Spontaneous Generation and the Origin of Life

by

Copyright © 2004
[Posted: April 26, 2004]

Summary

What Louis Pasteur and the others who denied spontaneous generation demonstrated is that life does not currently spontaneously arise in complex form from nonlife in nature; he did not demonstrate the impossibility of life arising in simple form from nonlife by way of a long and propitious series of chemical steps/selections. In particular, they did not show that life cannot arise once, and then evolve. Neither Pasteur, nor any other post-Darwin researcher in this field, denied the age of the earth or the fact of evolution.

Introduction

A recurring theme in antievolution literature is that if science cannot account for the origin of life, evolution is false, and that "spontaneous generation" was disproven, so therefore evolution is false. This syllogism fails, because evolution (that is, common descent and transmutation of species) occurs whether or not life arose by chance, law or design, but there is another more insidious mistake here. It is not true that "spontaneous generation" has been ruled out in all cases by science; the claims disproven were more restricted than that.

Hence this essay. We will look at the history of the idea, and then the disproofs, and finally the relation of the origin of life to evolutionary theory in general. As always, we start with the Greeks. Once we reach Pasteur, the implications of the debate to that point for evolution will be considered. Then we will look at the modern - post-Pasteur and post-Darwin - developments in Origins of Life research.

Early views on spontaneous generation

The first western thinker to suggest that life arose spontaneously was probably Anaximander, a Milesian philosopher (in what is now Turkey) who wrote in the 6th and 5th centuries before Christ (611-547 BCE). He believed that everything arose out of the elemental nature of the universe, which he called the "apeiron" or "unbounded". As part of his overall attempt to give natural explanations of things that had previously been ascribed to the agency of the gods, such as thunder, the heavens, and the earth, he gave the following account of life.

According to a late source, Hippolytus in the third century CE, for Anaximander's own works do not survive, Anaximander claimed that living creatures were first formed in the "wet" when acted on by the Sun, and that they were different then than they are now. In particular he claimed humans were originally a kind of fish, and that based on the observation humans took a long time to mature to independence, humans must have earlier been born mature like other animals, or they would not have survived. It was not a complete theory of evolution by any means, although Haeckel and Osborn claimed he was a "prophet" of Kant, Laplace, Lamarck and Darwin. Anaximander also claimed that spontaneous generation continued to this day, with eels and other acquatic forms being produced directly from lifeless matter. {Lloyd 17-18, Osborn 33-35}

Anaximenes, his pupil (588-524) thought that air was the element that imparted life, motion and thought, and supposed there was a primordial terestrial slime, a mixture of earth and water, from which the sun's heat formed plants, animals and human beings directly. {Osborn 35}

Xenophanes (576-480), the founder of the Eliatic School, traced the origin of man back to the transitional period between the fluid stage of the earth and the formation of land. He too held to a spontaneous generation of fully formed plants and animals under the influence of the sun. So too did Parmenides (b544).

Empedocles (495-435) accepted the spontaneous generation of life, but held that there had to be trials of combinations of parts of animals that spontaneously arose. Successful combinations formed the species we now see, unsuccessful forms failed to reproduce. Osborn {37-40} thought this was a kind of natural selection, but as only one form is successful for each lineage, and species remain unchanged thereafter, it is a tenuous analogy to make.

Democritus (b450) and Anaxagoras (500-428) also adopted a terrestrial slime account, although Anaxagoras thought that the germs (seeds) of plants existed in the air from the beginning, and of animals in the ether. {Osborn 42-43}

Aristotle

All these accounts rely on the innate (or natural; the Greek word is phusis, from which we get "physics") properties of the elements of the universe. Life is a result of the propensities of the world. In Aristotle (384-322) we get the most sophisticated of all these Greek views. He thought there were four elements and a fifth essence later called the "quintessence" or "ether" that occurred only beyond the moon, in the heavens. The four terrestrial elements are, of course, earth, air, fire and water, each of which is a principle of hot, cold, dry and wet {see the discussions in Toulmin and Goodfield 1962a and 1962b}.

He thought that the properties of living organisms were due to the mixture of these principles and elements in each part of the body, plus an animating force he called "pneuma", which got translated as "anima" in Latin, the word for "soul". There were, in fact, a number of souls, ranging from growth, to motion, sensation, to thinking, and finally in humans, to reason.

In the History of Animals Aristotle several times says that animals of some kinds arise directly from elements and the pneuma of the material:

"So with animals, some spring from parent animals according to their kind, whilst others grow spontaneously and not from kindred stock; and of these instances of spontaneous generation some come from putrefying earth or vegetable matter, as is the case with a number of insects, while others are spontaneously generated in the inside of animals out of the secretions of their several organs." 539a18-26

"As a general rule, then, all testaceans grow by spontaneous generation in mud, differing from one another according to the differences of the material; oysters growing in slime, and cockles and the other testaceans above mentioned on sandy bottoms; and in the hollows of the rocks the ascidian and the barnacle, and common sorts, such as the limpet and the nerites."547b18-22

"Other insects are not derived from living parentage, but are generated spontaneously: some out of dew falling on leaves, ordinarily in spring-time, but not seldom in winter when there has been a stretch of fair weather and southerly winds; others grow in decaying mud or dung; others in timber, green or dry; some in the hair of animals; some in the flesh of animals; some in excrements: and some from excrement after it has been voided, and some from excrement yet within the living animal, like the helminthes or intestinal worms." 551a1-10

"Other animalcules besides these are generated, as we have already remarked, some in wool or in articles made of wool, as the ses or clothes-moth. And these animalcules come in greater numbers if the woollen substances are dusty; and they come in especially large numbers if a spider be shut up in the cloth or wool, for the creature drinks up any moisture that may be there, and dries up the woollen substance. This grub is found also in men's clothes.

