Origin and History of Life

Learning Objectives

Summarize in general terms several hypotheses for how life is thought to have originated on Earth.
Discuss the emergence of the first complex organic molecules, genetic material, and cells.
Explain the theory of chemical selection that drove the development of RNA in early Earth’s history.
Describe major environmental events on early Earth leading to the formation of the first cells and multicellular organisms.
Define fossils and radiometric dating, outlining their contributions to our understanding of prehistoric life.
Detail the origins of hominids, differentiating the traits that evolved within various species and address common misconceptions about human evolution.

Age of the Universe and Earth

The Universe: Approximately 13.7 billion years ago (bya) - Originated with the Big Bang.
The Solar System: Formed about 4.6 billion years ago (bya).
The Earth: Estimated to be around 4.55 billion years old.
Life on Earth: Emerged between 4 and 3.5 billion years ago (bya).

Prerequisites for Life

Requirements to form a living cell include:
- Polymers
- Information transfer mechanisms
- Membrane structures to protect the cell
- Cellular properties (functional characteristics of living things).

Formation of Early Cells

Origin of Organic Molecules (Step 1)
- Conditions: Early Earth conditions were conducive to spontaneous formation of organic molecules.
- Organic Molecules: Defined as molecules containing carbon that are essential for life. The early atmosphere had minimal oxygen, rich in compounds such as water vapor, methane (CH4), ammonia (NH3).
Hypotheses for Organic Molecule Formation:
- Reducing Atmosphere Hypothesis: Based on geological data, suggest a primordial atmosphere rich in reducing gases.
  - Illustrated by the Stanley Miller experiment using a chamber apparatus to simulate this environment with electrical discharge mimicking lightning, resulting in the formation of amino acids, sugars, and nitrogenous bases.
  - Criticism: Some scientists argue the actual composition of the early atmosphere may have differed significantly.
- Extraterrestrial Hypothesis: Suggests that meteorites brought organic carbon to Earth, including amino acids and nucleic acid bases.
  - Opponents highlight that much organic material would likely be destroyed during atmospheric entry.
- Deep-Sea Vent Hypothesis: Proposes that biologically important molecules formed in extreme environments between hot vent water and cold ocean water.
  - Supported by findings of complex communities that do not depend on solar energy, instead deriving energy from chemicals in the vents.
Synthesis of Polymers (Step 3)
- Location: Prebiotic synthesis may have occurred near deep-sea vents rather than in aqueous solutions due to hydrolysis interference.
- Experimental evidence shows nucleic acid polymers and polypeptides can form on clay surfaces in shallow pools.
Formation of Boundaries (Step 4)
- Protobionts: Defined as aggregates of prebiotically produced molecules and macromolecules with four key characteristics:
  - A boundary that separates internal contents from the external environment.
  - Polymers that contain information.
  - Polymers with enzymatic functions.
  - Capacity for self-replication.
- Suggestion that living cells could have evolved from structures such as coacervates (droplets formed from the association of charged polymers) and liposomes (vesicles with lipid layers).
RNA World Hypothesis (Step 5)
- Proposes RNA was the first macromolecule as it can store information, self-replicate, and possess enzymatic functions (worked as ribozymes).
- Chemical selection explains how certain RNA molecules could replicate more successfully due to mutations conferring catalytic abilities.
- Chemical Selection Process:
  - Process illustrated through experimental work where a large pool of RNA was synthesized, with some molecules gaining functions that allowed for increased replication rates.

Evidence Supporting Hypotheses

Chemical Evolution: Demonstrated experimentally in the laboratory by Bartel and Szostak, starting with a large pool of RNA molecules and refining through successive generations to enhance catalytic abilities.
Radiometric Dating of Fossils: Fossils formed through sedimentation processes, and radiometric dating allows us to estimate fossil age using isotope decay processes, particularly carbon-14, with a half-life of about 5,700 years. Other isotopes include potassium-40, rubidium-87, and uranium-238.

Earth’s Geological History

Earth has experienced numerous geological eons and eras:
- Precambrian: Origin of life and early cellular structures.
- Paleozoic Era: Dominated by vertebrates, plants, and significant geological and biological transitions.
- Notable events include the Cambrian explosion, large terrestrial colonization by plants/animals, and several mass extinction events.
- Mesozoic Era: Rise of dinosaurs and mammals, characterized by a hot climate and significant evolutionary changes.
- Cenozoic Era: Post-dinosaur age leading to the spread of mammals, particularly hominids leading to humans, with the appearance of Homo sapiens approximately 200,000 years ago.

Hominid Evolution

Hominids include various species, beginning with ancestors like Australopithecus afarensis (“Lucy”) to more advanced species like Homo heidelbergensis and Homo sapiens.
Key Traits in Hominid Evolution: Bipedalism, larger brain size, and anatomical adaptations for efficient locomotion and survival.
Genetic Similarities and Variations: Humans share about 99.9% of genetic makeup, with greater variation found among other mammalian species.

Unpacking Human Evolution

Humans did not evolve from modern monkeys but share a common ancestral lineage. The evolution was not linear but branch-like, with many species interacting and coexisting.
Key evolutionary adaptations include bipedalism, brain development, and changes in dental structures. Current evidence shows that humans continue to evolve, mainly influenced by environmental pressures and social structures.