It is not in the least bit controversial to picture humans* within the context of the placental mammal group that we belong to, the primates. Nor is it unusual for primatologists, anthropologists or biologists of other sorts to compare the anatomy, social or sexual behaviour, lifestyles or cognitive abilities of humans with those of other primates (e.g., Morris 1967, Sarmiento 1995, 1998, de Waal 2002). Despite this, Homo sapiens is often missed out or ignored when authors review the living primates of the world**, a perhaps understandable omission given that these works are about ‘other’ animals, but a misleading one given that it really helps to see the human lineage as part of the group; as an important and conspicuous part of primate diversity.
* ‘Human’ as used throughout this article denotes Homo sapiens alone. When H. sapiens and its extinct relatives are being referred to, the term ‘hominin’ is used.
** I should note that several books that cover primate diversity do include our species within the roster: Walker’s Mammals of the World does, so does Redmond’s The Primate Family Tree and Dunbar and Barrett’s Cousins: Our Primate Relatives, for example.
In many respects, Homo sapiens is an ‘extreme’ primate. Our vocal abilities, cultural diversity and application of technology (this includes the use of fire) exceeds that of other primates, and it can be argued that it is unparalleled technological innovation that has enabled us to pursue lifestyles – those involving agriculture, livestock management, or regular travel at sea, for example – that are so different relative to those of other primate species. In many respects, however, we aren’t unusual at all and fit within the broad spectrum of primate diversity: our substantial anatomical and cultural diversity is what we would predict for a widespread primate that occurs in numerous habitats, many of the anatomical and behavioural traits once considered unique to humans are not unique at all, and our social systems fit within the spectrum present in related primate species. [Images below by Tropenmuseum of the Royal Tropical Institute and Agência Brasil.]
In the following text, I want to explore just a few ideas that arise when we compare ourselves to our close relatives. The more we learn about primate biology, history and evolution, the smaller the gaps become between Homo sapiens and other primate species, and the less unusual we appear. Indeed, most of our traditional ideas about human ‘uniqueness’ reflect the fact that the species that seemingly bridge the gap between us and our closest living relatives are extinct.
Little discussed today is the fact that humans are unusual apes in that we’re actually proportioned more like monkeys. In fact, our superficially ‘monkey-like’ anatomy goes some way to explaining why the idea of a ‘pre-pongid’ divergence of the human lineage was so popular among anatomists prior to the molecular age (see Bowler 1987, Sarmiento 1998). The idea that living (non-human) apes are all somewhat specialised and thus different from whichever species were ancestral to the human lineage has often been mentioned, and as more fossil apes have been discovered it has become clear that hominids and hominid-like hominoids possessed a diversity of body plans, some of which were more ‘monkey-like’ than ‘ape-like’ (e.g., Moyà-Solà et al. 2004).
However, fossils show that hominins evolved from among a radiation of species with 'ape-like' proportions; the idea that the common ancestor of the whole African ape clade was Bonobo-like overall has been proposed (Zihlman et al. 1978), and data from australopithecines shows that some, at least, had ‘ape-like’ long forelimbs and proportionally short hindlimbs, though they didn't have the same skeletal specialisations for climbing or knuckle-walking as extant African apes (McHenry & Berger 1998, Lovejoy et al. 2009, Berger 2013). The evolution of especially long hindlimbs and of forelimbs proportioned more like those of monkeys than apes are therefore hominin innovations evolved somewhere on the branch between certain australopithecine species and the earliest members of Homo.
Let’s get it out of the way: apes are monkeys
Having mentioned monkeys, there’s something we have to get out of the way. The idea that humans are apes is a familiar and generally accepted one among people who follow tree-based thinking. Furthermore, despite the traditional idea that humans are ‘different enough’ from other primates to warrant their own group, we recognise today that Homo, Australopithecus and their close relatives should be united with chimps, gorillas and other great apes in a group that shares a single common ancestor. This group is Hominidae, within which the African ape branch is termed Homininae (the hominines), within which the human branch is termed Hominini (the hominins). Together with their ‘lesser ape’ relatives the gibbons and siamangs (the hylobatids), hominids are part of a more include clade called Hominoidea.
So, humans are apes from the phylogenetic point of view. That is, in scientific parlance, ‘ape’ no longer means ‘non-human hominoid’ (you might argue that, in common parlance, ‘ape’ does indeed mean ‘non-human hominoid’, but common parlance does not dictate best practice). But, if humans are apes it’s time to bite the bullet and admit that humans, and apes as a whole, are also monkeys. Again, common parlance would have it that monkeys are small, typically tailed, and do not include apes but, from a tree-based, phylogenetic point of view, apes are a particular group of large, tailless monkeys.