A creature is also found in wax long laid by, just as in wood, and it is the smallest of animalcules and is white in colour, and is designated the acari or mite. In books also other animalcules are found, some resembling the grubs found in garments, and some resembling tailless scorpions, but very small. As a general rule we may state that such animalcules are found in practically anything, both in dry things that are becoming moist and in moist things that are drying, provided they contain the conditions of life." 557b1-13

and

"Some writers actually aver that mullet all grow spontaneously. In this assertion they are mistaken, for the female of the fish is found provided with spawn, and the male with milt. However, there is a species of mullet that grows spontaneously out of mud and sand.

From the facts above enumerated it is quite proved that certain fishes come spontaneously into existence, not being derived from eggs or from copulation. Such fish as are neither oviparous nor viviparous arise all from one of two sources, from mud, or from sand and from decayed matter that rises thence as a scum; for instance, the so-called froth of the small fry comes out of sandy ground. This fry is incapable of growth and of propagating its kind; after living for a while it dies away and another creature takes its place, and so, with short intervals excepted, it may be said to last the whole year through." 569a21-569b3 {listed in Lennox 233}

He gives a theoretical explanation in the Generation of Animals Bk 3, ch 11. After reiterating the claim that some, but not all, of various classes of organisms spontaneously generate out of matter, he explains why they do:

"All those which do not bud off or 'spawn' are spontaneously generated. Now all things formed in this way, whether in earth or water, manifestly come into being in connexion with putrefaction and an admixture of rain-water. For as the sweet is separated off into the matter which is forming, the residue of the mixture takes such a form. Nothing comes into being by putrefying, but by concocting; putrefaction and the thing putrefied is only a residue of that which is concocted. For nothing comes into being out of the whole of anything, any more than in the products of art; if it did art would have nothing to do, but as it is in the one case art removes the useless material, in the other Nature does so. Animals and plants come into being in earth and in liquid because there is water in earth, and air in water, and in all air is vital heat so that in a sense all things are full of soul. Therefore living things form quickly whenever this air and vital heat are enclosed in anything. When they are so enclosed, the corporeal liquids being heated, there arises as it were a frothy bubble. Whether what is forming is to be more or less honourable in kind depends on the embracing of the psychical principle; this again depends on the medium in which the generation takes place and the material which is included."

In short, things arise from nonliving matter because there is a "vital heat", a pneuma, which is already there, and the proportions of that and the other elements enclosed by the forming structure determine the kind of organism.

Aristotle denied that the universe, and the earth, had a beginning, so this is a process that happens all the time, not just at the beginning, as in the older Greek thinkers. Given Aristotle's influence on later thinking, particularly throughout the later middle ages, his ideas form a kind of background "default" which western thinkers had unless they consciously were set against him on a particular topic. For example, Francis Bacon said, in his New Atlantis (c1614), that it was possible for his "learned Jew" protagonist to

"... imitate and demonstrate meteors -- as snow, hail, rain, some artificial rains of bodies and not of water, thunders, lightnings; also generations of bodies in air -- as frogs, flies, and divers others."

In short, spontaneous generation was even possible in the air. But in all these post-Aristotle claims, only simple bodies and organisms could spontaneously generate, and certainly not humans. In this belief, Theophrastus (370-288 BCE) concurred, as did most writers throughout the middle ages through to the beginnings of modern biology in the seventeenth century, including the early Christian fathers Origen and Augustine.

Early modern biology and the challenges to spontaneous generation

William Harvey (1578-1657) published his De Generatione in 1651, in which he coined the oft-quoted phrase "ex ova omnia" (all [life] from eggs). However, despite that phrase, he did allow that there could be spontaneously generated life {Gasking 18-19}. Hence, despite the textbook myth, Harvey was not the first to reject spontaneous generation, although he did say that many cases of apparent spontaneous generation were due to invisible seeds being scattered and dispersed through the air.

Spontaneous generation of mice was reported by Johannes Baptista van Helmont (1579-1644), a physician and alchemist. He believed that mice arose when a flask of wheat and old rags was incubated in a warm dark closet. {Magner 267}

Francisco Redi (c1626-1697) demonstrated in 1668 that maggots did not, contrary to Aristotle, arise spontaneously, but from eggs laid by adult flies. Meat covered so that the flies could not reach it was free of maggots, while meat that flies could reach developed them. A member of the Academy of Experiments in Florence, he carried out several experiments on the matter, following the development of fly larvae form eggs, on different meats including lion meat, lamb, fishes and snakes. The results were published as Experiments on the Generation of Insects. He said, using "worm" to mean maggot:

"I began to believe that all worms found in meat derived from flies and not from putrefaction. I was confirmed by observing that, before the meat became wormy, there hovered over it flies of that very kind that later bred in it. Belief unconfirmed by experiment is vain. Therefore I put a [dead] snake, some fish, and a slice of veal in four large, wide-mouthed flasks. These I closed and sealed. Then I filled the same number of flasks in the same way leaving them open. Flies were seen constantly entering and leaving the open flasks. The meat and fish in them became wormy. In the closed flasks were no worms, though the contents were now putrid and stinking. Outside, on the covers of the closed flasks a few maggots eagerly sought some crevice of entry.

"Thus the flesh of dead animals cannot engender worms unless the eggs of the living be deposited therein." {Quoted from Singer 440}

He continued the experiments using gauze, with the same results.

Redi did not disprove spontaneous generation as such, as Magner notes, but his experiments did "shrink the battle from the generation of macroscopic creatures to the small new world of infusoria and animalcules discovered by van Leeuwenhoek" {Magner 267}. Despite this, though, he continued to believe gall insects were spontaneously generated. Later workers, such as Antonio Vallisnieri (1661-1730), showed in 1700 that gall wasps laid their eggs in the plants before the gall formed around the larvae, as had Marcello Malpighi (1628-1694) {Singer 441}, and Jan Swammerdam (1637-1680) in 1669, while Rene Antoine Ferchault de Réamur (1683-1757) in his Contributions to the History of Insects (1737-1748) showed that insects that had been thought to spontaneously generate in fact arose from eggs. {Gasking 62-63}

Subsequent proponents of spontaneous generation were typically epigeneticists in the generations debates, and opponents typically preformationists. These were two theoretical viewpoints on the nature of the generation of new organisms: preformationists believed that the pattern of generation was included in the seed, each embryo encapsulated in its parental embryo all the way back to creation, while epigeneticists believed with Aristotle that each embryo is formed out of an undifferentiated matter by an organising form. Hence, preformationists had to reject spontaneous generation, ex hypothesi.

