irritant to him and his fellow scientists than spontaneous generation.
Figure 2.4 Louis Pasteur, microbiologist. He took on the idea of spontaneous generation with an ingenious experiment.
Source: Reproduced with permission of Pal Nadar, https://commons.wikimedia.org/wiki/File:Louis_Pasteur_(1822_-_1895),_microbiologist_and_chemist_Wellcome_V0026980.jpg.
His response to this centuries-old question was an experiment of ingenious simplicity. He invented his famous swan neck flasks (Figure 2.5). These flasks contained a variety of liquids including yeast extracts, pepper water, and urine. He heated them until they boiled for several minutes. Then he cooled them. The swan neck flasks were a very simple alteration to the type of flasks used by Schwann. The swan necks prevented aerially transported entities from entering the broth. In contrast, the flasks with the swan neck removed were rapidly contaminated and microbes grew. This experiment showed that broths were contaminated by organisms in the air and that life could not spontaneously be generated in a sterile medium. He concluded his remarkable paper in which he reported the results with a sentence of extraordinary clarity and importance: “There exist continually in the air organized bodies which cannot be distinguished from true germs.” With this sentence, microbiology had truly been propelled to center stage in medicine and our understanding of the environment. Pasteur's experiment was a response to a French Academy of Sciences prize in 1859 that challenged scientists to disprove spontaneous generation.
Figure 2.5 Louis Pasteur's swan neck flasks and his experiment to disprove spontaneous generation. The swan necks prevent microbes from dropping directly into the flasks after the broth has been heat-sterilized. Flasks with open necks become colonized, and microbial growth occurs.
Despite the end of spontaneous generation as an idea for the emergence of life as a continuous process on Earth, we know that it must have happened once. When life first arose, there was a transition from abiotic chemistry to the first replicating molecules and cellular life. So, the problem of spontaneous generation still has not gone away. However, to avoid confusing it with the old obsolete ideas of spontaneous generation, the origin of life is often referred to today as abiogenesis. How did simple molecules come together to form more complex ones and ultimately a self-replicating organism? We will investigate this question in Chapter 12.
Focus: Astrobiologists: Mary Beth Wilhelm
Affiliation: NASA Ames Research Center, California, USA
What was your first degree? My first degree was in Geology with a concentration in Planetary Science at Cornell University. I was lucky to go to a school with such a broad range of research opportunities, and while I was an undergraduate, I was able to do research projects in microbiology, geology, and astronomy. All three fields greatly interested me, and I feel like I found a good balance in astrobiology. By getting involved in research as a student, you really get a sense of what it is like to be a professional scientist, and a chance to increase your technical knowledge, challenge yourself to be resourceful, and think creatively.
What do you study? I primarily study the preservation of the molecular biosignatures (or biomarkers) in the fossil record on Earth, particularly places that have properties analogous to Mars. I've done a lot of work in the Atacama Desert in Chile, which is one of the driest places on Earth, and in Antarctica. My particular subfield is at the intersection of geology and biochemistry. Studying these types of environments on Earth help me and other astrobiologists to interpret data from Mars and design the next set of tools to look for life on the Red Planet.
What science questions do you address? There are a few questions that drive my research: (i) How do the molecules that make up organisms get incorporated into the geological record? (ii) How do those molecules break down over time? Which structural features or patterns diagnostic of their biogenicity remain? (iii) What physical or chemical conditions lead to increased chances of preservation of biomarkers? (iv) What are the extreme limits of life in desert environments?
How did you get involved in astrobiology research? I have loved geology, astronomy, and planetary science since I was six years old. I went to Space Camp, had a telescope, and read a lot of books as a child. I remember I first read about astrobiology when I was about 11 years old in a Dan Brown novel. I was lucky to grow up near a NASA center (NASA Ames Research Center in Silicon Valley, California), and when I turned 16 I got a summer internship at NASA. I helped my mentor analyze over 40 000 images of Mars, studying features that resemble terrestrial water-carved gullies. I was lucky to find supportive and brilliant mentors, and even all these years later I still collaborate with them! Ever since then, I have always been looking for the next interesting research question to dive in on!
2.4 More Modern Concepts
With the formulation of the Periodic Table, it was clear that living things were made of the same atomic material as all other matter. However, that did little to dispel the notion that living things were different from other matter. In religious circles, the idea of the “soul” persisted, an Aristotelian idea that continues to be held by people to the present-day.
Rather than resorting to a materialistic explanation, a common explanation for life has been to attempt to describe the collected set of characteristics that distinguish living things. This approach remains widespread. In schools, seven distinguishing characteristics of life versus non-life are taught to pupils using the acronym MRS GREN (Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion and Nutrition), and this provides the explanation of what life is. Francis Crick (1916–2004), co-discoverer of the structure of deoxyribonucleic acid (DNA), suggested that self-reproduction, evolution, and metabolism were jointly sufficient for something to be described as alive. All such attempts to define life, and there are many others, are focused on circumscribing the key characteristics of living entities.
This attempt to define the characteristics of living things leads inexorably to a desire to define life itself. Indeed, if we think that life has a collected set of characteristics that establish it as unique, it follows that a definition can be written down that simply circumscribes these characteristics. It has become something of a long-term fascination for scientists and non-scientists alike to “define” life, to come up with a succinct summary of the essence of life.
One characteristic of life is its capacity for evolution (intriguingly it does not feature in the MRS GREN mnemonic taught to school pupils, but it is common in many other discussions on the requirements for something to be classified as alive). Evolution is meant here in the Darwinian sense, namely the process by which variation in a population of organisms, placed under environmental conditions, results in the selection of surviving organisms that pass their traits onto subsequent generations (“natural selection”). Attempts to capture the concept of evolution are a common theme in definitions of life. The combination of evolution, growth, and reproduction is bound up in one definition devised by NASA scientist Gerald Joyce that life is “a self-sustained chemical system capable of undergoing Darwinian evolution.”
However, we might question whether a shopping list of characteristics really is sufficient to circumscribe what life
