27.4 The Evolutionary History of the Animal Kingdom - Biology 2e _ OpenStax
Learning Objectives
Describe the features that characterized the earliest animals and when they appeared on Earth.
Explain the significance of the Cambrian period for animal evolution and changes in animal diversity.
Describe unresolved questions surrounding the Cambrian explosion.
Discuss implications of mass animal extinctions in evolutionary history.
Early Animal Origins
Animal Timeline
Earliest Animal Existence: Research ongoing about the origins and evolutionary history.
Questions include:
How long have animals existed?
What were the earliest members of the animal kingdom?
What organism was the common ancestor of early animals?
Pre-Cambrian Animal Life
Ediacaran Period (635 - 543 million years ago)
Final period of the late Proterozoic Neoproterozoic Era.
Fossils found in Ediacaran hills, Southern Australia.
Early life termed Ediacaran biota evolved from protists; no living representatives today.
Fossil Features
Impressions resembling feathers or coins.
Most Ediacaran fossils were just a few mm to cm long, some over a meter long.
Increasing evidence suggests more varied and complex animal species existed earlier.
Recent Discoveries
Discovery of Coronacollina acula fossils (560 million years old) indicates hard body parts.
Fossils of primitive, sponge-like creatures discovered (650 million years ago) could represent earliest animals.
Fossil evidence suggests some species evolved during the Cryogenian period.
The Cambrian Explosion
Cambrian Period Overview
Timeframe: 542 - 488 million years ago.
Fast expansion of new animal phyla; marks rapid evolution and diversification of animal life.
Key Fauna of the Cambrian
Included representatives of modern groups: echinoderms, mollusks, worms, arthropods, and chordates.
Anomalocaris: Top predator; over a meter long with compound eyes and spiky tentacles.
Trilobites: Dominant species; first animals to exhibit vision; now extinct.
Theories on Cambrian Explosion Causes
Various hypotheses proposed on why explosion occurred:
Environmental Changes: Increased atmospheric oxygen and oceanic calcium concentrations.
Habitat Availability: Expansion of continental shelves, providing more living space for diverse species.
Ecological Relationships: Changes in food web dynamics may have spurred evolutionary change.
Genetic Factors: Evolution of Hox control genes, allowing diverse morphologies.
Post-Cambrian Evolution and Mass Extinctions
Continued Evolution Post-Cambrian
Following the Cambrian, the Ordovician period saw the emergence of land plants, allowing aquatic species to invade terrestrial niches.
New adaptations in animals (limbed appendages in amphibians, epidermal scales in reptiles) developed in response to environmental changes.
Mass Extinction Events
Major events led to substantial loss of species diversity:
End of the Permian: Around 95% species extinction; trilobites and dominant reptiles disappeared.
The Mesozoic Era saw rapid evolution and diversification of dinosaurs, facilitated by climatic stability.
Another mass extinction at the end of the Cretaceous period due to meteoric impacts allowed mammals and birds to dominate post-extinction.
Extinction's Role in Evolution
Mass extinctions erase genetic lines, creating opportunities for new species to evolve and fill ecological niches.
Speciation increased in response to new environments (post-extinction recovery periods) and geographic changes.
Paleontology and Research
Role of Paleontologists
Study fossilized remains to understand prehistoric life and extinction patterns.
Require multidisciplinary knowledge: includes math, biology, geology, and ecology.
Mary Anning: Significant contributions in early paleontology; discovered major dinosaur fossils.
Extinction Intensity Graphs
Graph shows spikes of extinction intensity during key period transitions in Earth's history.
Learning Objectives
Describe the features that characterized the earliest animals and when they appeared on Earth.
Explain the significance of the Cambrian period for animal evolution and changes in animal diversity.
Describe unresolved questions surrounding the Cambrian explosion.
Discuss implications of mass animal extinctions in evolutionary history.
Early Animal Origins
Animal Timeline
Earliest Animal Existence: Research ongoing about the origins and evolutionary history of the animal kingdom. Key areas of inquiry include:
Duration of Existence: How long have animals existed?
Earliest Members: What were the earliest members of the animal kingdom? Evidence suggests sponges or similar organisms.
Common Ancestor: What organism served as the common ancestor to early animals? Molecular studies indicate a single-celled ancestor that eventually diversified into multicellular forms.
Pre-Cambrian Animal Life
Ediacaran Period (635 - 543 million years ago)
Considered the final period of the late Proterozoic Neoproterozoic Era.
Fossils found in Ediacaran Hills, Southern Australia, serve as critical evidence of early multicellular life.
Early life forms, collectively known as the Ediacaran biota, evolved from protists and are believed to lack living representatives today.
Fossil Features
Morphological Impressions: Fossils often exhibit impressions resembling feather-like shapes or disc-like coins, suggesting a variety of forms but not all were complex.
Size Variability: Most Ediacaran fossils ranged from a few millimeters to several centimeters in length, with some notable exceptions exceeding one meter.
Evidence suggests that more varied and complex animal species existed much earlier than previously thought, challenging established timelines of animal evolution.
Recent Discoveries
The discovery of Coronacollina acula fossils, estimated to be 560 million years old, suggests the earlier appearance of hard body parts among early animals, indicating a level of biological complexity not recognized before.
Recent findings of primitive, sponge-like organisms dating back 650 million years may represent some of the earliest ancestors of modern animals.