Like many people, I long resisted vociferously to this idea, insisting that the term ‘monkey’ has to be restricted to non-ape anthropoids (Anthropoidea is the clade that includes New World monkeys and both Old World monkeys and apes). But if birds are dinosaurs, whales are artiodactyls, and humans are apes then, dammit, apes are monkeys. This video from the brilliant AronRa helped sway my opinion, and you can help preach the message with this new t-shirt I’ve recently designed!
Bipedality, orthogrady, hips and hindlimbs
Let’s now look at various anatomical features of humans and see how they compare to those of other primates. For no particular reason we begin with habitual bipedal behaviour, something that occurs more regularly in some non-human primates than used to be thought, a fact that has only become better appreciated as more hours of field observation have been logged, as more species have been intensively studied, and as improved technology has allowed people to better record behaviour. There are several competing ideas on the distribution and early evolution of orthograde (= erect-bodied) bipedalism, with the idea that bipedality evolved in a terrestrial setting from quadrupedal, pronograde (= horizontal-bodied), knuckle-walking ancestors still being supported by some lines of evidence (e.g., Richmond & Strait 2000, Richmond et al. 2001).
However, the presence of both arboreal and terrestrial bipedal behaviour in gibbons and orangutans (Stanford 2006, Thorpe et al. 2006, 2007, b) and the presence of features associated with bipedal behaviour in fossil hominoids like Morotopithecus and Orrorin suggests that proficient bipedal abilities were present in some of the earliest hominids and hence inherited by hominines (the African ape clade) and hominins (australopithecines and Homo). Under this scenario, bipedalism evolved in an arboreal setting, and quadrupedal hominines (gorillas and chimps) evolved from more bipedal ancestors, knuckle walking evolving independently at least twice (see Dainton & Macho 1999).
Note that all of the skeletal features associated with bipedalism in humans – all of them – are present in various non-humans where they also assist in bipedal behaviours, or are associated with vertical climbing or quadrupedality (Sarmiento & Marcus 2000). We thus see such features as short hips, a prominent ischial spine, a wide sacrum, and a femur with a strong bicondylar angle in such taxa as spider monkeys, howler monkeys, orangutans and lorises (Sarmiento & Marcus 2000): the characters concerned are not unique to humans; rather, humans are unique in combining this diverse set of features.
A very special pelt
Humans appear somewhat weird in possessing several anatomical features that look unusual compared to those of other primate species. Perhaps the most famous aspect of human anatomical weirdness concerns our super-fine, especially short body hairs. Our pelage is (in average individuals of our species) so fine that it creates the impression of a naked-skinned appearance. As has been argued several times, humans are not ‘naked’ at all: according to Schwartz & Rosenblum (1981) our hair follicles are distributed at a density expected for an ape of our size.
Exactly what sort of selection led to our reduced pelt has been the source of substantial speculation (and virtually all of it really is just speculation): is it a product of neoteny (Morris 1967), were thermoregulatory pressures paramount (Wheeler 1992), was it to reduce parasite loads (Pagel & Bodmer 2003), or was it something to do with sexual selection (Darwin 1888) or, most radically, adaptation to an amphibious mode of life (Morgan 1997)? Based on the ecological context in which we evolved (tropical woodland-savannah complexes with watercourses and coasts within walking distance) and the presence within our species of sexually dimorphic integumentary features (read on), a combination of sexual selection and thermoregulation seems most likely.
The human face and nose
Other anatomical peculiarities include our hemispherical buttocks and our especially robust, fully adducted hallux (‘big toe’) (‘adducted’ means that it’s been moved to occupy a position close to the midline of the foot: the opposite anatomical term – referring to a position away from the midline – is ‘abducted’).
And yet more weirdness comes from the fact that we are flamboyant animals, our bodies acting as billboards that advertise sexual condition and quality, and our faces possessing features that function in visual display and social behaviour: eyebrows, the white sclera and sometimes blue, grey, green or hazel irides of our eyes, the beards and moustaches of males (in some, not all, populations) and long head hair.
Seen within the context of facial characters present across primates (not in apes alone), none of our facial features are remarkable: pale or unusually coloured eyes, beards and moustaches, pale and dark streaks, bars and circles around and above the eyes, and showy masses of head hair are present across monkeys and apes. The prominent, cartilage-supported, ‘hooded’ human nose looks unusual compared to that of other extant apes and presumably evolved as an aid to thermoregulation, filtration and water control (Elad et al. 1993, Churchill et al. 2004), crucial within the environments we evolved in: its evolution is ‘logical’ in a big-bodied, erect mammal that walks long distances in arid or semi-arid environments.
