Life on Earth seems to have evolved continuously for at least 3,800 million years, punctuated by intervals of mass extinction. The very earliest cells were probably archaea-like, tolerant of hot acidic conditions and an oxygen-free atmosphere. Continual changes in DNA generated novel organisms, which were subject to rigorous selection by the environment. Some survived and became established, altering the chemistry of their surroundings. The environment changed. Photosynthesis opened the door to the evolution of oxygen-using organisms and consumers. Symbiosis fused disparate genomes into new and ever more complex living forms. Single-celled eukaryotes emerged, then multicellular organisms, then animals and plants. Sexual reproduction appeared, food chains were born, the land was colonised. As a result, a huge diversity of species inhabits the world today.
Life on Earth has remained astonishingly tenacious. It continues to evolve. Notwithstanding widespread future extinctions, including that of Homo sapiens, it will go on evolving until increasing solar energy output boils the oceans dry. But how did it all begin?
The origin of life remains a fascinating and elusive topic. We have even less evidence about it than we have about the origins of photosynthesis or eukaryotes or sex, yet it is the focus of more speculation and debate. It is not a single problem but a constellation of problems, none of which has been fully solved, and different people think of it in different ways. Some authors have focused on the prebiotic formation of organic molecules, or of proteins, or nucleic acids, or membrane-like structures. Others take the view that to explain the origin of life, it is sufficient to explain the origin of molecular self-replication. We suggest that the phrase "origin of life" should denote the formation of a system fitting the characterisation of "livingness" that we established earlier (see the schematic diagram at the start of chapter 10).
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