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ReviewCite this article: Lane N. 2015 The unseenworld:
reflections on Leeuwenhoek (1677)
‘Concerning little animals’. Phil. Trans. R. Soc. B
370: 20140344.http://dx.doi.org/10.1098/rstb.2014.0344
One contribution of 18 to a theme issue
‘Celebrating 350 years of Philosophical
Transactions: life sciences papers’.
Subject Areas:microbiology
Keywords:Leeuwenhoek, animalcule, protozoa,
bacteria, eukaryote, tree of life
Author for correspondence:Nick Lane
e-mail: [email protected]
& 2015 The Authors. Published by the Royal Society under the
terms of the Creative Commons AttributionLicense
http://creativecommons.org/licenses/by/4.0/, which permits
unrestricted use, provided the originalauthor and source are
credited.
The featured article can be viewed at
http://dx.doi.org/10.1098/rstl.1677.0003.
Electronic supplementary material is available
at http://dx.doi.org/10.1098/rstb.2014.0344 or
via http://rstb.royalsocietypublishing.org.
The unseen world: reflections onLeeuwenhoek (1677)
‘Concerninglittle animals’
Nick Lane
Department of Genetics, Evolution and Environment, University
College London, London WC1E 6BT, UK
Leeuwenhoek’s 1677 paper, the famous ‘letter on the protozoa’,
gives the firstdetailed description of protists and bacteria living
in a range of environments.The colloquial, diaristic style conceals
the workings of a startlingly originalexperimental mind. Later
scientists could not match the resolution and clarityof
Leeuwenhoek’s microscopes, so his discoveries were doubted or even
dis-missed over the following centuries, limiting their direct
influence on thehistory of biology; but work in the twentieth
century confirmed Leeuwenhoek’sdiscovery of bacterial cells, with a
resolution of less than 1 mm. Leeuwenhoekdelighted most in the
forms, interactions and behaviour of his little ‘animal-cules’,
which inhabited a previously unimagined microcosmos. In
thesereflections on the scientific reach of Leeuwenhoek’s ideas and
observations,I equate his questions with the preoccupations of our
genomic era: whatis the nature of Leeuwenhoek’s animalcules, where
do they come from,how do they relate to each other? Even with the
powerful tools of modernbiology, the answers are far from
resolved—these questions still challengeour understanding of
microbial evolution. This commentary was written tocelebrate the
350th anniversary of the journal Philosophical Transactions of
theRoyal Society.
My work, which I’ve done for a long time, was not pursued in
order to gain the praiseI now enjoy, but chiefly from a craving
after knowledge, which I notice resides in memore than most other
men.
Leeuwenhoek, Letter of 12 June 1716
Leeuwenhoek is universally acknowledged as the father of
microbiology. Hediscovered both protists and bacteria [1]. More
than being the first to see thisunimagined world of ‘animalcules’,
he was the first even to think of look-ing—certainly, the first
with the power to see. Using his own deceptivelysimple,
single-lensed microscopes, he did not merely observe, but
conductedingenious experiments, exploring and manipulating his
microscopic universewith a curiosity that belied his lack of a map
or bearings. Leeuwenhoek(figure 1) was a pioneer, a scientist of
the highest calibre, yet his reputation suf-fered at the hands of
those who envied his fame or scorned his unschooledorigins, as well
as through his own mistrustful secrecy of his methods, whichopened
a world that others could not comprehend. The verification of
thisnew world by the natural philosophers of the nascent Royal
Society laid outthe ground rules that still delineate science
today, but the freshness andwonder, the sheer thrill of
Leeuwenhoek’s discoveries, transmit directlydown the centuries to
biologists today. Microbiologists and phylogeneticistscontinue to
argue about the nature of Leeuwenhoek’s little animals, if inmore
elaborate terms. Only now are we beginning to find answers—and
sur-prisingly uncertain answers—to the questions that drove
Leeuwenhoek:where did this multitude of tiny ‘animals’ come from,
why such variety insize and behaviour; how to distinguish and
classify them?
Leeuwenhoek’s 1677 paper [1] was not his first contribution to
PhilosophicalTransactions, nor was it his first mention of little
animals living in water.
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Figure 1. Portrait of Leeuwenhoek by Jan Verkolje, 1686, at age
54. Copyright & The Royal Society.
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The paper was translated (‘English’d’) from low Dutch,and
excerpted to half its original length by the redoubtableHenry
Oldenburg, first Secretary of the Royal Society andfounding editor
of Philosophical Transactions. Oldenburgcorresponded so widely
across Europe that he was imprisonedin the Tower for suspected
espionage in 1667, during theSecond Anglo-Dutch War (when the Dutch
colony of NewAmsterdam was renamed New York). Oldenburg
lateradopted a pseudonym, the anagram ‘Grubendol’, to
avertsuspicion (it would arouse mine). Among his regular
Dutchcorrespondents were the surgeon Regnerus de Graaf andstatesman
Constantijn Huygens, father of famed astrono-mer Christiaan
Huygens, both of whom wrote epistles toOldenburg introducing ‘the
exceedingly curious and indus-trious’ Leeuwenhoek, Huygens adding
the helpful note‘or Leawenhook, according to your orthographie’
[2]. Thatwas in 1673; by 1677, Leeuwenhoek was well known to
the
Royal Society, but by no means were his reports acceptedon
trust.