Our large nose – involving innovations in the form and size of the nasal, ethmoid, sphenoid and maxilla and position of the fleshy nostril openings – can thus be seen as the best solution a short-faced primate could make in adapting to these problems, a poor version of the enhanced noses present in other big mammals of similar environments, but one which occurred in step with pharyngeal shape and hence vocal communication. In the simple sense of what it looks like on the outside, the human nose is, again, not unique within primates but has precedent in the pendulous noses of some cercopithecid monkeys where hooded and pendulous noses almost certainly evolved within the regime of sexual display. While the human face undeniably reflects a complex evolutionary interplay between social signalling, environmental, dietary adaptation and our short-faced, big-brained anthropoid legacy, the general message behind its anatomy is that we are part of the spectrum of elaborate facial forms present across our group.
The human female
It sometimes seems assumed that the features of the human female body associated with sexuality are ‘unusual’ compared to the anatomy of non-human primates. The fact that signs of oestrus are ‘concealed’ by the human body (we don’t produce obvious sexual swellings) has been much discussed and usually taken as evidence that the evolution of our anatomy was driven by the maintenance of pair-bonds, and hence social cohesion, through sex (Lovejoy 1981). Some authors have argued that oestrus shouldn’t be considered ‘concealed’, but – rather – that human females display what looks like ‘permanent oestrus’ (Szalay & Costello 1991). [Photo above by hdptcar.]
Human breasts are not unprecedented when we look at the pectoral display structures of other primates (Geladas Theropithecus gelada, with their brightly coloured array of intermittently-present pink pectoral vesicles, are the classic example [adjacent photo by BluesyPete]), and those of average individuals don’t seem tremendously unusual in size or proportions compared to those of other hominids during parts of their reproductive cycles. Having said that, I don’t think there’s any indication that the breasts of bonobos, gorillas or orangutans play the same role in sexual attraction as they do in humans, and human breasts typically contain more fatty tissue that those of other primates. Most hypotheses put forward to explain the evolution of breasts posit that they are honest indicators of fat reserves and hence advertise reproductive potential in some way. Marlowe (1997) termed this the Nubility Hypothesis. The idea that they might deceive potential partners by enhancing perceived reproductive potential – the so-called Deception Hypothesis (Caro & Sellen 1989) – has also been put forward. [Woman of Willendorf image below by Matthias Kabel.]
The inevitable conclusion of this line of argumentation is that female bodies display a permanent sexual availability that evolved in step with bipedality, that the evolution of breasts and other female secondary sexual characteristics was adaptive as it enhanced access to resources acquired by males and acted as an anti-infanticidal strategy (because males would assume that children were the products of the numerous matings they had had with their ‘permanently available’ partners), and that it links to our reduced pelage (by making females, and sex, ‘sexier’).
However, there are several reasons why this view of humans as creatures that display a mock 'permanent oestrus' might be problematic, or at best over-simplified. One is that the expectation that humans should display oestrus swellings of any sort might be a mistake based on misleading comparisons made with chimps and various monkey species. Pawłowski (1999) argued that humans might lack oestrus swellings as an adaptation to our orthograde bipedality and choice of habitat, but also noted that the idea that swellings were ancestral for our lineage might be erroneous anyway (they aren’t present in gibbons, orangutans or gorillas and, within Hominoidea, might be unique to the Pan lineage).
Another problem with the idea that the female body advertises permanent sexual availability is that it implies or relies on the idea that females are sexually passive and effectively selected by males based on their reproductive potential; it more or less paints a picture of females as home-bound creatures that get protected and cared for by males if they’re plucked from the crowd on the basis of their good genes.
This scenario looks at odds with what we more typically see in other animals, including primates: females select males as mates and typically choose who mates with them and when, and it’s males who are the showy ones whose evolution has been driven by selection to be sexually attractive. A large body of literature suggests that these generalisations apply across primates, including lemurs, macaques, mandrills, orangutans, chimps and others; in fact, several studies emphasise the importance of intrasexual competition among females, female selection of mates, and the adaptive advantage of promiscuity among females (e.g., Smuts 1987, Paul 2002, Drea 2005). This view isn’t without controversy, however, since some workers have argued that female mate choice in primates is fundamentally limited by male aggression, especially in species with strong sexual dimorphism.
As for humans, there’s little doubt that women are acutely aware of the physical appearance and social status of other women, and some studies show that women are highly competitive – at least as competitive as men (Buss 1988) – when it comes to mate attraction. However, the ability of females to exercise choice is known to be limited by social conditions across primates (Keddy-Hector 1992), and if anything is clear from the conventions and constraints of culture, it’s that people don’t usually get to simply pick up who they like.
