The latest installment in the Star
Wars franchise, The Force Awakens, opens on December 18, and is expected to be
the movie event of the year! To get the hype machine running at hyper speed,
the merchandizing onslaught has begun. New Star Wars merchandise is everywhere:
lightsabers, bed sheets, shower curtains, action figures, r/c droids, breakfast
cereals, flamethrowers (okay… no flamethrowers).
My personal favorite is the menu of
Star Wars-themed sandwiches sold by a local
food truck, including the ever-so-delectable
Chuebakka. And as I sat eating my Chuebakka for lunch, I couldn’t help wonder –
as only a science geek would – if Chewbacca would eat this sandwich in the same
way that I do.
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| Chewbacca meets the "Chuebakka" |
Chewbacca is a wookiee from the
jungle planet Kashyyyk. Wookiees are known for their high intelligence, brute
strength, unwavering loyalty, and short tempers. According to Wookieepedia, George Lucas took his inspiration to create Chewbacca (and
the rest of the wookiees) from his dog, Indiana, who would ride shotgun in his car. As revealed in an
episode of Animal Planet’s Animal Icons (“Star Wars Creatures”), orangutans and
other suspensory primates also inspired Lucas’ creation. And the link to
primates, whether living, extinct, or legendary, is
undeniable. They are a species of tall, human-like bipeds (i.e., they walk one
two legs) with forward-facing eyes, a keen sense of smell, sharp teeth, and opposable
thumbs. Because they are a species of tree-climbers, they also possess a number
of mammalian anatomical specializations that enable them to live an arboreal
lifestyle, including disproportionately high strength for their body size, well-developed
back and calf muscles, and retractable claws.
The phylogeny or evolutionary relatedness
among animal groups can be ascertained by examining their shared ancestral
traits relative to their shared derived traits. For example, primates and dogs
are both considered mammals because they share a number of derived traits.
Among the most well known of these is the presence of mammary glands that
produce milk for newborns to suckle. Additional traits include the presence of
hair, sebaceous (oil) glands in the skin, and rearranged accessory bones of the
jaw, including those that became the bones of the middle ear: incus, stapes,
and malleus.
Evolution, and the paleontology
through which it is interpreted, is a historical science. This means that the
inferences one can make about the relatedness of one species to another are
rooted in the fact that life on Earth has changed over time and the different
species that make up Earth’s biodiversity share common ancestry.
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| Phylogeny of the vertebrates. Each node connecting two branches represents the common ancestor of the two descendent taxa. Major character transformations are indicated on the stems leading up to the nodes. For example, this phylogeny indicates that amniotic eggs evolved in the common ancestor of primates, rodents/rabbits, crocodiles, and dinosaurs/birds; hair, however, evolved sometime after amniotic eggs but prior to the common ancestor of primates and rodents/rabbits. Figure taken from University of California Museum of Paleontology's Understanding Evolution (http://evolution.berkeley.edu). |
In the 19th century,
geologist Charles Lyell recognized that processes that alter the Earth’s
landscape are gradual, and he postulated that these processes are uniform
through time. This idea has since been validated, as we know that plate
tectonics alter the shape of the Earth’s crust by small amounts each year, and
that over vast stretches of time (millions of years), these small incremental
changes leave the landscape looking profoundly different. Lyell’s understanding
of the geologic record is known as the Principle of Uniformitarianism, and it was profoundly influential to the writings of
Charles Darwin as he formulated his thoughts on biological descent with
modification (the underpinning of natural selection) while he sailed around the
world aboard the HMS Beagle.
Assuming that the principle of
uniformitarianism can be applied a long time ago in a galaxy far, far away, we are
presented with an opportunity to examine whether wookiees are in fact more
dog-like or more primate-like by looking closely at the morphological traits
that unite them relative to their own uniquely derived traits. And of course, whether
they are more dog-like or primate-like has important implications for whether
Chewbacca would eat a Chuebakka like I do. (Truth be told, my behavior is
probably better described as more pig-like than primate-like when I eat this
sandwich, even though my anatomy is undoubtedly primate).
The genera and species of the mammalian
order Primates (pronounced pry-may-tees)
are defined morphologically by the possession of large brains relative to body
size, stereoscopic vision (forward facing with overlapping visual fields),
reduced sense of olfaction (smell) compared to other mammals, and prehensile
hands and feet, which includes opposable thumbs and big toes in most species. The
reduced sense of smell in primates is considered an evolutionary tradeoff of
having a large brain and stereoscopic vision; primates are more reliant on
their sense of vision than their sense of smell in order to find food and avoid
predators. Most primates also have nails instead of claws on all or most of
their digits, and the ones that retain claws on a digit or two use them
primarily for grooming.
Dogs are members of the family
Canidae, which belongs to the mammalian order Carnivora. Within Carnivora, there are two distinct suborders: Caniformia,
which includes those animals that are more dog-like: bears, weasels, badgers,
raccoons, and pinnipeds (seals, sea lions, and walruses); and Feliformia, which
includes those animals that are more cat-like: cats, hyenas, aardwolves,
civets, genets, and mongooses. The defining characteristics of the Canivora
include a number of very specific morphologies of the skull, like how and where
certain bones of the skull are fused to the rest of the skull. Among the
Caniformia, the canids exhibit highly developed senses of smell, as evidenced
by extensive sensory mucosa covering a series of intricate turbinal bones in
the nasal cavity (as mentioned above, this is significantly less developed in
primates), and their claws are most definitely non-retractable. Like most
carnivorans, canids have specialized shearing blades, also called carnassials,
on their lower first molars and upper fourth premolars, which are highly
effective at slicing tough food objects like meat.
