What Constitutes Life?

Life’s definition is not universally agreed upon, though a common description describes it as a condition distinguishing living organisms from inorganic matter, which includes growth, reproduction, and ongoing change until death.

The scientific community has struggled to reach a consensus about what exactly constitutes life. For instance, the classification of viruses as living entities remains contentious.

If we cannot delineate life on Earth, how can we define it on other planets? This is a primary consideration.

To grasp our current understanding of life, we should revisit theories regarding the origin of biological entities on our planet.

The Earth took shape approximately 4.5 billion years ago. For eons, it faced asteroid bombardment, extreme temperatures, and water existed only as vapor. Life likely first appeared around 3.9 billion years ago when conditions became favorable enough for water to form oceans.

The Oparin-Haldane hypothesis posits that life arose from non-living matter through gradual chemical evolution. This theory suggests that simple organic molecules reacted to form essential components like amino acids and nucleotides, which accumulated in oceans. Over time, these building blocks combined to create more complex structures capable of self-sustenance and replication.

The Khan Academy explains that scientists believe the RNA world commenced when an RNA molecule capable of self-replication emerged, leading to the evolution of DNA and proteins. Proteins began to catalyze chemical reactions within cells, while DNA, being more stable than RNA, took on the role of genetic information storage.

Though not all aspects of this theory are unanimously accepted, experiments have shown that complex structures can form from the right chemical building blocks and energy sources.

Recent research indicates that life could potentially utilize radiation, including galactic cosmic rays, to support biological processes.

Understanding the Essence of Life

“All living things contain DNA within their cells. Indeed, nearly every cell in a multicellular organism holds a complete set of DNA necessary for that organism. DNA not only dictates the structure and function of living beings but also serves as the primary hereditary unit across all life forms,” according to Nature Education.

This brings us to the core question of how life — at its most fundamental level — acquired the ability to replicate.

There are two primary hypotheses regarding the emergence of self-replicating organisms: the genes-first hypothesis and the metabolism-first hypothesis.

• Genes-first hypothesis: The earliest life forms were self-replicating nucleic acids like DNA or RNA, with metabolic elements added later.
• RNA world hypothesis: Some scientists propose that RNA, rather than DNA, was the first genetic material, possibly originating from simpler RNA-like molecules capable of information storage.
• Metabolism-first hypothesis: This theory suggests that self-sustaining metabolic networks may have represented the earliest living organisms, potentially enabling the construction of complex molecules like proteins and nucleic acids.

Is DNA Essential for Life?

Based on our current understanding, the answer is affirmative. DNA encodes the necessary information to construct and sustain an organism, and all known life on Earth contains DNA within its cells.

Beyond defining structure and function, DNA acts as the primary hereditary unit, storing and passing information across generations.

Nonetheless, scientists propose that DNA and RNA may not be the sole means of information storage.

Research involving synthetic DNA has suggested that life could potentially evolve without it. Known as xeno nucleic acid (XNA), these synthetic genetic codes, while not found in nature, can perform some functions akin to their biological counterparts.

Richard Dawkins stated, “At the deepest level, all living things that have ever been examined share the same DNA code and many of the same genes.”

Although these XNA structures do not yet possess self-replicating capabilities, they function as catalysts, facilitating specific chemical reactions and assembling fragments of genetic code.

XNA enzymes can manipulate RNA similarly to natural enzymes, demonstrating that, like ancient RNA, XNA can catalyze reactions even if it cannot replicate itself as RNA does.

Perhaps DNA became the predominant molecule during evolution due to its superior properties at that time.

Concluding Thoughts on Life's Genetic Blueprint

Many aspects of life and heredity remain elusive, making it challenging to speculate about the existence of life on other planets.

Chemists recognize over 50 million distinct chemicals, with approximately half derived from carbon. This element exhibits an extraordinary capacity to bond with various atoms in myriad shapes and forms.

While carbon is not the only conceivable foundation for a living system, it serves as a versatile building block in chemistry. The double-helix structure of DNA efficiently carries and duplicates genetic information.

It is conceivable that on other planets, unique environmental conditions could give rise to life forms with different genetic architectures. The double helix may be supplanted by spherical, cubic, or ribbon-like structures.

However, a carbon-based informational framework akin to DNA is likely to be central to their biological makeup.

Dr. Ana Luiza Dias holds a Ph.D. in Psychobiology (Sleep Sciences) and specializes in Biotechnology at the Federal University of São Paulo, Brazil. She is dedicated to science, nature, and biotechnology, aiming to enhance people’s health and quality of life.

James Maynard is the founder and publisher of The Cosmic Companion. A New England native, he now resides in Tucson with his wife, Nicole, and their cat, Max.

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