EMBRYOLOGICAL EVIDENCE

MODEL 1: The various groups of living things are not related because they did not evolve from a common ancestor. Humans have not descended from other primates.

Because their origins were completely independent, it is not expected that human embryos display traits which are observed in other animals but which are not found or not useful in humans after birth.

MODEL 2: Living things have evolved from a common ancestor; humans have evolved from other primates. Human embryos should possess ancestral traits not found or not useful in humans after birth.

WHICH OF THESE TWO MODELS IS SUPPORTED BY THE EMBRYOLOGICAL EVIDENCE?

If evolution occurred, one would expect that human embryos display some traits not found in adult humans but that would have been present in the embryos of our ancestors. This is not expected if evolution did not occur.

Please look at the following chart to determine if human embryos possess traits that adult humans do not possess would lend support to human evolution from other animals.

ANCESTRAL TRAIT

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

HUMAN ADULT

SKELETAL SYSTEM

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

1 frontal bone

premaxillary bone holding upper incisors

no premaxillary bone (it fused to maxillary)

a number of smaller bones in place of occipital bone

occipital bone

a number of smaller bones in place of sphenoid bone

sphenoid bone

a number of smaller bones in place of temporal bone

temporal 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 inside temporal bone in middle ear

angular bone forms ring for eardrum

no separate angular bone; part of temporal

stapes has a different origin than malleus and incus

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

notochord primary longitudinal support

vertebral column provides longitudinal support

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

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

axis has a process known as dens

at least some of ribs have 2 heads

no ribs have 2 heads

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

secondary palate separating the nasal and oral cavities

elbow and knee face the same direction

elbow and knee face opposite directions

coracoid a separate bone from scapula

coracoid fused to scapula

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

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

paddle shaped limbs

limbs elongated and not paddle shaped

fingers and toes webbed

fingers and toes separate

NERVOUS SYSTEM

brain tubular, 3 regions

brain regions folded on themselves, 5 regions

pineal gland exposed

pineal not exposed

vomeronasal organ prominent

vomeronasal organ not prominent; may not be functional

accessory olfactory bulb in brain

no accessory olfactory bulb

cerebrum not folded

cerebrum folded

cochlea of inner ear not coiled

cochlea coiled

CARDIOVASCULAR SYSTEM

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

single aorta

tubular heart, 1 atrium and 1 ventricle

heart not tubular; 4 chambers

heart includes a separate truncus arteriosus

no truncus arteriosus (remnants compose part of aortic arch)

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

no sinus venosus (forms part of right atrium)

pacemaker in sinus venosus

pacemaker in right atrium

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 not nucleated

DIGESTIVE SYSTEM

cloaca

Cloaca

separate openings for urinary, digestive, and reproductive systems

URINARY AND REPRODUCTIVE SYSTEMS

pronephros

Pronephros

mesonephros functional as kidney

no mesonephros functional as 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 in pelvis (females) or outside body (males)

MUSCULAR SYSTEM

somites obvious

somites not as obvious

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

no diaphragm to separate thoracic and abdominopelvic body cavities

diaphragm

tail muscles

tail muscles modifed to reinforce body wall

GLANDS (Exocrine and Endocrine)

mammary tissue over broad area of ventral surface

mammary tissue limited to chest

thyroid and parathyroid separate structures

thyroid and parathyroid joined

adrenal medulla and cortex separate structures

adrenal medulla and cortex joined

EXTRAEMBRYOIC MEMBRANES

Most vertebrates lay eggs. In reptiles, birds, and monotremes, the embryos are surrounded by a layer of albumen, a shell membrane, and a shell. Marsupial embryos are surrounded by a zona pellucida, albumen, and a shell membrane. Placental embryos have a zona pellucida and a thin layer of albumen but have no trace of a shell or shell membrane (Mossman, p.31). In marsupials the keratinous shell membrane is usually lost after the embryo reaches the uterus but may last until birth in some species (Mossman, p. 55).

In addition to placental and marsupial mammals, there are vertebrates which give birth to live young. Most bony fish and amphibians lay eggs before they are fertilized while most cartilaginous fish, reptiles, and all birds lay eggs after they are fertilized. Even among these egg-laying groups, reproduction through live birth (viviparity) is known. There are viviparous cartilaginous fish, actinopterygian bony fish, sarcopterygian bony fish (coelocanths), amphibians, lizards, and snakes (Mossman, p.3). These methods of live birth are different enough to have evolved separately.