Oldenburg published several of Leeuwenhoek’s letters in1673 and
1674, which dealt with interesting but uncontentiousmatters, such
as the structure of the bee sting. Equivalentmicroscopic structures
of objects visible to the naked eye hadbeen illuminated by Robert
Hooke in his Micrographia nearlya decade earlier; indeed, it is to
Hooke that we owe the word‘cell’, which he used to denote the boxy
spaces (reminiscentof the small rooms in a monastery) that make up
the structureof cork [3]. From some of Leeuwenhoek’s slightly
waspishremarks in his early letters, he had almost certainly seena
copy of Micrographia on his visit to London in 1667 or1668, when
the book was practically a fashion accessory(‘the most ingenious
book I read in all my life’, wrote Pepys,who stayed up all night
with it; Pepys reputedly stayed upall night often, though rarely
with a book). Leeuwenhoek
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Figure 2. First and last pages of Leeuwenhoek’s 1676 letter to
Oldenburg, in the hand of a copyist. Copyright & The Royal
Society.
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first courted controversy in a letter of September
1674.Describing a nearby lake, Berkelse Mere, he noted that
itswater was very clear in winter ‘but at the beginning ormiddle of
summer it becomes whitish, and there are thenlittle green clouds
floating in it’ [4]. These clouds containedwispy ‘green streaks,
spirally wound serpent-wise, and orderlyarranged’—the beautiful
green alga Spirogyra. Then came Leeu-wenhoek’s first mention of
little animals: ‘among these streaksthere were besides very many
little animalcules . . . And themotion of most of these animalcules
in the water was soswift, and so various upwards, downwards and
round aboutthat ‘twas wonderful to see: and I judged that some of
theselittle creatures were above a thousand times smaller than
thesmallest ones I have ever yet seen upon the rind of cheese’(by
which he meant mites) [4].
Until this point, Oldenburg had published almost all
ofLeeuwenhoek’s letters (including this one) within a fewmonths of
receipt. Now, he drew pause. Of the next 12 letterssent by
Leeuwenhoek, only three were published, and nonethat touched on
animalcules. Oldenburg had every reasonto be suspicious; as
Leeuwenhoek wrote to Hooke a fewyears later ‘I suffer many
contradictions and oft-times hearit said that I do but tell fairy
tales about the little animals’[5]. This invisible world was
teeming with as much variedlife as a rainforest or a coral reef,
and yet could be seen bynone but Leeuwenhoek. No wonder Oldenburg
and hiscolleagues had doubts. Set against this background,
Leeu-wenhoek wrote his eighteenth letter to the Royal Society,dated
October 1676, the celebrated ‘letter on the protozoa’,
which Oldenburg excerpted, translated and published asthe 1677
paper. It opens with a bang: ‘In 1675 I discoveredliving creatures
in Rain water which had stood but fewdays in a new earthen pot,
glased blew [i.e. painted blue]within. This invited me to view this
water with great atten-tion, especially those little animals
appearing to me tenthousand times less than those represented by
Mons.Swamerdam and called by him Water fleas or Water-lice,which
may be perceived in the water with the naked eye’ [1].
Clifford Dobell, in his delightful biography published in1932
(300 years after Leeuwenhoek’s birth), notes that Olden-burg’s
translation is good but not perfect [6]. That’s notsurprising.
While Oldenburg knew the language, he had noknowledge of the
organisms themselves. In contrast, Dobellwas a distinguished
microbiologist, a Fellow of the RoyalSociety, and had the great
benefit of hindsight. His biographywas a labour of love, written
over 25 years, frequently in themiddle of the night, while carrying
out his own research onintestinal protozoa and other protists.
Dobell taught himselfDutch and translated Leeuwenhoek’s letters
painstakingly—written, as they were, in a colloquial Dutch no
longer in use,and in the beautiful but scarcely legible hand of a
copyist(figure 2). Dobell revelled in the precise beauty of
Leeuwen-hoek’s descriptions of Euglena, Vorticella and many
otherprotists and bacteria, which leapt off the page,
immediatelyrecognizable to this expert kindred spirit. Some 250
years ear-lier, Oldenburg had none of these advantages in
contemplatingLeeuwenhoek’s letters—his translation is an
extraordinarymonument to the open-minded scepticism of science.
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I sketch this background because the paper itself is unu-sual
even in Leeuwenhoek’s oeuvre, taking the form of adiary. On a
cursory reading, it seems almost embarrassinglynaive to the modern
ear—the earthen pot ‘glased blewwithin’ in the first sentence is a
good example (but see [7]for a discussion of Dutch prose style in
the seventeenth cen-tury). We learn that on ‘the 17th of this month
of June itrained very hard; and I catched some of that rain water
in anew Porcelain dish, which had never been used before, butfound
no living creatures at all in it’ [1]. On it goes, with pre-cise
but apparently irrelevant details. ‘In the open Court ofmy house I
have a well, which is about 15 foot deep, beforeone comes to the
water. It is encompassed with high walls,so that the Sun, though in
Cancer, yet can hardly shinemuch upon it. This water comes out of
the ground, whichis sandy, with such a power, that when I have
laboured toempty the well, I could not so do it but there
remainedever a foots depth of water in it. This water is in
Summertime so cold, that you cannot possibly endure your hand init
for any reasonable time’ [1]. And my favourite: ‘July 271676. I
went to the sea-side, at Schlevelingen, the windcoming from the Sea
with a very warm Sun-shine; and view-ing some of the Sea-water very
attentively, I discovereddivers living animals therein. I gave to a
man, that wentinto the Sea to wash himself, a new glass-bottle,
boughton purpose for that end, intreating him, that being on
theSea, he would first wash it well twice, or thrice, and thenfill
it full of the Sea-water; which desire of mine havingbeen complied
with, I tyed the bottle close with a cleanbladder’ [1].
