Bacteria have been around, as nearly as we can tell, for a couple of billion years. These single-celled animals reproduce asexually, by fission. That is, when a bacterium gets fat enough, it splits in two.
We can, if we like, say that the two bacteria are “new.” We might call them daughter cells. From the point of view of an individual daughter cell, however (if a bacterium can be said to have a point of view), it’s the same as it was before the split. The other daughter cell has split off from it. The other daughter cell would be thinking the same thing, of course — if bacteria could think. Each would feel that it was still the same creature it had been before the split.
One fascinating consequence of this is that the first original bacterium that assembled itself (somehow) from the primordial ooze two billion years ago is still alive. It has split untold trillions of times, and mutated beyond measure, but each of the daughter bacteria is still, in a basic way, the original bacterium. There is continuity. In fact, all of the bacteria alive today are the first, original bacterium.
Multi-celled organisms such as ourselves arose, of course, from single-celled organisms. Our first ancestors were colonies of bacteria, or of free-floating cells descended from bacteria. Most of the cells in a multi-celled organism become highly specialized. They become brain cells, or liver cells, or whatever. But in each generation, a few (the egg cells) remain in a sort of general-purpose, unspecialized state. So as to minimize the danger of harmful mutation, these reproductive cells divide very few times before being passed down to the next generation.
In each new generation, the egg cell begins splitting wildly. Its daughter cells take on myriad shapes. But again, a few are preserved in their original undifferentiated state so as to be passed on.
This unbroken lineage of egg cells over countless generations suggests a rather startling insight:
Every single cell in your body is two billion years old!
Just thinking about it makes me dizzy.