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psychoceramics: The Comparative Anatomy of Eating

The  Comparative  Anatomy  of  Eating  
by  Milton  R.  Mills,  M.D.  
Humans  are  most  often  described  as  "omnivores".    This  
classification  is  based  on  the  "observation"  that  humans  
generally  eat  a  wide  variety  of  plant  and  animal  
foods.    However,  culture,  custom  and  training  are  
confounding  variables  when  looking  at  human  dietary  
practices.    Thus,  "observation"  is  not  the  best  
technique  to  use  when  trying  to  identify  the  most  
"natural"  diet  for  humans.  While  most  humans  are  
clearly  "behavioral"  omnivores,  the  question  still  
remains  as  to  whether  humans  are  anatomically  suited  
for  a  diet  that  includes  animal  as  well  as  plant  
A  better  and  more  objective  technique  is  to  look  at  
human  anatomy  and  physiology. Mammals  are  anatomically  
and  physiologically  adapted  to  procure  and  consume  
particular  kinds  of  diets.    (It  is  common  practice  
when  examining  fossils  of  extinct  mammals  to  examine  
anatomical  features  to  deduce  the  animal's  probable  
diet.)  Therefore,  we  can  look  at  mammalian  carnivores,  
herbivores  (plant-eaters)  and  omnivores  to  see  which  
anatomical  and  physiological  features  are  associated  with  
each  kind  of  diet.    Then  we  can  look  at  human  
anatomy  and  physiology  to  see  in  which  group  we  
Oral  Cavity  
Carnivores  have  a  wide  mouth  opening  in  relation  to  
their  head  size. This  confers  obvious  advantages  in  
developing  the  forces  used  in  seizing,  killing  and  
dismembering  prey.  Facial  musculature  is  reduced  since  
these  muscles  would  hinder  a  wide  gape,  and  play  no  
part  in  the  animal's  preparation  of  food  for  
swallowing.    In  all  mammalian  carnivores,  the  jaw  joint  
is  a  simple  hinge  joint  lying  in  the  same  plane  as  
the  teeth.    This  type  of  joint  is  extremely  stable  
and  acts  as  the  pivot  point  for  the  "lever  arms"  
formed  by  the  upper  and  lower  jaws.    The  primary  
muscle  used  for  operating  the  jaw  in  carnivores  is  the  
temporalis  muscle.    This  muscle  is  so  massive  in  
carnivores  that  it  accounts  for  most  of  the  bulk  of  
the  sides  of  the  head  (when  you  pet  a  dog,  you  are  
petting  its  temporalis  muscles).    The  "angle"  of  the  
mandible  (lower  jaw)  in  carnivores  is  small.    This  is  
because  the  muscles  (masseter  and  pterygoids)  that  
attach  there  are  of  minor  importance  in  these  animals.  
  The  lower  jaw  of  carnivores  cannot  move  forward,  and  
has  very  limited  side-to-side  motion.    When  the  jaw  of  
a  carnivore  closes,  the  blade-shaped  cheek  molars  slide  
past  each  other  to  give  a  slicing  motion  that  is  very  
effective  for  shearing  meat  off  bone.  
The  teeth  of  a  carnivore  are  discretely  spaced  so  as  
not  to  trap  stringy  debris.    The  incisors  are  short,  
pointed  and  prong-like  and  are  used  for  grasping  and  
shredding.    The  canines  are  greatly  elongated  and  
dagger-like  for  stabbing,  tearing  and  killing  prey.  The  
molars  (carnassials)  are  flattened  and  triangular  with  
jagged  edges  such  that  they  function  like  serrated-edged  
blades.    Because  of  the  hinge-type  joint,  when  a  
carnivore  closes  its  jaw,  the  cheek  teeth  come  together  
in  a  back-to-front  fashion  giving  a  smooth  cutting  
motion  like  the  blades  on  a  pair  of  shears.  
The  saliva  of  carnivorous  animals  does  not  contain  
digestive  enzymes.    When  eating,  a  mammalian  carnivore  
gorges  itself  rapidly  and  does  not  chew  its  food.  
  Since  proteolytic  (protein-digesting)  enzymes  cannot  be  
liberated  in  the  mouth  due  to  the  danger  of  
autodigestion  (damaging  the  oral  cavity),  carnivores  do  
not  need  to  mix  their  food  with  saliva;  they  simply  
bite  off  huge  chunks  of  meat  and  swallow  them  whole.  
