Download Appendix 8-Eukaryotes

Appendix 8
The "Big Bang" Creation of Eukaryotes1
"An organic being is a microcosm, a little universe formed of a
host of self propagating organisms inconceivably minute, and as
numerous as the stars in heaven."
Charles Darwin (1887)2
"The origin of eukaryotes is a huge enigma and a major challenge
for evolutionary biology."
E. V. Koonin (2010)3
The most visible difference between prokaryotes and eukaryotes is the existence
of a nucleus which holds the cell's DNA (see figure). This is only one of many
important differences, some of which are mentioned below.
Prokaryotes and Eukaryotes
The sudden appearance of eukaryotes at about 1.8 Ga (Billion years ago), with
no clear predecessors and with major structural and genetic changes, has been
described as a "Biological Big Bang". Not only do the eukaryotes appear on the
scene suddenly and without warning or obvious predecessors, but even within
the major types of eukaryotes there is no clear line of antecedence—no clear
1
Koonin EV: The Biological Big Bang model for the major transitions in evolution. Biol Direct 2007, 2:21,
"Major transitions in biological evolution show the same pattern of sudden emergence of diverse forms at a
new level of complexity. ... In each of these pivotal nexuses in life's history, the principal 'types' seem to
appear rapidly and fully equipped with the signature features of the respective new level of biological
organization. No intermediate 'grades' or intermediate forms between different types are detectable."
2 Charles Darwin, Animals and Plants, p.ii, p399 (1887).
3 Koonin EV, "The origin and early evolution of eukaryotes in the light of phylogenomics" Genome Biology
2010, 11:209. He continues, "There is a sharp divide in the organizational complexity of the cell between
eukaryotes, which have complex intracellular compartmentalization, and even the most sophisticated
prokaryotes (archaea and bacteria), which do not... [R]econstructions show that the characteristic eukaryotic
complexity arose almost ‘ready made’, without any intermediate grades seen between the prokaryotic and
eukaryotic levels of organization. ... it is becoming increasingly clear that most or perhaps all of them
evolved from more complex ancestral forms by reductive evolution."
"archeotype". Indeed the first eukaryote appears to be an example of "the
magnates walk first"4—the (unknown) ancestor is more complex than any of the
descendents. This is a recurring theme in the development of species. 5
The first eukaryotic cells were single-celled (the protists), as bacteria are.
Multicellular structures formed by cyanobacteria are actually individual singlecelled microbes that live attached, and may have some cell specialization such
as the nitrogen-fixing heterocysts and akinetes. But the structural and transport
features of the proper cell have the potential for far more, and thus led in time to
multi-celled species, and eventually to the visible, multi-cellular plants and
animals.
There are six major groups of eukaryotes (see Wikipedia), and the inferred
"common ancestor" must have been more complex than any existing (very
complex) group. Many traits of eukaryotes are unique to them and have no
analog in bacteria.6
Features include:
•
Eukaryotes are a radical advance in organizational complexity over
all prokaryotes.
•
All eukaryotes display a large package of genes that are unknown—
let alone separately existing—in any prokaryotes.
•
The genes that do appear to be shared with prokaryotes seem to be
indiscriminately selected from both archaea and bacteria7. There is
no plausible scenario in which this might occur by natural evolution—
even given lateral gene transfer, which is well-established,
demonstrated in the laboratory, and can occur between unrelated
species.
The following table lists some of the Eukaryote innovations. The mitochondria are
the "powerhouse of the cell"—main producers of ATP, and use oxygen to do this:
this is the reason why all eukaryotes require oxygen (even plants—derived from
photosynthesis).
4
"Even the most widely accepted notion that eukaryotes originated from prokaryotes is problematic because
traits unique to eukaryotes, such as the nucleus, endomembrane system, cytoskeleton, and mitosis, are
found in all taxa with no intermediate stages left as signposts of their evolution."
5 See Appendix 4. This is sometimes called Reductive Evolution.
6 Gross and Bhattacharya, Uniting sex and eukaryote origins in an emerging oxygenic world, Biology Direct
2010, 5:53
7 bacteria and archaea are the two domains of bacteria, with archaea also called "extremophiles" because
they live under extreme conditions (such as near hot deep ocean vents). The name "archaea" implies
greater antiquity, but this is probably not the case because in fact archaea are intermediate between
bacteria and eukaryotes in genetic complexity. In my view the archaea are a later development of bacteria.
See Margulis, Kingdoms and Domains (2009). The ribosomes (which make proteins) of archaea are more
complex than the bacterial ribosomes, and seem to be closer to the eukaryote ribosomes.
Eukaryote Innovations
DNA
Mitochondria
Prokaryote
Single loop in cell body
None
cell division
binary fission
sexual
reproduction
Structure
None
membranebound
organelles
balloon-like: Shaped by
pressure9 See the range
of shapes displayed in
the Merck Manual.
absent
internal
transport
diffusion
programmed
death
absent
Eukaryote
non-looped chromosomes in nucleus
Membrane-bound organelle:
Consumes O2 to generate ATP energy
"batteries"
mitosis and cytokinesis (non-sexual cell division
with mitotic spindle for division of cell & nucleus)
Meiosis to form sperm and egg cells8
Cytoskeleton: shape & internal structure. See a
chart of the many ciliate shapes.
Organelles with controlled micro-environments
for specialized tasks: the nucleus, nucleolus,
mitochondria, golgi apparatus, endoplastic
reticulum, etc.
Kinesin (see Figure below) and other transport
motor molecules to carry food and waste along
microtubules within the cell (e.g. between
organelles).
Ubiquitin found in all eukaryotes. Directs protein
recycling (programmed cell death).
Kinesin
The invention of the eukaryotic cell came at a critical time in the history of the
Earth, at a time when most of the reduced minerals in the oceans and Earth crust
8
Two cell divisions resulting in four cells. Tetrad spores reflect this: "Envelope-enclosed spore tetrads are
taken as the earliest evidence of plant life on land" dated to 470 Ma during the Ordovician. See illustration in
the Chapter on Greening of the Land.
9 Osmotic or turgor pressure.
had oxidized and the atmosphere's oxygen content had reached a level that
could poison cyanobacteria.
be another example of starting with a "magnate".10 With the advent of detailed
gene studies, one might have hoped that the problem of complexity of the first
"proto-eukaryotes" might be solved, but such is not the case. Consider the
following statement regarding the identity of the very earliest eukaryotes,
summarizing a decade of intensive genetic research on the origin of eukaryotes:
"There are therefore no grounds to consider any group of
eukaryotes primitive... Rather it is becoming increasingly clear
that most or perhaps all of them evolved from more complex
ancestral forms by reductive evolution. Reductive evolution
refers to the evolutionary modality typical of parasites: they tend
to lose genes, organelles and functions when the respective
functionalities are taken over by the host. So the archezoan
(crown group) phylogeny seems to have been disproved, and
deep phylogeny and the theories of the origin of eukaryotes
effectively had to start from scratch."11
10
See Appendix 4.
Koonin EV: The origin and early evolution of eukaryotes in the light of phylogenomics Genome Biology
2010, 11:209
11