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The Beginning of Life as We Know It

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Supporting Document: Viral Eukaryogenesis

The Beginning of Life as We Know It

 

ZOOL3001 Assignment 1 Chris Lovell

#:33552468

 

!Eukaryogenesis:

The Beginning of Life as We Know It

In essence…we have two rival schools of biology based upon evolutionary

speculations: the classical or “traditional” biologists who believe that the

eukaryotic cell is the end-result of a gradual but continuous evolution from

the prokaryotic cell via the accumulation of mutational events, and the

endosymbiotic proponents whose thesis is that the eukaryotic cell is the

result of a sudden (or “revolutionary”) acquisition of whole prokaryotic

cells, followed by the symbiotic evolution of guests and hosts.

(Fredrick 1981:x)

 

So began Jerome Frederick in his opening statement to the New York Academy of

Sciences, chairperson of a conference held in January 1980 entitled “Origins and

Evolution of Eukaryotic Intracellular Organelles”. In the time since Fredrick made these

opening remarks about eukaryogenesis (the origin of the eukaryotic condition)

evolutionary biologists have (for the most part) come down upon one side of this once

contentious debate ¾ the endosymbiotic. And yet, for the most part, endosymbiosis (and,

for that matter, symbiosis in general) has remained in the shadows of mainstream

(populist) biological thought (like Dawkins 1976, 1986; Gould 1977, 1989, 1996; and

Wilson 1994). Eukaryogenesis (the evolution of the eukaryotic condition) is an event so

crucial to life as we see it on earth today that it is held, by those who have assumed the

responsibility of investigating it, as the crucial precursory event in the diversification of

life on earth. From the advent of the eukaryotic condition sprung everything we

commonly associate with life (and evolution) ¾ things like multicellularity, sexuality

and speciation (see Margulis and Sagan 1984, Paterson 1989) ¾ including, of course, our

own species. And yet, for all of its magnanimous evolutionary consequences there

remains a stupendous silence within the populist literature about the eukaryogenic theory

(symbiogenesis) that has, over the past twenty years or so, gained such widespread

acceptance. In this short essay I shall attempt to assess (critically) the alternative

explanations of eukaryogenesis, commenting less upon the biochemical, cellular,

molecular and microbial evidence (I realize that I do not possess the depth of knowledge

nor the expertise to critique the work of people like Lynn Margulis who’ve dedicated

their entire lives to investigating these phenomenon), and more upon how they have (or

have not) gained acceptance within biology, as Rose (1997:19) understands it: “Biology

¾ not the phenomena of life, but their scientific study ¾ is itself historically

constructed.”

The profound differences in cellular structure between prokaryotic (from the Greek ‘pro’

meaning ‘before’, and ‘karion’ meaning ‘nucleus’) and eukaryotic (from the Greek eu =

true) cells have been known to biologists for well over a century (in fact, the first

proponents for an endosymbiotic origin of eukaryotic intracellular organelles were two

scientists, Altmann and Schimper, working in the 1880s ¾ see Lang et al. 1999:352). It

is these differences (see Table 1) that have, since that time, begged evolutionary

biologists to elicit an evolutionary explanation in order that evolution, the theory closest

to the epistemological foundations of biology (again, the scientific body of knowledge,

the discourse, not the living systems themselves), might work to maintain its position

within biological discourse1.

Table 1: The major differences between prokaryotic and eukaryotic cellular structure and

function (see de Duve 1996; Knox et al. 1994; Kostianovsky 2000; Margulis 1970,1993)

1 For a discussion of ‘science as discourse’ see Smocovitis (1996) Chapter 4, “The New Contexualism:

Science as Discourse and Culture”, pp.73-94.