We therefore have a complex situation where several factors seemingly evolved in tandem: yes, breasts and other features of the human female body do attract male attention and male choice may have contributed to female appearance, but intrasexual competition among females and its link with social standing and hence mate choice may also have been important, perhaps more so. The fact that sexual size dimorphism in hominins is reduced relative to that of other hominids (men are, on average, approximately 15% heaver than women, whereas the size different in gorillas and orangutans is more like 50%) (Larsen 2003, Reno et al. 2003) suggests that a reasonably egalitarian sexual system was ancestral for humans: it was not males in charge all the way.
The human male
To return to that idea that human females are ‘unusual’ as goes our hypothesised ancestral condition, note that the opposite might also be true: male facial and body hair and the bulbous human penis all appear unusual compared to the conditions in other closest relatives. The sometimes thick and extensive beards and moustaches of human males (remember though: extensive facial hair is not a universal male trait of our species) seem to have roles in perceived attractiveness (Dixson & Brooks 2013), a fact which strongly suggests that sexual selection has driven their evolution. Similar claims can be made about other aspects of the male face and jaw.
How does the ornamented male face compare to that of related primates? Of course, comparative data across all the bits of anatomy we’re interested in have yet to be compiled. However, Weston et al. (2007) documented sexual dimorphism in the breadth-to-height ratio of the upper face in Homo sapiens and found the same sort of dimorphism to be present across the hominins they sampled. It is not present, however, in the faces of chimpanzees, leading them to suggest that, while hominins reduced the amount of sexual dimorphism present within their canine teeth, they compensated for it by increasing facial dimorphism.
Again, however, this doesn’t mean that humans are somehow doing anything especially different from what’s seen in other primates, since other forms of facial dimorphism are present in other lineages: in chimps, orangutans, capuchins and macaques, males have proportionally broader faces than females (Weston et al. 2004). This facial breadth is clearly accentuated in male orangutans by the massive cheek flanges they possess. Note, however, that researchers who have searched for this kind of facial dimorphism in certain human populations have failed to find it (Özener 2012) so it may not be universal.
In the end, the bodies of male and female humans both exhibit significant morphological novelties that almost certainly evolved within the context of sexual selection, and we should be perceived as showy primates highly reliant on visual signals of quality. As is the case with other comparisons made between Homo sapiens and other primates, we can better appreciate the evolutionary context of our showiness by looking beyond our nearest relatives – the relatively conservative chimps and gorillas – and at other hominids (orangutans), hominoids (gibbons) and catarrhines (Old World monkeys).
However, never forget that culture and convention are paramount in how humans present themselves to others – the evolutionary pressures that led to our anatomy, behaviour and sexual dimorphism are effectively over-ridden, to a greater or lesser degree, in numerous cultures [adjacent image by Zivya]. Does this mean that they no longer play a role in selection and hence evolution? Well, that's hard to answer. Meanwhile, in other cultures, dress, convention and tradition mean that sexual dimorphism is emphasised or accentuated, a pattern consistent with claims that anatomical and psychological evolution in humans is occurring faster than it was before.
Human social systems are impressively diverse but the existence and maintenance of family groups that belong to extended families and clans have to be considered most typical. Monogamy, polygyny and polyandry are all present in different cultures and it’s worth noting that many stereotypes about divisions of labour between gender groups don’t apply universally across our species. The diversity of social styles and cultures present across Homo sapiens is such a vast subject that I can’t begin to cover it here.
As an addendum to the discussion above about human anatomy and its role in sexual selection, I will end by saying that a major part of our flamboyance comes from the accoutrements we wear or attach to our bodies: tattoos and body paint, piercings and other forms of body modification, jewellery, styled and cut hair, and clothing. Again, the notion that humans have developed extreme versions of trends present within other primates receives support even here, because we know of chimpanzee populations that indulge in ephemeral fashions whereby blades of grass are worn as ear decorations (van Leeuwen et al. 2014). What does, of course, make humans very different from other primates is that, through our use of tools (especially blades), we modify the hair on our heads and bodies, and use clothing to enhance, modify or conceal parts of our bodies.
As I said right at the start of this article, the idea that we might better understand human biology and evolution by making comparisons with other primate species is hardly a novel or innovative idea – it has a long history extending back to Darwin and beyond. And it’s such a vast area with so many facets and relevant areas of investigation that it’s difficult to know where to start. Personally, I think it’s helpful to see us within the context of being ‘just another one of the primates’, but... as for whether this perspective downplays human uniqueness... well, we’re still weird. But then, so are so many of the other primates.
For previous Tet Zoo articles on Old World monkeys and other primates, do check out...
- Zihlman’s ‘pygmy chimpanzee hypothesis’
- Marmosets and tamarins: dwarfed monkeys of the South American tropics
- The amazing swimming Proboscis monkey (part I)
- Nasalis among the odd-nosed colobines or The “Nasalis Paradox” (proboscis monkeys part II)
- Old World monkeys of choice
- De Loys’ Ape and what to do with it
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