Map of life expectancy at birth from Global Education Project.

Thursday, August 03, 2006

You Karyote, You

Now that Kansas voters have put the so-called "opponents of evolution,"* as the Boston Globe headline had it, into the minority on the state Board of Education, this seems like a good time to ask why God designed life on earth to look exactly as if it had evolved over billions of years.

The vast majority of living organisms on earth are of a kind called prokaryotes. In fact, there are more prokaryotes living in your colon than there are cells in your body, by a substantial factor. That is possible because prokaryotes are much smaller than your cells, which are called eukaryotic cells. Prokaryotic cells consist of the bacteria which you have no doubt heard of, and the archaea, which resemble bacteria but have differences in their genetic transcription methods and lipid chemistry, and are often found in extreme environments -- very hot, very cold, very acidic, etc. However, some live in our bowels and give off gas that makes us fart. Unlike bacteria, archaea do not cause disease.

The prokaryotes are not only smaller than eukaryotic cells, they are much simpler. They lack the internal structure -- the membrane bound organelles of eukaryotes. Their biochemistry takes place in an undifferentiated soup. They have only a single main strand of DNA, unlike the multiple twinned chromosomes of eukaryotes, although many also have smaller fragments of DNA floating around in rings called plasmids. Our eukaryotic cells store their chromosomes in a central structure called a nucleus. They carry out metabolism in special compartments called mitochondria. Our cousins the plants also have special compartments called chloroplasts which carry out photosynthesis, making sugar from C02 and sunlight -- which is where we ultimately get our own sustenance.

Sadly, prokaryotes never came up with sex, per se. That's a eukaryotic innovation as well. However, it may be that prokaryotes have something even better in some ways -- they can exchange DNA by swapping plasmids, even among different species, and they can even pick up plasmids from dead cells. So they have all sorts of opportunities for picking up genetic information that we don't. On the other hand, without the patterned inheritance of sex, they have never been able to generate the informational complexity and developmental processes needed to create multicellular organisms. The closest they come is aggregations of slime, basically.

But here's the kicker. Some time around, oh, a billion years ago or more, there weren't any eukaryotes. But then, according to one possible scenario, one cell, presumably a large one as prokaryotes go, most likely an archaea, ingested another cell, probably of a kind called a proteobacterium, but didn't manage to digest it. Instead the proteobacterium started living and reproducing inside the big cell, and its progeny went along with the larger cell's descendants when it divided. Whether immediately or after a period of co-evolution, these two organisms developed a symbiotic relationship that benefited both. Today, the descendants of the proteobacterium live in our own cells, as the organelles called mitochondria. On some other occasion, apparently at around the same time (in geological terms, i.e. within a few millions or tens of millions of years) one of these double organisms developed a second symbiosis, with a type of prokaryote called a cyanobacterium, which is capable of photosynthesis, and its descendants became the chloroplasts of plants. It is possible, though probably less than 50/50, that a similar event created the nucleus of the eukaryotic cell.

Very soon after they originated, eukaryotes began to, ahh, do it. For early sexually reproducing organisms, however, that wasn't actually any fun. In the normal method of reproduction of a single cell, in the case of those fancy new eukaryotes, all of the chromosomes would reproduce themselves, the now four sets of chromosomes would move apart to form two sets of two, and the cell would then divide to form two cells, each with two full sets of chromosomes. (How there got to be two sets in the first place is a mystery, of course, but you need them for sexual reproduction.) In sexual reproduction, however, the chromosomes do not reproduce; they first go through a sort of swap meet in which segments may be exchanged between members of each pair, then the pairs are pulled apart and two new cells form each of which has only one set of chromosomes, instead of the usual two. These unusual cells are called gametes. Two gametes, presumably from different parents, then fuse, to form a new cell with a complete double set of chromosomes.

The mitochondria, by the way, kept their own DNA, and continued to reproduce the old fashioned way, just going along for the ride.

Note that you don't need sex organs, or any organs at all, to do this. Single cells can do it. But why do it? It seems like an awful lot of unnecessary trouble. To understand the point of all this, you need evolution.

More on that later.

* Being an opponent of evolution is sort of like being the cartoon version of King Canute, who purportedly ordered the tide to recede. Actually the original version of the story is that he did so to prove that kings are only human, not that he thought he would succeed.

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