Origin of Life on Earth – AP Biology College Board Revision

Understanding the Origin of Life on Earth

The Origin of Life on Earth is a topic of great interest and the subject of significant ongoing research.
It’s widely accepted that life began approximately 3.5 to 4 billion years ago, shortly after the Earth’s formation, but the exact process by which this occurred is still debated among scientists.
One prevailing theory is that life began in the ocean, in deep-sea vents or ‘hot springs’, where the right combination of chemical ingredients and energy sources were present.
This process is known as abiogenesis, the original evolution of life or living organisms from inorganic or inanimate substances.

Panspermia: This theory suggests life may have been carried to Earth from elsewhere in the universe, such as on a meteorite.
Primordial Soup Theory: Proposed by Oparin and Haldane, this theory states that life began in a ‘primordial soup’, a body of water containing a mix of organic molecules.
Miller-Urey Experiment: In 1953, Stanley Miller and Harold Urey conducted a landmark experiment demonstrating that complex organic molecules could be produced from simpler inorganic compounds, lending support to the primordial soup theory.

The fossil record, including the discovery of microbial mats called stromatolites, provides strong evidence for the existence of ancient life forms.
Understand that fossils of the oldest known life forms are microscopic, because complex, multicellular life had not yet evolved.
Microfossils, though controversial due to their tiny sizes and overlapping features with non-biological processes, suggest the existence of life at least 3.5 billion years ago.

Almost all forms of life use DNA as the molecule of inheritance, whereas RNA serves as the intermediary in protein synthesis.
However, RNA is capable of both storing information like DNA and catalysing its own synthesis, which has led to the RNA world hypothesis — the idea that life may have started with RNA.
This theory is further supported by the discovery of ribozymes, RNA molecules that can catalyse chemical reactions.
It’s suggested that DNA was favoured over time due to its greater stability, while proteins took over the catalytic roles of RNA due to their greater versatility.