Science's Genuine Controversies
What Is Life? Where Did It Come From?

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What is life? Where did it come from? These are probably the two most difficult questions in all of biology, and both are hotly debated.

The Biology 101 definition of "life" would include any self-replicating creature that can grow and evolve. But this is problematic because it excludes viruses and other obligate intracellular parasites that require a host to survive. And what about self-replicating pieces of DNA called transposons ("jumping genes"), or self-replicating proteins (e.g., prions, which can cause mad cow disease)? Do they qualify as life?

Even trickier than defining life is determining how it evolved in the first place. The early Earth was an inhospitable mix of inorganic compounds. In some way, life emerged, but how? The leading hypothesis is known as "RNA World," in which self-replicating RNA molecules served as the precursors of cellular life. But this idea, like all hypotheses of abiogenesis (and there are many such hypotheses), suffers from a lack of convincing evidence. In fact, bioinformatics professor Gustavo Caetano-Anollés believes that the RNA World hypothesis is incompatible with the rest of evolutionary biology.

All hypotheses of abiogenesis must address enormous gaps in our knowledge of the origin of life. For example, how do self-replicating, organic molecules arise from non-replicating, inorganic compounds? How do information-containing molecules, such as DNA, evolve? More fundamentally, how does information (e.g., the genetic code) evolve? How did these information-containing molecules become incorporated into primordial cell-like structures?

These represent just a handful of the big questions facing the field. Given the associated perplexities, they will surely keep researchers busy for decades to come.

(Image: Hydrothermal vent via Wikimedia Commons)

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