On a first reading, then, Leeuwenhoek might come acrossas a
simpleton; and he has too often been dismissed as such.One can only
smile at the image of Leeuwenhoek on thebeach, pressing his
pre-prepared bottles onto strangers. Butwhich details are
important? How should he have chartedthis abundant new world? We
need to appreciate severalpoints. This letter was intended to
defend his discoveries—‘merely so as to make my observations more
credible inEngland and elsewhere’ [8]. Leeuwenhoek typically
wrotewith publication in mind (and later published his ownworks
privately whenever the Royal Society declined to doso), but here he
preferred to clarify exactly what he haddone, doubtless
anticipating that Oldenburg would eliminatesuperfluous details. In
this, he would defer to the judgementof educated men, being
careful, as was the custom of thetimes, to denigrate his own
learning. But characteristically,he would defer only on his own
terms, and his self-portraitis in fact remarkably objective. ‘I
have oft-times beenbesought, by divers gentlemen, to set down on
paper whatI have beheld through my newly invented Microscopia: but
Ihave generally declined; first, because I have no style, orpen,
wherewith to express my thoughts properly; second,because I have
not been brought up to languages or arts,but only to business; and
in the third place, because I donot gladly suffer contradiction or
censure from others’ [9].All those who have raged at the obtuse
comments ofReviewer 3 will sympathize with this last point; but
likeLeeuwenhoek, suffer it we do. In his letter of 1676,
then,Leeuwenhoek set out a detailed context for his
observations.Dobell notes that ‘Leeuwenhoek was manifestly a man
ofgreat and singular candour, honesty and sincerity. He
wasreligiously plain and straightforward in all he did, and
there-fore sometimes almost immodestly frank in describing his
observations. It never occurred to him that Truth couldappear
indecent’ [10].
On a closer reading, the colloquial manner of Leeuwen-hoek’s
letter conceals the workings of his precise andmethodical mind.
Leeuwenhoek was acutely aware ofcontamination; he replenished
evaporated water with snow-water, the purest then available, making
every effort not tointroduce little animals from any other source.
He sampledwater from many different sources—his well, the sea,
rainwater, drain pipes, lakes—always taking care to clean
hisreceptacles. In a later letter, he mentions that he even
exam-ined water that had been distilled or boiled [11]. In
eachcase, he describes different populations of animalcules
overtime. Time is critical. Frequently, he observes nothing for
aweek, checking each day, before reporting a profusion oflittle
animals of diverse types, replicating themselves overseveral days
before dying back again. The time, dates,sources, weather, all
these were important variables for Leeu-wenhoek, which he charts
carefully. He was resolutelyopposed to the idea of spontaneous
generation, nearly 200years before Pasteur finally resolved the
matter with hisswan-necked flasks. Leeuwenhoek later described the
pro-creation of cells via copulation or schism to releasedaughter
cells in arresting detail. But his early disbelief ofspontaneous
generation is implicit in the comparisons ofhis 1677 paper, in his
care to avoid contamination, and hisestimation of rates of
growth.
Leeuwenhoek also reports experiments, adding pepper-corns to
water, both crushed and uncrushed (as wellas ginger, cloves, nutmeg
and vinegar, omitted fromOldenburg’s excerpts for Philosophical
Transactions). In theseinfusions, Leeuwenhoek observed an
astonishing prolifer-ation of tiny animals ‘incredibly small; nay,
so small, in mysight, that I judged that even if 100 of these very
wee animalslay stretched out one against another, they could not
reachthe length of a grain of course sand; and if this be true,then
ten hundred thousand of these living creatures couldscarce equal
the bulk of a course grain of sand’ [1]. Again,the colloquial
language deceives. In a clarification sent toConstantijn Huygens
and Hooke, Leeuwenhoek writes‘Let’s assume that such a sand-grain
is so big, that 80 ofthem, lying one against the other, would make
up thelength of one inch’ [12]. He goes on to calculate the
numberof animalcules in a cubic inch; for our purposes here,
hiscalculation puts the length of his ‘very wee animals’ at
lessthan 3 mm. Bacteria. (He later describes bacterial
motilityunequivocally [13]). He also notes that he deliberately
under-estimates the number of bacteria in a drop of water—‘for
thereason that the number of animalcules in so small a quantityof
water would else be so big, that ‘twould not be credited:and when I
stated in my letter of 9th October 1676, thatthere were upwards of
1 000 000 living creatures in onedrop of pepper-water, I might with
truth have put thenumber at eight times as many’ [14]. An innocent,
earlyexample of spinning data to sell to a journal?