Gottfried Willhelm Leibniz (1646-1716), a famous philosopher, mathematician and scientist, asserted that there were fundamental "living molecules" he called "monads", from which all things sprang. Although he held to a static view of nature, he did think that there was a scale of complexity, and that very simple organisms were comprised directly of monads. His views influenced a great many biologists after him. {Nordenskiold 128, Magner 267}

Needham and Spallanzini

The great French naturalist Georges Louis Leclerc, comte de Buffon (1707-1788, known as Buffon even by the French) and an English Catholic priest, the Abbé John Turberville Needham (1713-1781), an accomplished microscopist whom Buffon met on a trip to England, decided around 1738 to attempt to disprove the work of Louis Joblot (1645-1723). {Gasking 89-90} Joblot had tried to show that infusoria (simple organisms found in infusions into organic material, mostly ciliates) were not spontaneously generated by boiling a medium, and placing one part in a sealed vessel, the other in an open one. The sealed one did not become infused with these organisms. To prove the medium was still capable of supporting life, he exposed the sealed material to the air and it was soon teaming too.

When Needham repeated the experiments at the urging of Buffon, he found that, boiled or not, sealed or not, life arose in the vessels of broth. He concluded that there was a vegetative force in every bit of matter, just as Buffon's theory of there being an "interior mold" (moule interieur) for generation of larger organisms (i.e., an epigenetic view) predicted. The results were published in 1748 in the Philosophical Transactions of the Royal Society.

Abbott Lazzaro Spallanzani (1729-1799) disagreed, and set about disproving Needham and Buffon's results. He was also a professor at the universities of Reggio, Modena and Pavia, and his experimental work was of high standard. He reasoned that the minute organisms must have a more minute early stage of growth, and so decided that the problem could not be resolved through the use of a microscope. {Singer 442} In 1767, he published his account rebutting Needham and Buffon, saying:

"I sought to discover whether long boiling would injure or prevent the production of animalcules in infusions. I prepared infusions with eleven varieties of seeds, boiled for half an hour. The vessels were loosely stopped with corks. After eight days I examined the infusions microscopically. In all there were animalcules, but of differing species. Therefore long boiling does not of itself prevent their production". {Quoted in Singer 442}

So he tried excluding air, placing infusions in five series of flasks. One series was left open, the other four were sealed and raised to boiling, each series for 30 seconds longer than the first. After two days the open series was swarming, and the 30 second series contained smaller organisms, while the remainder contained almost none. He had shown that the duration of the boiling mattered, in that some organisms were more heat resistant than others. Boiling sealed vessels for a half to three-quarters of an hour, Spallanzani showed that no life would develop so long as the flask was kept sealed. {Singer 442-443}

When Needham objected that the heat had rendered the infusions themselves sterile - that is, incapable of supporting life - Spallanzini broke the necks of the flask, and the infusions soon showed the usual life. {Nordenskiöld 131}

This did not end the debate - others repeated the experiments with varying success or failure. Theodor Schwann (1810-1882), one of the founders of the Cell Theory, showed that air that had been heated would not cause putrefaction in a sterilised broth in 1836-1837, but the reason was ambiguous; it may have been heated (calcined) air was unable to support respiration. French chemist Joseph-Louis Gay-Lusssac (1778-1850) showed that Spallanzani's experiments included oxygen, which was necessary for fermentation and putrefaction, by proving that a frog could live in it. Others such as Franz Schultze (1815-1873), Heinrich Schroder (1810-1885) and Theodor von Dusch (1824-1890), all tried to resolve the matter, to no avail. {Singer 443, Magner 269-270} The arguments continued.

Despite the theoretical arguments, a French chef, Nicholas Appert (1750-1841) applied Spallanzani's results to food commercially, placing it in clean bottles, corking them slightly, and boiling them. These techniques were published in 1810, and founded the canning industry. {Magner 269} Another industrial matter came to the fore at this time - fermentation. The man to resolve this side of the debate was Louis Pasteur.

The Nineteenth century before Pasteur

Throughout the nineteenth century, there were believers in spontaneous generation. A major believer was Lorenz Oken (1779-1851), a follower of Goethe, who proposed (1809) a "sea-slime" theory of the origins of life, just as Anaximander had. He believed it occurred where land and sea met, and minute bladders of foam enclosed three life principles - feeding, respiration and digestion. However, he was not consistent and made many contradictory claims - such as Man being the offspring of a warm and gentle seashore in India. {Osborn 126-127}

Like Oken, Jean Baptiste de Lamarck (1744-1829), a student of Buffon's, also believed in spontaneous generation, in contradiction to his mentor, and for the first time made it a cornerstone of a theory of transmutation (1809). However, unlike Darwin's later theory, Lamarck's assumed that each species was the result of an independent event of spontaneous generation, and that they were continuing to the present day - each species was advanced just as much as it had been in existence. He wrote:

"In the waters of the ancient world, an at the present time, very small masses of mucilaginous matter were collected. Under the influences of light, certain elements, caloric and electric, entered these little bodies. These corpuscles became capable of taking in and exhaling gases; vital movements began, and thus an elemental plant or animal sprang into existence. Possibly higher forms of life, such as infest the intestines, originate in this way. Nature is thus always creating." {1802, quoted in Osborn 178}

Just preceding Lamarck, Darwin's own grandfather, Dr Erasmus Darwin (1731-1802), wrote in his scientific poem The Temple of Nature (1802), much appreciated at the time but not much later:

"Hence without parents, by spontaneous birth,
Rise the first specks of animated earth."

and

"Organic life beneath the shoreless waves
Was born and nurs'd in ocean's pearly caves;"

and so on. These initial forms of life are primitive and minute, and larger forms evolve later. In the Zoonomia (1794), he conjectured about the first life form:

"Shall we conjecture that one and the same kind of living filament is and has been the cause of organic life?"

