The Recipe of Life: Exploring the Origins and Evolution of Living Organisms
The Recipe of Life
There are 90 natural chemical elements, but only about 20 are essential for life. These elements are found in both the Earth’s crust and the human body. Living matter consists primarily of hydrogen (98%), oxygen, and carbon.
Essential Elements for Life
Water
Water is a vital solvent that facilitates chemical reactions and ensures their speed is sufficiently high.
Carbon
Carbon’s oxidation number of IV allows it to form four bonds, enabling the creation of long chains and thousands of different compounds, including carbohydrates, proteins, and nucleic acids.
Energy
Living organisms obtain energy from chemical reactions involving organic matter, which is primarily produced through photosynthesis.
Energy Acquisition in Living Organisms
Autotrophs
Autotrophs transform inorganic matter into organic matter using solar energy.
Heterotrophs
Heterotrophs obtain organic matter by consuming organisms that have already produced it.
Defining Life
Historically, the definition of life has evolved. Aristotle believed life depended on a soul, while mechanistic views saw no distinction between living organisms and machines. Vitalists proposed a “vital impulse” as a non-supernatural force distinguishing living beings.
Characteristics of Life
- Made up of cells
- Surrounded by cellular membranes
- Cells nourish, reproduce, and interact with their environment
- Molecular structure relies on proteins formed according to instructions in nucleic acids
- Ability to reproduce
Early Life and Its Origins
Early Earth
The early Earth was a hot sphere bombarded by ionizing radiation. Unlike the Sun, which is mainly hydrogen and helium, Earth had an abundance of heavier elements like calcium, magnesium, and iron. Cosmic radiation caused these elements to recombine, forming complex molecules. Eventually, a unique combination of molecules led to the first cell and the emergence of life.
Archaea
The first organisms likely resembled archaea, which thrive in extreme environments like volcanic areas and the Antarctic.
Water on Earth
Water on Earth likely came from impacts with smaller celestial bodies rather than condensation from the early solar system.
Carbon Source
Early Earth was not carbon-rich. Volcanic eruptions released carbon dioxide, which was assimilated by living beings.
Early Earth Scenarios
Scenario 1
- Hot interior
- Oceanic planet with submarine volcanism
- First organisms appeared near volcanic vents, assimilating carbon dioxide
Scenario 2
- Atmosphere without oxygen or ozone
- Harmful UV radiation reached the surface
Spontaneous Generation vs. Abiogenesis
The idea of spontaneous generation, where life arises from non-living matter, persisted until the 19th century. The invention of the microscope challenged this notion, leading to the concept of abiogenesis, where life arises from pre-existing organic molecules.
Miller-Urey Experiment
The Miller-Urey experiment simulated early Earth’s reducing atmosphere and produced organic molecules, supporting abiogenesis.
Hypotheses on the Origin of Life
Metabolic Hypothesis
Molecules interacted within membrane-bound units, eventually leading to self-replication.
RNA World Hypothesis
RNA molecules, capable of both catalysis and information storage, played a crucial role in early life.
Panspermia
Panspermia proposes that life originated elsewhere in the universe and was brought to Earth. While evidence suggests organic molecules exist in space, the question of how life initially formed remains unanswered.
Dating the Origin of Life
Sedimentary rocks containing carbon isotopes suggest life existed as early as 3.85 billion years ago.
Evolutionary Theories
Lamarckism
Lamarckism proposed that organisms adapt to their environment through acquired characteristics, which are then passed on to offspring. This theory has been largely discredited.
Darwinism
Darwin and Wallace proposed the theory of evolution by natural selection, where organisms with advantageous traits are more likely to survive and reproduce, leading to the gradual change of species over time.