The ubiquity of organisms raises obvious questions: how did life start and what led to all these different types of organisms? At one point, people believed that these two questions had a single answer, but we now recognize that they are really two quite distinct questions and their answers involve distinct mechanisms. An early view held by those who thought about such things was that supernatural processes produced life in general and human beings in particular. The articulation of the Cell Theory and the Theory of Evolution by Natural Selection, which we will discuss in detail in the next chapter, together with the accumulation of data enables us to conclude quite persuasively that life had a single successful origin and that various natural evolutionary processes generated the diversity of life.
But how did life itself originate? It used to be widely accepted that various types of organisms, such as flies, frogs, and even mice, could arise spontaneously, from non-living matter.35 Flies, for example, were thought to appear from rotting flesh and mice from wheat. If true, on-going spontaneous generation would have profound implications for our understanding of biological systems. For example, if spontaneous generation based on natural processes was common, there must be a rather simple process at work, a process that (presumably) can produce remarkably complex outcomes. In contrast, all bets are off if the process is supernatural. If each organism arose independently, we might expect that 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. However, we know this is not the case, since all organisms are clearly related and can be traced back to a single ancestor, a conclusion to which we return, repeatedly.
A key event in the conceptual development of modern biology was the publication of Francesco Redi’s (1626–1697) paper entitled “Experiments on the Generation of Insects” in 1668. He hypothesized that spontaneous generation did not occur. His hypothesis was that the organisms that appeared had developed from "seeds" deposited by adults. His hypothesis led to a number of clear predictions. One was that if adult flies were kept away from rotting meat maggots (the larval form of flies) would never appear no matter how long one waited. Similarly, the type of organism that appeared would depend not 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 contained meat. One set of flasks were exposed directly to the air and so to flies, the other was sealed with paper or cloth. Maggots appeared only in the flasks open to the air. Redi concluded that organisms as complex as insects, and too large to pass through the cloth, could arise only from other insects, or rather eggs laid by those insects–that life was continuous.
The invention of the light microscope and its use to look at biological materials by Antony van Leeuwenhoek (1632-1723) and Robert Hooke (1635-1703) led to the discovery of a completely new and totally unexpected world of microbes or microscopic organisms. We now know these as the bacteria, archaea, a range of unicellular photosynthetic and non-photosynthetic eukaryotes.36 Although it was relatively easy to generate compelling evidence that macroscopic (that is, big) organisms, such as flies, mice, and people could not arise spontaneously, it seemed plausible that microscopic and presumably much simpler organisms could form spontaneously.
The discovery of microbes led a number of scientists to explore their origin and reproduction. Lazzaro Spallazani (1729-1799) showed that after a broth was boiled it remained sterile, that is, without life, as long as it was isolated from contact with fresh air. He concluded that microbes, like larger organisms, could not arise spontaneously but were descended from other microbes, many of which were floating in the air. Think about possible criticisms to this experiment – perhaps you can come up with ones that we do not mention!
One obvious criticism was that it could be that boiling the broth destroyed one or more key components that were necessary for the spontaneous formation of life. Alternatively, perhaps fresh air was the "vital" ingredient. In either case, boiling and isolation would have produced an artifact that obscured rather than revealed the true process. In 1862 (note the late date, this was after Charles Darwin had published On the Origin of Species in 1859), Louis Pasteur (1822-1895) carried out a particularly convincing set of experiments to addressed both of these concerns. He sterilized broths by boiling them in special "swan-necked" flasks (→). What was unique about his experimental design was the shape of the flask neck; it allowed air but not air-borne microorganisms to reach the broth. Microbes in the air were trapped in the bended region of the flask’s neck. This design enabled Pasteur to address a criticism of previous experiments, namely that access to air was necessary for spontaneous generation to occur. He found that the liquid, even with access to air, remained sterile for months. However, when the neck of the flask was broken the broth was quickly overrun with microbial growth. He interpreted this observation to indicate that air, by itself, was not necessary for spontaneous generation, but rather was normally contaminated by microbes. On the other hand, the fact that the broth could support microbial growth after the neck was broken served as what is known as a “positive control” experiment; it indicated that the heating of the broth had not destroyed some vital element needed for standard growth to occur. We carry out positive control experiments to test our assumptions; for examine, if we are using a drug in a study, we first need to test to make sure that the drug we have is actually active. In Pasteur’s experiment, if the boiled broth could not support growth (after the flask was broken) we would not expect it to support spontaneous generation, and so the experiment would be meaningless. We will return to the description of a “negative control” experiment later.37
Of course, not all, in fact, probably not any experiment is perfect. For example, how would one argue against the objection that the process of spontaneous generation normally takes tens to thousands, or millions, of years to occur? If true, this objection would invalidate Pasteur’s conclusions. Clearly an experiment to address that particular objection has its own practical issues. Nevertheless, the results of various experiments on spontaneous generation have led to the conclusion that neither microscopic nor macroscopic organisms could arise spontaneously, at least not in the modern world. The problem, at least in this form, became uninteresting to working scientists.
Does this mean that the origin of life is due to a supernatural event? Not necessarily. Consider the fact that living systems are complex chemical reaction networks. In the modern world, there are many organisms around, essentially everywhere, who are actively eating complex molecules to maintain their non-equilibrium state, to grow and, to reproduce. If life were to arise by a spontaneous but natural process, it is possible that it could take thousands to hundreds of millions of years to occur. We can put some limits on the minimum time it could take from geological data using the time from when the Earth’s surface solidified from its early molten state to the first fossil evidence for life, about 100 to 500 million years. Given the tendency of organisms to eat one another, one might argue (as Darwin did →) that once organisms had appeared in a particular environment they would suppress any subsequent spontaneous generation events – they would have eaten the molecules needed for the process to occur. But, as we will see, evolutionary processes have led to the presence of organisms essentially everywhere on Earth that life can survive – there are basically no welcoming and sterile places left within the modern world. Here we see the importance of history. According to the current scientific view, life could arise de novo only in the absence of life; once life had arisen, the conditions had changed. The presence of life is expected to suppress the origin of new forms of life. Once life was present, only its descendants could survive.