2.4: Spontaneous generation and the origin of life

The diversity of organisms, estimated to be in the millions of distinct species^[52] raises two obvious questions: how did life start and why are there so many different types of organisms? At one point, people believed that these questions had a single answer. We now recognize that they are distinct questions and their scientific answers involve distinct mechanisms. Originally supernatural processes were thought to be necessary to "create" the different "forms" of life (including human beings). In the light of Cell and Evolution theories, and the accumulation of molecular (gene/genome sequence) it seems a certainty that life on Earth had a single successful origin.

How then did life originate? It was once believed by some that organisms, such as flies, frogs, and mice, arose spontaneously from non-living matter.^[53] If true, spontaneous generation would have profound implications for our understanding of biological systems. It would, presumably, involve a rather simple process. All bets are off if the origin of life process is supernatural. If each organism arose independently, we might expect that, at the molecular level, details of each would be unique, since they presumably arose independently from different stuff and under different conditions compared to other organisms. This does not appear to be the case; all known organisms use similar molecular mechanisms and are composed of structurally similar cells.

He who experiments increases knowledge. He who merely speculates piles error upon error.

- Arabic epigraph quoted by Francisco Redi.

A key event in the development of modern biology was the publication of Francesco Redi’s (1626-1697) paper “Experiments on the Generation of Insects”. His hypothesis (an observation-informed guess) was that spontaneous generation did not occur.^[54] He thought that the organisms that appeared had developed from "seeds" deposited by adults. One prediction of his hypothesis was that if adult flies were kept away from rotting meat maggots, the larval form of flies, would not appear no matter how long one waited and that the type of organism that appeared would not depend on the type of rotting meat, but rather on the type of adult fly that had access to the meat. To test his hypothesis Redi set up two sets of flasks both of which contained fresh meat. One set of flasks were sealed with paper or cloth so that no flies could enter, the "control" condition. The experimental condition was exposed to the air and so to flies. Maggots appeared only in the flasks open to the air (and flies). Redi concluded that organisms as complex as insects, and too large to pass through the cloth, could arise only from other insects, or rather from the eggs laid by those insects. Life came from life.

The invention and use of the light microscope by Antony van Leeuwenhoek (1632-1723) and Robert Hooke (1635-1703) led to the discovery of a completely new and unexpected world of organisms, known as microbes.^[55] While it was relatively easy to generate compelling evidence that macroscopic (that is, big) organisms, such as flies, mice, and people did not arise spontaneously, it seemed plausible that microscopic, and presumably much simpler, organisms could. Some began to study such "microorganisms". Lazzaro Spallanzani (1729-1799) found that when a broth was boiled and isolated from contact with the air, it remained sterile. He concluded that microscopic organisms, like larger macroscopic organisms, did not arise spontaneously but were descended from other microbes, many of which were floating in the air.

A key aspect of scientific thinking is to think carefully about the limitations of a particular observation or experiment. Are there aspects of Spallanzani's experiment that would leave open the possibility of the spontaneous origin of microbes? One possibility was that boiling the broth destroyed a key component that were necessary for the spontaneous generation, or perhaps exposure to fresh air was the "vital" ingredient. Boiling and isolation could produce an unintended effect that obscured rather than revealed the process. Louis Pasteur (1822-1895) carried out a particularly convincing set of experiments to address both concerns. He sterilized broths by boiling them in special "swan-necked" flasks. What was unique about his experimental design was the shape of the neck. Microbes in the air were trapped in the bent region of the flask’s neck (←); the result was that air but not air-borne microorganisms to reach the broth. This design enabled Pasteur to address these concerns. He found that the boiled broths, even with access to air, remained sterile for months, but when the flask neck was broken (red arrow) the broth was quickly overrun with microbial growth. He conclusion? Air, by itself, was not necessary for spontaneous generation, but rather contained microbes. The fact that the broth could support microbial growth after the neck was broken served as a “positive control”; it indicated that the heating of the broth had not destroyed some vital element needed to support growth. Positive control experiments to test whether specific assumptions are correct. For example, if we are using a drug in a study, we need to establish (rather than take someone's word for it) that the sample of the drug we are using is active. In Pasteur’s experiment if, after the neck was broken, the boiled broth could not support growth we would not expect it to support spontaneous generation, and so the experiment would be meaningless. We will return to the description of “negative control” experiments later.^[56]

No experiment is perfect, nor does it have to be for science to work. How would you argue against the idea that spontaneous generation takes thousands to millions of years to occur? A direct experiment would have its own practical issues (can you identify some?). So far, many studies have led to the consistent conclusion that neither microscopic nor macroscopic organisms can arise spontaneously in nordern world.

It is often said that all the conditions for the first production of living organisms are now present. But if (and oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc. present, that a proteine compound was formed, ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed.

- Charles Darwin (1887).

But we assume that spontaneous generation occurred at least once. What explains its absence of in the modern world? Living systems involve complex chemical reaction networks. In the modern world, there are many organisms around, essentially everywhere; these organisms are eating complex molecules to maintain their living state. Given the tendency of organisms to eat one another, one might argue, as did Darwin →, that once organisms had appeared in a particular environment they would suppress subsequent events – they would have eaten the molecules needed for spontaneous generation to occur. There are no welcoming and sterile, that is, life-less places left for new forms of life to arise.

Here we see the importance of history. It appears that a new form of life could arise only in the absence of life. Studies of Earth's history put some limits on the time it took for life to appear, sometime after the Earth’s surface solidified from its early molten state to the first fossil evidence for life, about 100 to 500 million years. Once life arose conditions changed.Once organisms were present, only there descendants could survive. In such a system, history matters.

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