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INTRODUCTION
EMBRYOLOGY AND EVOLUTION SOMITES

     Embryos and fetuses are not little adults.  In humans, the embryonic brain does not form as a miniature adult brain; the occipital bone does not form as a miniature occipital bone; the kidneys do not form as miniature adult kidneys.  Many of the characteristics of human embryos are more similar to the characteristics the adults of more primitive vertebrates rather than those of adult humans.  This observation long preceded Haekel’s notion that an embryo actually recapitulated evolutionary ancestry in order to achieve its adult form.  Geoffrey argued that some embryonic structures were vestigial since 1829 and the notion that embryos recapitulate stages in which they are equivalent to other organisms since 1811.  In 1793, Kielmeyer, one of Cuvier’s teachers, compared the development of life on earth to the stages of fetal development (Hall, p. 42-52).  While embryological development does not recapitulate phylogenetic history, embryos can retain structures from ancestral embryos even though these structures may be lost or highly modified in the adult forms.  The following chart depicts some of these similarities.


 

 

ANCESTRAL TRAIT

 

TRAIT IN AT LEAST SOME PART OF THE DEVELOPMENT OF HUMAN EMBRYO/FETUS

 

HUMAN ADULT

 

 

SKELETAL SYSTEM

 

 

bone in skull while rest of skeleton is cartilaginous

 

bone in skull while rest of skeleton is cartilaginous

 

bony skeleton (although the bone of the skull and clavicle formed through a different process than that of the rest of the skeleton)

 

 

2 frontal bones

 

2 frontal bones

 

1 frontal bone

 

 

premaxillary bone holding upper incisors

 

premaxillary bone holding upper incisors

 

no premaxillary bone (it fused to maxillary)

 

 

a number of smaller bones in place of occipital bone

 

a number of smaller bones in place of occipital bone

 

occipital bone

 

 

a number of smaller bones in place of sphenoid bone

 

a number of smaller bones in place of sphenoid bone

 

sphenoid bone

 

 

a number of smaller bones in place of temporal bone

 

a number of smaller bones in place of temporal bone

 

temporal bone

 

 

a number of smaller bones in place of ethmoid bone

 

a number of smaller bones in place of ethmoid bone

 

ethmoid bone

 

 

malleus and incus (quadrate and articular) in jaw region

 

malleus and incus (quadrate and articular) in jaw region

 

malleus and incus inside temporal bone in middle ear

 

 

angular bone forms ring for eardrum

 

angular bone forms ring for eardrum

 

no separate angular bone; part of temporal

 

 

stapes has a different origin than malleus and incus

 

stapes has a different origin than malleus and incus

 

3 middle ear bones side by side; no separate origin apparent

 

 

notochord primary longitudinal support

 

notochord primary longitudinal support

 

vertebral column provides longitudinal support

 

 

notochord a hollow tube between nerve chord and gut; stretches from brain to tail

 

notochord a hollow tube between nerve chord and gut; stretches from brain to tail

 

remnants of notochord in center of intervertebral disks between vertebrae

 

 

vertebrae composed of separate bones: paired neural arches over notochord, paired pleurocentra on either side of notochord, and intercentra

 

vertebrae composed of separate bones: paired neural arches over notochord, paired pleurocentra on either side of notochord, and intercentra

 

each vertebra is a separate bone (the pleurocentra have fused and replaced the notochord to form the body; the neural arches have fused and have joined the pleurocentra, intercentra have formed part of atlas and capitulum of ribs)

 

 

pleurocentrum of atlas separate from axis

 

pleurocentrum of atlas separate from axis

 

axis has a process known as dens

 

 

at least some of ribs have 2 heads

 

at least some of ribs have 2 heads

 

no ribs have 2 heads

 

 

cervical, lumbar, and sacral ribs

 

cervical, lumbar, and sacral ribs

 

no cervical, lumbar, and sacral ribs (small remnants form transverse processes of cervical and lumbar vertebrae and part of sacrum)

 

 

no secondary palate, choanae exist as the continuation of the nasal cavity into the oral cavity

 

no secondary palate, choanae exist as the continuation of the nasal cavity into the oral cavity

 

secondary palate separating the nasal and oral cavities

 

 

elbow and knee face the same direction

 

elbow and knee face the same direction

 

elbow and knee face opposite directions

 

 

coracoid a separate bone from scapula

 

coracoid a separate bone from scapula

 

coracoid fused to scapula

 

 

hip made of 3 separate bones: ilium, ischium, and pubis

 

hip made of 3 separate bones: ilium, ischium, and pubis

 

hip composed of one solid bone (the 3 original bones fuse)

