Ch 22 - History of life on Earth - concepts & terms

Chapter 22 – The History of Life on Earth

Chapter Outline

• 1. The fossil record

• 2. History of life on Earth

Section 22.1 Learning Outcomes

• Describe how fossils are formed

• Explain how radiometric dating is used to estimate the age of a fossil

• List several factors that affect the completeness of the fossil record

22.1 The Fossil Record

• The Earth is estimated to be 4.55 billion years old.

• For the first ~500 million years, conditions were too hot for liquid water to exist.

• By 4 billion years ago, Earth had cooled sufficiently to form solid outer layers and oceans.

• Fossils are preserved remains of past life on Earth.

• The oldest fossils are about 3.5 billion years old, resembling modern cyanobacteria.

Fossils Are Formed Within Sedimentary Rock

• Sedimentary rocks form as particles of older rocks settle in rivers, lakes, and oceans.

• Most fossils are created when organisms are buried quickly.

• Hard parts of organisms are replaced by minerals during sedimentary formation, creating fossil impressions.

• Fossil ages can be inferred from their locations in rock layers; lower layers are typically older than upper ones.

Radioisotopes Provide a Way to Date Fossils

• Radiometric dating determines the age of samples by analyzing radioactive isotopes.

• Radioisotopes are unstable isotopes that decay at a constant rate.

• The half-life is the time needed for a radioisotope to decay to half its initial amount.

Several Factors Affect the Completeness of the Fossil Record

• The fossil record is incomplete but provides key insights into the types of life that existed on Earth.

Section 22.2 Learning Outcomes

• Describe environmental changes affecting life on Earth.

• Explain the cell structure and energy use of first living organisms during the Archaean eon.

• Detail how eukaryotic cells originated from bacterial and archaeal unions.

• Describe features of multicellular organisms from the Proterozoic eon.

• Outline major events in species diversity throughout the Paleozoic, Mesozoic, and Cenozoic eras.

• Identify the emergence of major groups like prokaryotes, eukaryotes, invertebrates, and plants.

22.2 History of Life on Earth

• The geological timescale records Earth’s history and major events from its creation to now.

• Divided into four eons (Hadean, Archaean, Proterozoic, Phanerozoic) and further into eras and periods.

• Transitions often correspond to significant changes in life forms rather than a fixed number of years.

Many Environmental Changes Have Occurred Since Earth's Origin

• Species have evolved over 4 billion years due to genetic changes and environmental alterations.

• Environmental factors that can lead to flourishing organisms or extinction include:

  • Temperature changes (e.g., Ice Age)

  • Atmospheric composition changes

  • Continental drift

  • Natural disasters (floods, glaciations, eruptions, meteorite impacts)

  • These changes can cause mass extinctions.

Prokaryotic Cells Arose During the Archaean Eon

• The Hadean eon was followed by the Archaean eon, when Earth cooled down.

• During the Archaean, microbial life thrived in primordial oceans.

• All forms of life were prokaryotic for over 1 billion years; the atmosphere contained minimal oxygen, supporting anaerobic organisms.

Biologists Are Undecided About Whether Heterotrophs or Autotrophs Came First

• Living organisms utilize two main energy acquisition strategies:

  • Heterotrophs: acquire energy from organic molecules they consume.

  • Autotrophs: harness energy directly from light or inorganic molecules.

• The debate persists regarding whether the first life forms were heterotrophic or autotrophic.

• Autotrophic cyanobacteria appear in the oldest fossils, possibly due to their growth patterns.

• Cyanobacteria create layered structures called stromatolites.

The Origin of Eukaryotic Cells Involved a Union Between Bacterial and Archaeal Cells

• Eukaryotic cells emerged during the Proterozoic era, having distinct nuclei and organelles.

• DNA analyses suggest eukaryotic genomes derive from separate bacterial and archaeal ancestors.

• Major metabolic genes originate from ancient bacterial sources, while genes for gene expression from archaeal sources.