New fossil evidence indicates that certain species may have evolved during the Cryogenian period, adding complexity to our understanding of early animal development.
The Cambrian Explosion
Cambrian Period Overview
Timeframe: 542 - 488 million years ago.
Characterized by an explosive expansion of new animal phyla; represents a crucial point of rapid evolution and diversification of life forms on Earth.
Key Fauna of the Cambrian
Included early representatives of modern groups: echinoderms, mollusks, worms, arthropods, and chordates.
Anomalocaris: A top predator of the Cambrian seas, measuring over a meter long with large compound eyes and segmented spiky appendages for capturing prey.
Trilobites: Dominated the seas and were notable for being the first animals to exhibit vision; their diversity and abundance in Cambrian strata underscore their ecological prominence, despite being extinct today.
Theories on Cambrian Explosion Causes
Various hypotheses have been proposed for the Cambrian explosion:
Environmental Changes: Increases in atmospheric oxygen levels and oceanic calcium concentrations potentially supported new metabolic processes.
Habitat Availability: Expansion of continental shelves likely provided more ecological niches and living space for diverse species to thrive.
Ecological Relationships: Altered dynamics within food webs may have stimulated evolutionary pressures leading to diversification.
Genetic Factors: The evolution of Hox genes allowed for the development of varied body plans, leading to greater morphological diversity.
Post-Cambrian Evolution and Mass Extinctions
Continued Evolution Post-Cambrian
Following the Cambrian, the Ordovician period marked the emergence of land plants, creating new opportunities for aquatic species to invade previously unoccupied terrestrial niches.
Significant adaptations arose in animals, such as limbed appendages in amphibians and epidermal scales in reptiles, in response to environmental challenges and opportunities.
Mass Extinction Events
Major extinction events resulted in substantial losses of biodiversity:
The End of the Permian Extinction: Responsible for the extinction of around 95% of all species, including dominant trilobites and many reptilian groups.
During the Mesozoic Era, a unique period of rapid evolution and diversification of dinosaurs occurred, aided by a relatively stable climate.
The Cretaceous-Paleogene Extinction Event: Triggered by catastrophic meteoric impacts, permitted mammals and birds to excel and diversify in the aftermath of the extinction.
Extinction's Role in Evolution
Mass extinctions frequently obliterate established genetic lines, simultaneously creating ecological openings for new species to evolve and fill vacated niches.
Speciation tends to increase noticeably during recovery periods following mass extinctions, particularly as geographical and environmental changes prompt adaptation and diversification.
Paleontology and Research
Role of Paleontologists
Paleontologists play a vital role in studying fossilized remains, attempting to reconstruct prehistoric life and analyze extinction patterns.
This field necessitates a multidisciplinary approach, requiring expertise in math, biology, geology, and ecology, for a holistic understanding of life through time.
Mary Anning: Notable for her significant contributions in early paleontology, including the discovery of some of the first major dinosaur fossils, reflecting the importance of women in science historically.
Extinction Intensity Graphs
Graphs represent spikes of extinction intensity correlated with key transitions in Earth's geological history, illustrating patterns that inform our understanding of biodiversity loss and recovery through time.
Key Terms and Definitions
Early Animal Origins
Earliest Animal Existence: Research ongoing about how long animals have existed and their evolutionary history.
Common Ancestor: The organism that served as the ancestral root for early animals, likely a single-celled ancestor that diversified into multicellular forms.
Pre-Cambrian Animal Life
Ediacaran Period: The last period of the late Proterozoic Neoproterozoic Era (635 - 543 million years ago) notable for early multicellular life known as Ediacaran biota.
Ediacaran Biota: Early life forms that evolved from protists, believed to lack living representatives today.
Coronacollina acula: Fossils estimated to be 560 million years old, indicating the earlier appearance of hard body parts among early animals.
The Cambrian Explosion
Cambrian Period: A significant period (542 - 488 million years ago) characterized by the rapid evolution and diversification of new animal phyla.
Anomalocaris: A top predator of the Cambrian seas, measuring over a meter long, characterized by large compound eyes and segmented spiky appendages.
Trilobites: Dominant Cambrian species known for being the first animals to exhibit vision; extinct but notable for their ecological prominence.
Theories on Cambrian Explosion Causes
Environmental Changes: An increase in atmospheric oxygen and oceanic calcium concentrations that supported new metabolic processes.
Habitat Availability: Expansion of continental shelves that provided new ecological niches for various species.
Ecological Relationships: Altered food web dynamics that stimulated evolutionary pressures leading to diversification.
Genetic Factors: Evolution of Hox genes that facilitated the development of varied body plans.
Post-Cambrian Evolution and Mass Extinctions
Ordovician Period: Period following the Cambrian marked by the emergence of land plants, allowing aquatic species to explore terrestrial niches.
End of the Permian Extinction: Major extinction event responsible for around 95% species extinction, affecting trilobites and many reptilian groups.
Cretaceous-Paleogene Extinction Event: Event that allowed mammals and birds to diversify post-extinction due to catastrophic meteoric impacts.
Paleontology and Research
Role of Paleontologists: Scientists who study fossilized remains to reconstruct prehistoric life and extinction patterns, requiring multidisciplinary knowledge.
Mary Anning: Early paleontologist known for significant contributions, including the discovery of major dinosaur fossils.
Extinction Intensity Graphs: Graphical representations showing spikes of extinction during key transitions in Earth's history, highlighting patterns in biodiversity loss and recovery.