Prokaryotes

· Cells small (1-10µm across)

· Nucleoid, not membrane bound

· One set of genes (haploid), arranged

into a circular chromosome

· Cell division mainly via binary

fission or budding

· All are microbes: unicellular,

colonial, filamentous and mycelial

forms; no extensive tissue

differentiation

·

No packaged membrane-bound

enzyme sacs, no heritable organelles

·

Rigid, thick cell external cell wall

· Some have simple flagella

·

No phagocytosis, pinocytosis or

cyclosis

· Include anaerobes (killed by

oxygen), microaerobic and aerobic

forms

Eukaryotes

· Cells large (10-100µm across)

· Nucleus bound by a double

membrane

· Two sets of genes (i.e., diploid,

except gametes), chromosomes in

strands

· Cell division via mitosis

· Some microbes, mostly large

organisms: unicellular, colonial,

mycelial and multi-cellular forms;

extensive tissue differentiation in

multicellular forms

· Contain an endomembrane system

including rough and smooth

endoplasmic reticulum, the Golgi

complex, lysosomes, secondary

granules, peroxomes, microbodies,

hydrogenosomes, chloplasts and

mitochondria

· Complex internal cytoskeleton

· Except fungi, most have

undulipodia: i.e. 9(2)+2 ‘flagella’

or cilia

· Phagocytosis, pinocytosis, cyclosis

present

· All are aerobic (need oxygen to live),

exceptions are secondary

modifications

Microfossil, stromatolite and chemical fossil evidence suggest an origin for the

prokaryotic cell of between 3 and 3.5 billion years ago and of the eukaryotic cell between

1 and 1.5 billion years ago (Margulis 1993:120-121, 156). Neither eukaryogenic

explanation disagrees that the eukaryotic condition evolved from an ancestral prokaryote

(a position supported by the universality of certain cellular components and metabolic

pathways). Rather they differ in how they explain the transition from an ancestral

prokaryote to a eukaryotic cell. In order that a theory maintain itself within the scientific

discourse, it must be able to offer an explanation of the evidence at hand. Of course, as

time goes by, the evidence at hand changes, and so the theory must either account for the

new evidence or change its mode of explanation.

This is exactly what has happened with eukaryogenic explanations. It is traditional within

the scientific literature to pit one theory against the other, each facing the other in mortal

combat, one destined to oust the other (following the narrative structure of Western myth

¾ see Landau 1991, Smocovitis 1996:87). However, what actually happens within

scientific discourse is nothing like this. It is very rare (perhaps nonexistent) for one

theory to completely replace (conquer) the other. What usually (perhaps always) happens

is that new theories appropriate certain elements from old explanations, rearranging them

into new discursive structures to produce new modes of explanation (or theoretical

synthesis). This is the case for eukaryogenic explanations. The endosmbiotic theory never

ousted the traditional viewpoint, rather it retained certain aspects of its explanation,

discarded some and invented others as it was (and continues to be) reformulated. The

reformulation has, in fact, never ended and, so long as new evidence continues to be

sought and found, never will.

According to Smocovitis (1996:196) it was Borghoorn and Tyler whom introduced

cellular evolution into the continuum when Precambrian (i.e. older than 600 million

years) microfossils were ‘discovered’ in 1954, using a new imaging technology, electron

microscopy. In light of this evidence the origin and evolution of cellular structure began

to be investigated in earnest. The initial explanations pertaining to eukaryogenesis were

formulated in terms of the current evolutionary rhetoric: organisms evolve by the

accumulation of mutational change and their natural selection (Kostianovsky 2000:63).

According to this theory, all of the differences between prokaryotic and eukaryotic cells

could be accounted for in terms of the accumulation of single step mutational changes

within an ancestral protoeukaryotic cell. As the evidence mounted it became clear this

theory was not adequate to explain the differences between prokaryotes and eukaryotes.

How could the similarities between mitochondrial division and prokaryotic division be

accounted for? Why was it that certain organelles contained genetic material? And why

was that genetic material haploid and circular?