However, neither Oken, Lamarck nor Darwin had much impact on the academic world of their day. In France only Étienne Geoffroy (1725-1810) championed his views, while in Britain, only Robert Grant in Edinburgh, later a teacher and friend of Charles Darwin's, continued to present evolutionary views.

Pasteur, fermentation, contagion, and proving a negative

In the period following the increasingly evolutionary views on life, there were two urgent problems that had to be resolved that touched on spontaneous generation. Both were heterogenesis issues (life arising from the degraded or putrified products of other life) and not abiogenesis (a word coined much later in the century by T. H. Huxley, as we shall see later). Heterogeny was a major problem in two ways not related to evolution - one was the issue of the origin of diseases, and in particular of parasitic worms and flukes; and the other was the cause of fermentation. The former is a public health issue, exemplified by the cholera pandemics of 1831, 1848, 1853 and 1861 in England, while the latter was a matter of great concern in the viticulture and brewing industry of France and elsewhere. Let us consider fermentation first.

There were two theories as to the origin of microorganisms in fermentations, and the process of fermentation itself. One was put forward in 1836 by a French engineer, Charles Cagniard-Latour (1777-1859), that yeast, recognised as the active ingredient in fermentation, was made up of minute organisms that caused the fermentation directly through what we now call their metabolic processes (a term that was coined only a few years later by Schwann, one of the discoverers of the cell theory). {Nordenskiöld 431, Singer 339} The other, proposed by the famous chemist Antoine-Laurent Lavoisier in 1789, and championed by Justus von Liebig (1802-1873), was that fermentation was caused by a chemical process - of the action of air on grape juice (Joseph-Louis Gay Lussac, 1778-1850, in 1810); or of "a compound of nitrogen in a state of putrefaction or decay" that caused a similar condition in other bodies (Liebig, 1840) {Farley 49}. Of course, the chemical explanation, although bolstered by the work of Liebig's student Friedrich Wöhler (1800-1882) in the synthesis of the organic compound urea in 1828, had a problem - in yeast during fermentation, microorganisms were found to grow. Therefore, either they grew as a result of the chemistry, that is, spontaneous generation, or they were infections that took advantage of these products.

In 1837-8, three researchers independently found that yeast were living organisms: Cagniard-Latour, Friedrich Kützing, and Schwann. The first two established that yeast causes the decomposition of sugar when alive, and not when dead. Schwann, trying to prove that spontaneous generation did not occur on meat, showed that the air in the flasks used to prove that meat would not putrefy when boiled was still "vital" by using it to grow yeast on boiled cane sugar. When they did not cause fermentation, he examined the yeast and concluded it was an "articulated fungi" and concluded, without real warrant, that alcoholic fermentation occurs when yeast (or as he called it, "the sugar fungus", or Zuckerpilz) uses sugar and nitrogenous substances for its growth, incidentally converting these elements to alcohol. He was right, but that was not known then, and Liebig rejected his evidence. So although there was a minority view that yeast were living organisms causing fermentation, the majority view remained that chemical fermentation occurred, and spontaneous generation was the cause of yeast cells.

Contagion, on the other hand, was equally critical. It might be thought that the success in antiseptic techniques introduced by Joseph Lister (1827-1912) in 1865 proved that contagion was caused by pathogens, he himself allowed, though he did not agree, that the "septic particles" might be cell products, not cells. {Farley 83} In any case, this was after Pasteur. More to the point was John Snow's (1813-1858 ) work on tracing the source of cholera in London in 1849 and 1855 to certain wells. Although it was thought that the epidemic was due to transmission, most British contagionists thought that the contagion was due to "nonorganismic particles", a view that can be traced back to Galen (130-200), who thought diseases were transmitted by "miasmas". So it stood when Pasteur undertook his research.

Theodor Schwann's work, in particular, had asserted that cells might form out of cell products such as the extracellular material he called the "Cytoblastema", while his co-theorist Matthias Schleiden (1804-1881) thought that all cells formed from structures within existing cells. Hugo von Mohl (1805-1872), though he thought that cells were in general formed by direct division, still said in passing that free cell formation might occur independently of "life of the parent plant in the creation of parasitic fungi, yeast cells, etc., both in the decomposing fluid of cells and in the excreted or expressed juices". {Farley 53}

The famous pathologist and cytologist Rudolph Virchow (1821-1902) agreed with Robert Remak (1815-1865) who said in 1852 that the occurrence of free cell formation was as improbable as spontaneous generation. Virchow considered spontaneous generation as "heresy, or devil's work" in 1855, and much later asserted that Schwann had reinvigorated the old doctrine of generatio aequivoca, as spontaneous generation was known. {Farley 199n} In its place, Virchow asserted that any kind of life required a matrix, a prior organisation:

"Life does not reside in the fluids as such, but only in their cellular parts; it is necessary to exclude cell-free fluids from the realm of the living and intercellular material of cell-containing fluids as well. ... Life will always remain something apart, even if we should find out that it is mechanically aroused and propagated down to the minutest detail." {quoted in Farley 54}

Virchow famously propounded the dictum Omnis cellula e cellula (all cells from cells), but he never was able to provide an "absolute demonstration" of this "established principle"; for a very good reason - you can neither prove a universal negative nor a universal positive with a finite or limited set of observations. Even as Virchow was attacking the non-cellular origin of cells, cell theory itself was being modified to accommodate the idea of a "protoplasm", which will become important in the period after Pasteur.

Other combatants over spontaneous generation at this time included Christian Ehrenberg (1795-1876; opposed) and Felix Dujardin (1801-1862, in favour), among others. {Farley 55-56}. One interesting side debate was whether or not parasitic worm and liver flukes formed through irritations in the tissues of the sufferers or due to infection. In Britain, around 240 papers were published on this subject, which was clouded by the fact that, as it turned out, many such parasites have distinct forms in alternating generations, and so the infecting parasites were not recognised as being the same species. The discovery of the alternation of generations was made by Japetus Steenstrup (1813-1897), a Danish zoologist, in 1842.

