History of Life on Earth

Key Terms and Concepts

• Abiotic synthesis: The formation of organic molecules from inorganic substances in the absence of life. This process is hypothesized to have occurred on early Earth, leading to the origin of life.

• Hydrothermal vents: Openings in the seafloor that emit hot, mineral-rich water. These environments may have provided the energy and chemicals necessary for the abiotic synthesis of organic molecules.

• Replication: The process by which molecules, such as nucleic acids, make copies of themselves. Replication is essential for heredity and evolution.

• Protocells: Simple, cell-like structures with a membrane that can maintain an internal environment different from their surroundings. Protocells are considered a possible intermediate stage in the origin of life.

• Geological eons and eras: Large divisions of Earth's history, such as the Hadean, Archaean, Proterozoic, and Phanerozoic eons, and the Paleozoic, Mesozoic, and Cenozoic eras.

• Aerobic organisms: Organisms that require oxygen for cellular respiration.

• Anaerobic organisms: Organisms that do not require oxygen and may even be poisoned by it.

• Prokaryotes: Single-celled organisms lacking a nucleus, including Bacteria and Archaea.

• Eukaryotes: Organisms whose cells contain a nucleus and other membrane-bound organelles.

• The oxygen revolution: A period about 2.4 billion years ago when oxygen produced by photosynthetic prokaryotes accumulated in the atmosphere, dramatically altering Earth's environment.

• Serial endosymbiosis: The theory that eukaryotic cells originated through a sequence of endosymbiotic events, where ancestral prokaryotes engulfed other cells that became organelles (e.g., mitochondria, chloroplasts).

• Multicellular organisms: Organisms composed of more than one cell, with cells specialized for different functions.

• Ediacaran biota: Early multicellular life forms that lived during the Ediacaran period (~635–541 million years ago).

• Cambrian explosion: A period (~541 million years ago) marked by a rapid diversification of animal life and the appearance of most major animal phyla.

• Continental drift: The movement of Earth's continents over geological time due to plate tectonics.

• Continental (tectonic) plates: Large sections of Earth's lithosphere that move and interact at their boundaries, causing earthquakes, volcanic activity, and continental drift.

• Earth’s mantle: The thick layer of rock between Earth's crust and core, involved in tectonic movements.

• Pangea: A supercontinent that existed about 250 million years ago, formed by the merging of all major landmasses.

• Mass extinction: A rapid decrease in the diversity and abundance of life on Earth, often due to catastrophic events.

• Adaptive radiation: The rapid evolution of many diverse species from a common ancestor, often following environmental changes or mass extinctions.

• Regional adaptive radiation: Adaptive radiation occurring in a specific geographic area, often after colonization of new habitats.

• Developmental genes (Homeotic genes, Hox genes): Genes that control the development of body structures and organization during embryogenesis.

• Heterochrony: Evolutionary change in the timing or rate of developmental events.

• Paedomorphosis: The retention of juvenile features in the adult form of an organism, often due to changes in developmental timing.

Origin of Life and Abiotic Synthesis

The origin of life is hypothesized to have occurred through a series of stages, beginning with the abiotic synthesis of small organic molecules.

• Abiotic synthesis experiment: The classic Miller-Urey experiment simulated early Earth conditions by combining water, methane, ammonia, and hydrogen gases with electrical sparks. This produced amino acids and other organic molecules, supporting the hypothesis that life's building blocks could form spontaneously.

• Hydrothermal vents: These environments may have provided the necessary energy and chemicals for the synthesis of organic molecules, possibly leading to the formation of protocells.

Fossilization and Early Life Forms

• Fossilization: Hard-bodied organisms (e.g., those with shells or bones) are more likely to be preserved as fossils than soft-bodied organisms.

• Earliest fossils: The oldest known fossils are stromatolites, layered structures formed by cyanobacteria, dating back over 3.5 billion years.

Key Events in the Evolution of Life

Major milestones in the history of life include:

• Origin of prokaryotes (~3.5 billion years ago)

• Oxygen revolution (~2.4 billion years ago)

• Origin of eukaryotes (~1.8 billion years ago)

• Origin of multicellular organisms (~1.2 billion years ago)

• Ediacaran biota (~635–541 million years ago)

• Cambrian explosion (~541 million years ago)

• Colonization of land by plants, fungi, and animals (~500–400 million years ago)

The Oxygen Revolution

• Caused by photosynthetic cyanobacteria releasing oxygen as a byproduct.

• Oxygen accumulated in the atmosphere, leading to the extinction of many anaerobic organisms and enabling the evolution of aerobic respiration.

Evolution of Eukaryotes

• Serial endosymbiosis: Eukaryotes are thought to have evolved when ancestral prokaryotes engulfed other cells that became mitochondria and chloroplasts.

• Evidence includes similarities between these organelles and certain prokaryotes (e.g., double membranes, their own DNA).

Multicellularity

• All multicellular organisms are eukaryotes; prokaryotes are unicellular or form simple colonies.

• Multicellularity allowed for cell specialization and the evolution of complex life forms.

The Cambrian Explosion

• Marked by the rapid appearance of most major animal groups (phyla).

• New adaptations included hard shells, predation, and complex body plans.

Continental Drift and Plate Tectonics

• Movement of tectonic plates reshaped continents and oceans, influencing climate and the distribution of organisms.

• Formation and breakup of supercontinents like Pangea led to isolation and diversification of species.

Colonization of Land

• First colonizers were likely photosynthetic organisms (cyanobacteria, algae), followed by plants, fungi, and animals.

• Adaptations for life on land included waterproof coverings, support structures, and reproductive strategies independent of water.

Mass Extinctions

• Permian extinction: Occurred ~252 million years ago; the largest mass extinction, eliminating about 96% of marine species.

• Cretaceous extinction: Occurred ~66 million years ago; led to the extinction of non-avian dinosaurs and many marine species, likely caused by an asteroid impact.

Effects of Mass Extinctions and Adaptive Radiations

• Mass extinctions remove dominant groups, opening ecological niches for surviving lineages to diversify (adaptive radiation).

• Example: After the Cretaceous extinction, mammals diversified and became the dominant terrestrial animals.

Developmental Genes and Evolutionary Change

• Changes in developmental genes (e.g., Hox genes) can lead to major changes in body form and structure.

• Heterochrony: Alterations in the timing of developmental events can produce new features (e.g., paedomorphosis in salamanders, where adults retain juvenile traits).

Summary Table: Major Events in the History of Life

Additional info:

• Developmental genes such as Hox genes are highly conserved and play a critical role in determining the body plan of animals.

• Adaptive radiations can also occur when organisms colonize new environments, such as Darwin's finches on the Galápagos Islands.