20.2: Thinking about Life's Origins- A Short Summary of a Long History
By all accounts, the earth must have been a very unpleasant place soon after its formation! For that reason, the period from 4.8 to 4.0 billion years ago is called the Hadean Eon, after Hades, the hell of the ancient Greeks! Until recently, geological, geochemical and fossil evidence suggested that life arose between 3.8 and 4.1 billion years ago. The 2017 discovery of 3.95 billion year-old sedimentary rocks in Labrador with evidence of life, points to an even earlier origin of life, (see From Canada Comes the Oldest Evidence of Life on Earth ). In fact, questions about life’s origins are probably “as old as the hills…” or at least as old as the ancient Greeks! We only have records of human notions of life’s origins dating from biblical accounts and, just a bit later, from Aristotle’s musings. While Aristotle did not suggest that life began in hell, he and other ancient Greeks did speculate about life’s origins by spontaneous generation , in the sense of abiogenesis ( life originating from non-life ). He further speculated that the origins of life were gradual.
Later, the dominant theological accounts of creation in Europe in the middle ages muted any notions of origins and evolution. While a few mediaeval voices ran counter to strict biblical readings of the creation stories, it was not until the Renaissance in the 14th-17th century that an appreciation of ancient Greek humanism was reawakened, and with it, scientific curiosity and the ability to engage in rational questioning and research.
Many will recall that Louis Pasteur in the mid-19th century put to rest any lingering notions of life forming from dead (e.g., rotten, or fecal) matter. He showed that life would not form in sterilized nutrient solutions unless the broth was exposed to the air. Fewer know that much earlier, Anton Van Leeuwenhoek, the 17th century microscopist who first described bacteria and animalcules , mostly protozoa in pond water, had already tested the notion of spontaneous generation. By observing open and sealed containers of meat over time, he became convinced that ‘large’ animals like fleas and frogs do not arise de novo from putrid meat or slime. He also declared that insects come from other insects, and not from the flowers that they visited.
No lesser light than Charles Darwin suggested in 1859 that life might have begun in a " warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, &c., present, that a proteine compound was chemically formed ready to undergo still more complex changes. " He even realized that these chemical constituents would not have survived in the atmosphere and waters of his day, but must have done so in a prebiotic world. In On the Origin of Species , he referred to life having been ‘created’. There, Darwin was not referring to a biblical basis of creation; he clearly meant that life originated “ by some wholly unknown process " at a time before which there was no life. Finally, Pasteur’s 1861 contribution was the irrefutable, definitive proof that ‘invisible’ microbial life likewise did not arise by spontaneous generation. Thus for creatures already on earth, they could only arise by biogenesis ( life - from - life ), the opposite of abiogenesis , a term that now applies to only the first origins of life!
Among Darwin’s friends and contemporaries were Charles Lyell and Roderick Murchison, both geologists who understood much about the slow geological changes that shaped the earth. Darwin was therefore familiar with the concept of extended periods of geological time, amounts of time he believed was necessary for the natural selection of traits leading to species divergence.
Fast-forward to the 1920s when J.H.B.S. Haldane and A. Oparin offered an hypothesis about the life’s origins based on notions of the chemistry and physical conditions that might have existed on a prebiotic earth . Their proposal assumed that the earth’s atmosphere was hot, hellish and reducing (i.e., filled with inorganic molecules able to give up electrons and hydrogens). There are more than a few hypotheses for which chemicals were already present on earth, or that formed when the planet formed about 4.8 billion years ago. We’ll start our exploration with Oparin and Haldane’s reducing atmosphere . Then we look at the possibility that life began under non-reducing conditions (with passing reference to a few other ideas).