evolution and development of the skull

modules and networks

modular development and evolution

when and how during development and evolution? •  increase in brain size •  decrease in facial size

human “candidate genes”

•  FOXP2 [forkhead box P2] (Lai et al. 01) –  some alleles associated with speech and language disorder –  humans have specific allele, different from great apes –  “human” allele also in Neanderthals (Krause et al. 07)

•  MYH [myosin heavy chain expression] (Stedman et al. 04) –  less expressed in humans (MYH16 inactivation) compared to great

apes

•  ASPM [abnormal spindle-like microcephaly-assoc.] (Mekel Bobrov et al. 05) MCPH1 [microcephalin] (Evans et al. 05) –  some alleles involved in primary microcephaly –  some alleles show signs of recent selective sweeps in humans,

but they are not related to brain size

skull: development, evolution, function

human neonate skull (CT reconstruction)

neurocranium desmocranium: intramembranous (desmal)

splanchnocranium chondrocranium: enchondral (cartilaginous)

Meckel’s cartilage

ear ossicles

developmental modules or developmental network?

skull development and evolution branchial (pharyngeal) arch transformation

Reichert's theory

eye

stapes

incus

malleus

proc. styl.

hyoid

mandibula

maxilla

squa- mosum

MC: Meckel's cartilage PQ: palatoquadratum TR: trabeculae

nasal bones

larynx

synapsid (mammal-like reptile)

mammal

Liem et al. 04

angular

Reichert's theory

branchial arches and circulation

Liem et al. 04

branchial arches and circulation Li

em e

t al.

04

frog human

external (ventral) carotid

internal (dorsal) carotid

carotid pathways

most lemuriforms

cheirogaleids loriforms

tarsiers anthropoids

Geissmann, 03 Gray’s Anatomy

ce/ci: carotis ext./int. s: a. stapedia p: a. promontorii pa: a. pharyngea ascendens

branchial arches and human anatomy

arch" skeletal elements" muscles" arteries" cranial nerves"

1" maxilla, mandible, incus, malleus"

mastication" -" V trigeminus"

2" stapes, styloid process, "body and lesser horns of hyoid"

facial expression" -" VII facialis"

3" body and greater horns of hyoid "

stylopharyngeus" int. carotis"

IX glossopharyngeus"

4-6" laryngeal cartilages" palatine, pharyngeal, laryngeal, trapezius, sternomastoid"

4: aorta, "subclavia"5,6: -"

X vagus + acc.cran."XI acc.spin."

gene expression patterns during early head development

Kuratani 04

genetics of craniofacial development

San

taga

ti &

Rijl

i, 03

congenital malformations

Franceschetti (Treacher-Collins) syndrome Apert/Crouzon (1st branchial arch) syndrome

skull evo-devo and human craniofacial malformations

FGFR-2 gene mutations lead to congenital malformations of head and limbs: „1st+2nd branchial arch disorders“

•  e.g. Apert syndrome (acrocephalo-syndactyly) –  craniosynostoses –  underdevelopment of maxilla –  low position of ears –  syndactyly

•  e.g. Franceschetti (Treacher Collins) syndrome –  underdevelopment of mandible, zygomatic bones and

external/middle ears

developmental reorganization during evolution

•  heterotopy: spatial reorganization –  deposition vs. resorption –  size, shape, arrangement of dep. and res. fields

•  heterochrony: temporal reorganization –  sequence of growth events (onset - duration - offset) –  local rates of growth and development

speciation and development

heterochrony, heterotopy, allometry

definitions

•  ontogeny: structural (form) change from conception to death

•  form: size and shape •  growth: change in size •  development: change in shape •  allometry: size-related change (or variation) in shape

speciation through ontogenetic reorganization

•  heterotopy: spatial reorganization of ontogeny, change in directions of growth and development

–  e.g. migration patterns of NCC –  e.g. nr. and arrangement of dental cusps –  e.g. spatial arrangement of depository and resorptive bone growth fields

•  heterochrony: temporal reorganization of ontogeny, change in rates of growth and development

–  e.g. proliferation rates of NCC –  e.g. onset, duration, and offset of dental cusp formation –  e.g. local rates of bone resorption and deposition

heterochrony example: humans and chimps

cranial size

dental age

birth adult

neurocranial size

dental age

human

chimp

birth adult

splanchnocranial size

dental age

heterochrony example: humans and chimps

cranial shape: splanchno/neuro size

dental age birth adult

log (cranial size)

human

chimp

log (cranial shape)

ontogenetic allometry

birth

adult

heterochrony: paedomorphosis

phenotype

developmental time

adult descendant similar in shape to juvenile ancestor

adult descendant juvenile ancestor

heterochrony: peramorphosis

phenotype

juvenile descendant adult ancestor

juvenile descendant similar in shape to adult ancestor

developmental time

heterochrony: pattern and process

•  different heterochronic processes can result in similar heterochronic patterns

•  example: paedomorphy of the human head can result from –  delayed facial growth –  decelerated facial growth –  early cessation of facial growth –  advanced brain growth –  accelerated brain growth –  long duration of brain growth –  … or a combination of these processes è all result in small-faced, large-brained heads