According  to  evolutionary  theory,  the  anatomical  features  
consistent  with  an  herbivorous  diet  represent  a  more  
recently  derived  condition  than  that  of  the  carnivore.  
  Herbivorous  mammals  have  well-developed  facial  
musculature,  fleshy  lips,  a  relatively  small  opening  
into  the  oral  cavity  and  a  thickened,  muscular  tongue.  
  The  lips  aid  in  the  movement  of  food  into  the  mouth  
and,  along  with  the  facial  (cheek)  musculature  and  
tongue,  assist  in  the  chewing  of  food.    In  herbivores,  
the  jaw  joint  has  moved  to  position  above  the  plane  
of  the  teeth.    Although  this  type  of  joint  is  less  
stable  than  the  hinge-type  joint  of  the  carnivore,  it  
is  much  more  mobile  and  allows  the  complex  jaw  motions  
needed  when  chewing  plant  foods.    Additionally,  this  
type  of  jaw  joint  allows  the  upper  and  lower  cheek  
teeth  to  come  together  along  the  length  of  the  jaw  
more  or  less  at  once  when  the  mouth  is  closed  in  
order  to  form  grinding  platforms.    (This  type  of  joint  
is  so  important  to  a  plant-eating  animal,  that  it  is  
believed  to  have  evolved  at  least  15  different  times  
in  various  plant-eating  mammalian  species.)    The  angle  
of  the  mandible  has  expanded  to  provide  a  broad  area  
of  attachment  for  the  well-developed  masseter  and  
pterygoid  muscles  (these  are  the  major  muscles  of  
chewing  in  plant-eating  animals).  The  temporalis  muscle  
is  small  and  of  minor  importance.    The  masseter  and  
pterygoid  muscles  hold  the  mandible  in  a  sling-like  
arrangement  and  swing  the  jaw  from  side-to-side.  
  Accordingly,  the  lower  jaw  of  plant-eating  mammals  has  
a  pronounced  sideways  motion  when  eating.    This  lateral  
movement  is  necessary  for  the  grinding  motion  of  
The  dentition  of  herbivores  is  quite  varied  depending  
on  the  kind  of  vegetation  a  particular  species  is  
adapted  to  eat.    Although  these  animals  differ  in  the  
types  and  numbers  of  teeth  they  posses,  the  various  
kinds  of  teeth  when  present,  share  common  structural  
features.    The  incisors  are  broad,  flattened  and  
spade-like.  Canines  may  be  small  as  in  horses,  
prominent  as  in  hippos,  pigs  and  some  primates  (these  
are  thought  to  be  used  for  defense)  or  absent  
altogether.    The  molars,  in  general,  are  squared  and  
flattened  on  top  to  provide  a  grinding  surface.    The  
molars  cannot  vertically  slide  past  one  another  in  a  
shearing/slicing  motion,  but  they  do  horizontally  slide  
across  one  another  to  crush  and  grind.    The  surface  
features  of  the  molars  vary  depending  on  the  type  of  
plant  material  the  animal  eats.    The  teeth  of  
herbivorous  animals  are  closely  grouped  so  that  the  
incisors  form  an  efficient  cropping/biting  mechanism,  and  
the  upper  and  lower  molars  form  extended  platforms  for  
crushing  and  grinding.    The  "walled-in"  oral  cavity  has  
a  lot  of  potential  space  that  is  realized  during  
These  animals  carefully  and  methodically  chew  their  
food,  pushing  the  food  back  and  forth  into  the  
grinding  teeth  with  the  tongue  and  cheek  muscles.  
  This  thorough  process  is  necessary  to  mechanically  
disrupt  plant  cell  walls  in  order  to  release  the  
digestible  intracellular  contents  and  ensure  thorough  
mixing  of  this  material  with  their  saliva.    This  is  
important  because  the  saliva  of  plant-eating  mammals  
often  contains  carbohydrate-digesting  enzymes  which  begin  
breaking  down  food  molecules  while  the  food  is  still  
in  the  mouth.  
