In recent days the Internet science community has got its collective nickers in a mighty twist. The disciples of the Aquatic Ape Hypothesis (AAH) have dared to hold an international congress in London and the, oh so irresponsible, press has wasted precious print and cyber space reporting on this frivolity. For those not in the know the Aquatic Ape Hypothesis claims that certain aspects of human evolution can only be explained by assuming that a group of anthropoids, the future humans, lived for a substantial period in water. My evolutionary theory expert friends assure me that this hypothesis is a festering heap of non-scientific nonsense, as was forcibly expressed by Henry Gee in this Guardian blog post. So far so good but a couple of science writers thought it would be a good idea to draw parallels between the AAH and one or other historical scientific theory with disastrous results.

The first of these was Ben Richmond writing on The Mother Board in his post How the Aquatic Ape Theory Keeps Floating On. Mr Richmond chose to equate the AAH with the Ptolemaic geocentric theory. He wrote:

But the aquatic apes theory is more like Ptolemaic models of the cosmos that Copernicus overthrew. These models of the solar system had to be ever more complex to keep the Earth in the middle and also account for the incongruous movement of the planets. In the end Copernicus’s moving the Sun to the center of the solar system simplified the model…

This paragraph is of course a complete myth that has absolutely nothing to do with what really happened in the sixteenth century. As this is a standard myth that gets repeated time and time again I shall briefly sketch, not for the first time, the true facts of the story.

The Ptolemaic models developed in the Middle Ages, first by Islamic astronomers and then, most recently before Copernicus, by Peuerbach actually became simpler not more complex. The Peuerbachian geocentric model in use as Copernicus published his De revolutionibus was actually simpler than Copernicus’ heliocentric model. The contemporary astronomers hoped that Copernicus’ model would at least deliver more accurate data on the positions of the heavenly bodies, the, at the time, principle function of mathematical astronomy but the Copernican system based on the same data as the Ptolemaic systems was just as inaccurate as its predecessors. Heliocentricity only “overthrew” geocentricity as Kepler developed his actually simpler and much more accurate elliptical astronomy in the first quarter of the seventeenth century.

Ed Yong, notable science writer, chose a similar tactic and compared, in a tweet, the AAH not with geocentricity but with the phlogiston theory of eighteenth century chemistry.

Ed Yong tweeted:

After the recent shadow biosphere piece & this wk’s aquatic apes one, I look forward to the Obsever’s exposés on phlogiston & cold fusion

Rebecca Stewart praised his audacity in another tweet:

Phlogiston is sooo underrated!

Karen James crowned him a master tweeter for this piece of brilliance

That last tweet shows why ‪@edyong209 is considered a master tweeter. I suppose we shouldn’t be surprised; tweeting is writing, after all.

This comparison is just as wrong as Richmond’s and casts a poor light on Ed Yong for having made the comparison. Yong’s tweet and those of his supporters imply, at least unintentionally, that AAH and the phlogiston theory are equivalent in their scientific status a complete fallacy. The AAH is a highly dubious hypothesis that does not according to Henry Gee, even agree with the know facts of evolution theory whereas the phlogiston theory was an important scientific research programme, which played an important role in the evolution of chemistry in the eighteenth century.

It is very easy from our standpoint in the twenty-first century to pour scorn on the theory of phlogiston, which viewed whiggishly, that is in comparison to our current knowledge of chemistry, can in some of its aspect appear more than somewhat ridiculous. However viewed within the context of the situation in which it was born the phlogiston theory made a great deal of sense.

Phlogiston arose at the end of the seventeenth century when the dominant theory of matter was still the four-element theory of the ancient Greeks. This had been supplemented by the tria prima concept of the Paracelcians. On top of the four Greek elements of earth, water, fire and air Paracelsus had added the principles of mercury, sulphur and salt. It is important to realise that these are principles involved in the composition of substances rather than substances themselves. In an analogy based on the combustion of a piece of wood Paracelsus compared the smoke to mercury, the flame to sulphur and the ash to salt. This analogy is important, as combustion alongside distillation was one of the two principle methods of chemical investigation available to alchemists in the Early Modern Period.

Whilst rejecting Paracelcian alchemy the German physician Johann Joachim Becher (1635 – 1682) borrowed the tria prima replacing the mercury, sulphur and salt with three forms of the element earth:

terra fluida or mercurious earth, which contributed fluidity, subtility, volatility and metallicity to substances.

terra pinguis or fatty earth, which produced oily, sulphureous and combustible properties.

terra lapidea or vitreous earth, which was the principle of fusibilty.

Becher published this theory in his Physica Subterranea in 1667. For Becher his terra pinguis played an essential role in combustion.

Another German physician Georg Ernst Stahl (1659 – 1734) took up Becher’s theory, in 1718, renaming terra pinguis, phlogiston, from the Greek meaning inflammable, using this principle to explain both combustion and corrosion (rusting). Hypothesising that all inflammable materials contain phlogiston, which is consumed or used up during combustion. The important point is that the phlogiston theory as developed by Stahl readily explained the known facts of combustion.[1]

Working within the phlogiston research programme, in particular English chemists such as Joseph Black, Daniel Rutherford, Henry Cavendish, James Watt and Joseph Priestley isolated and discovered a whole range of elemental and compound gasses and furthered the evolution of chemistry over the next sixty or seventy years. In fact it was using the discoveries of the phlogistonists that Lavoisier and others were able to produce the synthesis that became known as modern chemistry. This dependence on the phlogistonists was so great that the great German nineteenth century chemist Justus Freiherr von Liebig stated in his third Familiar Letter on Chemistry: “He discovered no new body, no new property, no natural phenomenon previously unknown; but all the facts established by him were the necessary consequences of the labours of those who preceded him.”[2]

Supplanted by the Lavoisier’s synthesis phlogiston became an obsolete theory one that viewed from the new standpoint of the facts that it had discovered was no longer able to explain the available evidence a fate it was soon to share with Lavoisier’s own contribution to the evolution of chemistry.

Above I said that viewed whiggishly, that is in comparison to our current knowledge of chemistry, can in some of its aspect appear more than somewhat ridiculous. Interestingly the, now, Cambridge historian of chemistry Hasok Chang has written a brilliant paper titled We Have Never Been Whiggish (About Phlogiston)[3], in which he takes a new whiggish look at the phlogiston theory and its successor. I recommend this paper to anybody wishing to equate the phlogiston theory to the AAH.

Unfortunately it is common practice for people largely ignorant of the history of science to equate modern pieces of idiocy with obsolete theories from the evolution of science that having served their purpose and having been superseded by other theories are then considered as suitable subjects for ridicule. This is wrong, with a probability approaching certainty all of the scientific theories that we use today will someday themselves be superseded by better or more explanatory theories and become in their turn obsolete. This will not make them ridiculous but they will become part of a long chain of theories that have facilitated the evolution of science. Theories that no matter how strange they might appear to us from our privileged position of hindsight should be treated with the respect that they deserve for their important contribution to the advancement of knowledge.

[1] This very brief account of the origins of the phlogiston theory is largely taken from, William H. Brock, The Fontana History of Chemistry, Fontana Press, London, 1992, pp. 78-86. [2] I owe this Liebig quote to regular Renaissance Mathematicus reader and commentator Arjen Dijksman (@materion) to whom I’m very grateful. [3] Hasok Chang, We Have Never Been Whiggish (About Phlogiston), Centaurus, Vol. 51, 2009, pp. 239-264.