Animals emerged during the Ediacaran period, evolving from soft-bodied organisms to diverse forms with specialized tissues and organs. This set the stage for the Cambrian explosion, a rapid diversification of animal life that shaped Earth's ecosystems.

The Cambrian period saw the appearance of major animal phyla, hard body parts, and complex behaviors. This evolutionary burst, along with subsequent mass extinctions, profoundly influenced animal diversity and laid the groundwork for modern ecosystems.

The Earliest Animals and the Cambrian Explosion

Characteristics of earliest animals

• Multicellular eukaryotes emerged during Ediacaran period (~635-541 million years ago)
  • Soft-bodied, sessile or slow-moving organisms (Dickinsonia, Spriggina, Charnia)
• Multicellularity allowed for cell and tissue specialization
• Extracellular matrix provided structural support and cell adhesion
• Simple nervous systems developed for coordination and response to stimuli
• Basic sensory organs emerged, such as light-sensitive eyespots
• Diversification of body plans, including radial and bilateral symmetry

Impact of Cambrian period

• Cambrian period began ~541 million years ago
  • Rapid increase in animal diversity and complexity known as Cambrian explosion lasted 20-25 million years
• Most major animal phyla appeared, including chordates, arthropods, and mollusks
• Hard body parts evolved, such as shells, exoskeletons, and mineralized spines
• Sensory organs diversified, including compound eyes and antennae
• More complex nervous systems and behaviors developed
• Predator-prey relationships and arms races emerged
• Significant increase in animal diversity set stage for evolution of more complex ecosystems
• Fossil record provides evidence of early animal diversification during this period

Debates on Cambrian explosion

• Causes of Cambrian explosion remain topic of scientific debate and research
• Proposed explanations for rapid diversification of animals:
  • Increased atmospheric oxygen levels supported larger and more complex body plans
  • Changes in ocean chemistry (increased calcium concentration) facilitated formation of hard body parts
  • New feeding strategies and habitats created ecological opportunities
  • Genetic and developmental innovations emerged (Hox genes, signaling pathways)
• Some argue Cambrian explosion may be preservational artifact due to increased fossilization potential of hard body parts
  • However, sudden appearance of diverse animal phyla in fossil record suggests genuine evolutionary event

Effects of major extinctions

• Several mass extinctions have significantly impacted animal diversity and evolutionary trajectories
• End-Ordovician extinction (~444 million years ago)
  • Global cooling and sea level drop eliminated many marine species
  • Disappearance of some trilobite and brachiopod lineages
• Late Devonian extinction (~375-360 million years ago)
  • Global cooling, ocean anoxia, and volcanic activity triggered extinction
  • Decline of reef-building organisms and armored fish
• End-Permian extinction (~252 million years ago)
  • Most severe mass extinction, eliminating >90% of marine species and 70% of terrestrial vertebrate species
  • Massive volcanic eruptions, global warming, and ocean acidification caused extinction
  • Rise of dinosaurs and mammals in following Triassic period
• End-Cretaceous extinction (~66 million years ago)
  • Asteroid impact and volcanic activity caused extinction
  • Non-avian dinosaurs and many marine reptiles went extinct
  • Radiation of mammals and birds in Paleogene period

The Importance of Mass Extinctions in Shaping Animal Evolution

• Mass extinctions have played crucial role in shaping evolutionary history of animals
  • Eliminate dominant groups, freeing up ecological niches for new lineages to diversify
  • Surviving lineages often undergo adaptive radiations, leading to emergence of novel body plans and ecological roles
• Recovery from mass extinctions can take millions of years
  • Ecosystems are restructured and new evolutionary innovations arise during recovery
• Understanding causes and consequences of past mass extinctions provides insights into resilience and adaptability of animal life in face of global change

Evolutionary Processes and Patterns in Animal Kingdom

• Natural selection drives adaptation and speciation in animal populations
• Phylogeny reconstructs evolutionary relationships between animal groups
• Convergent evolution results in similar traits evolving independently in different lineages
• Cladistics uses shared derived characteristics to determine evolutionary relationships among organisms