Stomach  and  Small  Intestine  
Striking  differences  between  carnivores  and  herbivores  
are  seen  in  these  organs.    Carnivores  have  a  capacious  
simple  (single-chambered)  stomach.    The  stomach  volume  
of  a  carnivore  represents  60-70%  of  the  total  capacity  
of  the  digestive  system.    Because  meat  is  relatively  
easily  digested,  their  small  intestines  (where  absorption  
of  food  molecules  takes  place)  are  short  --  about  
three  to  five  or  six  times  the  body  length.    Since  
these  animals  average  a  kill  only  about  once  a  week,  
a  large  stomach  volume  is  advantageous  because  it  
allows  the  animals  to  quickly  gorge  themselves  when  
eating,  taking  in  as  much  meat  as  possible  at  one  
time  which  can  then  be  digested  later  while  resting.  
  Additionally,  the  ability  of  the  carnivore  stomach  to  
secrete  hydrochloric  acid  is  exceptional.    Carnivores  
are  able  to  keep  their  gastric  pH  down  around  1-2  
even  with  food  present.    This  is  necessary  to  
facilitate  protein  breakdown  and  to  kill  the  abundant  
dangerous  bacteria  often  found  in  decaying  flesh  foods.  
Because  of  the  relative  difficulty  with  which  various  
kinds  of  plant  foods  are  broken  down  (due  to  large  
amounts  of  indigestible  fibers),  herbivores  have  
significantly  longer  and  in  some  cases,  far  more  
elaborate  guts  than  carnivores.    Herbivorous  animals  
that  consume  plants  containing  a  high  proportion  of  
cellulose  must  "ferment"  (digest  by  bacterial  enzyme  
action)  their  food  to  obtain  the  nutrient  value.    They  
are  classified  as  either  "ruminants"  (foregut  fermenters)  
or  hindgut  fermenters.    The  ruminants  are  the  
plant-eating  animals  with  the  celebrated  
multiple-chambered  stomachs.    Herbivorous  animals  that  
eat  a  diet  of  relatively  soft  vegetation  do  not  need  
a  multiple-chambered  stomach.    They  typically  have  a  
simple  stomach,  and  a  long  small  intestine.    These  
animals  ferment  the  difficult-to-digest  fibrous  portions  
of  their  diets  in  their  hindguts  (colons).    Many  of  
these  herbivores  increase  the  sophistication  and  
efficiency  of  their  GI  tracts  by  including  
carbohydrate-digesting  enzymes  in  their  saliva.    A  
multiple-stomach  fermentation  process  in  an  animal  which  
consumed  a  diet  of  soft,  pulpy  vegetation  would  be  
energetically  wasteful.  Nutrients  and  calories  would  be  
consumed  by  the  fermenting  bacteria  and  protozoa  before  
reaching  the  small  intestine  for  absorption.  The  small  
intestine  of  plant-eating  animals  tends  to  be  very  long  
(greater  than  10  times  body  length)  to  allow  adequate  
time  and  space  for  absorption  of  the  nutrients.  
The  large  intestine  (colon)  of  carnivores  is  simple  and  
very  short,  as  its  only  purposes  are  to  absorb  salt  
and  water.    It  is  approximately  the  same  diameter  as  
the  small  intestine  and,  consequently,  has  a  limited  
capacity  to  function  as  a  reservoir.  The  colon  is  
short  and  non-pouched.    The  muscle  is  distributed  
throughout  the  wall,  giving  the  colon  a  smooth  
cylindrical  appearance.    Although  a  bacterial  population  
is  present  in  the  colon  of  carnivores,  its  activities  
are  essentially  putrefactive.  
In  herbivorous  animals,  the  large  intestine  tends  to  be  
a  highly  specialized  organ  involved  in  water  and  
electrolyte  absorption,  vitamin  production  and  absorption,  
and/or  fermentation  of  fibrous  plant  materials.    The  
colons  of  herbivores  are  usually  wider  than  their  small  
intestine  and  are  relatively  long.    In  some  
plant-eating  mammals,  the  colon  has  a  pouched  appearance  
due  to  the  arrangement  of  the  muscle  fibers  in  the  
intestinal  wall.  Additionally,  in  some  herbivores  the  
cecum  (the  first  section  of  the  colon)  is  quite  large  
and  serves  as  the  primary  or  accessory  fermentation  
What  About  Omnivores?  
One  would  expect  an  omnivore  to  show  anatomical  
features  which  equip  it  to  eat  both  animal  and  plant  
foods.    According  to  evolutionary  theory,  carnivore  gut  
structure  is  more  primitive  than  herbivorous  adaptations.  
  Thus,  an  omnivore  might  be  expected  to  be  a  
carnivore  which  shows  some  gastrointestinal  tract  
adaptations  to  an  herbivorous  diet.  
