Origin and History of Life: Biological Diversity (BSC 2011)

Origin and History of Life

Introduction

The study of the origin and history of life explores how life began on Earth, the major transitions in biological evolution, and the environmental and genetic factors that have shaped the diversity of living organisms. This topic integrates evidence from geology, paleontology, molecular biology, and evolutionary developmental biology to reconstruct the timeline and mechanisms of life's emergence and diversification.

Stages in the Origin of Life

Key Stages in the Origin of Life

• Stage 1: Synthesis of Organic Molecules - Simple organic molecules such as nucleotides and amino acids are thought to have formed spontaneously on the primitive Earth. - Hypotheses for the origin of these molecules include synthesis in the atmosphere, delivery by meteorites (extraterrestrial hypothesis), and formation at deep-sea hydrothermal vents (deep-sea vent hypothesis).

• Stage 2: Polymerization into Macromolecules - Monomers polymerized to form larger molecules such as proteins and nucleic acids. - Polymerization is more favorable on surfaces such as clay or at hydrothermal vents, as aqueous solutions tend to favor hydrolysis over polymerization.

• Stage 3: Formation of Boundaries (Protocells) - Aggregates of molecules became enclosed by membranes (e.g., lipid bilayers), forming protocells that maintained distinct internal environments.

• Stage 4: Origin of Self-Replicating Molecules - The emergence of molecules capable of self-replication, such as RNA, enabled inheritance and evolution.

Why RNA Was Likely the First Genetic Material

• Information Storage: RNA can store genetic information.

• Self-Replication: Some RNA molecules can catalyze their own replication.

• Enzymatic Function: RNA molecules called ribozymes can catalyze chemical reactions.

• Transition to DNA/Protein World: DNA eventually took over information storage due to its stability, while proteins became the main catalysts due to their greater catalytic potential.

Fossil Record and Dating

Fossils and Their Formation

• Fossils are preserved remains or traces of past life, typically found in sedimentary rocks.

• Organisms are more likely to be fossilized if they have hard body parts and are buried quickly.

• Older fossils are found deeper in the rock layers.

Radiometric Dating

• Uses the decay of radioactive isotopes to estimate the age of rocks and fossils.

• Half-life is the time required for half of a radioactive isotope to decay.

Key Equation:

$N = N_0 \left(\frac{1}{2}\right)^{t/T_{1/2}}$

Where: N = remaining quantity of isotope N0 = initial quantity t = elapsed time T1/2 = half-life

Common Radioisotopes Used in Dating

Radioisotope	Decay Product	Half-life (years)	Useful Dating Range (years)
Carbon-14	Nitrogen-14	5,730	100–30,000
Potassium-40	Argon-40	1.3 billion	100,000–4.5 billion
Rubidium-87	Strontium-87	47 billion	10 million–4.5 billion
Uranium-235	Lead-207	704 million	10 million–4.5 billion
Uranium-238	Lead-206	4.5 billion	10 million–4.5 billion

Biases in the Fossil Record

The fossil record is incomplete and biased due to several factors:

Factor	Description
Anatomy	Organisms with hard body parts are more likely to be preserved.
Size	Larger organisms are more likely to be found as fossils.
Number	Species that were abundant or widespread are more likely to be preserved.
Environment	Marine or near-water species are more likely to fossilize than inland species.
Time	Organisms that lived recently or for long periods are more likely to be found.
Geology	Certain chemical conditions favor fossilization.
Paleontology	Researcher interest and search locations introduce bias.

Major Events in the History of Life

Timeline of Life on Earth

• Earth forms: ~4.6 billion years ago (BYA)

• First prokaryotes: ~3.5–4 BYA

• First eukaryotes: ~2 BYA

• Multicellular eukaryotes: ~1.5 BYA

• Animals: less than 1 BYA

• Plants colonize land: before animals

• Humans: very recent in Earth's history

Major Environmental Changes

• Climate and temperature fluctuations

• Atmospheric composition changes (e.g., rise of oxygen)

• Continental drift and landmass formation

• Floods, glaciations, volcanic eruptions, and meteor impacts

These changes have driven evolutionary innovation and mass extinctions.

Mass Extinctions and Adaptive Radiations

• Mass extinctions are periods when large numbers of species go extinct in a short time.

• Often followed by adaptive radiations, where surviving groups diversify to fill ecological niches.

Evolution of Eukaryotic Cells and Multicellularity

Endosymbiotic Theory

• Eukaryotic cells likely evolved from a symbiotic relationship between ancestral prokaryotic cells (bacteria and archaea).

• Mitochondria and chloroplasts originated as free-living bacteria that were engulfed by a host cell.

• Evidence includes double membranes and their own DNA in these organelles.

Origin of Multicellularity

• Multicellularity may have evolved by aggregation of unicellular organisms or by cells remaining together after division.

• Colonial forms likely served as intermediates.

Evolutionary Developmental Biology (Evo-Devo)

Role of Developmental Genes

• Comparing embryonic development helps infer evolutionary relationships.

• Developmental genes control cell division, migration, differentiation, and death.

• Spatial and temporal variation in gene expression leads to morphological diversity.

Examples in Cetaceans (Whales and Dolphins)

• Early embryos have hind limb buds, which are resorbed in later development due to gene regulation.

• Migration of the nose opening during development results in the blowhole position in adults.

Summary Table: Major Events in the History of Life

Event	Approximate Time (BYA)
Earth forms	4.6
First prokaryotes	3.5–4
First eukaryotes	2
Multicellular eukaryotes	1.5
Animals	<1
Plants colonize land	<1
Humans	<0.01

Additional info:

• Some details about the timing and mechanisms of early life stages are still speculative and based on experimental models.

• The "RNA world" hypothesis is widely accepted but not definitively proven.