Pouchet and Pasteur

The Director of the Natural History Museum in Rouen, Félix Archimède Pouchet (1800-1872), began presenting a series of papers in 1855 to the Academy of Sciences in Paris, purporting to prove spontaneous generation, and to show not only that it happened, but under what circumstances. He named his subject heterogenesis, which was the title of a massive volume he published in 1859. Like Buffon and Needham, Pouchet thought that heterogenesis was not accidental, but due to the vital force of the materials, which had to be pre-existing organic matter. According to him, the causal factors involved were organic matter, water, air, and the right temperature. {Magner 270}

Pouchet's results showed, he claimed, that although in the animal kingdom, all life arose from eggs, those eggs arose, at times, by spontaneous generation. He wrote,

"Spontaneous generation does not produce an adult being; it proceeds in the same manner as sexual generation which, as we shall show, is initially a completely spontaneous act by which the plastic force brings together in a special organ the primitive elements of the organism." {quoted in Farley 97}

In other words, Pouchet thought that sexual generation was a spontaneous act caused by a vital force as much as spontaneous generation. He held that this occurred by divine providence rather than chance. Spontaneous generation had been previously attacked for being irreligious, as the event was due to the chance recombination of molecules. Pouchet's version was divinely guided. He thought that both the original act of creation was divinely guided, and so too were subsequent events. Hence, Pouchet was trying to wrest spontaneous generation from the materialists. "The law of heterogenesis," he wrote, "far from weakening the attributes of the Creator, can only augment Divine Majesty." {Farley 98} As Farley notes, Pasteur and subsequent accounts of the debate overlooked Pouchet's orthodox theism and piety.

Moreover, Pouchet's account was based on the origination of new life from the organic material of old life, not from non-living matter:

"The succession of life on the surface of the globe links matter in a narrow circle from which it cannot escape. It is successively attracted and repelled by these incessant phenomena. But the organic particles, sometimes intimately united to form organisms, and sometimes free in space, are no less animated with a latent life, which seems to wait only for their grouping to be visibly manifested. It seems that for organic molecules, there is no death ... only a transition to a new life." {quoted in Farley 98}

The obvious exception to this is, of course, the first divine creation. All else required a "force plastique", a molding power.

But Louis Pasteur (1822-1895) objected to the idea of spontaneous generation. The French Academy of Science offered the Alhumbert Prize of 2500 francs to whoever could shed "new light on the question of so-called spontaneous generation". Pasteur won it in 1862 for his famous essay in 1861, "Mémoire sur les corpuscules organisés qui existent dans l'atmosphère", published in their Annales the next year. In this he described a series of elegant experiments designed to disprove Pouchet's major claim that there were no organisms introduced into his flasks. Agreeing that neither the air used, nor the water contained germs in Pouchet's experiment, and that he had sufficiently sterilised the flask and materials with heat, he focused on another item in the experiment - the mercury trough in which Pouchet cooled the flask. In this, Pasteur claimed, dust, carrying germs, had settled and this introduced germs into Pouchet's sealed flask.

Pasteur could not, of course, merely argue that Pouchet might have made this mistake, he had to show that if properly carried out, no germs would spontaneously develop. So he had flasks made with a series of differing shapes designed to allow the movement of air, but not of dust that would carry germs, into the flask containing sterilised broth. The liquid remained clear for months. As one biographer notes,

"The observer had a choice between only two hypotheses: placing the origin of germs either in solid particles (fragments of wool or cotton, starches) that float in the atmosphere, or in spores of molds or the eggs of infusoria. Pasteur said: "I prefer to think that life comes from life rather than from dust." {Debré 161}

Subsequent debate and experiment involved sampling air from the ceilings of cathedrals by Pouchet, and from a balloon by Pasteur, and mountaintops by both. A competition in June 1864 between the two overseen by a Pasteur-inclined committee of the Academy was won by Pasteur when Pouchet walked out claiming bias and misprocedure. Pasteur was held to have shown that spontaneous generation did not exist, and became a hero in French society. But had he shown this?

Strictly, Pouchet had shown that hay infusions would generate even when boiled, because, as it was shown a while later, hay had heat resistant spores. {Geison 131} Had he stayed in the competition, he may very well have won (although not because he was right about spontaneous generation). More worrying to us moderns is that it transpires, now that Pasteur's notebooks have become available (they were made available only in the 1970s, and an index published only in 1985), that Pasteur repeatedly ignored positive results in experiments, claiming that they were due to error rather than spontaneous generation; in fact only 10% of his experiments gave his desired result. {Geison 130}

Even so, Pasteur was correct - modern life, including fungi and infusorians, did not arise from non-living matter, whether or not that matter was organic or elemental. The debate over his experimental technique matters only to historians, although Geison's 1995 book caused an enormous furore in France, where Pasteur is something of a secular saint.

In his later years, Pasteur was forced to modify some of his views (not about spontaneous generation). He had thought that microorganisms retained their virulence indefinitely. But in 1881, he was forced to admit that virulence could attenuate spontaneously (and he made it the foundation of his anti-rabies vaccine). Debré says, "And now, at the age of sixty, Pasteur was once again facing facts that did not fit in which his concepts. Attenuated virulence conflicted with his biological philosophy. He had to renounce his dogmas and enter the debate on the evolution of species." He had to choose between Darwin's view that selection was in operation, or Lamarck's that the environment directly influenced the species of organism, and chose Lamarck. But he did accept transmutation of species, as is demonstrated by his comment quoted in Hilaire Cuny's biography, from Pasteur {Cuny, 122, from Pasteur 434}:

"Virulence appears in a new light which cannot but be alarming to humanity; unless nature, in her evolution down the ages (an evolution which, as we now know, has been going on for millions, nay, hundreds of millions of years), has finally exhausted all the possibilities of producing virulent or contagious diseases - which does not seem very likely."