This  is  exactly  the  situation  we  find  in  the  Bear,  
Raccoon  and  certain  members  of  the  Canine  families.  
  (This  discussion  will  be  limited  to  bears  because  
they  are,  in  general,  representative  of  the  anatomical  
omnivores.)  Bears  are  classified  as  carnivores  but  are  
classic  anatomical  omnivores.    Although  they  eat  some  
animal  foods,  bears  are  primarily  herbivorous  with  
70-80%  of  their  diet  comprised  of  plant  foods.    (The  
one  exception  is  the  Polar  bear  which  lives  in  the  
frozen,  vegetation  poor  arctic  and  feeds  primarily  on  
seal  blubber.)  Bears  cannot  digest  fibrous  vegetation  
well,  and  therefore,  are  highly  selective  feeders.  
  Their  diet  is  dominated  by  primarily  succulent  lent  
herbage,  tubers  and  berries.  Many  scientists  believe  the  
reason  bears  hibernate  is  because  their  chief  food  
(succulent  vegetation)  not  available  in  the  cold  
northern  winters.    (Interestingly,  Polar  bears  hibernate  
during  the  summer  months  when  seals  are  unavailable.)  
In  general,  bears  exhibit  anatomical  features  consistent  
with  a  carnivorous  diet.    The  jaw  joint  of  bears  is  
in  the  same  plane  as  the  molar  teeth.    The  temporalis  
muscle  is  massive,  and  the  angle  of  the  mandible  is  
small  corresponding  to  the  limited  role  the  pterygoid  
and  masseter  muscles  play  in  operating  the  jaw.    The  
small  intestine  is  short  (  less  than  five  times  body  
length)  like  that  of  the  pure  carnivores,  and  the  
colon  is  simple,  smooth  and  short.    The  most  prominent  
adaptation  to  an  herbivorous  diet  in  bears  (and  other  
"anatomical"  omnivores)  is  the  modification  of  their  
dentition.    Bears  retain  the  peg-like  incisors,  large  
canines  and  shearing  premolars  of  a  carnivore;  but  the  
molars  have  become  squared  with  rounded  cusps  for  
crushing  and  grinding.    Bears  have  not,  however,  
adopted  the  flattened,  blunt  nails  seen  in  most  
herbivores  and  retain  the  elongated,  pointed  claws  of  a  
An  animal  which  captures,  kills  and  eats  prey  must  
have  the  physical  equipment  which  makes  predation  
practical  and  efficient.  Since  bears  include  significant  
amounts  of  meat  in  their  diet,  they  must  retain  the  
anatomical  features  that  permit  them  to  capture  and  
kill  prey  animals.    Hence,  bears  have  a  jaw  structure,  
musculature  and  dentition  which  enable  them  to  develop  
and  apply  the  forces  necessary  to  kill  and  dismember  
prey  even  though  the  majority  of  their  diet  is  
comprised  of  plant  foods.    Although  an  herbivore-style  
jaw  joint  (above  the  plane  of  the  teeth)  is  a  far  
more  efficient  joint  for  crushing  and  grinding  
vegetation  and  would  potentially  allow  bears  to  exploit  
a  wider  range  of  plant  foods  in  their  diet,  it  is  a  
much  weaker  joint  than  the  hinge-style  carnivore  joint.  
  The  herbivore-style  jaw  joint  is  relatively  easily  
dislocated  and  would  not  hold  up  well  under  the  
stresses  of  subduing  struggling  prey  and/or  crushing  
bones  (nor  would  it  allow  the  wide  gape  carnivores  
need).    In  the  wild,  an  animal  with  a  dislocated  jaw  
would  either  soon  starve  to  death  or  be  eaten  by  
something  else  and  would,  therefore,  be  selected  
against.    A  given  species  cannot  adopt  the  weaker  but  
more  mobile  and  efficient  herbivore-style  joint  until  it  
has  committed  to  an  essentially  plant-food  diet  test  it  
risk  jaw  dislocation,  death  and  ultimately,  extinction.  
What  About  Me?  
The  human  gastrointestinal  tract  features  the  anatomical  
modifications  consistent  with  an  herbivorous  diet.  
  Humans  have  muscular  lips  and  a  small  opening  into  
the  oral  cavity.    Many  of  the  so-called  "muscles  of  
expression"  are  actually  the  muscles  used  in  chewing.  
