Origins and Evolution - Chapter 17
Chapter 17: Origins and Evolution
Chapter Overview
Origins of Life
The nature of the earliest cells
The divergence of microbes from common ancestors, modified by gene transfer and symbiosis
The mechanisms of microbial evolution as it unfolds in nature and in the laboratory
Forming the First Cells
Evolution: Phylogeny and Gene Transfer
Natural Selection and Adaptation
Microbial Species and Taxonomy
Symbiosis and the Origin of Mitochondria and Chloroplasts
17.1 Origins of Life
Before the first cells could evolve, several fundamental conditions were required:
Essential Elements: Necessary to compose organic molecules.
Continual Source of Energy: Primarily provided by nuclear fusion reactions within the Sun.
Temperature Range Permitting Liquid Water: Essential for metabolic reactions to occur.
Prebiotic Soup Theory: Proposes that the fundamental biomolecules of life arose spontaneously through the condensation of reduced inorganic molecules.
Early Metabolism: Involved anaerobic oxidation-reduction reactions.
Evidence for Early Life
Evidence includes stromatolites, microfossils, and biosignature molecules:
Stromatolites: Earliest forms of life with clear fossil evidence are bacterial communities known as stromatolites. These are bulbous masses of sedimentary layers of limestone (CaCO₃) that are approximately 3.4 billion years old.
17.2 Elements of Life
Formation of Major Elements: The major elements of biomolecules were formed through nuclear reactions within stars, which exploded into supernovas before the birth of our solar system.
Formation of the Solar System: Individual planets coalesced out of matter attracted by gravitational forces.
Geological Composition of Earth
Layers of the Earth:
Core: Rich in iron.
Mantle: Also rich in iron.
Outer Crust: Thin layer that supports the biosphere—the total sum of all life on Earth.
The biosphere generates oxidants, most notably O₂.
Volcanic Activity: Releases gases like carbon dioxide (CO₂) and nitrogen, forming the first atmosphere, which was thin and primarily composed of CO₂.
As Earth developed living organisms, they filled the atmosphere with gaseous N₂ and O₂, ultimately determining the composition of Earth's atmosphere.
Geological Evidence for Early Life
Evidence for life in the geological record is termed biosignature or biological signature.
Biosignature Significance: Found earlier than the oldest fossils, yet their nonbiogenic explanations limit their significance, leading researchers to seek corroborating evidence.
Hadean Eon (4.6 to 4.0 Gyr ago): Named for Hades, the ancient Greek world of the dead, marked by meteor bombardment that likely killed off incipient life multiple times before living microbes became established.
Earliest Geological Evidence
Evidence of Microbial Life: Generally accepted at 4.0–2.5 Gyr ago during the Archaean eon, where meteor bombardment was less frequent. Key evidence includes isotope ratios, stromatolites, and microfossils.
Microfossils: The earliest accepted are dated at 2.0 Gyr ago and include filamentous prokaryotes and colonial cyanobacteria. More recent strata (1.2 Gyr ago) reveal larger fossil cells comparable to modern eukaryotes like algae.
Biosignatures
Isotope Ratios: Provide quantitative evidence of biosignatures altered by biological activity.
Chemical Biosignatures: Cyanobacterial hopanoids are durable, steroid-like membrane molecules indicating the presence of life.
17.3 Carbon Isotope Deletion and Banded Iron Formations (BIFs)
The evolution of the first oxygenic phototrophs, cyanobacteria that split water to form O₂, is an extraordinary event.
Evidence for O₂ in the Biosphere: Comes from the oxidation states of minerals, especially those containing iron (Fe).
Banded iron formations (BIFs) show periods of alternating oxygen-rich and anoxic conditions through layers of iron oxide (Fe₂O₃) and silicon dioxide (SiO₂).
Distribution of Banded Iron Formations: Common sources of iron ore around the world linked to fluctuating O₂ levels in the atmosphere.
