Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 32.2 The History of Animals Spans More Than Half a Billion Years
The evolutionary journey of animals began over 700 million years ago and spans several geologic eras. From soft-bodied Ediacaran life to the explosion of complex body plans in the Cambrian and the rise of mammals in the Cenozoic, animals have continually diversified through extinction events, environmental shifts, and developmental innovation.
Early Origins and Ediacaran Life
Fossil steroids suggest sponges existed ~710 million years ago.
Molecular clock studies date the common animal ancestor to ~770 million years ago.
Closest relatives: choanoflagellates—unicellular protists with genetic and morphological similarities to sponge cells.
The transition to animals involved:
New forms of cell adhesion (e.g., cadherins)
Evolving cell-signaling mechanisms
Ediacaran biota (~560 million years ago):
Early multicellular eukaryotes with diverse soft-bodied forms.
Some resemble sponges, mollusks, and cnidarians.
May include early embryos and the first evidence of predation (e.g., Cloudina boreholes).
Increased ecological complexity likely set the stage for later radiations.
The Cambrian Explosion
Occurred 535–525 million years ago.
Sudden appearance of fossils from many extant animal phyla:
Arthropods, chordates, echinoderms, and more.
Many had bilateral symmetry and complete digestive tracts.
Hypotheses for Cambrian diversification:
Predator-prey dynamics → new body structures (claws, armor).
Rising atmospheric oxygen → support for active metabolisms.
Developmental gene evolution, especially Hox genes and miRNAs.
Paleozoic Era (541–252 Million Years Ago)
Animals diversified and moved onto land:
Arthropods were first terrestrial animals (~450 mya).
Vertebrates followed around 365 million years ago.
Early land interactions:
Fossilized fern galls suggest co-evolution with insects by ~302 mya.
Rise of amphibians and amniotes (reptiles, mammals).
Mesozoic Era (252–66 Million Years Ago)
Animal groups expanded into new niches:
Reefs, aquatic reptiles, dinosaurs, birds, and mammals evolved.
Late Mesozoic: Angiosperms and insects diversified rapidly, influencing each other’s evolution.
Cenozoic Era (66 Million Years Ago–Present)
Mass extinction ended the Mesozoic (e.g., non-avian dinosaurs).
Mammals filled empty ecological roles:
Large herbivores and predators emerged.
Global climate cooled → new adaptive landscapes.
Among primates, some species adapted to grasslands, giving rise to human ancestors.
In a Nutshell
The animal kingdom has evolved through more than 700 million years of dramatic change—from sponge-like ancestors to dinosaurs to humans. Fossils, genes, and embryology trace a complex path shaped by mass extinctions, atmospheric shifts, and developmental breakthroughs that continue to shape life on Earth today.