Life Science: Origin of Life and Characteristics of Living Things

Life Science

The branch of science that studies living organisms and life processes—including how living things are structured, how they function, how they grow and reproduce, how they interact with their environment, and how they have changed over time.

Age of the Earth

The scientifically estimated age of planet Earth, which is said to be over $4.5\text{ billion years}$ old.

Zircon Crystal

A naturally occurring mineral commonly found in igneous, metamorphic, and sedimentary rocks that is very resistant to weathering; estimated at $4.3\text{ billion years old}$, it is the oldest material found on Earth.

Gigaannum (Ga)

A unit of geological time equal to one billion ($10^9$) years, used to date major Earth events such as the emergence of cellular life around $3.8\,\text{Ga}$.

Prokaryotes

Single-celled organisms lacking a membrane-bound nucleus with which life on Earth initially began approximately $3.8\,\text{Ga}$ ago.

Arthropods

Invertebrates with jointed legs that make up about $75\%$ of all animals on Earth, playing key ecological roles as pollinators, recyclers of nutrients, scavengers, and prey.

Homo sapiens

The biological species representing modern humans, believed to have evolved around $200\,000\text{ years ago}$.

Theory of Catastrophism

The geological and biological theory asserting that Earth's landscape and fossil record have been shaped by sudden, short-lived, major violent events such as volcanoes, earthquakes, floods, and cosmic collisions.

Georges Cuvier and Alcide Dessalines d'Orbigny

French scientists who supported the Theory of Catastrophism, proposing that successive natural catastrophes eliminated past organisms and each subsequent creation consisted of new, distinct life-forms.

Theory of Abiogenesis

The hypothesized natural process and starting point of life on Earth describing how chemistry gave rise to biology without invoking supernatural causes or pre-existing organisms.

Spontaneous Generation Theory

The historical doctrine related to abiogenesis proposing that living organisms could arise naturally and spontaneously from non-living or decaying matter.

Stanley Miller’s Experiment

A $1953$ experiment that demonstrated how passing an electric spark (simulating lightning) through a mixture of simple atmospheric gases resulted in the abiotic synthesis of amino acids.

Amino Acids

Organic molecules synthesized in Miller's $1953$ prebiotic experiment, recognized as the essential building blocks of proteins and living tissues.

Theory of Biogenesis

The foundational biological principle stating that living things can arise only from other pre-existing living things ("life comes from pre-existing life").

Francesco Redi

An Italian scientist who performed controlled experiments with covered and uncovered meat flasks, helping to disprove spontaneous generation by showing that maggots do not spontaneously generate from meat.

Louis Pasteur's Broth Experiment

A landmark scientific experiment in which boiled nutrient broth stayed sterile in swan-neck flasks until tilted, conclusively proving that microorganisms come from other microorganisms rather than non-living broth.

S-shaped (Swan-neck) Tube

A curved flask opening designed by Louis Pasteur that allowed air into the flask while trapping airborne dust and microorganisms in the bend to keep the boiled broth sterile.

Cell

The fundamental structural and functional unit of all living organisms; all organisms are composed of one or more cells.

Unicellular Organism

An organism consisting of only a single cell that conducts all vital life processes independently, such as a bacterium.

Multicellular Organism

An organism composed of multiple coordinated cells that evolved over a billion years after the first single-celled life appeared.

Robert Hooke

The English scientist credited with the discovery and naming of the cell after observing non-living cork tissue under a microscope.

Matthias Schleiden

The German botanist who contributed to the cell theory by determining that all plant tissues are composed of cells.

Theodor Schwann

The German zoologist who contributed to the cell theory by establishing that all animals are composed of cells.

Rudolf Virchow

The German physician and scientist who contributed to cell theory with the principle that all living cells originate from pre-existing cells.

Eukaryotes