So, looking back on the descriptions
of wookiee anatomy detailed at Wookieepedia (the definitive authority, after
all), it appears that wookiees possess a mosaic of derived traits that align
them with both primates and dogs. This is not surprising as both of these
groups inspired Lucas’ imagination when he created Chewbacca. However, the
preponderance of evidence suggests that wookiees are more similar in morphology
to primates than they are to dogs. On the one hand/paw, they are arboreal, bipedal, and
have stereoscopic vision – all primate characteristics. On the other hand/paw,
they have a strong sense of smell, sharp pointed teeth, and claws instead of
nails – all canid features. Yet, wookiee claws are retractable, which is a shared,
derived trait present only in the carnivoran family Felidae (cats, Feliformia),
and so it is not a canid trait. With the majority of evidence suggesting a
closer link to primates than to canids, the wookiee retractable claw is clearly
a derived trait, as they are not present in any canid common ancestors, at
least as we understand mammalian evolution on Earth. Obviously Kashyyyk would
have a completely different evolutionary history…but hey, who’s counting?
So then, operating within the context
that wookiees are more primate-like than dog-like, it is now possible to
examine whether or not Chebacca’s chuebakka would undergo the same process of
ingestion and deglutition (swallowing) as mine. Spoiler alert: probably not!
Primate vocal anatomy and physiology
have been described in greater detail in a previous entry at The 'Scope. But
a brief review here is in order. In the neck, there are two passageways: one
for air and one for food. The airway (yellow line/arrow) begins in the nose and
mouth and ends after traveling through the pharynx by passing anteriorly to the
laryngeal cartilages and into the trachea and lungs. The foodway (blue
line/arrow) begins in the mouth and ends after also traveling through the
pharynx by passing posteriorly to the esophagus and into the stomach. Notice
that the two passageways cross each other. And then remember that it “could
be bad” when the streams are crossed.
During a normal swallow, the hyoid bone
and larynx move closer to the base of the skull (higher in the neck), while the
epiglottis attached to the top of the thyroid cartilage (larynx) folds/flips
backward to direct food toward the esophagus (digestive tract) rather than into
larynx (respiratory tract).
In most
living mammals, including non-human primates, the hyoid and larynx are positioned high
in the neck (very close to the head) throughout postnatal development, as
are their connections to most of their contiguous structures such as the tongue,
pharyngeal constrictor muscles, and
the supra- and infra-hyoid strap
musculature
that moves the hyolaryngeal complex during swallowing. This configuration
enables the epiglottis (blue
structure) to form a seal with the soft palate (pink structure),
safeguarding against accidental aspiration of
a bolus of food. This means that most mammals can swallow and breathe at
the same time without choking. In humans,
these structures are positioned similarly high in the neck at birth (diagram on left), but they descend gradually during postnatal development (diagram on right), reaching
a unique position further down in
the neck than in any other mammal
(and perhaps wookiees?).
The caudal position of the human hyoid and
larynx creates a supralaryngeal vocal tract, which forms two dynamically functioning and equal sized
"tubes" – a horizontal tube extending from the posterior pharyngeal
wall to the lips, and a vertical
tube extending from the vocal folds to the soft
palate. The ontogenetic descent of the hyoid and larynx in humans, along with the ability of the tongue to alter the cross-sectional area of both of
these “tubes”, enables production of a wide range of vocalizations necessary for the evolution of quantal
speech (which we know wookiees cannot produce). However, this adaptation comes at an expense to the swallowing function, as the lower position of the hyo-laryngeal complex in the neck prevents the epiglottis
from forming a seal with the soft palate. Thus, the risk
of aspirating a food bolus increases markedly in humans, making coordination between respiration and swallowing
even more critical to survival.
So back to the original question… would Chewbacca have ingested and processed his Chuebakka in the same manner that I do? Well… it seems unlikely. It is well-documented (again on Wookieepedia) that wookiees speak a number of dialects that do not require vocal nuance, and due to their unique vocal apparatus, they are incapable of speaking Galactic Basic Standard, the most prevalent and most used language in the galaxy. If wookiees have a hyo-laryngeal complex that is situated high up in the neck near the base of the skull, morphology shared by all non-human primates, it would suggest that they are capable of swallowing and breathing simultaneously. This in turn would suggest that while I would have to wait to swallow chuebakka before alerting the world (or those immediately nearby) about how great it tastes, Chewbacca himself could swallow and breathe and presumably roar. At. The. Same. Time. And this makes me insanely jealous.
Contributed by: Jason Organ, Ph.D.
Follow Jason on Twitter.
Laitman JT, & Reidenberg JS (1997). The human aerodigestive tract and gastroesophageal reflux: an evolutionary perspective. The American journal of medicine, 103 (5A) PMID: 9422615
Lieberman DE, McCarthy RC, Hiiemae KM, & Palmer JB (2001). Ontogeny of postnatal hyoid and larynx descent in humans. Archives of oral biology, 46 (2), 117-28 PMID: 11163319
Crompton AW, German RZ, & Thexton AJ (2008). Development of the movement of the epiglottis in infant and juvenile pigs. Zoology (Jena, Germany), 111 (5), 339-49 PMID: 18387794
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