What was the Earth like when life first appeared? It was probably very hot; there would have been liquid water, but it was probably close to boiling point over most of the surface. The planet's crust was thin; volcanoes and earthquakes were commonplace. Meteorite storms and comet collisions were regular events. Thunderstorms were probably ubiquitous and virtually continuous. The atmosphere was mainly carbon dioxide and nitrogen, mixed with gases emanating from volcanoes. There would have been virtually no free oxygen. Because there was no free oxygen there was no ozone layer, so if and when the cloud cover was not too thick, ultraviolet radiation penetrated freely to the surface.
We can only guess about the Earth's first inhabitants. They were presumably small prokaryotes with the simplest possible genomes. They probably resembled archaea rather than bacteria because they had to survive in a very hot and acidic environment. Oxygen would have killed them; oxygen is very damaging to living materials. A large percentage of present-
day organisms tolerate it, even require it, only because their cells have elaborate mechanisms for rendering it harmless. When life first appeared on the planet there might have been some free hydrogen in the atmosphere. (Hydrogen is quickly lost from the atmospheres of small planets because it is such a light gas; strong gravitational fields such as those of Jupiter and Saturn are needed to retain it.) If so, the first organisms might have used hydrogen as a fuel source. Otherwise they probably used hydrogen sulphide released from volcanic vents, or similar chemical reductants. How many "species" of these earliest organisms there were, how many arose independently from non-living sources, we cannot guess. But since comparative DNA sequencing evidence points to a single common ancestor, all but one of these "species" must have vanished as though they had never been.
Because the earliest organisms were probably archaea-like, some people conjecture that the prokaryotes that inhabit deep ocean vents today must resemble them. These deep ocean vent organisms are therefore living fossils of the very earliest life. As we mentioned in chapter 12, they tolerate high temperatures, high pressures and high acidity and utilise hydrogen sulphide. The possibility is reasonable, but it would be a mistake to equate modern archaea with the pioneers of terrestrial life. Modern archaea seem to require products of photosynthesis, which reach them from the ocean surface. Geological evidence shows there was no oxygen in the atmosphere when life began. Photosynthesis produces oxygen, so if there was no oxygen in the atmosphere, there was no photosynthesis. Therefore, modern archaea are not identical with the earliest organisms. Atmospheric oxygen became detectable later, and the amount increased slowly over the succeeding two thousand million years.
The earliest organisms probably obtained their energy from chemical sources such as hydrogen sulphide, but could they have used sunlight? Some modern archaea and bacteria utilise solar energy not by photosynthesis, but by a simpler mechanism that does not produce oxygen. Sunlight activates a membrane protein, which pumps hydrogen ions out of the cell. The resulting hydrogen ion gradient is used to manufacture ATP, the "common energy currency" of metabolism (chapter 4). This simple light-driven hydrogen ion pump operates in many present-day archaea and bacteria, so it might have a very ancient origin. The mitochondria and chloroplasts of eukaryotic cells, which were once free-living prokaryotes, also use hydrogen ion pumps to couple their energy sources to ATP synthesis.
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