I assume people like to learn about science for the same reasons I do: to learn about the
world that they live in so that they can understand how it reflects on who and what they are.  I
admittedly feel that when I learn something new about science I am excited, in part,
because I am indirectly learning something new about me.

This essay is dedicated to you.  I may not know you personally, but if I can safely assume
that you are a product of the planet earth, and that you have the ability to read this essay
then I can draw many general, as well as specific, conclusions about you.  Did you know that
you are a eukaryote… a eumetazoan… a bilaterian… a deuterostome?  I will define these,
and other, taxonomic characteristics, in a specific chronological order, to shed light on your
evolutionary heritage.

-The energy and matter in your body came out of the big bang.  The hydrogen atoms within
your body were formed in space by highly energetic light waves.  All of the other atoms
within your body were formed in a star, either by the thermonuclear fusion of lighter atomic
elements or by a supernova.

- You evolved from a protobiont, an inorganic molecular system that eventually gave rise to
the processes of replication and heredity.

-You are a eukaryote. Your cells are eukaryotic, meaning that they are highly complex cells
that evolved from a symbiosis between less complex, yet highly specialized prokaryotic
cells.  Eukaryotic cells can comprise single-celled or multi-cellular organisms, however,
prokaryotic cells comprise only singled celled organisms.  “Karyotic” comes from the greek
word karyon, meaning kernel, which refers to the cell nucleus.  “Pro” means coming before
and so prokaryotes do not have a nucleus.  “Eu,” on the other hand means true, and
eukaryotes contain a true nucleus in each cell.

-You are a heterotroph, this means that you cannot synthesize sunlight or abiotic chemical
energy into a source of calories, you must eat organic compounds to stay alive.  
Fundamentally, this distinguishes us from plants.

- You are an opisthokont, roughly speaking.  Strictly speaking though, you evolved from the
group opisthokonts, which gave rise to all animals and fungi and also many species of
protists (single-celled eukaryotes).  The opisthokonts are distinguished from other early
groups by the existence of a single posterior flagellum.  Interestingly, this flagellum, or
threadlike propeller, has persisted into the human species and expresses itself there on the
tail end of human sperm.

-You are a eumetazoan.  “Metazoan” in this case means animal and “eu” means true.  
Humans, along with most animals, are recognized as “true animals” because they have
organs and are composed of multiple tissue types.  Some animals, recognized as
parazoans, do not have these qualities and they are commonly called sponges.  In fact,
sponges have no internal organs, muscles or nerves and they simply filter water through
their bodies to obtain nutrients.  They represent an intermediary form between animals and

-You are a bilaterian.  Bilaterally symmetrical animals (all land animals, fish, worms…) can
only be divided in half in one way and have both halves be symmetrical.  Many other
animals, such as jellyfish and starfish (which are not really fish) can be divided in half in
many ways (radial symmetry), and both halves will always be symmetrical.  This radial
symmetry came first and we, bilaterians, took advantage of our bilateral symmetry to move
more effectively through the water.  Bilateral symmetry gave us a front end and a back end,
which allowed us a reason to focus our sensory structures at the front.  Nerves accumulated
near the front end to help process sensory stimuli and this is how the earliest brains initially
formed (a process called encephalization).

-You are a coelomate, this means that you belong to a minority of animals whose internal
organs and digestive tract are separated from the rest of the body by an internal cavity (a
coelom) enabling them to move independently of the rest of the body.  This allowed animals
to move, using their muscles, quickly and effectively without disturbing their internal milieu.  
Acoelomate animals do not have this cavity.  Very simple modern, acoelemate worms of
the phylum platyhelminthes, who have muscles, organs, and a bilateral nervous system,
probably approximate a distant acoelomate ancestor of ours.

-You are a deuterostome.  Deuterostomes, another small minority in the animal kingdom,
are differentiated from protostomes by an early developmental milestone.  Early in animal
embryologic development, the ball of cells that is the embryo folds in on itself (gastrulation).  
This folding creates a pocket inside the ball of cells that later becomes the digestive tract.  
The pocket opens up to the outside via a small pore.  This pore becomes the mouth in
protostomes (mouth first) and it becomes the anus in deuterostomes (mouth second).  All
chordates (defined below) along with echinoderms (starfish, brittle stars, sea cucumbers,
and sea urchins) are deuterostomes.  The protostomes include insects, crustaceans and
mollusks and many worms.  Interestingly, despite the fact that our human line usually
features the newest, most innovative adaptations, new molecular data show that
deuterostomes probably came first, and that the protostomes evolved from deuterostomy,
the ancestral condition.

-You are a chordate.  Chordates are deuterostomes that have four features: 1) a notochord,
2) a post anal tail, 3) a hollow, dorsal nerve cord and 4) pharyngeal slits.  The pharyngeal
slits serve as gills for ocean dwelling chordates.  You might be surprised to learn that all
humans, despite the fact that they don’t have visible pharyngeal slits after birth, are included
in the chordate group because they have the slits early in fetal development.  In other words,
early on, you used to have gills.  The invertebrate chordate that we are most closely related
to would be the lancet (amphioxus).  They have a notochord, but no vertebrae, they are also
without distinct fins, distinct eyes or a distinct head.  The simple light sensors near their front
end probably represent a very early form of the vertebrate eye.