But the natural philosophers of the Royal Society, in
pio-neering the methods we still use in science today, were
noteasily spun. Leeuwenhoek’s letter had been read aloud
overseveral sessions and attracted great interest, verging on
con-sternation. Oldenburg wrote to Leeuwenhoek, asking himto
‘acquaint us with his method of observing, that othersmay confirm
such Observations as these’, and to providedrawings [15].
Leeuwenhoek declined, throughout his
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(a) (b)
Figure 3. (a) Rotifers, hydra and vorticellids associated with a
duckweed root, from a Delft canal. From Leeuwenhoek [16]. (b)
Bacteria from Leeuwenhoek’s mouth;the dotted line portrays
movement. From Leeuwenhoek [17]. Copyright & The Royal
Society.
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life, to give any description of his microscopical methods
(‘forreasons best known to himself’, said Hooke; though sciencehas
hardly resolved the issue of intellectual property sincethen). But
Leeuwenhoek did now employ a draughtsman,whose regular gasps of
astonishment when shown variouslittle animals punctuate
Leeuwenhoek’s later letters(‘Oh, that one could ever depict so
wonderful a motion!’).Some of these limner’s drawings are shown in
figure 3.Leeuwenhoek also sent eight testimonies from gentlemenof
repute—a Lutheran minister, a notary and a barrister,among others.
It is striking to the modern reader that noneof these gentlemen
were natural philosophers acquaintedwith the methods of science;
but according to the historianSteven Shapin, it was the bond of the
gentleman thatcounted. The practice of signed testimonies from
gentlemenwas common in the seventeenth century; the fact that
Leeu-wenhoek called upon eight such testimonies attests to
theunprecedented character of his findings, but also perhaps tohis
lower social standing [18].
No doubt all this was helpful, but it was counteredby letters
from others such as Christiaan Huygens (son ofConstanijn), then in
Paris, who at that time remained scepti-cal, as was his wont: ‘I
should greatly like to know how muchcredence our Mr Leeuwenhoek’s
observations obtain amongyou. He resolves everything into little
globules; but for mypart, after vainly trying to see some of the
things which hesees, I much misdoubt me whether they be not
illusions ofhis sight’ [19]. The Royal Society tasked Nehemiah
Grew,the botanist, to reproduce Leeuwenhoek’s work, but Grewfailed;
so in 1677, on succeeding Grew as Secretary, Hookehimself turned
his mind back to microscopy. Hooke tooinitially failed, but on his
third attempt to reproduceLeeuwenhoek’s findings with pepper-water
(and otherinfusions), Hooke did succeed in seeing the
animalcules—‘some of these so exceeding small that millions of
millionsmight be contained in one drop of water’ [20] (actually
farless precise than Leeuwenhoek). He went on to write ‘Itseems
very wonderful that there should be such an infinite
number of animalls in soe imperceptible quantity of matter.That
these animalls should be soe perfectly shaped andindeed with such
curious organs of motion as to be ableto move nimbly, to turne,
stay, accelerate and retard theirprogresse at pleasure. And it was
not less surprising tofind that these were gygantick monsters
[protozoa] in com-parison of a lesser sort which almost filled the
water[bacteria]’ [21].
Unlike Leeuwenhoek, Hooke gave precise details of
hismicroscopical methods, and demonstrated them before thegathered
fellows, including Sir Christopher Wren, later pub-lishing both his
methods and observations in Microscopium(1678) [20]. He even taught
himself Dutch, so that he couldread the letters of the ‘ingenious
Mr Leeuwenhoek’. Asnoted by the microscopist Brian J. Ford [22] and
microbiologistHoward Gest [23], Hooke was a central and too-often
over-looked figure in the history of microbiology: his earlier
bookMicrographia (1665) most likely inspired Leeuwenhoek tobegin
his own microscopical studies. Without Hooke’s sup-port and
verification—a task beyond several of the bestmicroscopists of the
age, including Grew—Leeuwenhoekmight easily have been dismissed as
a charlatan. Instead,through Hooke’s impressive demonstrations, and
withthe direct support of the patron of the Royal Society,
KingCharles II, Leeuwenhoek was elected a Fellow in 1680.Others had
independently changed their view of Leeuwen-hoek in the interim,
but that did little to alter the course ofevents. Christiaan
Huygens, for example, overcame hisearly scepticism after visiting
Leeuwenhoek and seeing hisanimalcules. He went on to grind his own
lenses, observingvarious protists himself [24]. Indeed, Huygens
made anumber of pioneering observations, but these remained
inmanuscript and were unpublished until the turn of thetwentieth
century [25].
Ironically, Hooke’s admirable comments on the construc-tion of
microscopes might have undermined Leeuwenhoek’slater reputation.