The similarities between the prokaryotic cell and certain of the eukaryotic intracellular

organelles (namely the mitochondria and chloroplasts) had been known since the end of

the 19th century, however it was not until the late 1960s that the endosymbiotic theory,

originally formulated by Mereschkowsky earlier that century to account for the origin of

plastids (Kostianovsky 2000:65), was once again launched by Margulis using modern

biochemical, genetic, cytological, and paleontological information as evidence (Margulis

1970:5). According to Margulis’s synthesis eukaryotes were borne from a stepwise

symbiotic union of prokaryotic cells, where one prokaryote eventually engulfed another.

The other cellular features we associate with eukaryotes evolved (according to this

synthesis) during and after the symbiotic event (Roger 1999:S147). Her early synthesis

outlined how eukaryotic cells came to possess many of their features (intracellular

organelles, organelle genomes and the division of mitochondria and plastids, for

example), but did little to elaborate upon how (mechanistically) one ancestral prokaryote

could engulf another ¾ an event that is rather central to the theory.

It was left to other early theorists like de Duve and Stanier, also working in the late 60s,

to postulate the development of an early phagocytic protoeukaryote (a eukaryotic

precursor capable of engulfing voluminous bodies) prior to endosymbiosis (Roger

1999:S147, see also de Duve 1996). This protoeukaryote, according to de Duve (who

called it the urkaryote ¾ see de Duve 1991:62) needed to develop certain eukaryotic

features in order to phagocytose: the cytoskeleton and the endomembrane system. Having

developed these features the urkaryote had the structural capability of ingesting

prokaryotic cells that, later, became endosymbiotic bacterial organelles. Taylor dubbed

this new synthesis the serial endosymbiosis theory (SET) (Lang et al. 1999:352) which

was, in effect, a synthesis of Margullis’s endosymbiosis, de Duve’s urkaryote and the

traditional formulation: natural selection (since each new characteristic had to have, after

all, conferred to the organism some selective advantage in order that it spread throughout

the population, no matter if that characteristic was the result of mutational change or of

symbiosis).

In no way, then, did the SET synthesis oust the traditional approach, rather it built upon a

foundation set by earlier thinkers, modifying certain aspects, discarding others. And of

course the synthesis didn’t stop there. There are still many issues left to resolve, and

always more evidence to explain. In what order did these early evolutionary events occur

(see Roger 1999)? Are the mitochondria and chloroplasts of monophyletic or

polyphyletic origin (see Moreira et al. 2000, Palmer 2000, Penny and O’Kelly 1991)?

Did the origin of the mitochondrion contribute to the origin of the nuclear genome (see

Lang et al. 1999)? Are undulipodia of symbiotic origin (see Margullis 1993: Ch.9)?

One of the most interesting points about the development of eukaryogenic theory

remains: Why, given the current widespread acceptance of SET, have the symbiotic

aspects of evolution remained largely an ignored topic within mainstream evolutionary

biology? Perhaps, I would suggest, it has something to do with the history of the

discipline. Evolutionary relationships (phylogeny and cladistics) are traditionally

expressed in terms of the “branching tree” model. Evolutionary biology traditionally

explains evolution as ‘branching’ of this ‘tree of life’. The model doesn’t readily

predispose itself to the idea that branches sometimes weave themselves back into one

another, made particularly difficult if those branches are imagined to be on opposite sides

of the tree. This is, of course a problem within biology (the discipline) not with life itself.

Evolutionary biology traditionally concerns itself with examining the differences between

living organisms, how they arise, and how those differences confer selective advantages,

concentrating more upon competition and less upon exchange. In doing so the tendency is

to paint the picture of an organism, alone in an inhospitable world, pitted against, never

aided by, other organisms (again, a healthy articulation of the myth structure of Western

culture, and how we imagine our own species to be placed in the world). It is ironic (yet

historically consistent, given the nature of our society) that this should be the case since

natural selection actually works to minimize intraspecial competition (which is exactly

why, if two organisms occupy the same ecological niche, one of them will disappear from

the community). Intense competitions, where they are seen, are always short-lived

phenomena that, very soon, eliminate themselves. Of course, this is why symbioses are

such successful adaptations: they work to minimize the competition between organisms.