  The  muscular  and  agile  tongue  essential  for  eating,  
has  adapted  to  use  in  speech  and  other  things.    The  
mandibular  joint  is  flattened  by  a  cartilaginous  plate  
and  is  located  well  above  the  plane  of  the  teeth.  
  The  temporalis  muscle  is  reduced.  The  characteristic  
"square  jaw"  of  adult  males  reflects  the  expanded  
angular  process  of  the  mandible  and  the  enlarged  
masseter/pterygoid  muscle  group.    The  human  mandible  can  
move  forward  to  engage  the  incisors,  and  side-to-side  
to  crush  and  grind.  
Human  teeth  are  also  similar  to  those  found  in  other  
herbivores  with  the  exception  of  the  canines  (the  
canines  of  some  of  the  apes  are  elongated  and  are  
thought  to  be  used  for  display  and/or  defense).    Our  
teeth  are  rather  large  and  usually  abut  against  one  
another.    The  incisors  are  flat  and  spade-like,  useful  
for  peeling,  snipping  and  biting  relatively  soft  
materials.    The  canines  are  neither  serrated  nor  
conical,  but  are  flattened,  blunt  and  small  and  
function  Like  incisors.    The  premolars  and  molars  are  
squarish,  flattened  and  nodular,  and  used  for  crushing,  
grinding  and  pulping  noncoarse  foods.  
Human  saliva  contains  the  carbohydrate-digesting  enzyme,  
salivary  amylase.    This  enzyme  is  responsible  for  the  
majority  of  starch  digestion.    The  esophagus  is  narrow  
and  suited  to  small,  soft  balls  of  thoroughly  chewed  
food.    Eating  quickly,  attempting  to  swallow  a  large  
amount  of  food  or  swallowing  fibrous  and/or  poorly  
chewed  food  (meat  is  the  most  frequent  culprit)  often  
results  in  choking  in  humans.  
Man's  stomach  is  single-chambered,  but  only  moderately  
acidic.  (Clinically,  a  person  presenting  with  a  gastric  
pH  less  than  4-5  when  there  is  food  in  the  stomach  
is  cause  for  concern.)  The  stomach  volume  represents  
about  21-27%  of  the  total  volume  of  the  human  GI  
tract.    The  stomach  serves  as  a  mixing  and  storage  
chamber,  mixing  and  liquefying  ingested  foodstuffs  and  
regulating  their  entry  into  the  small  intestine.    The  
human  small  intestine  is  long,  averaging  from  10  to  11  
times  the  body  length.    (Our  small  intestine  averages  
22  to  30  feet  in  length.    Human  body  size  is  
measured  from  the  top  of  the  head  to  end  of  the  
spine  and  averages  between  two  to  three  feet  in  length  
in  normal-sized  individuals.)  
The  human  colon  demonstrates  the  pouched  structure  
peculiar  to  herbivores.    The  distensible  large  intestine  
is  larger  in  cross-section  than  the  small  intestine,  
and  is  relatively  long.    Man's  colon  is  responsible  
for  water  and  electrolyte  absorption  and  vitamin  
production  and  absorption.    There  is  also  extensive  
bacterial  fermentation  of  fibrous  plant  materials,  with  
the  production  and  absorption  of  significant  amounts  of  
food  energy  (volatile  short-chain  fatty  acids)  depending  
upon  the  fiber  content  of  the  diet.    The  extent  to  
which  the  fermentation  and  absorption  of  metabolites  
takes  place  in  the  human  colon  has  only  recently  begun  
to  be  investigated.  
In  conclusion,  we  see  that  human  beings  have  the  
gastrointestinal  tract  structure  of  a  "committed"  
herbivore.    Humankind  does  not  show  the  mixed  
structural  features  one  expects  and  finds  in  anatomical  
omnivores  such  as  bears  and  raccoons.    Thus,  from  
comparing  the  gastrointestinal  tract  of  humans  to  that  
of  carnivores,  herbivores  and  omnivores  we  must  conclude  
that  humankind's  GI  tract  is  designed  for  a  purely  
plant-food  diet.  