Hooke made various types of microscope.He much preferred using
larger instruments with two
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(c) (d)
Figure 4. (a) Replica of a single-lens microscope by Leeuwenhoek
(Image by Jeroen Rouwkema. Licensed under CC BY-SA 3.0 via
Wikimedia Commons). (b,d)Photomicrographs taken using simple
single-lens microscopes including one of Leeuwenhoek’s originals in
Utrecht, by Brian Ford (Copyright & Brian J. Ford). (b)
Anair-dried smear of Ford’s own blood through the original van
Leeuwenhoek microscope at Utrecht, showing red blood cells and a
granulocyte with its lobed nucleus(upper right; about 2 mm in
diameter). (c) Spiral bacteria (Spirillum volutans) imaged through
a replica microscope with a lens ground from spinel; each
bacterialcell is about 20 mm in length. (d ) The intestinal protist
parasite Giardia intestinalis imaged through a replica soda-glass
produced by Brian Ford [28,29].
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lenses, but in the Preface to Micrographia [26] he also
describedhow to make ‘simple’ microscopes with a single lens—what
became known as a Leeuwenhoek microscope [27].The lens is produced
by melting Venice glass into thin threads,containing little
globules, which are then ground andpolished, and mounted against a
needle hole pricked througha thin plate of brass (figure 4). ‘If .
. . an Object, plac’dvery near, be look’d at through it, it will
both magnifie andmake some objects more distinct than any of the
great Micro-scopes. But because these, though exceeding easily
made,are yet very troublesome to be us’d, because of their
smallness,and the nearness of the Object; therefore to prevent both
ofthese, and yet have only two refractions, I provided me aTube of
Brass’ [26]. In 1678, Hooke reiterated his dislike ofsingle-lens
microscopes: ‘I have found the use of them offen-sive to my eye,
and to have much strained the sight, whichwas the reason why I
omitted to make use of them, thoughin truth they make the object
appear much more clear and dis-tinct, and magnifie as much as the
double Microscopes: nay tothose whose eyes can well endure it, ‘tis
possible with a singleMicroscope to make discoveries much better
than with adouble one, because the colours which do much disturbthe
clear vision in double Microscopes is clearly avoided andprevented
with the single’ [20].
It seems that Hooke’s aversion to simple single-lensmicroscopes
passed on down the generations, but not his
appreciation of their merits. The compound microscope, withits
refractive aberrations, became the tool of choice, andLeeuwenhoek’s
microscopes were quietly forgotten, theiroblivion hastened by
Leeuwenhoek’s own secrecy, notwith-standing his gift of 13
microscopes, with correspondingspecimens, to the Royal Society on
his death in 1723 at theage of 90. Leeuwenhoek had actively
discouraged teachinghis methods, for reasons that are troubling
today in an agewhen education is open to all. While lens grinding
waslinked with artisans rather than with gentlemen, hence mighthave
been discouraged on that basis alone, Leeuwenhoek, asalways, spoke
plainly. In a letter to Leibnitz, he wrote ‘Totrain young people to
grind lenses, and to found a sort ofschool for this purpose, I
can’t see there’d be much use: becausemany students at Leyden have
already been fired by my dis-coveries and my lens grinding . . .
But what’s come of it?Nothing, as far as I know: because most
students go there tomake money out of science, or to get a
reputation in the learnedworld. But in lens grinding, and
discovering things hiddenfrom our sight, these count for nought.
And I’m satisfied toothat not one man in a thousand is capable of
such study,because it needs much time, and spending much money;
andyou must always keep on thinking about these things, if youare
to get any results. And over and above all, most men arenot curious
to know: nay, some even make no bones aboutsaying: What does it
matter whether we know this or not?’
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[30]. Most scientists, I imagine, would see themselves as
thatone man in a thousand; it is our task today to persuadeothers
that it does indeed matter, not for any immediate benefit,but for
the sake of curiosity and its unknowable contribution tothe sum of
human knowledge and wellbeing.
The dominant use of compound microscopes over the fol-lowing
centuries meant that the brief blaze of Leeuwenhoek’sdiscoveries
was nearly extinguished until the great compoundmicroscope makers
of the early-nineteenth century, notablyJoseph Bancks (who also
produced some high-poweredsingle-lens microscopes, used by Robert
Brown in his discoveryof Brownian motion and cytoplasmic streaming,
and byDarwin aboard the Beagle). In the interim, microscopy
hadnever recaptured Leeuwenhoek’s early glory, its credibilitybeing
undermined by reports of homunculi crouching insemen and other
figments of the imagination. The concept ofpreformation was called
into serious question from the 1740s,beginning with Abraham
Trembley’s work on the regenerationof freshwater polyps [31]. In
the 1750s, Linnaeus scarcelytroubled himself with the
classification of microbes; hedumped the whole lot into the phylum
Vermes (‘worms’),genus Chaos (formless). The damaging accusation of
seeingthings that were not there, combined with Linnaeus’s
insinuatedabsence of structure, meant that few believed
Leeuwenhoekcould have seen cells as small as bacteria; even the
empatheticDobell struggled to conceive what magical form of
lightingLeeuwenhoek must have employed to view his specimens.Only
the galvanizing work of Brian J. Ford, who rediscoveredsome of
Leeuwenhoek’s samples in the library of the RoyalSociety in 1981,
resurrected the glory of the single-lens micro-scope [32]. Ford
photographed Leeuwenhoek’s originalspecimens using one of his
surviving microscopes in Utrecht,and demonstrated a remarkable
resolution of less than 1 mm[33] (figure 4). That left little scope
for disbelief: plainly,Leeuwenhoek really did see much of what he
claimed.
