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08/14/2003
Weak Walls Make Good Ancestors
About 4 billion years ago life, in the form of single-cells known
as “prokaryotes”, made its debut on Earth. Today, there are still
lots of prokaryotes around, the numerous varieties of bacteria
being prime examples. For 2 billion years, prokaryotes were the
whole ball of wax. There were no critters with more than one
cell. But things were about to change. Another kind of cell
appeared upon the scene. This type of cell is known as a
“eukaryote”. Its appearance proved to be a truly revolutionary
development that led to the evolution of forms of life that had
more than one cell. All of us animals and the plants around us
are composed of eukaryotic cells.
The typical eukaryotic cell is quite different from the prokaryotic
cell. For one thing, consider their sizes. The typical prokaryotic
cell is about a micron in size, which means it takes about 25,000
of them lined up to make an inch. Eukaryotic cells are typically
10 to 30 larger in size. When it comes to their volume, however,
eukaryotic cells are roughly 10,000 times larger than prokaryotic
cells. With this much larger volume, it’s not a surprise that the
innards of the eukaryotic cell are much more complex than a
prokaryotic cell. One major difference is that the eukaryotic cell
has a nucleus with its well-organized chromosomes containing
the DNA strands that determine our identities. The prokaryote
has no nucleus. There’s just one chromosome, a single strand of
DNA.
Obviously, without the eukaryote, you and I would not exist.
How did the eukaryotes come into being? That’s one of the most
fundamental questions we can ask if we’re concerned about our
roots. I found one expert’s answer to this question last week
while browsing through a Scientific American special issue
given to those who renew their subscriptions early. This issue is
titled “Earth from the Inside Out” and contains an article by
Christian de Duve that first appeared in the April 1996 Scientific
American. De Duve shared the 1974 Nobel Prize in Physiology
or Medicine for his work on the structure and functioning of the
cell. His article, “The Birth of Complex Cells”, puts forth his
view as to how the evolution of eukaryotic cells took place.
To appreciate the feat that we have to explain, let’s take a closer
look at our eukaryotic cell. In addition to the nucleus, it also has
an internal skeletal structure with various sections or
compartments containing so-called “organelles”. These
organelles are specialized structures such as the mitochondria
that power the cell or the ribosomes that manufacture the
proteins for various functions. The cell may contain thousands
of these organelles, which are about the size of single prokaryotic
cells. We’ll see shortly that this particular size is no accident.
Let’s start with our prokaryotic cell with its single strand of DNA
and some ribosomes to manufacture enzymes. The prokaryotic
cell has a substantial wall, smooth and not very flexible. De
Duve proposes that the eukaryotes evolved from the prokaryotes
through the development of a special kind of ancestral cell. In
his theory he makes three major assumptions. First, he assumes
that the ancestral cell had to eat and that it fed off the debris and
discharges of other organisms. This seems sensible to me.
Second, he assumed that it digested its food. This seems obvious
but we have to clarify something. The prokaryotes fed by
digesting their food outside the cell wall using the above-
mentioned enzymes. In other words, their food was predigested
before they ate it.
The third assumption was key to the whole scenario. This is that
the ancestral cell had lost its prokaryotic ability to make its
sturdy cell wall. I’m thinking that some prokaryotic cells
suffered a mutation that left them defective in their wall-making
ability. The result was a flimsy, flexible membrane. In a rough
and tumble world, this could be a fatal defect. But there must
have been nooks and crannies where things were pretty peaceful
and some of these defective cells survived.
Suppose you’re one of these ancestral cells. You find that
instead of a smooth wall, your boundary with the outside world
is more like a coastline, with its inlets and bays. These folds and
convolutions provided you with a distinct advantage. There is
now more surface area for you to take in predigested food. So,
naturally you eat more and we all know what happens. You get
fatter and fatter. Without any serious predators that can match
your size, your children and succeeding generations get bigger
and bigger through natural selection.
So, fast forward who knows how long, maybe a million years?
Now assume that you’re one of these ancestral cells that have
gotten so big that a prokaryote can fit in one of your bays (folds).
In fact, suppose that prokaryotic cell wanders in and gets
trapped. What happens if it gets trapped and the your membrane
closes and fuses together? You might find that the trapped cell is
a tasty morsel and you “eat” it! This is reminiscent of a type of
cell that’s around today, the phagocyte. We have phagocytes
that do indeed search out invaders and trap and destroy them, to
our advantage if the invaders are threatening bacteria or viruses.
But remember that food has to be predigested, so maybe the
prokaryotic cell doesn’t get eaten or maybe only a bit of it suffers
that fate. Fast forward again and by this time the ancestral cells
are routinely trapping prokaryotes and taking them inside. If
you’re one of these ancestor cells, you might suddenly realize
that, hey, this prokaryote is making enzymes that predigest food
and now I have one inside me. Could this be an advantage? I
could trap and bring the food inside and have the prokaryote
digest it for me. If I keep it alive, I can consume more food and
I’ll grow even bigger. In fact, why not trap some more of these
critters to do my work and eventually I won’t have to make any
enzymes but let them do all the work?
OK, that may be giving our little ancestor more credit than it
deserves but, over the next millions of years more prokaryotic
cells get incorporated and tiny tubules develop to make skeletal
compartments for the different kinds of “internalized”
prokaryotic cells. These become the organelles with their
different functions. Another consequence of the folding and
isolation of different sections of the membrane is that sections of
DNA attached to the membrane also get folded up and isolated,
thus forming precursors to the nucleus. Over time, the packets of
DNA fuse together and DNA from the now permanent former
prokaryotic cells gets incorporated into the nucleus. During this
time, our ancestral cell develops flagella, those whiplike strands
that are used to propel the cell around. Now it’s able to move
about and become a hunter, searching out food. It’s becoming a
true phagocyte.
Up until about 2 billion years ago, there was little or no oxygen
in the atmosphere. However, cyanobacteria appeared upon the
scene and they had the annoying habit of emitting oxygen.
Pretty soon there was what de Duve terms “the oxygen
holocaust”. In many, perhaps most cells, the oxygen led to
formation of peroxides, superoxides and hydroxyl compounds,
which were toxic. De Duve believes that many cells died out,
with those remaining being those hiding in oxygen-free areas and
those that evolved ways of coping with the new threat.
De Duve postulates that at this point precursors of mitochondria
and another organelle called peroxisome came to the rescue and
were gathered into the evolving eukaryotic cells. These two
organelles detoxify oxygen, the mitochondria by turning the
oxygen into water. The peroxisomes take care of the oxygen
toxicity by other chemical reactions. The cyanobacteria that
started the crisis itself turned out to be a precursor to the trapped
and internalized chloroplasts in some of eukaryotic cells. Thus
the cyanobacteria’s ability to utilize sunlight to produce oxygen
became the keystone in photosynthesis and we have plants as a
result! Altogether, it took about 3 billion years from the
appearance of the first prokaryotes to the beginnings of animal
and plant life.
Lest you think this scenario is woven out of thin air, De Duve
does cite various pieces of evidence that support this view of
eukaryote evolution. Thinking about it, aren’t we all sort of like
our ancestral cells? We also gobble up and internalize certain
prokaryote bacteria and employ them to our benefit. For
example, colonies of bacteria in our guts help us in digesting and
processing our waste. Bless those prokaryotes, at least the good
ones.
Allen F. Bortrum
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