Unit 3:  The origin and evolution of life                          1   2   3   4   5   6 

1.4. The biological evolution

a) The first cells

The early cells were prokaryotes very simple. They were similar to the actual cyanobacteria.  They appeared 3,500 million years ago. It has found fossils of these microorganisms, the stromatolites (they are accumulations of layers of bacteria). These prokaryotes were heterotrophs and feed on organic matter that was very abundant in oceans.

The primitive cells originate the first true bacteria. These bacteria performed photosynthesis but without oxygen (photosynthetic anaerobic autotrophs). Their vital activity consumed carbon dioxide and released oxygen to atmosphere. 

Due to this fact, the primitive atmosphere was enriching progressively in oxygen. 2,500 million years ago the atmosphere was already as it is now (rich in oxygen and with an ozone layer).

b) The cellular evolution

The change of composition in the atmosphere provoked the extinction of many species that cannot survive with oxygen (anaerobic organisms) and the appearance of other many ones (aerobic organisms). The heterotrophic bacteria appeared in this time.

The evolution of these early cells is little known, but it is thought that some prokaryotic cells evolved to give origin to the first eukaryotic cells about 1,000 million years ago.

Probably, as much by folding of the membrane (autogenous theory) as by establishing symbiosis with other bacteria (endosymbiotic theory), were forming the different cellular organelles. 

It seems that autotrophic and heterotrophic eukaryotic cells appeared at the same time. These cells had to be very similar to actual unicellular algae and protozoa. They are the ancestors of all eukaryotic living beings.

Animation: Endosymbiosis (McGraw Hill)

c) From unicellular to multicellular

With the pass of time, single-cell living beings started to associate in clusters.

In the beginning, they were just groups of equal and self-sufficient cells joined by some mucilaginous substance or enveloped by a common cover. But little by little, these groups became more complex and cells reached some work division transforming into colonies.

Later the specialised cells of these colonies remained together after the cell division developing in this way the tissues and the first multicellular living beings.

1.9. Why is supposed that the first living beings were anaerobic heterotrophic

         unicellular  prokaryotic cells?

1.10. Are endosymbiosis theory and autogenous theory incompatible? Why?