Amniotes (reptiles, birds, and mammals) all possess four extra-embryonic membranes: the yolk sac, allantois, amnion, and chorion. In all amniotes, these extra-embryonic membranes develop much faster than the embryo itself—establishing these membrane is a priority for the developing embryo. In the illustrations used in this chapter, the amnion will be depicted with a light blue, the chorion with red (and the chorionic cavity with a light red), the yolk sac with yellow, and the allantois with green. In some illustrations, the maternal tissue will be represented with purple.

In the following illustration of a oviparous lizard (egg-laying; the egg shell is not included in the drawing), the primitive amniote condition is seen. Development occurs in the fluid of the amnion. The yolk sac provides the nourishment for the developing embryo and fetus and the allantois collects wastes which develop during development. The chorion performs gas exchange with the outside world through the egg shell.

This set of extra-embryonic membranes are seen in lizards which give birth to live young as well.

YOLK SAC

Some placental mammals, such as rodents and some edentates, bats, and insectivores, have a persistent yolk sac (Mossman, p. 59). Some mammals such as rodents, rabbits, moles, shrews, armadillos and some bats develop an early choriovitelline placenta and the yolk sac may invert towards the uterine tissue (Mossman, p. 84). In carnivores, rodents, and some insectivores (soricoids), the yolk sac is large and has a significant vascular supply (Mossman, p. 126).

In 2-4% of people, part of the vitelline duct persists. If complete, it may discharge fecal material into the umbilical area. It may contain pancreatic tissue (Sadler, p. 251).

ALLANTOIS

The placenta in placental mammals is a chorioallantoic placenta. Although the endodermal portion of the allantois may be rudimentary and eventually degenerate, the blood vessels supply the chorion. One type of lizard and three marsupials are also known to possess chorioallantoic placentas (Mossman, p.40, 54).

The allantois seems to be a precocious urinary bladder which grow rapidly (Mossman, p.118). Without the allantois, amniote embryos would not have been able to grow larger than marsupial infants (Mossman, p. 118). The allantois persists as a separate sac in some marsupials, especially those which are born very young. In carnivores and perissodactyls, there is a large allantoic cavity while in anthropoids the allantoic cavity and duct are only rudimentary. Some eutherian mammals may still use the allantois to collect waste (Mossman, p. 119). True moles (insectivores) possess the most primitive eutherian fetal membrane system which includes a large allantois (Mossman, p.168-9).

In humans, part of the allantois may persist as an urachal fistula, cyst, or sinus (Sadler, p. 270).

PHARYNGEAL ARCHES

Pharyngeal arches are one of 4 primary characteristics that all chordates share (along with a notochord, a dorsal nerve chord, and a post-anal tail). Although they are more obvious in the adults of more primitive vertebrates (in fish they contain the gills) than in humans, they are still a major developmental characteristic of early human embryos. They were formerly called branchial arches (branchia is the Greek word for gill)—even animals which lacked gills develop these arches a pattern equivalent to those that possess gills. These arches have cartilages (most of which degenerate), their own aortic arches for a blood supply (most of which degenerate), and grooves or slits between them (which later fuse). Human embryos have 4 visible pharnygeal arches (the first arch has two important subdivisions) and 2 more which are not visible externally.

Each pharyngeal arch possesses a bar of cartilage, an aortic arch, and a cranial nerve (or branch of a cranial nerve).

POUCHES

The cleft of the first pouch forms the external auditory meatus, the middle ear cavity, and the Eustachian (pharyngotympanic) tube. The tissue which separates the outer and inner parts of the cleft forms the tympanic membrane.

The second pouch forms the palatine tonsil. The tonsillar fossa in adults is a remnant of the pouch.

The third pouch forms the inferior parathyroid gland (the dorsal portion) and the thymus (the ventral portion). When the thymus descends, the inferior parathyroid gland descends with it.

The fourth pouch forms the superior parathyroid glands. The ventral portion of this pouch may form thymic tissue which later disappears.

The fifth pouch forms the ultimobranchial body which will contribute to the thyroid gland (it contributes the parafollicular cells or C cells which secrete calcitonin). Remnants of the ultimobranchial body can form fistulas and cysts (Sadler, p. 305-8).

NERVES

The innervation of the pharyngeal pouches is simple with cranial nerves V (two branches), VII, IX, and X innervating the first four arches (and parts of X innervate the 6^th arch); this establishes the innervation pattern of many structures of the head and neck.

ABNORMALITIES

Some individuals have cysts which develop at the sites of the first pharyngeal grooves. These may form branchial sinuses which open to the outside of the throat or rarely, they may open into the pharynx. (Moore, p. 227).

Other abnormalities in development can result in remnants of branchial cartilages that do not degenerate, masses of thymus tissue which remain in the neck or are connected to the parathyroid glands, parathyroid glands that do not descend, and accessory thyroid glands.