Foundations of Life: Characteristics, Classification, and Evolution

Characteristics of Life

Defining Life: What Does It Mean to Be Alive?

Biologists use a set of common features to distinguish living things from non-living matter. These characteristics help define what it means to be alive and are used to classify and study organisms.

• Organized Structure: All living things are composed of one or more cells, which are considered the basic units of structure and function in organisms.

• Regulation or Homeostasis: Living organisms maintain a stable internal environment despite changes in the external environment.

• Metabolism (Use of Energy): Life requires energy to carry out processes such as growth, reproduction, and maintenance. Organisms obtain energy through various means, such as photosynthesis or cellular respiration.

• Growth, Reproduction, and Death: Living things grow, reproduce to create new individuals, and eventually die.

• Response to Environment: Organisms can sense and respond to stimuli in their environment.

• Evolutionary Adaptation: Populations of living organisms evolve over generations through changes in genetic information, leading to adaptations that enhance survival and reproduction.

Are Viruses Alive?

Viruses are submicroscopic infectious agents that contain genetic material (DNA or RNA) and can evolve over time. However, they lack many characteristics of life, such as independent metabolism and cellular structure, and cannot reproduce without a host cell. The debate over whether viruses are alive centers on which traits are considered essential for life.

• Support for Viruses as Living: Contain genetic material, can evolve, and have an organized structure.

• Arguments Against: Cannot maintain homeostasis, do not metabolize or grow, cannot reproduce independently, and lack cellular structures.

Example: Influenza virus requires a host cell to replicate and does not carry out metabolic processes on its own.

Scientific Inquiry and the Nature of Science

How Science Works

Science is a systematic process for understanding the natural world through observation, experimentation, and reasoning. It relies on testable hypotheses and theories.

• Hypothesis: A testable, falsifiable statement that explains a phenomenon or answers a scientific question.

• Theory: A well-substantiated explanation of some aspect of the natural world, based on a body of evidence and repeatedly tested hypotheses.

• Limitations of Science: Science cannot answer questions about supernatural phenomena, moral values, or subjective experiences.

Classification of Life

Broad Categories of Life

Living organisms are classified into broad categories based on cellular structure and evolutionary relationships.

• Prokaryotes: Organisms without a membrane-bound nucleus. Includes Bacteria and Archaea.

• Eukaryotes: Organisms with a membrane-bound nucleus. Includes Animals, Plants, Fungi, and Protists.

The Three Domains of Life

All life is classified into three domains based on genetic and cellular differences.

Domain	Cell Type	Examples
Bacteria	Prokaryotic	Escherichia coli, Cyanobacteria
Archaea	Prokaryotic	Halophiles, Thermophiles
Eukarya	Eukaryotic	Animals, Plants, Fungi, Protists

Unity and Diversity of Life

Unity: Shared Features

Despite the diversity of life, all organisms share certain fundamental features, such as the universal genetic code (DNA), similar cellular structures, and common metabolic pathways.

• Universal Genetic Code: All known organisms use DNA as their genetic material.

• Cellular Organization: All life is cellular in nature, whether prokaryotic or eukaryotic.

Diversity: Evolutionary Adaptation

Diversity arises from evolutionary changes that occur as populations adapt to different environments over time.

• Evolution: The process by which populations of organisms change over generations through genetic variation and natural selection.

• Speciation: The formation of new species as populations become reproductively isolated and diverge genetically.

Evolution by Natural Selection

Darwin's Theory of Evolution

Charles Darwin proposed that evolution occurs by natural selection, a process in which individuals with advantageous traits are more likely to survive and reproduce.

• Variation: Individuals in a population vary in their traits, often due to mutations.

• Heritability: Some traits are heritable and can be passed to offspring.

• Overproduction: More individuals are born than can survive.

• Selection: Individuals with traits that confer a survival or reproductive advantage are more likely to pass those traits to the next generation.

Example: Darwin's finches on the Galápagos Islands evolved different beak shapes to exploit different food sources, leading to speciation.

Speciation and Adaptive Radiation

Speciation can occur when populations become isolated and adapt to different environments, sometimes resulting in the rapid emergence of many new species (adaptive radiation).

Chemical Foundations of Life

Essential Elements for Life

Life is based on a small subset of elements that are abundant and chemically suitable for building complex molecules.

• Major Elements: Oxygen (O), Carbon (C), Hydrogen (H), and Nitrogen (N) make up about 96% of the human body mass.

• Other Essential Elements: Calcium (Ca), Phosphorus (P), Potassium (K), Sulfur (S), Sodium (Na), Chlorine (Cl), and Magnesium (Mg).

• Trace Elements: Elements required in very small amounts, such as Iron (Fe), Iodine (I), and Zinc (Zn).

Element	Approximate % of Body Mass	Role in Organisms
Oxygen (O)	65%	Component of water, cellular respiration
Carbon (C)	18.5%	Backbone of organic molecules
Hydrogen (H)	9.5%	Component of water and organic molecules
Nitrogen (N)	3.3%	Component of proteins and nucleic acids
Calcium (Ca)	1.5%	Bone structure, signaling
Phosphorus (P)	1.0%	Component of nucleic acids, ATP
Potassium (K)	0.4%	Nerve function, fluid balance
Sulfur (S)	0.3%	Component of proteins
Sodium (Na)	0.2%	Nerve function, fluid balance
Chlorine (Cl)	0.2%	Fluid balance
Magnesium (Mg)	0.1%	Enzyme function

Example: Iodine deficiency can cause goiter, an enlargement of the thyroid gland.

Additional info: The chemical properties of these elements, such as their ability to form stable covalent bonds, make them especially suitable as building blocks for life.