Facial  Muscles  
CARNIVORE:  Reduced  to  allow  wide  mouth  gape  
HERBIVORE:  Well-developed  
OMNIVORE:   Reduced  
HUMAN:  Well-developed  
Jaw  Type  
CARNIVORE:  Angle  not  expanded  
HERBIVORE:  Expanded  angle  
OMNIVORE:  Angle  not  expanded  
HUMAN:  Expanded  angle  
Jaw  Joint  Location  
CARNIVORE:  On  same  plane  as  molar  teeth  
HERBIVORE:  Above  the  plane  of  the  molars  
OMNIVORE:  On  same  plane  as  molar  teeth  
HUMAN:  Above  the  plane  of  the  molars  
Jaw  Motion  
CARNIVORE:  Shearing;  minimal  side-to-side  motion  
HERBIVORE:  No  shear;  good  side-to-side,  front-to-back  
OMNIVORE:  Shearing;  minimal  side-to-side  
HUMAN:  No  shear;  good  side-to-side,  front-to-back  
Major  Jaw  Muscles  
CARNIVORE:  Temporalis  
HERBIVORE:  Masseter  and  pterygoids  
OMNIVORE:  Temporalis  
HUMAN:  Masseter  and  pterygoids  
Mouth  Opening  vs.  Head  Size  
Teeth:  Incisors  
CARNIVORE:  Short  and  pointed  
HERBIVORE:  Broad,  flattened  and  spade  shaped  
OMNIVORE:   Short  and  pointed  
HUMAN:      Broad,  flattened  and  spade  shaped  
Teeth:  Canines  
CARNIVORE:  Long,  sharp  and  curved  
HERBIVORE:  Dull  and  short  or  long  (for  defense),  or  none  
OMNIVORE:  Long,  sharp  and  curved  
HUMAN:  Short  and  blunted  
Teeth:  Molars  
CARNIVORE:  Sharp,  jagged  and  blade  shaped  
HERBIVORE:  Flattened  with  cusps  vs  complex  surface  
OMNIVORE:  Sharp blades  and/or  flattened  
HUMAN:  Flattened  with  nodular  cusps  
CARNIVORE:  None;  swallows  food  whole  
HERBIVORE:  Extensive  chewing  necessary  
OMNIVORE:  Swallows  food  whole  and/or  simple  crushing  
HUMAN:  Extensive  chewing  necessary  
CARNIVORE:  No  digestive  enzymes  
HERBIVORE:  Carbohydrate digesting  enzymes  
OMNIVORE:   No  digestive  enzymes  
HUMAN:      Carbohydrate  digesting  enzymes  
Stomach  Type  
CARNIVORE:  Simple  
HERBIVORE:  Simple  or  multiple  chambers  
OMNIVORE:  Simple  
HUMAN:  Simple  
Stomach  Acidity  
CARNIVORE:  Less  than  or  equal  to  pH  1  with food  in stomach  
HERBIVORE:  pH  4  to  5  with  food  in  stomach  
OMNIVORE:   Less  than  or  equal  to  pH  1  with  food  in stomach  
HUMAN:  pH  4  to  5  with  food  in  stomach  
Stomach  Capacity  
CARNIVORE:  60%  to  70%  of  total  volume  of  digestive  tract  
HERBIVORE:  Less  than  30%  of  total  volume  of digestive  tract  
OMNIVORE:  60%  to  70%  of  total  volume of  digestive  tract  
HUMAN:  21%  to  27%  of  total  volume  of  digestive  tract  
Length  of  Small  Intestine  
CARNIVORE:  3  to  6  times  body  length  
HERBIVORE:  10  to  more  than  12  times  body  length  
OMNIVORE:  4  to  6  times  body  length  
HUMAN:  10  to  11  times  body  length  
CARNIVORE:  Simple,  short  and  smooth  
HERBIVORE:  Long,  complex;  may  be  sacculated  
OMNIVORE:   Simple,  short  and  smooth  
HUMAN:      Long,  sacculated  
CARNIVORE:  Can  detoxify  vitamin  A  
HERBIVORE:  Cannot  detoxify  vitamin  A  
OMNIVORE:   Can  detoxify  vitamin  A  
HUMAN:      Cannot  detoxify  vitamin  A  
CARNIVORE:  Extremely  concentrated  urine  
HERBIVORE:  Moderately  concentrated  urine  
OMNIVORE:  Extremely  concentrated  urine  
HUMAN:  Moderately  concentrated  urine    

CARNIVORE:  Sharp  claws  
HERBIVORE:  Flattened  nails  or blunt  hooves  
OMNIVORE:  Sharp  claws  
HUMAN:  Flattened  nails  

Ernest and politically incorrect

"Few of our ancestors were perfect ladies
and gentlemen; the majority, in fact, 
weren't even mammals." -Robert Anton Wilson