-You are a vertebrate.  Vertebrates are chordates whose notochord is replaced by a bony
vertebral column.  The lancets mentioned above evolved into animals like the modern day
lampreys which have well developed fins and eyes and which later evolved into animals like
the modern day hagfish which are true vertebrates.  You and I are descendents of early
vertebrate fish that would have been very similar to the modern hagfish.    

-You are a gnathostome.  Vertebrates can be divided into two types; gnathostomes, with
jaws and agnathans which are jawless fish. Members of agnatha include the lampreys and
hagfish which we saw in the last section. Gnathostomata includes sharks, rays, skates and
bony fish.

-You are roughly a type of bony fish (osteichthyes).  Not all fish are bony fish.  Sharks,
skates and rays have skeletons that are composed only of cartilage (chondrichthyes).  Early
in fetal development the cartilaginous skeletons of bony fish are mineralized and turned into
bone.  Bony fish, in the strict sense (excluding you and their other non-fish descendents),
are the most successful type of vertebrates on the earth.  The first fish to breathe air, which
are probably approximated by modern day lungfish, probably did so to survive in stagnant,
de-oxygenated ponds.

-You are a tetrapod.  This means that you have four limbs.  The first four limbed animal to
climb onto the land would have been a fish that used its fins to move about.  The ability to
move on dry land would have allowed these early lungfish to escape water bound predators
and also to access new types of food.  Some of these fish became more and more adept at
breathing air, and at moving about on the ground.  Over millions of years, adaptations that
increased terrestrial mobility transformed fish into amphibians.

-You are roughly an anthracosaur.  Although it would be more correct to say that you
descend from a group of reptile like amphibians, anthracosauria.  Unlike reptiles, however,
this group still layed eggs in the water to ensure that they would not dry out from exposure to
the air.

-You are an amniote.  The amniotes evolved from amphibians, but differ from them in their
ability to lay their eggs on land due to the air-tight shell that keeps their eggs from drying
out.  The amnion and outer shell are two of several membranes protecting amniote eggs
from dessication.  These membranes allowed early reptiles to travel far from water and
“explore” the vast surface which had already been pioneered by land plants and insects (the
terrestrial descendents of crustaceans).  All reptiles, birds and mammals are considered

-You are roughly a synapsid.  The synapsids (also known as mammal-like reptiles) together
with the sauropsids (including all dinosaurs, modern birds and modern reptiles) encompass
all of the descendants of amphibeans.  The sauropsids (which means “lizard faced”) and
the synapsids (which are also known as theropsids meaning “beast faced”) can be
distinguished from one another by the presence of hair, and the structure of the heart, blood-
vessels and the forebrain.

-You are a eutherian or placental mammal.  This means that the mammalian females of your
species do not lay eggs (like monotremes), nor do their young climb out of the womb into a
pouch to finish their gestation (like marsupials).  All eutherian, or placental mammals are
born live after detaching from the placenta.  Placentalia includes all living species of
placental mammals but eutheria includes all living placentals as well as their extinct
placental ancestors.  

You are a haplorhine. Monkeys (haplorhines) can be distinguished from their monkey-like
forebears, the prosimians (strepsirhines) by the shape of their nostrils.  The first primates to
evolve from a small, shrew-like, mammalian ancestor were prosimians like the lemur, the
loris and the tarsier.  Prosimians are strepsirhine, meaning split nostril, and they have the
same nostril shape as a dog.  They can be distinguished from haplorhines by their long
snouts, wet noses, lower brain to body weight ratio, and more acute sense of both hearing
and smell.  All monkeys are haplorhines (simple nosed) and the structure of their nose, in
relation to their gums and lips, allows them to make more complex facial expressions.  
Haplorhines are also known to have excellent depth perception and very acute vision.

-You are a catarrhine.  Ancient monkeys are divided into two groups, catarrhini (African)
and platyrrhini (South American), due to the continental split between the Old World (Africa)
and the New World (South America).  New World monkeys are distinguished from Old
World monkeys by their flat noses.  “Platy” in platyrrhine means flat and “rrhine” means
nose.  “Cata” in catarrhine means down, and Old World monkeys are thought to have down
turned noses.  Apes (bigger, smarter, tailless monkeys) evolved from African monkeys, but
no apes evolved in the New World.  Humans descended from apes and thus can be
referred to as catarrhine, or Old World, primates.  

You are a hominid.  Hominids are apes that acquired the ability to walk on two feet.  
Humans (Homo sapiens) are the only living species of bipedal ape.

I hope that this journey through phylogenetic time has been fun.  I will leave you with the
taxonomic classifications for humans.  Now you should be more familiar with each taxon.

Kingdom        Animalia
Phylum           Chordata
Class              Mammalia
Order              Primates
Family             Hominidae
Genus             Homo
Species          Homo sapiens

phylogeny: noun
The evolutionary history of a group of organisms as depicted in a family tree

taxon: noun (plural, taxa)
A taxonomic category, such as species or phylum

taxonomy: noun
The science or technique of identifying, describing or classifying animals
Know Yourself... Taxonomically
Organization for the Advancement of   
Interdisciplinary Learning
Essay 15  01/05/05  Phylogeny, Taxonomy, Zoology