Ernst Haeckel's 'Pedi­gree of Man'

Pic­tures Con­sid­ered #42

by Christoph

It's ex­actly 150 years since the pic­ture con­sid­ered in this post was pub­lished in the 'Gen­eral Mor­pho­lo­gy of Or­gan­isms' by Ernst Haeckel (1866). "Phy­logeny is the evo­lu­tion­ary his­tory of or­gan­isms," ex­plains Uwe Hoßfeld, co-au­thor of a re­cent pa­per in Na­ture that cel­e­brates this an­ni­ver­sa­ry. "Be­cause Haeckel was the first ac­tu­ally to de­fine this term, in that same work, he was also the only per­son ca­pa­ble of rep­re­sent­ing the first tree of life of this kind." But be­fore you start feel­ing all too com­fort­able within the beau­ti­fully drawn twigs and branches of Haeckel's tree I should bet­ter men­tion its four mi­nor flaws, and one more fun­da­men­tal flaw in par­tic­u­lar (see Fig­ure).

From Ernst Haeckel's 'Gen­eral Mor­phol­ogy of Organ­isms', 1866 (cropped). Source

1.  You no­tice Homo sapi­ens right on top in the fig­ure. To­day, bi­ol­o­gists have no rea­son­able in­di­ca­tion for an evo­lu­tionary mas­ter­plan that would, by ne­ces­sity, lead from 'prim­i­tive' uni­cel­lu­lar or­gan­isms to mul­ti­cel­lu­lar or­gan­isms ca­pa­ble of pon­der­ing, for ex­am­ple, how life evolved. Yet, seven years af­ter Charles Darwin's 'On the Ori­gin of Species', it was still out­right scan­dalous to pro­pose that the crown & glory of cre­ation (I'm re­fer­ring to Gen­e­sis 1:26–31 here) is a mem­ber of the pri­mates. De­nying Ho­mo sapi­ens the top po­si­tion in the Tree of Life would have been per­ceived as a dou­ble hu­mil­i­a­tion then, and Haeckel avoided this prob­a­bly as a con­ces­sion to Zeit­geist. That he did not deem a promi­nent po­si­tion i­n the Tree of Life nec­es­sary for H. sapi­ens can be safely de­du­ced from his al­ter­na­tive de­pic­tion of the tree com­pri­sing Planta, Pro­tista, and An­i­malia on equal lev­els (also from 1866).

2.  Did you no­tice that the Tree of Live Life is de­void of fo­liage? In win­ter time, this is the com­mon phe­no­type of oak trees (Quer­cus robur L) as Haeckel might have seen them aplenty in Jena, Ger­many. In­tu­itively, one would rather think of ex­tant spe­cies as the leaves on the branches of the Tree of Life. But Dar­win also ig­nores the leaves: "The green and bud­ding twigs may rep­re­sent ex­ist­ing species; and those pro­duced dur­ing for­mer years may rep­re­sent the long suc­ces­sion of ex­tinct species" ('On the Ori­gin of Species', 1st Ed., Chap­ter IV., 'Nat­ural Se­lec­tion', p. 129). Maybe Dar­win im­plic­itly thought of ex­ist­ing in­di­vid­u­als as the lea­ves on the 'species twigs' but we can only guess what led Haeckel to omit the leaves. Maybe for the more triv­ial rea­son that fo­liage usu­ally hides the branches of a tree from be­com­ing vis­i­ble at a dis­tance (and proudly mak­ing all the draw­ings him­self, Haeckel would have had prob­lems with the nu­mer­ous name tags for all the taxa, too).

3.  You see Mon­era – prokary­otes in mod­ern terms – and Amoe­bae (pro­tists) lumped to­gether at the un­struc­tured base of the tree's stem. This is un­der­stand­able given the (al­most) com­plete lack of in­sight into the world of the small things in Haeckel's time. But the base of the tree drawn with the im­mense di­ver­sity of uni­cel­lu­lar life that we know now ex­ists in mind would eas­ily span two or more full pages, mak­ing the 'tree' sim­ile ap­pear, hm..., ridicu­lous.

