Origin and Evolution of Life

How did we get here? How did life go from a single self-replicating molecule in a primordial soup to the 8.7 million species alive today — from bacteria to blue whales, from fungi to philosophers? Evolution is the story of that transformation. It's the central organizing principle of biology — nothing in biology makes sense except in the light of evolution. This chapter covers where life came from, how it changes over time, and how we know all of this. The origin of life is still partly mysterious, but we have a good working hypothesis. The early Earth (4.6 billion years ago) had a reducing atmosphere — methane, ammonia, hydrogen, and water vapor, but no free oxygen. Lightning, UV radiation, and volcanic heat provided energy. In the 1920s, Oparin and Haldane independently proposed that organic molecules could have formed from inorganic precursors under these conditions. In 1953, Stanley Miller and Harold Urey tested this — they simulated early Earth conditions with sparks and within a week found amino acids and other organic compounds formed spontaneously. Later experiments produced sugars, lipids, and even nucleotides. The RNA world hypothesis suggests that RNA, which can both store information and catalyze reactions, was the first molecule of life. Darwin's theory of evolution by natural selection is the bedrock of modern biology. Darwin observed that organisms produce more offspring than can survive, individuals vary in their traits, and some variations give advantages in survival and reproduction. Those with advantageous traits survive and reproduce more, passing those traits to the next generation. Over immense timescales, this process transforms species. The peppered moth is a textbook example: before the Industrial Revolution, light moths were common (camouflaged on lichen-covered trees), dark moths were rare. After pollution darkened tree trunks with soot, dark moths became more common because they were better camouflaged. Evidence for evolution is overwhelming. Homologous organs (same embryonic origin, different functions — like human arm, bat wing, whale flipper) show common ancestry. Analogous organs (different origins, similar functions — like butterfly wing and bird wing) show convergent evolution. Vestigial organs (wisdom teeth, appendix, coccyx) are evolutionary leftovers. Fossils provide a timeline of life's history. Embryological development shows striking similarities across vertebrates. And molecular evidence is the clincher — we share 98% of our DNA with chimpanzees. The Hardy-Weinberg principle describes a non-evolving population — one where allele frequencies stay constant across generations. The conditions for Hardy-Weinberg equilibrium are: no mutation, no gene flow, random mating, no natural selection, and infinitely large population size. Since these conditions are never all met in nature, real populations always evolve. Speciation is how one species splits into two. It requires reproductive isolation — populations must stop interbreeding. Allopatric speciation happens when a physical barrier separates populations. Sympatric speciation happens without physical separation. Darwin's finches in the Galapagos are a classic example of adaptive radiation. Human evolution is our own story. We share a common ancestor with chimpanzees about 6-7 million years ago. The lineage: Australopithecus afarensis (Lucy, walked upright, 3.2 mya) to Homo habilis (first toolmaker, 2.4 mya) to Homo erectus (mastered fire, first to leave Africa, 1.8 mya) to Neanderthals to Homo sapiens (modern humans, 300,000 years ago).