Although he shortly afterwards refers to "the myriad species of Creation", it is clear that he accepted the reality of evolution. Moreover, he characterised the interaction between microbes and hosts as a "struggle for existence" (a phrase, it must be remembered, invented by the Swiss botanist Alphonse de Candolle, and borrowed by Darwin). However, I doubt he accepted that evolution occurred by natural selection, as the French rarely did until the 1950s and Jacques Monod's writings. However, he was not a creationist, at least at this point in his life.

Moreover, much has been made about Pasteur's faith. It is often claimed that he was a devout Catholic, but it seems he was very lax in his religious devotion, reading through church services as a student, and not attending church much during his life. He was, it must be said, opposed to the philosophical vogue of radical materialism in France, from which the spontaneous generation debate sprang, but he was hardly a model believer. Even so, despite claims made by Farley and Geison that Pasteur allowed his research to be guided by his a priori philosophy, he did turn out to be correct that the growths of germs were caused by pre-existing germs, and that fermentation was due to yeast.

Summary so far

So we must ask - what did Pasteur prove? Did he prove that no life can ever come from non-living things? No, he didn't, and this is because you cannot disprove something like that experimentally, only theoretically, and he had no theory of molecular biology to establish this claim. What he showed was that it was highly unlikely that modern living organisms arose from non-living organic material. This is a much more restricted claim than that primitive life once arose from non-living non-organic material.

So far we have seen that neither Redi, Spallanzani nor Pasteur disproved the origination of life in all cases, only in particular cases. Moreover, we have seen that the claims "all life from eggs", "all cells from cells" and "all life from life" are generalisations not fully supported by the experimental evidence available at the time they were made.

Evolution and abiogenesis

[Note: from this point on, dates are not given for scientists. Biographical details can be found in Farley's book, or in a history such as Singer's.]

The reaction to Pasteur was almost unanimous, at least in France: heterogenesis was a dead issue. But what Pasteur showed and what was drawn from it as a philosophical moral were different things. French science had since the time of the philosophes had a strong materialistic flavour. Many objected to this, and Pasteur was immediately pressed into service against it. In 1873, Father H. de Valroger saw spontaneous generation as a necessary belief of atheism, and wrote that the "action of the creator has been necessary for the production of the first living beings", while on the other side, Felix Isnard wrote in 1879 that "one must submit to rigorous proof of reasoning and accept as truth only that which is demonstrated by science", and that reason forced us to accept abiogenesis, and admit that heterogenesis was also still possible. {Farley 119}

In other countries, where Darwin's Origin, published the same year as Pasteur's studies, had an influence, which in France it tended not to, abiogenesis was still regarded as a viable notion. Although Darwin added the phrase "by the Creator" into his final paragraph in the second (1860) edition, and Huxley had also publicly stated that life may have been originally created, this was never understood to be part of the evolutionary mindset, and it was not long before people began to speculate on how life began. Darwin himself did, in a letter to his botanist friend Joseph Hooker in 1871, he wrote:

"It is often said that all the conditions for the first production of a living organism are now present, which could ever have been 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, &c., present, that a proteine (sic) compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly absorbed, which would not have been the case before living creatures were found."

In print, though, he restrained from speculation, noting that "In what manner the mental powers were first developed in the lowest organisms, is as hopeless as how life itself first originated. These are problems for the distant future, if they are ever to be solved by man." (Descent of Man, chapter 2, 1871). In an essay to the Atheneum in 1863, Darwin wrote upon heterogeny "as the old doctrine of spontaneous generation is now called", in which he noted that a "mass of mud with matter decaying and undergoing complex chemical changes is a fine hiding-place for obscurity of ideas". He argued that while it is true that at one time "there must have been a time when inorganic elements alone existed on our planet", "our ignorance is as profound on the origin of life as on the origin of force or matter", and denies that the theory of evolution requires that life continuously arises. So-called "primitive" life forms as Foraminifera are well adapted to their conditions, and are not evidence of on-going heterogenesis: "the nature of life will not be seized on by assuming that Foraminifera are periodically generated from slime or ooze". {Barrett 2:78ff}

T. H. Huxley, although agreeing with Darwin that Pasteur had shown that heterogeny was not continuously occurring, expressed the belief that life was formed from a fluid found in cells called "protoplasm", which was the clay that life molded, as it were {Huxley 1868}. Many thought he meant to claim that life arising from inorganic life was evolutionary "doctrine", although he denied this. It was to no avail. In 1870, he returned to the question and coined the term "abiogenesis" {Huxley 1870} to contrast to "biogenesis", the doctrine that all life arises from life. He concurred with Pasteur for all "known forms of life". He noted that heterogenesis and abiogenesis were often confused with each other, but that disproving the one did not thereby disprove the other. Again, though, he did not argue in its favour.

In 1868, Huxley had re-examined some oceanic mud dredged up in 1857 with a more powerful microscope than the one he used in his first examination, and thought he saw in it "a new form of ... simple animated beings" that Ernst Haeckel had previously called Urschleim (original slime), and he called it Bathybius Haeckelii. Protoplasmists were now able to say the oceanic floor was covered in protoplasmic Urschleim. All Huxley would say in 1870, though, was that abiogenesis was theoretically possible but "that I see no reason for believing that the feat has been performed yet". {Farley 74f} The Darwinists seemed to line up behind Pasteur, on this and other questions (Huxley was a medical doctor, and he accepted the contagion theory of disease).

William Thompson, later Lord Kelvin, whose physical contributions included a denial that there had been enough time for natural selection to cause evolution (later shown by the discovery of radioactivity to be wrong), said in his presidential address to the British Association for the Advancement of Science in 1871:

"I confess to being deeply impressed by the evidence put before us by Professor Huxley, and I am ready to adopt, as an article of scientific faith, true through all space and all time, that life proceeds from life, and from nothing but life.