So what is Leeuwenhoek’s legacy? Most of his discoverieswere
forgotten, and only rediscovered in the nineteenth cen-tury, 150
years later, being then interpreted in the context ofthe newly
developing cell theory, with little reference back toLeeuwenhoek
himself. In this regard Leeuwenhoek’s legacy isanalogous to that of
Gregor Mendel, likewise rediscovered ata time when others were
exploring similar ideas. Leeuwen-hoek’s work, of course, ranged far
beyond microbiology. Inall, he sent around 200 letters to the Royal
Society, 112 ofwhich were published, touching on many aspects of
biologyand even mineralogy. He remains the most highly
publishedauthor in the journal. He is considered to be the founder
ofmany fields, but none of them more important than his
aston-ishing discoveries in microbiology, and none conveyed
withsuch delight. Leeuwenhoek was captivated by his
animalcules.‘Among all the marvels that I have discovered in
nature’, hewrote, ‘these are the most marvellous of all’ [34]. His
exhilara-tion in discovery, combined with a fearless and
surefootedinterpretation of unknown vistas, is for me
Leeuwenhoek’strue legacy. It is a spirit effervescent in many later
pioneers ofmicrobiology, indeed in science more generally. And
manyof the problems that beset Leeuwenhoek troubled them too.
Take the ultrastructure of cells, especially protists.
Leeu-wenhoek could clearly see ‘little feet’ (cilia) and also
thebudding offspring of cells, but he saw much more thanthat. I’m
struck by this passage in the 1677 paper, describingan ‘egg-shaped’
animalcule (which Dobell tentatively ident-ified as the ciliate
Colpidium colpoda [35]): ‘Their body did
consist, within, of 10, 12, or 14 globuls, which lay
separatefrom each other. When I put these animalcula in a dry
place,they then changed their body into a perfect round, andoften
burst asunder, & the globuls, together with someaqueous
particles, spred themselves every where about, with-out my being
able to discern any other remains. Theseglobuls, which in the
bursting of these creatures did flowasunder here and there, were
about the bigness of the firstvery small creatures [bacteria]. And
though as yet I couldnot discern any small feet in them, yet me
thought, theymust needs be furnished with very many . . . ’
[1].
While the ‘globuls’ in C. colpoda were probably mostly
foodvacuoles, as well as the macronucleus, Leeuwenhoek’s
com-parison with bacteria leaves open the tantalizing
possibilitythat he had even seen organelles such as
mitochondria,which with a diameter of 0.5–1 mm would have pushed
hismicroscopical resolution to the limits. Some 250 years
later,this equivalence between intracellular ‘globuls’ and
free-living bacteria was pursued by the early-twentieth
centurypioneers of endosymbiotic theory, notably the
RussianKonstantin Mereschkowski, Frenchman Paul Portier andAmerican
Ivan Wallin, the latter pair independently going sofar as to argue
that mitochondria could be cultivated [36].The idea of
‘symbiogenesis’ was famously ridiculed by theAmerican cell
biologist E.B. Wilson, who summed up the pre-vailing attitude: ‘To
many, no doubt, such speculations mayappear too fantastic for
present mention in polite biologicalsociety; nevertheless, it is
within the range of possibility thatthey may some day call for
serious consideration’ [37]. Anotherhalf-century was to elapse
before Lynn Margulis and othersdemonstrated that mitochondria and
chloroplasts do indeedderive from bacterial endosymbionts [38]; and
even then, notwithout a fight. I doubt that the idea of
endosymbiosiswould have shocked Leeuwenhoek; nor would he have
beenmuch surprised by the contemptuous disbelief of manybiologists
over decades.
Another unifying theory came from biochemistry, and fit-tingly
drew inspiration from Leeuwenhoek’s hometown ofDelft (described by
the Earl of Leicester, once Governor-General of the Netherlands, as
‘another London almost forbeauty and fairness’). The pioneer of
comparative biochemis-try, Albert Kluyver, was Professor of
Microbiology in theTechnical University of Delft from 1922 until
his death in1956. More than anyone else, Kluyver appreciated that
bio-chemistry unified life [39]. He realized that different typesof
respiration (he cites sulfate reduction, denitrification
andmethanogenesis) are fundamentally equivalent, all involvingthe
transfer of electrons from a donor to an acceptor. Heappreciated
that all forms of respiration and fermentationare united in that
they all drive growth by means of phos-phorylation. Such parallels
made the startling differencesbetween cells explicable, a discovery
he cherished as ‘highlyedifying to the scientific mind’ [40]. He
expressed this unityin the awkward phrase ‘From elephant to butyric
acid bacter-ium—it is all the same’, later paraphrased, more
memorablybut without attribution, by François Jacob and
JacquesMonod as ‘that old axiom ‘what is true for bacteria is
alsotrue for elephants’’. Kluyver, in a seminal passage,
recog-nized that the fundamental unity of biochemistry ‘opensthe
way for a better appreciation of evolutionary develop-ments which
have taken place in the microbial world, sincethe antithesis
between the aerobic and anaerobic mode oflife has been largely
removed’ [40].
