Life on Mars?
Box 2 | Cyanobacteria: The simple things of life
Cyanobacteria, also known as blue-green algae, started out on Earth quite a while ago. Their fossils have been found in rocks that are about 2800 million years old.
They are still with us. Indeed, blue-green algal blooms are becoming increasingly evident in Australia's rivers and lakes as human activities create conditions in which they thrive.
Although commonly referred to as blue-green algae, cyanobacteria are not actually algae. Cyanobacteria, and bacteria in general, are prokaryotic life-forms. This simply means that their cells do not have distinct nuclei – their genetic material mixes in with the rest of the cell. This characteristic is distinctive of bacteria and archaea; all other life-forms on Earth consist of eukaryotic cells in which the genetic material is contained inside a membrane.
Bacteria (and archaea) are hardy creatures. They can survive in hot, cold, salty, acidic and alkaline environments in which eukaryotes would perish. Despite this, they have a bad image: after all, bacteria cause many diseases in humans, some of them fatal.
Yet, without them we may not be here at all. Most scientists believe that microbes were the earliest life-forms, simple creatures that fed on carbon compounds that were accumulating in Earth's early oceans. In the harsh conditions that were present then, no other organism could have survived.
Slowly, other microbes evolved that could use the sun's energy to manufacture their own food. Cyanobacteria then went a step further: they started to extract hydrogen from water during photosynthesis, releasing oxygen as a by-product. Over time, enough oxygen accumulated in the Earth's atmosphere to allow the evolution of oxygen-breathing organisms.
But we may owe bacteria more than the air we breathe. It is possible that eukaryotic cells, of which humans are made, evolved from bacteria about 2 billion years ago. The theory is that larger prokaryotic cells started 'swallowing' smaller ones. Eventually, the small cells became the membrane-enclosed nuclei of the larger cells, and eukaryotic cells came into existence.
Regardless of how it happened, the evolution of eukaryotic cells was a significant milestone in the history of life on Earth. As conditions became more favourable, ever more complex organisms began to evolve.
Over three billion years later, we have reached a point in our own evolution where we can peer down a microscope at perhaps a thousand of these tiny life-forms drifting in a drop of water. Are we looking at our ancestors?
Boxes
Box 1. Mars: Earth's cool cousin
Box 3. ET, can you speak up?
Related site
Cyanobacteria: The mothers of our atmosphere? (NASA University of NSW Pilbara Education Project, Australia)
Posted November 2008.