 

 

paddle shaped limbs

 

paddle shaped limbs

 

limbs elongated and not paddle shaped

 

 

fingers and toes webbed

 

fingers and toes webbed

 

fingers and toes separate

 

 

NERVOUS SYSTEM

 

 

brain tubular, 3 regions

 

brain tubular, 3 regions

 

brain regions folded on themselves, 5 regions

 

 

pineal gland exposed

 

pineal gland exposed

 

pineal not exposed

 

 

vomeronasal organ prominent

 

vomeronasal organ prominent

 

vomeronasal organ not prominent; may not be functional

 

 

accessory olfactory bulb in brain

 

accessory olfactory bulb in brain

 

no accessory olfactory bulb

 

 

cerebrum not folded

 

cerebrum not folded

 

cerebrum folded

 

 

cochlea of inner ear not coiled

 

cochlea of inner ear not coiled

 

cochlea coiled

 

 

CARDIOVASCULAR SYSTEM

 

 

cardinal veins a major drainage system; prominent posterior, common, and subcardinal veins

 

cardinal veins a major drainage system; prominent posterior, common, and subcardinal veins

 

no cardinal veins (although remnants of them have formed parts of azygous and common iliac veins)

 

 

paired dorsal aortae

 

paired dorsal aortae

 

single aorta

 

 

tubular heart, 1 atrium and 1 ventricle

 

tubular heart, 1 atrium and 1 ventricle

 

heart not tubular; 4 chambers

 

 

heart includes a separate truncus arteriosus

 

heart includes a separate truncus arteriosus

 

no truncus arteriosus (remnants compose part of aortic arch)

 

 

heart includes a separate bulbis cordis

 

heart includes a separate bulbis cordis

 

no bulbis cordis (right forms part of pulmonary trunk and walls of ventricles; left forms aortic vestibule)

 

 

heart includes a separate sinus venosus

 

heart includes a separate sinus venosus

 

no sinus venosus (forms part of right atrium)

 

 

pacemaker  in sinus venosus

 

pacemaker  in sinus venosus

 

pacemaker in right atrium

 

 

6 pairs of aortic arches, one for each pharyngeal arch

 

6 pairs of aortic arches, one for each pharyngeal arch

 

no aortic arches, remnants contribute to a few arteries

 

 

red blood cells nucleated (at least first ones produced)

 

red blood cells nucleated (at least first ones produced)

 

red blood cells not nucleated

 

 

DIGESTIVE SYSTEM

 

 

Cloaca

 

Cloaca

 

separate openings for urinary, digestive, and reproductive systems

 

 

URINARY AND REPRODUCTIVE SYSTEMS

 

 

Pronephros

 

Pronephros

 

 

 

mesonephros functional as kidney

 

mesonephros functional as kidney

 

no mesonephros functional as kidney

 

 

testes empty into kidney

 

testes empty into kidney

 

testes do not release sperm into a kidney

 

 

ducts of mesonephros (both for urine and sperm)

 

ducts of mesonephros

 

in females ducts degenerate; in males form ducts of reproductive system

 

 

gonads located by mesonephros high in abdomen

 

gonads located by mesonephros high in abdomen

 

gonads in pelvis (females) or outside body (males)

 

 

MUSCULAR SYSTEM

 

 

somites obvious

 

somites obvious

 

somites not as obvious

 

 

somites extend into a significant tail

 

somites extend into a significant tail

 

tail much reduced, a number of somites have degenerated

 

 

pharynx composed of a series of separate pouches separated by grooves (each with a nerve, aortic arch, and cartilage)

 

pharynx composed of a series of separate pouches separated by grooves (each with a nerve, aortic arch, and cartilage)

 

pharynx not composed of a series of pouches (earlier structures highly modified)

 

no diaphragm to separate thoracic and abdominopelvic body cavities

 

no diaphragm to separate thoracic and abdominopelvic body cavities

 

diaphragm

 

 

 

 

tail muscles

 

tail muscles

 

tail muscles modifed to reinforce body wall

 

 

GLANDS (Exocrine and Endocrine)

 

 

mammary tissue over broad area of ventral surface

 

mammary tissue over broad area of ventral surface

 

mammary tissue limited to chest

 

 

thyroid and parathyroid separate structures

 

thyroid and parathyroid separate structures

 

thyroid and parathyroid joined

 

 

adrenal medulla and cortex separate structures

 

adrenal medulla and cortex separate structures

 

adrenal medulla and cortex joined

 

 

 