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eubacteria archaebacteria
eukaryotes
Figure 5. A tree of life drawn by Bill Martin in 1998,
reflecting whole genomes. The tree shows the chimeric origin of
eukaryotes, in which an archaeal host cellacquired bacterial
endosymbionts that evolved into mitochondria; and the later
acquisition of chloroplasts in Plantae. Reproduced with permission
from [51].Copyright 1999 & John Wiley & Sons, Inc.
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The unity of biochemistry, then, gave the first insightsinto the
evolution of the astonishing variety of ‘little animals’,which
until then had remained a mystery, their provenanceas wholly
unknown as in Leeuwenhoek’s time. Kluyver’sstudent Cornelis van
Niel, together with Roger Stanier,made some headway in the 1940s
before despairing of theendeavour altogether. By the time they
published theirfamous essay ‘The concept of a bacterium’ in 1961
they nolonger cared to defend their own earlier taxonomic
systems[41]; they sought only to distinguish bacteria
(prokaryoticcells, lacking a nucleus) from larger eukaryotic
protists,all of which have a nucleus. In this, they were
remarka-bly perspicacious, commenting: ‘The differences
betweeneukaryotic and prokaryotic cells are not expressed in
anygross features of cellular function; they reside rather
indifferences with respect to the detailed organization of the
cellularmachinery’ [41]. They cite the examples of respiration
andphotosynthesis, found in both eukaryotic and prokaryoticcells:
‘But in the prokaryotic cell, these metabolic unitprocesses are
performed by an apparatus which alwaysshows a much smaller degree
of specific organization. Infact, one can say that no unit of
structure smaller than thecell in its entirety is recognizable as
the site of either metabolicunit process’ [41]. This is a beautiful
insight, worthy ofLeeuwenhoek himself. In eukaryotes, respiration
and photo-synthesis are conducted in mitochondria and
chloroplasts,respectively, and continue perfectly well in isolation
fromthe rest of the cell, as all the soluble enzymes needed
areconstrained within the bioenergetic membranes of theorganelle.
In bacteria, by contrast, the enzymes requiredare split between the
cell membrane (whether invaginatedor otherwise) and the cytosol,
making the bacterium asa whole the indivisible functional unit.
This distinctionapplies as much to cyanobacteria (classed as algae,
not bac-teria, by Ernst Haeckel and later systematists) as to
otherbacteria. Stanier and van Niel therefore argued that
bacteriaare a single (monophyletic) group, all similar in their
basic plan, but insisted that any further attempts to
definephylogeny were hopeless.
The timing was unfortunate. Francis Crick had alreadyadvocated
the use of molecular sequences as a wonderfullysensitive
phylogenetic signal, writing in 1958: ‘Biologistsshould realize
that before long we shall have a subject whichmight be called
‘protein taxonomy’—the study of amino acidsequences of proteins of
an organism and the comparison ofthem between species. It can be
argued that these sequencesare the most delicate expression
possible of the phenotypeof an organism and that vast amounts of
evolutionaryinformation may be hidden away within them’ [42].
Soonafterwards, Zuckerkandl & Pauling [43] formalized the
argu-ment with sequence data; and a mere two decades later,Carl
Woese published his first tree of life [44]. Woese [45]was soon
dismissing Stanier and van Niel as epitomising thedark ages of
microbiology, when microbiologists had givenup any prospect of a
true phylogeny. Woese’s tree was basedon ribosomal RNA. He showed
that prokaryotes are notmonophyletic at all, but subdivide into two
great domains,the bacteria and archaea. Later work, which used
othermethods to ‘root’ the tree [46], portrayed the eukaryotes as
a‘sister group’ to the archaea [47]. For the first time, it
seemedpossible to reconstruct the evolutionary relationships
betweenLeeuwenhoek’s animalcules in an evolutionary tree of
life.Woese and his co-workers went so far as to argue that theterm
prokaryote was obsolete, being an invalid negative defi-nition
(i.e. prokaryotes are defined by the absence of a nucleus;[48]).
The three domains tree is still the standard text bookview. Even
so, for all its revolutionary appeal, Woese’s treeis the apotheosis
of a reductionist molecular view of evolution,based on constructing
trees from a single gene. It is ironic that,later in life, Woese
called for a more holistic biology, whilerefusing to countenance
the limitations of his single-genetree [49].
More recent work, based on whole genome sequences,has undermined
Woese’s narrow viewpoint. While the
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sisterhood of archaea and eukaryotes is upheld for a core
ofinformational genes—genes involved in DNA
replication,transcription and translation—it is not at all true for
mostother genes in eukaryotes, which are more closely related
tobacteria than archaea. Woese’s iconic tree is therefore
pro-foundly misleading, and should be seen strictly as a tree ofone
gene only: it is not a tree of life. We cannot infer whata cell
might have looked like, or how it might have lived inthe past, on
the basis of its ribosomal genotype. Eukaryotesare now plainly seen
to be genomic chimeras, apparentlyformed in a singular
endosymbiosis between an archaealhost cell and a bacterium around
1.5 billion years ago [50].This chimerism cannot be depicted on a
normal branchingphylogenetic tree, because endosymbiosis involves
fusionof branches, not bifurcation, producing instead a
strikingcomposite tree, depicted beautifully (and presciently, as
thisis still accurate) by Bill Martin in 1998 [51] (figure 5).
