Archaea 3.9 bya?

Based on their structure and biochemistry we believe the earliest cells were similar to the living Archaea (which used to be called the Archaebacteria). Archaea are probably living fossils, similar to the earliest cells.

Halobacteria image by NASA

We believe that Archaea are ancient because:

►    they can survive in environments similar to those found on the young Earth: hot springs, sea vents releasing sulfide-rich gases, boiling mud around volcanoes

►    they are mostly anaerobic

►    the RNA of their ribosomes is different to that of bacteria

►    Unlike bacteria, they have no peptidoglycan in their cell walls

►    the fats in their membranes are unique

Anaerobic Bacteria

Archaea include the methane forming, the salt loving and the heat loving bacteria. Most of these are anaerobic, that is, they do not need a supply of oxygen to live. Indeed, they die if they are put in contact with the air which, of course, contains oxygen.

This is one reason to believe that the archaea evolved at a time when the Earth's atmosphere was very different from now, when there was no free oxygen.

Methane-Forming Archaea

The Methanobacteria hide away in places where there is no oxygen, such as marshes, when we call the methane they produce ‘marsh gas'. They also thrive in sewage treatment plants and the intestines of animals.

Salt-Loving Archaea

Before the invention of refrigeration and freezing, people used to preserve their food by salting it. This killed the bacteria. However there are some Archaea, the Halobacteria, which can only live in water with a high salt concentration! They produce a pink scum on salt flats. Note that, unusually for archaea, they are aerobic, which means that they need oxygen to survive.

Heat-Loving Archaea

Another way to kill normal bacteria is to cook food. But heat does not kill some Archaea. Thermoacidophilic Archaea can only live in water which is very hot (70 - 75 °C) and as acidic as concentrated sulphuric acid. They are found in hot springs.