Life

Defining Life: A Journey Through Time and Science

Life is a quality that distinguishes matter with biological processes from non-biological matter. It is characterized by homeostasis, organisation, metabolism, growth, adaptation, response to stimuli, and reproduction. The study of life has been ongoing since ancient times, with various philosophical definitions proposed, such as self-organizing systems. Life exists everywhere on Earth in different environments, including harsh ecosystems occupied by extremophiles.

The Origin and Evolution of Life

Life is believed to have originated at least 3.5 billion years ago, resulting from a universal common ancestor that evolved into all existing species through extinct species and fossil records. Modern classification began with Carl Linnaeus’s binomial nomenclature system in the 1740s. Living organisms are composed of biochemical molecules, including proteins and nucleic acids, which carry genetic information and serve as machinery for chemical processes.

The Cell: The Fundamental Unit of Life

The cell is the fundamental unit of life, ranging from single cells in smaller organisms to multicellular structures in larger ones. Life is unique to Earth but extraterrestrial life is thought possible, with artificial life being simulated and explored by scientists and engineers.

Definitions of Life: Philosophical and Scientific Perspectives

The definition of life has long been a challenge for scientists and philosophers, due in part to its processual nature and lack of knowledge about extraterrestrial life. Philosophical and legal definitions have also struggled with distinguishing living things from non-living entities, leading to over 123 proposed definitions. Life is considered a characteristic that preserves, furthers or reinforces its existence in the given environment, characterized by homeostasis, organisation, metabolism, growth, adaptation, response to stimuli, and reproduction.

From Materialism to Vitalism

Some of the earliest theories of life were materialist, holding that all that exists is matter, and that life is merely a complex form or arrangement of matter. Empedocles (430 BC) argued that everything in the universe is made up of a combination of four eternal ‘elements’ or ‘roots of all’: earth, water, air, and fire.

Democritus (460 BC) was an atomist; he thought that the essential characteristic of life was having a soul (psyche), and that the soul, like everything else, was composed of fiery atoms. Plato, in contrast, held that the world was organised by permanent forms, reflected imperfectly in matter; forms provided direction or intelligence, explaining the regularities observed in the world.

The mechanistic materialism that originated in ancient Greece was revived and revised by the French philosopher René Descartes (1596–1650), who held that animals and humans were assemblages of parts that together functioned as a machine. This idea was developed further by Julien Offray de La Mettrie (1709–1750) in his book L’Homme Machine. In the 19th century, advances in cell theory encouraged this view.

The evolutionary theory of Charles Darwin (1859) is a mechanistic explanation for the origin of species by means of natural selection. At the beginning of the 20th century, Stéphane Leduc (1853–1939) promoted the idea that biological processes could be understood in terms of physics and chemistry, and that their growth resembled that of inorganic crystals immersed in solutions of sodium silicate.

Vitalism: The Life-Principle Theory

Vitalism is the belief that there is a non-material life-principle. This originated with Georg Ernst Stahl (17th century) and remained popular until the middle of the 19th century. It appealed to philosophers, anatomists, chemists, and others who believed in a fundamental difference between organic and inorganic material. The idea was disproved by Friedrich Wöhler’s synthesis of urea from inorganic materials in 1828. Later discoveries, such as Hermann von Helmholtz’s demonstration that no energy is lost in muscle movement, led to the abandonment of vitalistic theories.

The Diversity and Extremes of Life

Microorganisms have dominated Earth’s habitable environment for most of its existence, influencing the physical-chemical environment through metabolism and evolution. These changes affected subsequent life forms, such as the release of oxygen by cyanobacteria, which induced global environmental changes, ultimately leading to the evolution of major animal and plant species.

The biosphere is the global sum of all ecosystems, a closed system largely self-regulating. Organisms exist everywhere, including soil, hot springs, inside rocks, deep ocean, and high atmosphere. Life forms thrive in extreme environments like the Mariana Trench and Antarctica’s ice below 800m depth. Microbes survive in extreme conditions and are extremely adaptable.

Classification of Life

The first classification of organisms was made by Aristotle (384–322 BC), who grouped living things into two categories: plants and animals, based on their ability to move. Animals were divided into five groups: viviparous quadrupeds (mammals), oviparous quadrupeds (reptiles and amphibians), birds, fishes, and whales. Bloodless animals were divided into five groups, including cephalopods, crustaceans, insects, shelled animals, and ‘zoophytes.’

In the late 1740s, Carl Linnaeus introduced binomial nomenclature for species classification. Fungi were initially classified as plants but later reassigned to their own kingdom in Whittaker’s five-kingdom system, due to their evolutionary relationship with animals.

The Core Elements of Life

All life forms require six core chemical elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. These elements are necessary for nucleic acids, proteins, and lipids, which make up living matter. Carbon is the most abundant element and forms stable covalent bonds, allowing for a wide variety of organic molecules.

Deoxyribonucleic acid (DNA) is a molecule that carries genetic instructions for growth, development, and reproduction in all known living organisms and many viruses. Most DNA consists of two biopolymer strands coiled around each other to form a double helix, with nucleotides joined by sugar-phosphate bonds. The base pairing rules A-T and C-G create hydrogen bonds between nitrogenous bases, allowing each strand to recreate the other.

Cells are the basic unit of structure in every living thing, arising from pre-existing cells by division. The activity of an organism depends on its cells’ total activity and energy flow within and between them. Cells contain hereditary information that is carried forward as a genetic code during cell division.

The Evolutionary Journey

Evolution is the change in heritable characteristics of biological populations over successive generations. It results in the appearance of new species and often the disappearance of old ones. Evolution occurs when evolutionary processes such as natural selection (including sexual selection) and genetic drift act on genetic variation, resulting in certain characteristics increasing or decreasing in frequency within a population over successive generations.

The diversity of life on Earth results from a dynamic balance between genetic opportunity, metabolic capability, environmental challenges, and symbiosis. Microorganisms have dominated Earth’s habitable environment for most of its existence, influencing the physical-chemical environment through metabolism and evolution. These changes affected subsequent life forms, such as the release of oxygen by cyanobacteria, which induced global environmental changes, ultimately leading to the evolution of major animal and plant species.

Artificial Life: The Future of Biology

Artificial life is the simulation of any aspect of life through computers, robotics, or biochemistry. Synthetic biology combines science and biological engineering to design new biological functions and systems not found in nature, with goals including processing information, manipulating chemicals, producing energy, providing food, and enhancing human health and the environment.

Life on Earth is a fascinating journey that continues to unfold. From the first simple organisms to the complex ecosystems we see today, life has shown incredible adaptability and resilience. As we continue to explore the depths of our planet and beyond, the mysteries of existence remain as captivating as ever. The quest for understanding life’s essence drives us forward, pushing the boundaries of what we know and can achieve.