"How then, did life originate on Earth? Tracing the physical history of the Earth backwards, on strict dynamical principles, we are brought to a red-hot melted globe on which no life could exist. Hence when the Earth was first fit for life, there was no living thing on it. ... Science is bound, by the everlasting law of honour, to face fearlessly every problem which can fairly be presented to it. If a probable solution, consistent with the ordinary course of nature, can be found, we must not invoke an abnormal act of Creative Power."

Thompson then suggested the first scientific idea of panspermia, or the seeding of life on earth. {Basalla et. al 125-127}

However, others were not so circumspect. Henry Charlton Bastian (1837-1915), one of the younger Darwinians, was convinced that life had spontaneously generated, and still did {Strick}. He declared that belief in abiogenesis did not commit one to denying the contagion theory of disease, although he denied that contagion was caused by living forms, but by organic but nonliving "fragments" from which living forms arose, a view admitted as legitimate by Pasteur, although Pasteur did not believe it. Bastian was opposed by, of all people, Huxley and John Tyndall, two of the older Darwinians, and adherents to the living contagion theory. Tyndal agreed with Lister that contagion was caused by living organisms, and that this mattered medically. Many other evolutionists, such as Herbert Spencer and William Thistleton-Dyer opposed spontaneous generation (abiogenesis) on the grounds that even the simplest organisms and protoplasm were too complex to arise directly from inorganic matter. Other forms of opposition came from vitalists such as William Carpenter, whom Darwin was criticising in his Heterogeny paper, and Lionel Beale. Vitalists opposed abiogenesis, but not heterogenesis, as a rule. John Tyndall, a friend of Darwin's and Huxley's, published a series of experiments in 1876 in which he claimed to show that positive results such as Bastian's were due solely to experimental error. {Farley chapters 5, 7}

Ernst Haeckel, whose monist philosophy combined Goethe and Darwin, asserted that spontaneous generation at the beginning of life was "a logical postulate of scientific natural history", and he regarded heterogenesis as "only of subordinate interest in the history of creation". As noted above, he thought that the simplest living material was protoplasm, and that the simplest organisms were little more than bags of protoplasm, so that "the deep chasm which was formally and generally believed to exist between organic and inorganic bodies is almost or entirely removed, and the way is paved for the conception of spontaneous generation" {Farley 75-77}

Others, such as Heinrich Bronn, rejected the idea until and unless it could be explained, and in general the debate was split between vitalists, who thought there was something special that made dead matter live, and materialists, who thought that life was merely chemistry in a different form. The idea and debate died, as no real progress was made, from the 1880s through to the second decade of the 20th century, although research on the contagion theory, or germ theory as it came to be known, continued apace, along with histological studies of cell growth, division and behaviour. Free cell formation, heterogenesis, and the chemical contagion theory all seemed to have disappeared. The only doctrine that still seemed to have some interest was abiogenesis, but little was done on it. Pasteur and Virchow appeared to have carried the day, and the Darwinians concurred with everyone else, although Bastian published a series of books between 1904 and 1911 still promoting his ideas. The notion that the simplest life was as simple as Haeckel had declared was shown, for modern single celled organisms, not to be true. They were in fact very complex, and the gap between non-life and life opened up again. And then closed, as biochemistry developed, leading Ben Moore, first professor of biochemistry at Liverpool University to remark in 1921:

"The territory of this spontaneous production of life lies not at the level of bacteria, or animalculae," rather it lies "at a level of life lying deeper than anything a microscope can reveal, and possessing a lower unit than the living cell" - in other words, it was chemical, not organismic. {Farley 155}

In the 1920s, the chemical theory of contagion got a boost, when viruses were discovered that were unfilterable even by earthernware filters. Viruses had been discovered in the 1890s in research on mosaic disease of tobacco and foot and mouth disease in cattle. Increasing interest was also shown in colloid chemistry (mixtures of chemicals that do not dissolve in each other) as a possible source of the activity of cellular material, or protoplasm. Proteins formed colloids in water, it was discovered, and they had a molecular weight in excess of anything previously discovered. Viruses were thought to be proteins. Enzymes in particular, which catalysed reactions in other proteins, were considered important. Some, such as Harvard biochemist Leonard Troland, saw the first life form as an autocatalytic protein enzyme, or in simple terms, a protein that caused reactions that generated more copies of itself. {Farley 157-159}

Felix D'Hérelle commenced studies in 1917 on dysentery in which he discovered bacteriophages, viruses that appeared to eat cells, that could lyse, or rupture, dysentery bacteria. He argued that viruses were living principles, parasites of bacteria, but not cellular, which seemed to overthrow as a general rule Virchow's dictum. He himself thought that the original living thing was a virus, and that it was composed of protein as a "micella" or thread. He thought that plant and animal life each arose separately from these micellae. The view was influential. {Farley 160-162}

At the same time, genetics was getting well underway, and Hermann Joseph Muller, the American geneticist, stated in 1921 that genes and viruses were the same thing, only viruses were "little else than the gene". In 1935, Wendell Stanley showed that viruses, in this case the same tobacco mosaic virus studied in the 1890s, was an autocatalytic protein that made us of the cellular machinery for reproduction. Further work showed it was not a pure protein, but a mixture of protein and nucleic acid. For this, he won the 1946 Nobel Prize for chemistry, showing that a pure chemical substance could behave as if alive. {Magner 318f}

In the meantime, though, a young Russian biochemist named Aleksandr Oparin gave a lecture on the origins of life in 1922, published as a booklet in 1924, that was to have a major impact on future research and ideas. To his ideas and the work that followed until the present day, we now turn.

