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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