Chimpanzees are our closest living relatives. Our lineages split a mere 7 - 14 million years ago. This may be orders of magnitude longer than the entire of human civilisation, but it's only a blink of an eye when it comes to evolution. Whilst we have changed a lot since then, chimp anatomy has remained distinctly ape-like. As such, many have argued they're good models for our earlier, more ape-like ancestors such as Lucy.
At the same time, we've found embarrassingly few chimpanzee fossils. Literally just a handful of teeth from late in their evolution. This makes it difficult for us to confirm if chimp anatomy really would make good models for our ancestors.
Fortunately, we've got a whole bunch of living chimps to examine ( although less than one might like, with less than half a million wild chimps left). So some researchers set out to do just that, conducting the most thorough dissection of chimps ever. Their results confirmed that we are surprisingly similar to our more furry cousins.
Separating man from ape
The research in question started with the dissection of 7 bonobos. Don't worry, they didn't achieve this sample size by going on a bonobo killing spree. Rather, frozen cadavers that had naturally accumulated at the Antwerp zoo (because animals sometimes die) were dissected at the local university. Crucially, this means that they were all (relatively speaking) fresh. As such, this research represents the largest comprehensive dissection of fresh bonobos.
For reference, a bonobo is a species of chimp that split from their common counterparts (literally, the other main species of chimp is called " the common chimpanzee ") around 2 million years ago. Bonobos have a bit of a reputation as the friendly chimp. They're generally less violent than their common counterparts (although not complete pacifists) and seem to be more sexual too. Females are higher ranked in society, often using sex to bond.
Now, at this point I could go into length anatomical descriptions of all the various differences the researchers found between humans and bonobos. But that would be rather long and tedious. Thankfully, the authors did put these differences in handy pictures, highlighting the differences between our species. So I will post those instead.
This slideshow requires JavaScript.
Now, those of you with an astute eye might notice some parts of the anatomy missing. This study only examined the head, neck, and limbs. However, these are also the parts of the body that have undergone some of the most dramatic changes within human evolution. Despite this, there are still fairly few muscle differences between us and the chimp species. In fact, when you tally it all together the researchers only found 28. Of these, only 20 were substantial changes (i.e., the development of a new muscle or loss of an existing one).
Chimp anatomy is ancient
I said this research started with the dissection of 7 bonobos. That was a bit of a lie. The team have actually been dissecting any ape carcass they could get their hands on since 2011. As such, between them and other primate anatomists, we've now got data on oranguatans, gorillas, common chimps, and gibbons.
Crucially, this makes it possible to start investigating how ape muscles have evolved. 166 traits were identified by these dissections. Similarities and differences between these traits were used to help build up a family tree of the apes. For example, humans and both species of chimp shared 3 unique traits not present in any other ape. So we're clearly closely related. But bonobos share a further 2 unique traits with chimps, meaning those two species are more closely related to each other than either is to humans.
This tree rightly points out that there are many differences between chimps and humans. However, most of these are changes that accumulated on the human branch. This would indicate that chimp evolution has been relatively stagnant and they would be more similar to our last common ancestor. So if you want to know what we used to look like, check out a chimp.
As a great aside, the family tree above was generated from this muscle data. It agrees perfectly with family trees based on genetics and other features. This sort of agreement between different data sets is one of the great pieces of evidence for evolution.
Bonobo evolution?
The other big revelation of this muscle family tree can be found on the bonobo line. Don't let the absence of boxes fool you. There were still 7 traits that differed between chimps and bonobos (although only 2 of these are substantial changes). However, none of those changes was unique to bonobos.
Instead, it was the chimps who developed unique features. This means that the differences between chimps and bonobos stem from chimps evolving. Bonobos have remained fairly consistent since the split. Thus, if you want to look for a species that closely represents an ancient chimpanzee, bonobos might be your best bet.
The ancient chimpanzee is also an interesting concept. As mentioned before, what separates that from humans are mostly changes that accumulated on the human lineage. Thus, if you wanted to look at what the first members of the human lineage looked like before any such changes occurred then they would probably look surprisingly similar to ancient chimps. And those ancient chimps would look a lot like bonobos. This could have fairly important implications when looking for models of what our ancestors were like.
But it's worth remembering that this study only examined muscles in certain parts of the body. To suggest that bonobos are identical to our ancestors in all ways is a bit of a reach. So sadly, we can't conclude they were as sex crazed as modern bonobos, simply that their face muscles were probably similar.
This is still an important discovery for understanding our evoltion, even if the headlines aren't quite as attention grabbing as "confirmed: Lucy wanted sum fuk".
References
Diogo, R. & Wood, B. Comparative anatomy and phylogeny of primate muscles and human evolution (Taylor and Francis, Oxford, 2012).
Diogo, R., Molnar, J.L. and Wood, B., 2017. Bonobo anatomy reveals stasis and mosaicism in chimpanzee evolution, and supports bonobos as the most appropriate extant mode l for the common ancestor of chimpanzees and humans. Scientific Reports, 7(1), p.608.