• A symbiotic relationship between bacteria and archaea likely formed eukaryotic cells.

Evolution of Internal Membranes

• An archaeal species may have gained the ability to invaginate its membrane, forming internal compartments.

• Bacterial genes transferred to the archaeal host, ultimately forming the nuclear genome.

Origin of Mitochondria and Chloroplasts

• Mitochondria potentially evolved from a bacterium similar to modern alphaproteobacteria.

• Mitochondria and chloroplasts arose from distinct endosymbiotic events involving cyanobacteria.

Emergence of Multicellular Eukaryotes and Animals

• Multicellular eukaryotes likely emerged around 1.5 billion years ago.

• Simple multicellular organisms originated through aggregation or by daughter cells sticking together after division.

• The first multicellular animals appeared towards the end of the Proterozoic era, mainly invertebrates with bilateral symmetry for locomotion.

Phanerozoic Eon – Paleozoic Era

• The Phanerozoic eon began 543 million years ago, marked by extensive multicellular life.

• Divided into three eras: Paleozoic, Mesozoic, and Cenozoic.

Paleozoic Era Overview

• Covers about 300 million years, subdivided into six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian.

Cambrian Period (543-490 mya)

• Climate: Generally warm and humid.

• Cambrian explosion led to rapid animal species diversification; major invertebrate types emerged.

Ordovician Period (490-443 mya)

• Early/mid climate: Warm and wet.

• Marine life flourished; late period saw glaciation and mass extinction.

Silurian Period (443-417 mya)

• Stable climate; marine life diversified with the rise of terrestrial plants and animals.

Devonian Period (417-354 mya)

• Dry climate in northern regions; significant increase in terrestrial plant species and emergence of amphibians.

Carboniferous Period (354-290 mya)

• Cooler, swampy climate; large trees and flying insects appear.

• Key evolutionary innovation: amniotic egg for reptiles.

Permian Period (290-248 mya)

• Formation of the supercontinent Pangaea; drastic climate fluctuations.

• Major extinction event: 90-95% of marine and many terrestrial species went extinct due to volcanic eruptions and climate shifts.

Phanerozoic Eon – Mesozoic Era

• The Mesozoic era follows the Permian extinction, divided into three periods: Triassic, Jurassic, Cretaceous.

• Dominant land plants: gymnosperms; major reptile diversification occurs.

Triassic Period (248-206 mya)

• Early reptiles thrived, leading to dinosaurs' emergence.

Jurassic Period (206-144 mya)

• Gymnosperms remained dominant; dinosaurs & reptiles continued to thrive, while the first birds appeared.

Cretaceous Period (144-65 mya)

• Dominance of dinosaurs persisted; appearance of flowering plants.

• Ended with mass extinction due to a comet impact; caused rapid climate changes.

Phanerozoic Eon – Cenozoic Era

• The Cenozoic era spans the last 65 million years, marked by significant diversification of mammals and flowering plants.

Tertiary Period (65-1.8 mya)

• Rapid mammal diversification follows the Cretaceous extinction; angiosperms become dominant vegetation.

• Emergence of hominoids around 7 million years ago.

Quaternary Period (1.8 mya - present)

• Characterized by alternating Ice Ages; several large mammals became extinct, including some hominins.

• Homo sapiens emerged around 200,000 years ago and spread globally.

Chapter 22 Summary

22.1 The fossil record

• Fossils are formed in sedimentary rock, and radioisotopes date them.

• Completeness of fossil record is influenced by various factors.

22.2 History of life on Earth

• Geological timescale organizes Earth's history into eons and examines environmental impacts on species evolution.

• Prokaryotic cells arose in the Archaean eon; the origins of eukaryotic cells highlighted symbiotic evolution.

• Phanerozoic eon: the Paleozoic era marked diversification, then the Mesozoic era showcased dinosaurs rising and falling, before mammals and flowering plants thrived in the Cenozoic.