EVOLUTION FROM PROKARYOTES TO EUKARYOTES Vent Hypothesis.

(Origin of life) One of the speculations trying to explain the origin of life is that it may have begun deep in the oceans, in underwater hot springs called hydrothermal vents. Vents could have supplied the energy and raw materials for the origin and survival of early life forms.

Evidence.

A group of bacteria, called archaeobacteria, that

 Tolerate temperatures up to and

 Seem to have undergone less evolutionary change than any other living species supports this vent hypothesis.

Evolution of Photosynthesis.

The nutrients produced in the primitive environment would have limited early life. If life were to continue, another source of nutrients was needed. Photosynthesis, probably freed living organisms from a dwindling supply of nutrients. Release of Oxygen H2S.The first photosynthetic organisms probably used hydrogen sulfide as a source of hydrogen for reducing carbon dioxide to sugars.

H2O.

Later, water served this same purpose, and oxygen liberated by photosynthetic reactions began to accumulate in the atmosphere.

Environmental changes.

Earth and its atmosphere slowly began to change due to oxygen accumulation.

 Ozone in the upper atmosphere began to filter ultraviolet radiation from the sun. About 420 million years ago, enough protective ozone had built up to make life on land possible.

 Reducing atmosphere slowly became an oxidizing atmosphere, and

 Some living organisms at least began to utilize oxygen. Ironically, the change from a reducing atmosphere to an oxidizing atmosphere also meant that life could no longer arise abiotically.