4.  The hi­er­ar­chi­cal or­der of the branches in Haeckel's tree im­plies a timely or­der of taxa evolv­ing from their an­ces­tors. An ex­plicit time scale is not given but all branches above the Mone­ra/Amoe­bae stem roughly re­flect the un­fold­ing of life forms – in this tree here only An­i­malia, but Haeckel's more com­plete tree also in­cludes the Pro­tista and Planta – since the Cam­brian ex­plo­sion. This is com­pa­ra­ble to set­ting the be­gin­ning of hu­man his­tory on ex­actly Oc­to­ber 12, 1492, Colum­bus' ar­rival in the Amer­i­cas.

Given these lat­ter two 'dis­tor­tions' of scales in Haeckel's tree I won­der whether any bi­ol­o­gist to­day would think of a tree as a suit­able sim­ile for Life on Earth in the first place.

But there is one more caveat for us­ing the Tree of Life metaphor. In a liv­ing tree, metabo­lites flow through its phloem, from the roots up to the tini­est branches, the twigs and the leaves, and back. Thus, Haeckel's or­gan­is­mal phy­lo­ge­netic tree in­sin­u­ates a ma­te­r­ial con­nec­tion be­tween the Mo­ne­ra and H. sapi­ens, as species and as mem­bers of these species, in­di­vid­ual or­gan­isms. How­ever, all our an­ces­tors are dead. And all an­ces­tors of all other liv­ing or­gan­isms are... dead, de­com­posed, fos­silized (maybe). Even bac­te­ria do not thrive and di­vide on end, as it has been shown for Cau­lo­bac­ter: in­di­vid­ual cells even­tu­ally cease di­vid­ing af­ter some 120 gen­er­a­tions and fi­nally die. There is no way to res­cue the fail­ing 'tree' metaphor. Charles Dar­win thought, for some time, about the 'Coral of Life' as a suit­able sim­ile. In corals, bud­ding polyps sit on top of the dead and min­er­al­ized skele­tons of their par­ents. There is no other ma­te­r­ial con­nec­tion than these skele­tons, no flow of metabo­lites be­tween the polyps on dif­fer­ent branches of the coral stem. Re­gret­tably, Charles Dar­win skipped this idea in fa­vor of the Tree of Life, prob­a­bly be­cause corals were just too ex­otic for 19th cen­tury Britons.

For fair­ness, Haeckel's Tree of Life should not be com­pared to mod­ern "trees" (you can eas­ily try to 'build' one your­self here ). His tree is ex­plic­itly meant as an or­gan­is­mal tree while all mod­ern ver­sions, start­ing with the 3‑­Do­main-Tree by Carl Woese, are 'gene trees' (de­spite be­ing of­ten cal­led 'Tree of Life' ). The only "flow" be­tween suc­ces­sive gen­er­a­tions that we re­ally know of is that of the in­for­ma­tion con­tained in the ge­netic ma­te­r­ial – and it is uni­di­rec­tional, De­scent with mo­di­fi­ca­tion (Dar­win) – but this does not al­low for a straight­for­ward or­gan­is­mal phy­logeny 1). In such mod­ern "trees", in­di­vid­ual genes (ri­bo­so­mal RNA), genomes, or con­cate­nated pro­tein se­quences are used as proxy for an or­gan­ism, and the "branch­ing" is cal­cu­lated by in­tri­cate al­go­rithms that as­sess the de­gree of re­lat­ed­ness among se­quences of the re­spec­tive data sets. This ap­proach is as far as it can get from any metaphoric 'tree'. Be­sides, high de­gree of hor­i­zon­tal gene trans­fer (HGT) – es­pe­cially but not ex­clu­sively among Prokary­otes – has led re­searchers into an al­most im­per­vi­ous thicket. So, to­day bi­ol­ogy is prob­a­bly far­ther away from an or­gan­is­mal Tree of Life than at Darwin's and Haeckel's time. We just have more data.

1) tack­ling or­gan­is­mal phy­logeny would re­quire solv­ing the mantra Om­nis cel­lula e cel­lula (Vir­chow, 1855), or its more mod­ern para­phrase Om­nis mem­brana ex mem­brana (Blo­bel, 1999). Briefly: we know (prob­a­bly) all the genes en­cod­ing the pro­teins that make the build­ing blocks for cell mem­branes, we know the path­ways for how these build­ing blocks get in­cor­po­rated into ex­ist­ing mem­branes to make them grow. But this toolkit does not al­low us to get de novo mem­brane syn­the­sis go­ing: al­ways a pre-ex­ist­ing mem­brane is nec­es­sary, think of the My­coplasma genome trans­plant by the Craig Ven­ter lab.

 

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