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Biological membranes evolved around 4-3.5 billion years ago and represent the second stage in the evolution of life on planet Earth, after complex organic molecules (which developed around 4 billion years ago). These complex organic molecules, including RNA, lipids and proteins, formed the first membranes. RNA molecules served as templates to bind amino acids into polypeptides. These complex macromolecules are ampiphilic (i.e. have both hydrophilic and hydrophobic ends) and in water orient themselves to form macrospheres (with hydrophilic ends pointing outwards and hydrophobic ends pointing inwards). These primitive membranes were somewhat selective and allowed the entry of water soluble molecules and prohibited entry of fat-soluble molecules. Simple chemical reactions taking place inside these membranes set in motion the first cellular reproduction.
Around 3.5-2.5 billion years ago, prokaryotic heterotrophic cells began to emerge. These cells had no nucleus and no membrane bound organelle. They took molecules such as glucose and proteins, directly from the environment to generate energy and synthesize molecules required for growth. Eventually their food sources ran out and they were replaced by autotrophic cells.
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