Symbioses are, typically, based upon exchange, and exchange (as every economist

knows) is one sure way of reducing competition (which is why laws have to be passed

limiting the exchange of information between competing companies).

Seen in this light, the endosymbiotic origin of eukaryotic organelles is simply another

incredible evolutionary innovation conferring such a degree of evolutionary success to

eukaryotes because it works to minimize the level of competition between the eukaryote

and its endosymbiotic bacterial organelles by maximizing their level of exchange. The

adaptation works (it seems) in much the same way that the next major evolutionary

transition (see Szathmary and Smith 1995), multicellularity, works: to minimize

intracellular competition between individual eukaryotic cells by maximizing the level of

intracellular exchange.

References

Dawkins, R.

1976 The Selfish Gene. Oxford: Oxford University Press.

1986 The Blind Watchmaker. London: Penguin Books.

de Duve, C.

1991 Blueprint for a Cell: The Nature and Origin of Life. Burlington: Neil Patterson.

1996 The Birth of Complex Cells. Scientific American 274:50-57.

Gould, S.J.

1977 Ever Since Darwin. New York: W.W. Norton and Company.

1989 Wonderful Life: The Burgess Shale and the Nature of History. London: Penguin Books.

1996 Life’s Grandeur: The Spread of Excellence from Plato to Darwin. London: Jonathan Cape.

Fredrick, J.F.

1981 Evolutionary Theories: Dogmas or Speculations? In J.F. Fredrick (ed.) Origins and

Evolution of Eukaryotic Intracellular Organelles, pp. ix-x. New York: The New York Academy

of Sciences.

Kostianovsky, M.

2000 Evolutionary Origin Of Eukaryotic Cells. Ultrastructural Pathology 24:59-66.

Knox, B., P. Ladiges and B. Evans

1994 Biology. Sydney: McGraw-Hill Book Company.

Landau, M.

1991 Narratives of Human Evolution. New Haven: Yale University Press.

Lang, B.F., M.W. Gray and G. Burger

1999 Mitochondrial Genome Evolution and the Origin of Eukaryotes. Annual Review of Genetics

33:351-397.

Margulis, L.

1970 Origin of Eukaryotic Cells. London: Yale University Press.

1993 Symbiosis in Cell Evolution: Microbial Communities in the Archean and Proterozoic Eons,

2nd Edition. New York: W.H. Freeman and Company.

Margulis, L. and D. Sagan

1984 Evolutionary Origins of Sex. Oxford Surveys in Evolutionary Biology 1:16-47.

Moireira, D., H. Le Guyader and H. Phillippe

2000 The Origin of Red Algae and the Evolution of Chloroplasts. Nature 405:69-72.

Palmer, J.D.

2000 A Single Birth of all Plastids. Nature 405:32-33.

Paterson, H.E.H.

1989 Updating the Evolutionary Synthesis. Biology Forum 82(3-4):371-375.

Penny, D. and C.J. O’Kelly

1991 Seeds of a Universal Tree. Nature 350:106-107.

Roger, A.J.

1999 Reconstructing Early Events in Eukaryotic Evolution. The American Naturalist 154

(Supplement):S146-S163.

Rose, S.

1997 Lifelines: Biology, Freedom, Determinism. London: Penguin Books.

Smocovitis, V.B.

1996 Unifying Biology: The Evolutionary Synthesis and Evolutionary Biology. Princeton:

Princeton University Press.

Szathmary, E. and J.M. Smith

1995 The Major Evolutionary Transitions. Nature 374(6519):227-232

Wilson, E.O.

1994 The Diversity of Life. London: Penguin Books.

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