The History of Life on Earth

The Geologic Record

The geologic record divides Earth's history into a series of eons, eras, periods, and epochs, providing a timeline for major biological and geological events.

• Earth's Age: Approximately 4.6 billion years old.

• Geologic Time Scale: Organizes Earth's history into hierarchical units (eons, eras, periods, epochs).

• Perspective: If Earth's history were scaled to 46 years, humans would have existed for only about 4 hours, and the industrial revolution would have begun just seconds ago.

Formation of Earth & Early Earth

Earth formed about 4.6 billion years ago and was initially inhospitable to life. Over time, conditions stabilized, allowing life to emerge.

• Hadean Eon: Early Earth was extremely hot and hostile.

• Oldest Earth Materials: Zircon crystals from Jack Hills, Australia (4.4 billion years old) and rocks from Acasta River, Canada (4.03 billion years old) are the oldest known fragments of Earth.

• Life-Supporting Conditions: Earth became cooler and more stable, eventually supporting life about 4 billion years ago.

Building Blocks of Life

The origin of life required the formation of organic molecules and the assembly of these molecules into more complex structures.

• Amino Acids: Fundamental components of proteins, can form spontaneously under certain conditions.

• Abiotic Synthesis: The formation of organic molecules from inorganic precursors without biological intervention.

• Self-Assembly: Lipids can spontaneously form vesicles, creating primitive cell membranes.

• RNA World Hypothesis: Suggests that RNA was the first genetic material, capable of storing information and catalyzing reactions.

Example: Laboratory experiments have shown that amino acids and nucleotides can form under simulated early Earth conditions.

Oldest Evidence of Life & Photosynthesis

Fossil evidence provides insight into the earliest life forms and the development of photosynthesis, which dramatically altered Earth's environment.

• Stromatolites: Layered structures formed by cyanobacteria, dating back over 3.5 billion years.

• Photosynthesis: The process by which organisms convert light energy into chemical energy, producing oxygen as a byproduct.

• Oxygen Revolution: The accumulation of oxygen in the atmosphere (~2.4 billion years ago) led to the extinction of many anaerobic organisms and the formation of banded iron formations (BIFs).m

Example: Banded iron formations are sedimentary rocks with alternating layers of iron-rich minerals, evidence of early photosynthetic activity.

Phylogeny and the Tree of Life

Classification of Living Things

Biologists classify organisms to organize the diversity of life and reflect evolutionary relationships.

• Taxonomy: The science of naming, describing0, and classifying organisms.

• Linnaean System: Hierarchical classification: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

• Binomial Nomenclature: Each species is given a two-part scientific name (Genus species), e.g., Homo sapiens.

Expressing Relationships as Trees

Evolutionary relationships are depicted as branching diagrams called phylogenetic trees.

• Phylogeny: The evolutionary history of a species or group of species.

• Phylogenetic Tree: Diagram showing evolutionary relationships; branches represent lineages, and nodes represent common ancestors.

• Clade: A group consisting of an ancestor and all its descendants (monophyletic group).

Example: The Tree of Life diagram shows the relationships among major groups such as Bacteria, Archaea, and Eukarya.

Cladistics and Homology

Cladistics is a method of inferring evolutionary relationships based on shared derived characteristics (synapomorphies).

• Homology: Similarity due to shared ancestry (e.g., vertebrate forelimbs).

• Analogy: Similarity due to convergent evolution, not common ancestry (e.g., wings of bats and insects).

• Synapomorphy: A shared, derived character unique to a particular clade.

• Types of Taxa:

  • Monophyletic: Includes ancestor and all descendants.

  • Paraphyletic: Includes ancestor and some, but not all, descendants.

  • Polyphyletic: Includes taxa with different ancestors.

Inferring Phylogenies

Phylogenies are inferred using morphological, molecular, and fossil data.

• Character Tables: Used to record the presence (1) or absence (0) of traits among taxa.

• Outgroup Comparison: An outgroup is used to infer the ancestral state of characters.

• Molecular Phylogenetics: DNA and protein sequences provide additional data for constructing phylogenies.

Example: Molecular clocks estimate divergence times based on the rate of genetic mutations.

Molecular Clocks

Molecular clocks use the rate of genetic mutations to estimate the timing of evolutionary events.

• Constant Rate: Some genes evolve at a relatively constant rate, allowing estimation of divergence times.

• Calibration: Fossil evidence is used to calibrate molecular clocks.

Equation:

Summary Table: Types of Taxonomic Groups

Key Terms

• Stromatolite: Layered sedimentary formation created by cyanobacteria.

• Banded Iron Formation (BIF): Sedimentary rock with alternating iron-rich layers, evidence of early oxygen production.

• Clade: A group of organisms that includes an ancestor and all its descendants.

• Synapomorphy: A shared, derived trait unique to a clade.

• Homology: Similarity due to shared ancestry.

• Analogy: Similarity due to convergent evolution.