The Common Ancestor

A section through a eucaryotic cell, with its prominent nucleus, in which the long DNA molecules (carrier of genetic instructions) lie tangled up. Other prominent cell components or organelles are floating in the endoplasmic fluid between the nucleus and the outer cell wall. These organelles include the mitochondria (responsible for energy budget), lysosomes (digestion), the reticulum and the Golgi apparatus. The human body is made of eucaryotic cells, about 100,000,000,000 of them. (Yes, 100 billion. As many as there are stars in the galaxy. The human body also hosts bacteria, mainly in the guts. That number is even larger.)

By modern standards, Earth was a very nasty place 4 billion years ago. The first 100 million years or so were marked by severe melting and re-melting of the rocks on the surface of the planet. After this process slowed, a solid crust began to form; but the continued bombardment from comets and asteroids kept on tearing up its face for another several hundred million years. The changes on the surface that began to take place in this time led to an environment capable of supporting organic compounds. Water was being released from the hot rocks below the crust (a process referred to as "outgassing"). This process went on for some time, and may still be active to some extent. However, the water emitted today in volcanoes, geysers and undersea vents is mostly recycled surface water.

If we are to reconstruct the Origin of Life we are first bound to admit that there was a privileged period on Earth when life forms first appeared, and that later forms then evolved from these earlier ones. If this is so, several questions arise. Was there just one life form at the beginning among many possible other ones? Did the form or forms that became the ancestor or ancestors to all succeeding life forms emerge by competition or by luck? As a single survivor? Or as an alliance between several different early life forms? How do we get from self-replicating molecules to self-replicating cells? Is this such a rare event that it might only have happened once, or are there other places in the universe where it did or could have happened? Are we, that is Life on Earth, extremely special?

There are no fossils from the earliest history of Earth that will give us clues to these questions. However, we might pick up such clues from the things living organisms have in common. A common ancestor should have had those very attributes.

First, there is the trivial fact, already mentioned, that organisms have pretty much the same elementary composition. The chemistry of Life is carbon chemistry, in combination with hydrogen, oxygen and nitrogen, and some sulfur and phosphorus. As a rule, these elements are quite readily available on Earth. Everyone uses them, and everyone uses them in pretty much the same way. Given that we have the same general chemical machinery, the pathogenic microbes we call germs can try to take ours over for their purposes. In response, we (our defensive cells) eat them. We know their chemistry (in our genes) and we know how to deal with it.

Second, all living organisms exist as cells or agglomeration of cells. A cell has a boundary between itself and the outside world. In single-celled organisms (which make up the great bulk of organisms on our planet) the cell walls are relatively thick and robust. With a few notable exceptions, single cells are too small to see with the unaided eye. We call them "microbes" for that reason. Most of them are various types of bacteria. (The biggest bacteria in fact can be seen by the naked eye; they occur in black marine muds in anaerobic environments and make a several mm long sheath, a housing wherein they live.) Other single-celled organisms are "eucaryotic" cells, with their genetic material collected in a true nucleus ("eu" means good, "caryos" means core; both Greek). Typical examples are abundant in puddles, rivers, lakes, and in the ocean, among the microscopic "plankton" organisms suspended in the water. Among these are all sorts of cells moving by means of rotating whips and hair-like protuberances.

We don't know how the first cell wall was made, but we can readily appreciate the usefulness of the invention. Once the genetic instructions were enclosed in a protected environment, it was like having a patent on whatever successful adaptations were developed. Favorable mutations that gave the individual an increased chance for survival would pay off in a big way. This information would be passed on to its offspring genetically.

Walled-in organisms could readily face new challenges, through adaptation to a changing environment. Some, indeed many, of these environmental changes would have been produced by the activities of the living organisms themselves. Thus, Life had to play catch-up with the very changes it wrought. (We call this the "Gaia-Playing-Catch-Up" hypothesis, or "Catch-up" hypothesis for short.) The main example for "catch-up" is the increasing availability for oxygen during the Archean and Proterozoic, which led to the evolution of oxygen-using microbes, and ultimately to the ascent of the energy-hungry eucaryotes.

If there ever was an alternative chemistry, or an alternative to cell structure, or an alternative to passing on information to the next generation by transferring RNA and DNA, we don't know about it. The winners write the history books.