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The first eukaryotic cells

First, what are eukaryotic cells? This picture depicts the difference between ‘procaryotic cells’ (bacteria) and ‘eukaryotic cells’ (almost everything else).

[life’s previous phase, bacteria]

You can see at once that eukaryiotic cells are far more complicated than bacteria.

The colour picture below shows that the cell membrane is much more complicated than that of a bacterium too. It comes from Scientific American October 85. The top of the picture is the outside. The phospholipids are still in there holding everything together. But notice the cholesterol, the little yellow cylinders. Reduce the amount in your diet by all means. But don’t cut it out altogether!

Eukaryotic cells are normally a thousand times larger than bacteria, that’s to say ten times bigger in each direction.

All larger organisms, including us, are made of countless millions of eukaryotic cells. Having said that though, our bodies also play host to ten times as many bacteria, mostly doing sterling work on our behalf in places not mentioned in polite society.

Eukaryotic cells must have started as self-contained live-alone organisms. And there are still plenty of single-celled eukaryotes around today.

When did eukaryotic cells first appear? Theories abound on this. I’ve put them down as first appearing 2.1 thousand million years ago, because that seems to be when they first appear in the fossil record. Some scientists undoubtedly believe that that’s too early. Others think they evolved very much earlier.

How did eukaryotic cells first appear? Theories abound about that too. It’s striking that some of the ‘organelles’ look very much like captured bacteria. The chloroplasts are the little factories that carry out the photosynthesis in green plant cells (other cells don’t have them). They show every sign of having descended from free-living cyanobacteria (more).

The mitochondria are the cell’s power stations. Whereas the chloroplasts absorb sunlight and give off oxygen, the mitochondria absorb oxygen and convert energy-rich materials into little battery molecules (ATP). The rest of the cell uses these battery molecules to power various chemical reactions. This is exactly what ‘aerobic’ bacteria do. And the mitochondria also look as though they were once free living bacteria.

So most scientists conclude that the bacteria must have evolved first – followed either quickly or after thousands of millions of years, by the eukaryotes. Later still, probably within the last thousand million years or so, individual eukaryotes started to experiment with clubbing together to form larger organisms.

[Click for the next developments, during the late Proterozoic.]