     The similarities of embryological features provides overwhelming evidence for evolution since there is no reason to predict that these similarities would exist if evolution had not occurred.  Of all of the ways that an occipital bone could form, there is no reason that it would form from the fusion of the same elements present in adult synapsid reptiles if humans were completely unrelated to synapsid reptiles.  Of all of the ways that a pharynx could be formed, there is no reason that land living humans should begin their pharyngeal development in the same way as fish with gills do.  There is no reason that the primitive pronephros and mesonephros should form in the same way as they do in fish if adult humans rely only on a metanephric kidney.  There is no reason that blood vessels equivalent to those in fish should form which would have to be completely reworked to form the adult vasculature.  There is no reason that human embryos should have a yolk sac or allantois given that nutrient and waste exchange occurs at the placenta.  There is no reason that an uninterrupted notochord should form in an embryo which is about to form a vertebral column. 

     Some have argued that these structures cannot be considered as vestigial evolutionary remnants if there is a function that they perform.  Does the human embryonic notochord help to induce the nervous system?  Yes.  Do remnants of the embryonic notochord still exist as the nucleus pulposus in the intervertebral disks which help cushion human steps?  Yes.  BUT IT’S A NOTOCHORD!  There are many ways that a nerve chord could be induced to development—all bilateral animals which are not chordates do so, in fact.  There are many possible sources of tissue which could form between neighboring vertebrae.  All humans once had a notochord as an embryo stretching from head to tail, just like in the most primitive chordates.   Human vertebrae formed as a series of small bones around the notochord (a pair of neural arches, a pair of pleurocentra, perhaps remains of intercentra) that gradually replaced it—in the same way that is evident in the fossil record.  In the modern evolutionary model, higher vertebrates which lack notochords as adults evolved from vertebrates which had notochords as their primary support rods.  The observation that human embryos have notochords supports this--whether or not these embryonic notochords have other functions.  The question is not “Do embryonic notochords have a function?”  but “Why is it a notochord which performs this function?” 

     The same can be said of other structures.  If you look in the head region of an early vertebrate embryo, you can’t miss the prominent pharyngeal arches.  Do these arches have a function?  Yes.  Pharyngeal arches contribute cartilage to the larynx and hyoid, small segments of embryonic blood vessels to adult vessels, tissue to the thymus and thyroid, etc.—BUT THEY ARE PHARYNGEAL ARCHES!   There are many ways a small hyoid bone could develop or blood vessels to enter the head could develop or a thymus could develop, etc.  One of the first things I did as an embryo was to dedicate a large number of my embryonic cells into making a yolk sac.  Does the human yolk sac have a function?  Yes--my first blood cells and spermatogonia and oogonia are made there—BUT IT IS A YOLK SAC!!  Blood cells can be made from lots of places, especially in fetal development, a yolk sac isn’t necessary.  The reproductive system would make much more sense if spermatogonia and oogonia actually formed in the gonads where they will exist later in development rather than being formed outside the embryonic body and having to travel to the gonads.  Regardless of these odd functions in the production of blood and reproductive cells—it is a yolk sac.   The question is not “Do embryonic pharyngeal arches and yolk sacs have a function?”  but “Why is it pharyngeal arches and a yolk sac which performs these functions?” 

     For each step which should occur in normal embryonic development, there is the possibility that a mistake occurs.  Because human embryos develop characteristics unlike those of human adults, they must undergo extra developmental steps which must modify these early features.  With these extra steps come additional possibilities for birth defects.  The development of the yolk sac increases an embryo’s susceptibility to herniation of the intestines.  The development of the thymus in the neck of the embryo creates the possibility that its migration to the thorax will not occur normally and will result in remnants of the thymic tissue left in the neck.  The formation of the primordial germ cells which will produce gametes outside the gonads (in the yolk sac and base of the allantois) necessitates that they migrate to the gonads, although this does not always occurs successfully.  The formation of the testes near the thorax requires that they descend into the scrotum but this process is not completed in many males, resulting in sterility.  The kidneys begin development in the pelvis and in some individuals, there are abnormalities resulting from their ascent.   A large number of abnormalities can be traced to a problem in converting an early developmental stage into the normal adult morphology such as pharyngeal slits which do not fully close, a palate which remains cleft, notochord remnants which persist (and may even cause a cancer known as a chordoma), composite bones which do not fuse normally, heart chambers which are not fully separate, persistent embryonic vessels such as the ductus arteriosus, and many others.  Thus, an understanding of the evolutionarily retained features of the embryo are important in understanding human birth defects.

     Comparative embryology reveals unexpected similarities in the development of animals and provides great evidence for their shared, evolutionary ancestry.