BillMartin and I have since argued that the singular endosym-biosis
at the origin of eukaryotes, which gave rise tomitochondria,
increased the energy available per gene ineukaryotic cells by a
breath-taking three to five orders of mag-nitude [52]. That
overcame the pervasive energetic constraintsfaced by bacteria,
enabling a massive expansion in cellvolume and genome size, and
permitting the evolution ofmany eukaryotic traits from the nucleus
to sex and phagocyto-sis (all of which were first reported by
Leeuwenhoek himself).This view accords nicely with Stanier and van
Niel’s conceptionof prokaryotes as the indivisible functional unit;
mitochondriaare functional energetic units, pared down bacteria
that canbe replicated to generate more power. It might be that
eukar-yotes had to evolve by way of an endosymbiosis, for
thesebioenergetic reasons.
Even in the absence of endosymbiosis, the idea of a
truephylogenetic tree of life is undermined by the prevalence of
lat-eral gene transfer in both bacteria and archaea.
Informationalgenes, including ribosomal RNA, are generally
inherited verti-cally, giving a robust phylogenetic signal, but
such genesaccount for barely 1% of a bacterial genome, and much of
therest is passed around between cells by lateral gene
transfer,confounding deep phylogenetic signals. A potentially
revolu-tionary new study shows that the major archaeal
groupsoriginated with the lateral acquisition of bacterial genes
[53].Ironically, the unity of biochemistry—Kluyver’s edifyingguide
to evolution—is the root problem: the universality ofthe genetic
code, intermediary metabolism and energy conser-vation (e.g. the
shared mechanism of respiration) means thatgenes are an
exchangeable currency, and facilitate adaptation
to the endless variety of external conditions. Again, the
linkbetween the ribosomal genotype of a prokaryotic cell and
itsphenotype—the way it makes a living—is forever changing.Ford
Doolittle notes that pervasive genetic chimerism meansthat ‘no
hierarchical universal classification can be taken asnatural’ [54];
the universal tree of life is a human foible andnot a true
representation of the real world. As Doolittleobserves, ‘Biologists
might rejoice in and explore, rather thanregret or attempt to
dismiss, the creative evolutionary role oflateral gene transfer’
[54]. The tree of life promises a hierarchi-cal order, and takes
authority from Darwin himself, but inmicrobes at least it is not
sustained by the very geneticsequences that made such phylogeny
possible. ‘Early evolutionwithout a tree of life’ [55] might seem
an alarming vista tomany, but Leeuwenhoek would surely have felt at
home. Hewas happiest without a compass.
Perhaps that, more than anything else, is the lesson westill
need to learn from Leeuwenhoek today. There is adanger of
complacency in biology, a feeling that the immensecomputational
power of the modern age will ultimatelyresolve the questions of
biology, and medical research morebroadly. But pathophysiology
stems from physiology, andphysiology is a product of evolution,
largely at the level ofcells. The eukaryotic cell seems to have
arisen in a singularendosymbiosis between prokaryotes, and
eukaryotes sharea large number of basic traits, few of which are
known in any-thing like the same form in bacteria or archaea. We
know ofno surviving evolutionary intermediates between prokar-yotes
and eukaryotes. We know almost nothing aboutwhich factors drove the
evolution of many basal eukaryotictraits, from the nucleus to
meiosis and sex, to cell death—traits first observed by
Leeuwenhoek. Why did meiosis andsex arise from lateral gene
transfer in bacteria? Why did thenucleus evolve in eukaryotes but
not in bacteria or archaea?What prevents bacteria from engulfing
other cells by phago-cytosis? There is no agreement on the answers
to thesequestions, nor more broadly to a question that might
easilyhave been asked by Leeuwenhoek himself—why is lifethe way it
is? Some of us have argued that eukaryotic evol-ution is explicable
in terms of the detailed mechanisms ofenergy conservation, with an
allied requirement for endo-symbiosis leading to conflict and
coadaptation betweenendosymbionts and their host cells [56]. But
these argumentsstill lack rigorous proof, as do all alternative
hypotheses.In the meantime, we have at best an unreliable map ofthe
land that enchanted Leeuwenhoek. We should rejoiceand explore.
Author profile
Nick Lane is a Reader in Evolutionary Biochemistry in the
Department of Genetics, Evolutionand Environment at University
College London. His research is on the role of bioenergetics inthe
origin of life and the early evolution of cells, focusing on the
importance of endosymbiosisand cellular structure in determining
the course of evolution. Nick was awarded the inauguralUCL
Provost’s Venture Research Fellowship in 2009, the BMC Research
Award for Genetics,Genomics, Bioinformatics and Evolution in 2011,
and the Biochemical Society Award for 2015.He leads the UCL
Research Frontiers Origins of Life programme and was a founding
memberof the UCL Consortium for Mitochondrial Research. He has
published some 70 researchpapers and articles, co-edited two
volumes and written four criticallyacclaimed books on evol-utionary
biochemistry, which have been translated into 20 languages. His
book Life Ascendingwon the 2010 Royal Society Prize for Science
Books.
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The unseen world: reflections on Leeuwenhoek (1677) 'Concerning
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