead proposes that modern reference taxa should be used to provide limits and guidelines. Parallels can be drawn between chimps and early hominids because of their geographic range and the environment in which they live(d). Modern chimps also use both trees and the ground to move about, show little difference in size between the sexes and live in a multi-male society (albeit one that may be slightly more violent than that of early hominids).
To deal with the issue of moving about, although early hominids did not knuckle walk we do know that both chimps and hominids used both trees and the ground. Given that they also inhabit similar environments, we can speculate that they spent similar amounts of time using each, possibly even for the same purposes: trees to eat and sleep, the ground for a large part of moving about during the day.
Stanford’s interpretation of the modern chimp diet is also different from that of Sayers’ team, as Stanford acknowledges that chimps do include animal protein in their diet. This mainly comes from rodents and occasionally from small monkeys, in an opportunistic manner. We are fairly confident that A. ramidus was similarly omnivorous, so perhaps they initially included animal protein when the fates allowed it too. We obviously took up the practice a bit more keenly to end up in our present-day situation of the meat-loving ape.
Conceptual vs. Referential Doctrine.
So I’m left in a position where I can see the merit of both sides. I agree that a conceptual model is useful because you can apply as much knowledge as possible to try to get a rounded picture. I like using knowledge. But I can also see Stanford’s point about the danger of such models being inconsistent within themselves, and requiring reference species with which to construct the wider conceptual model.He even talks down his own side of the argument like a good scientist should; he acknowledges the shortcomings of his own work, and argues in favour of a limited function of referential models.
But wait! Sayers’ article argues in favour of critically analysing and applying what knowledge you have in the context of your fossils. Stanford argues that reference models that apply all aspects of a reference species to your fossil of interest are not useful. If you combine stanford’s limited reference models, you get a conceptual model. after all, what’s the use of only using a single limited reference species and then saying ‘oh, we can only tell you a very small amount because our reference species isn’t wholly applicable’?
USE ANOTHER SPECIES TO PROVIDE A MORE WHOLESOME IMAGE!
Wow. Erm, sorry about that. As I was trying to say, i think that these two articles essentially agree but are approaching the problem from different angles; Sayers saying what’s bad about referential models and advocating a higher-level compromise, while Stanford is clinging to find the useful bits of referential models.
But what do you think? Maybe I’m not placing enough importance on the fact that internal inconsistencies in conceptual models can put you on completely the wrong track.
I welcome your thoughts in the comments!