Proposed Time Line for the Origin and Evolution of Life
Earth formed during the Hadean eon approximately 4.5 Gyr ago.
Environment primarily reducing until cyanobacteria introduced O₂ to the atmosphere.
Multicellular animals and plants evolved around 0.6 Gyr ago when O₂ levels were sufficient.
17.4 Forming the First Cells
Early life models focus on:
Environment where the first cells formed.
Metabolism used by first cells for energy generation.
Hereditary material involved in early cells.
The Prebiotic Soup and Metabolism
Prebiotic Soup: Small organic molecules arose abiotically from simple reduced chemicals ignited by lightning, leading to complex macromolecules capable of self-replication and membrane compartmentalization.
Early Oxidation-Reduction Reactions: Early life forms likely gained energy through reactions involving oxidized minerals and hydrogen gas (H₂).
The RNA World Model
The RNA world model proposes that RNA fulfilled all informational and catalytic roles of DNA and proteins at early stages.
Genome sequences reveal numerous catalytic and structural RNAs.
Ribozymes, catalytic RNA molecules, exhibit enzymatic properties similar to those of proteins.
Early cells likely composed of RNA enzymes (ribozymes) which evolved to incorporate protein subunits.
Key Remnants: Original RNA may persist as nucleotide cofactors like NADH.
Unresolved questions regarding early life focus on Earth's temperature, the role of CH₄ in the early atmosphere, and the source of Earth's first cells.
17.5 Evolution: Phylogeny and Gene Transfer
Clades: Branching groups of related organisms sharing a common ancestor, indicating phylogenetic divergence.
Phylogeny: Full description of branching divergence of a species.
Mechanisms of Evolution:
Random mutations during chromosome replication.
Natural selection favors organisms that produce more offspring.
Reductive evolution considers the loss or mutation of DNA encoding unselected traits.
Molecular Clocks
Molecular clocks provide temporal information from macromolecular sequences based on new random mutations during DNA replication.
Genes that remain consistent across evolutionary time include components involved in transcription and translation, such as small subunit rRNA (SSU rRNA).
Phylogenetic Analysis: Requires alignment of homologous sequences, with multiple differences needed for generating phylogenetic trees.
Maximum Parsimony: The tree requiring the fewest changes is the best fit.
Maximum Likelihood: Probability-based assessment of how likely a tree would yield observed DNA sequences.
Phylogenetic Trees
Used to estimate evolutionary divergence, inferring the length of time since species shared a common ancestor.
Types of Trees:
Rooted: Indicates common ancestor.
Unrooted: Does not indicate common ancestry.
17.6 Microbial Species and Taxonomy
Defining Prokaryotic Species: Unlike eukaryotes, prokaryotes reproduce asexually, which leads to debates surrounding their classification and identification.
Criteria for Defining Species: Proposals include SSU rRNA similarity ≥ 95%, average nucleotide identity (ANI) ≥ 95%, and shared ecological traits.
A Pangenome comprises core genome genes present in all isolates and accessory genes found in some but not others, with open pangenomes being common in nature.
Classification and Nomenclature
Taxonomy describes distinct life forms and organizes them into different categories based on shared traits (classification, nomenclature, and identification).
As new microorganisms are discovered, they may be termed emerging, and can be designated as unclassified or uncultured organisms.
Naming a Species: New species must be isolated and grown in pure culture; these organisms are known as isolates, and candidate species are derived from these isolates.
Evolution of Endosymbiosis
Symbiosis: Involves the intimate association between unrelated species; can be mutualistic or parasitic.
Endosymbiosis: A more intimate form where one organism grows within another. This concept implicates the evolution of mitochondria and chloroplasts from ancient symbiotic relationships.
Chloroplasts evolved from cyanobacteria, while mitochondria evolved from an alphaproteobacterium.
Endosymbiotic Bacteria: Essential for growth in some parasites, indicating potential for targeted antibiotic treatments in diseases.