T he phylogeny of life is normal-ly depicted graphically using agenealogical tree. A single

common ancestor produces descen-dants which are not identical, whichare exposed to natural selection andwhich, in the course of time, pro-duce brand-new species. Nobodytoday doubts that this process es-sentially underlies the natural di-versity of the living world. HenceDarwin’s „The Origin of Species“contains only one illustration, inwhich the evolution of species is de-picted diagrammatically in the formof a tree. Using this system, livingorganisms can be linked to eachother and to their extinct ancestorsby lines – lines of descent. Accord-ing to Darwin, the steady successionof variation amongst descendants,followed by selection over theunimaginably long span of theEarth’s history, would more thansuffice to explain the differencesand similarities observed in livingorganisms.

Just seven years after Darwin hadpublished his work, the young ErnstHaeckel drew a series of familytrees depicting the phylogenetic re-lationships between the most im-portant groups of organisms thenknown. Here, too, the medium ofthe family tree, bearing larger andsmaller branches and series of rami-fications, was employed.

Anyone opening a modern biolo-gy textbook will find not only familytrees derived from visible morpho-logical characters, but also othersbased on the comparison of gene se-quences – so-called molecular phy-logenies (family histories). Today’s

molecular family trees, even if theyare less original than Darwin’s andless aesthetic than Haeckel’s, nev-ertheless offer basically the same fa-miliar picture: deep-seated ramifi-cations followed by ever more splin-tering, right up to the final twigs,which bear our contemporaryspecies. Evolutionary biologiststake a special delight in disputingthe order of these bifurcations, andespecially of those branches whichlie close to the roots of the familytree.

In the age of genome analysis, bi-ologists are rapidly learning thatgenome sequences contain muchmore information than can currentlybe effectively analyzed. Moreover,genome sequences provide evi-dence of gene exchanges far be-yond the confines of individual

species. So what has science learntfrom genome sequences thus far?

First, that Darwin was essentiallyright: there has indeed been aprocess of evolution, and mankindis a part of it. Second: all organismsare interrelated. This does not meanthat life must have originated onlyonce, but that organisms alive todaydo have a single common origin,since the construction, mode of op-eration and the language of thegenes – the genetic code – are fun-damentally identical in all organ-isms. Third: trees of life such asthose drawn by Darwin and Haekelcannot even begin to describe thehistorical process of evolution if weonce start on the phylogeny of uni-cellular eukaryotes (protists). This isbecause the twigs and branches inthe tree of life can sometimes com-bine in such a way that two organ-isms will bring forth a single new or-ganism differing fundamentallyfrom its two predecessors. Thisunion of two different cells to form asingle, more complex, one withnovel characteristics is known as“endosymbiosis”.

The fundamental concept that theendosymbiotic union of two quitedifferent cells could be involved inthe genesis of new organisms is over100 years old. Probably the firstproperly formulated symbiotic hy-pothesis for cell evolution camefrom the Russian biologist Constan-tin Mereshkowsky. Between 1905and 1910 Mereshkowsky publisheda series of highly original and pas-sionately argued papers presentinghis hypothesis regarding the evolu-tionary origin of plastids – the com- 13

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The Biological Pathto ReunificationThe familiar form of the family tree is not applicable to all living organisms.Evolutionary history entails not only the divergence of lineages, but also mergersof distinct unicellular organisms (prokaryotes and protists) to form new cell types

Life Sciences

A historical depiction of plastid division ina fern cell from the year 1846: “... a in itsnormal state, b an oblong chlorophyllbubble with an amylum (starch) granule,c a chlorophyll bubble which has dividedthrough a wall into two daughterbubbles”.

partments of plant cells that containchlorophyll for photosynthesis –from free-living bacteria. In hisopinion, the various attributesshared between some photosyn-thetic bacteria (cyanobacteria) andplastids could mean only one thing:plastids are so similar to cyanobac-teria because they are directly de-scended from them. However, sev-eral decades had to pass before sci-

entists became accostomed to theidea that a free-living cell could,through endosymbiosis, become aclearly delineated compartment (or-ganelle) of another free-living cell.It was not until around 25 years agothat endosymbiosis was (again)taken seriously as a mechanism forthe genesis of plastids and of mito-chondria, the powerhouses of thecell which generate the vital energy

supplies. In 1980, as a result of genesequencing analysis it finally gainedgeneral acceptance. Today weknow that both organelles are in-deed descended from bacteria.

