EARLY ANIMALS, EVOLUTION, & DEVELOPMENT

Choanoflagellates & animals

Choanoflagellates likely share a common ancestor with animals because they look almost identical to sponge collar cells and have very similar DNA, suggesting evolutionary relatedness

Cambrian Explosion

Rapid evolution of many animal phyla due to higher oxygen levels, evolution of predators, and new ecological niches; occurred in the Phanerozoic eon

First animals evolved

Animals first evolved in the Proterozoic eon (Precambrian)

Precambrian

Includes Hadean, Archean, Proterozoic; grouped together because life was simple and fossil record is limited

Fossil

Preserved remnant or impression of an organism; formed by burial, compression, petrification, or preservation in amber/ice

Fossil dating

Relative (index fossils in layers) and absolute (radiometric dating using isotope decay)

Fossil record bias

Favors organisms with hard parts, abundant species, marine life, and organisms that were buried quickly

Three eras & dominance

Paleozoic: early animals, vertebrates move to land
Mesozoic: reptiles/dinosaurs dominate
Cenozoic: mammals and birds dominate

End of Paleozoic

Permian extinction (~252 MYA); caused by volcanic CO₂ → global warming + ocean acidification → massive die-off

End of Mesozoic

K/Pg extinction (~66 MYA); asteroid impact → dust blocks sunlight → collapse of food chains

K/T boundary evidence

Iridium layer, Chicxulub crater, tektites, global ash layer

Hox genes

Control body plan (placement, number, size of structures); allow complex body organization and diversity

Animal characteristics

Multicellular, eukaryotic, heterotrophic, no cell wall, internal digestion, have unique tissues (muscle + nervous)

Parazoa vs Eumetazoa

Parazoa (sponges): no true tissues
Eumetazoa: all other animals with true tissues

Diploblastic animals

Have 2 germ layers (ectoderm + endoderm); example: cnidarians

Triploblasctic animals

Have 3 germ layers (ectoderm + mesoderm + endoderm) example: vertebrates

Protostome vs Deuterostome

Protostome: mouth first, spiral cleavage, determinate
Deuterostome: mouth second, radial cleavage, indeterminate

Coelom formation

Protostomes: split from solid mass
Deuterostomes: form from pockets

Lancelets

Cephalochordates; suspension feeders with all chordate traits as adults

Urochordates

Tunicates; chordate traits in larvae, lose them as adults and become sessile

Zygote

Fertilized diploid egg

Cleavage

Rapid cell division without growth

Blastula

Hollow ball of cells

Gastrula

stage where germ layers form

Blastopore

Opening to outside (what turns into butt or mouth)

Archenteron

Primitive gut