Modern origins of life research

Oparin's book The Origin of Life in 1924, in which he proposed a chemical theory of the origin of life, was not published in English until 1936. Prior to that, it had been relatively uninfluential except in his native Soviet Union. Oparin (1894-1980) was personally well regarded in the Soviet Union, and was elected early to the Academy of Sciences. He was also unfortunately involved in the Lysenkoist debacle in Soviet genetics, and declared overtly that his views were compatible with the "dialectical materialism" of Soviet Leninism. However, despite this, it appears that the more important influence was the impact of colloid chemistry, then making great strides. {Farley 162-165}

Oparin's hypothesis was this: gels arose out of colloidal solutions which reacted in a way to cause more gels to be formed of the same chemical constitution. As the material in the surrounding watery medium diminished,"the more strongly and bitterly the struggle for existence was waged", so that gels either became "cannabilistic" or evolved to become autotrophs (organisms that metabolise non-living material, such as algae). {Farley 163}

He reasoned that if the early atmosphere lacked free oxygen, which is a product of plant respiration, simple organic compounds formed by vulcanism or lightning, containing the chemical elements that make up life - Carbon, Hydrogen, Oxygen, and Nitrogen - would not be destroyed, but would accumulate, forming a broth of organic molecules. {Schopf 121}

Before Oparin's work became known, the English biochemist J. B. S. Haldane, who had since 1923 been working on enzymes, wrote his paper in 1929, published in The Rationalist Annual, on the origin of life, in which he stated that, as a result of biochemistry, "since his [Pasteur's] death the gap between life and matter has been greatly narrowed", and, influenced by d'Hérelle, thought that the bacteriophage was a "step beyond the enzyme on the road to life, but it is perhaps an exaggeration to call it fully alive". The precursors of life were like viruses, due to anaerobic fermentation for millions of years. {Farley 163-164}

Despite many arguments, largely theoretical but with some experimental work, spontaneous generation remained a viable option for the origins of life - for abiogenesis - but it was a very confused field. What caused it to change and become focused was the publication on 23 April 1953 of Crick and Watson's Nature paper on the structure of DNA. Three weeks later, a graduate student at the University of Chicago named Stanley Miller published a paper in Science, on 15 May, entitled "A production of amino acids under possible primitive earth conditions".

Miller was a doctoral student of Nobel laureate Harold C. Urey (a chemist who discovered deuterium), after he heard a lecture by Urey in which he noted in passing that earth's primordial hydrogen-rich (reducing) atmosphere would have been favourable for the formation of simple organic molecules. {Schopf 123} He decided, with Urey's permission, to test this, assuming an atmosphere of molecular hydrogen (H₂), methane (CH₄), ammonia (NH₃) and water vapour (H₂O). Neither Urey nor Miller knew at this point that this was in line with Oparin's hypothesis, but as he prepared for the experiments, Miller read Oparin and mulled it over, along with Urey's hypotheses on the formation of the solar system. {Schopf 125}

He passed the atmosphere through a glass retort, continuously cycling it for several days, while exposing it to heat, electrical arcing, and cooling. After two days, the "ocean" (a flask of water through which the gases were passed) became pale yellow, and on analysis this turned out to be glycine, the simplest amino acid. They repeated the experiment for a week, and in the final yellow-brown solution, Miller detected seven amino acids, including three (glycine, anine and aspartic acid) found in modern living systems. In a period of three and a half months, Miller had confirmed Urey's and Oparin's hypotheses on the formation of the precursor molecules of life.

The claim was never that life had been made, but only that the necessary molecules for life could form spontaneously. Since Wöhler synthesised urea in 1828, this was becoming an inevitable conclusion - the molecular nature of life was more and more widely accepted and applied. Now there was no need to think that organic molecules had to come from organic systems. Later experiments use a more realistic atmosphere, replacing methane with carbon monoxide or dioxide (CO or CO₂), or ammonia with molecular nitrogen (N₂), with similar results.

An alternative to the Oparin-Miller model was proposed by Günter Wächtershäuser, who suggested that carbon oxides released from deep sea vents could stabilise on iron-sulphates, reacting with molecular hydrogen to form organic monomers (simple molecular units) from which life could form. Others have included the roles of clay substrates as catalytic templates for molecules to form on before there were genes, the formation of organic molecules in space (now well-established) seeding the early earth, and a formal model by Manfred von Eigen of how chemical reactions might generate copies of themselves - the hypercycle.

Sidney Fox successfully synthesised coascervate "cells" (a coascervate is a mixture of colloids that can, like lipids in modern cells, form a layer that will enclose molecules, but which can allow monomers to pass across it). These will, under some conditions, divide as they "grow" to form new cells.

There has been considerable progress made in recent years - the "Modern origins of life references" gives citations, but it is a complex and rapidly moving field.

Conclusions

1. In the initial period of biology it was assumed that life was a special substance, and that it could generate living beings directly. As research into the lifecycles of animals, plants and diseases progressed, it became obvious that modern living forms were always observed to form from existing living forms, and that cells always came from existing cells.
2. At the same time, it became increasingly obvious that the gap between living things at the chemical level and non-living molecules was decreasing, until it became clear in the mid-20th century that all processes of living things were chemical, and there was no "vital principle" needed for life.
3. Opposition to abiogenesis has sometimes been due to philosophical or religious principles, but also the state of scientific knowledge at the time. However, it is not feasible now, with our increasing knowledge of the chemistry of life and of prebiotic earth.
4. None of the people who did crucial experiments on spontaneous generation disproved abiogenesis. At best, they strongly confirmed the hypothesis that modern organisms (mice, maggots, or germs) did not arise in ordinary cases out of nonliving material. Most of the experiments against spontaneous generation were posed against heterogenesis, the doctrine that life could form from the decayed products of living organisms.
5. Pasteur did not disprove the origin of life by natural means, and the saying "all cells from cells" was not intended to cover the initial period of life on earth. Darwin did not propose a theory of the origin of life in the beginning.
6. Evolutionary theory was not proposed to account for the origins of living beings, only the process of change once life exists. However, many have thought that the theory of evolution logically requires a beginning of life, which is true. Of those, many have thought that a natural account of the origin of life is necessary, and some have proposed models which have borne up or not as research proceeds.

Acknowledgements

Thanks to Frank Lovell, John Pieret, Thomas Münster and Mark Isaak for contributions and corrections. Thanks to Adrien Delcour for the reference to Pasteur's comment.

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