The crucial clue emerged fromthe genome analyses of plastids andmitochondria. Viewed from the or-ganisation of their genomes and thesimilarity of their DNA sequences,there can be no doubt that thegenomes of the plastids are descen-dants of bacterial genomes.

Scientists are thus convinced thatplastids and mitochondria wereonce independent organisms. Butjust because these cell organelles, inturn, stem from free-living cells, theorigin of cells possessing mitochon-dria or plastids cannot solely be de-scribed by a system of divergentbranching. Indeed, cells with plas-tids and mitochondria are mixturesof diverse, once free-living, organ-isms which had a more ancient com-mon ancestor. In the course of theEarth’s history, the forebears ofthese ancestors developed along di-vergent paths, but later, as a resultof endosymbiosis, they became re-united within the confines of a sin-gle cell.

The principle of endosymbiosisthus stands opposed to Darwin’sideas, who initially depicted phy-logeny in the form of a branchingtree. When, however, two cells (hostand symbiont) merge through en-dosymbiosis to form a single cell,then the resulting cell has two an-cestors, not one. This means that thetree of life differs fundamentally inits structure from a true tree, in thesense that although the branches ofthe family tree are normally diver-gent, they occasionally merge aswell. All biologists agree about thistoday, although there is some dis-agreement as to how many differentendosymbioses have occurred dur-ing evolution. With each endosym-biosis a new type of cell is createdwith the characteristics of both part-ners, but with overall characteristicswhich clearly differ from those of itstwo predecessor cells.

The origin of plastids from a bac-terium was a critical event in thehistory of life because it marks theorigin of all contemporary plants.But we still know very little about14

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the concrete details of this event. Onthe basis of microfossil finds it maybe assumed that this took placesome 1.5 billion years ago. Usingphotosynthesis, bacteria are able toutilise sunlight to produce sugar,their food, from carbon dioxide. Thisprocess requires adenosine triphos-phate, the universal energy carrierof all cells, which they also producethrough photosynthesis. In contrastto this, protists that do not possessany plastids, have to eat to acquiretheir sugar through ingestion. Usingthe oxygen in their mitochondria,the sugar is oxidised to water andcarbon dioxide. Should such a graz-ing protist ingest a cyanobacterium

which generally accepted answershave yet to be found.

Modern biology needs the princi-ple of endosymbiosis to explain theobserved diversity of modern celltypes and cell organelles. But thequestion as to just how much cellevolution can be attributed to en-dosymbiosis, and how much wouldbe better explained by Darwin’sconcepts, will be the subject of in-tense study, but also the subject ofvigorous debate during the comingyears.

Prof. William MartinUniversität Düsseldorf 15

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with its food, it would be able tocover its sugar requirement throughphotosynthesis generated by its en-dosymbiont. For the host cell, giventhe presence of sunlight, a symbiontsuch as this would provide a sheerinexhaustible source of nutrition.However, via endosymbiosis a new,light-dependent (phototrophic)lifestyle is conferred upon the host,creating a dependence upon its newsymbiont. The characteristics of twocells will have then become unitedin a single, new type of cell. Derivedfrom two independent and highlydifferent kinds of cells, it was the

single common ancestor of our mod-ern flora.

Looking back from the presentinto the history of life, the emer-gence of the plastids from a bacteri-um lies in the distant, distant past.But further back still lies the symbi-otic origin of mitochondria frombacteria. Just when this event mighthave taken place, which selectiveadvantages could have played arole in the emergence of mitochon-dria, and in particular, what sort of ahost cell might have ingested theoriginal mitochondrion, are impor-tant questions in cell evolution, for

Three model concepts for the course ofevolution – left-hand side: The tree of life

by Ernst Haekel; right: the origins ofspecies according to Charles Darwin and amodern diagram as may be drawn on the

basis of molecular sequence data.

Eubakterien

KnallgasbakterienGram

PositiveProteobakterien

Cyanobakterien

Flavobakterien

Thermotogales

Thermoacidophile

MethanbildnerHalophile

CiliatenTiere

Pilze

Pflanzen

Schleimpilze

Flagellaten

Trichomonaden

Microsporidia

Diplomaden

Archaebakterien Eukaryoten