Chapter 14, Problem 31A

Summary Introduction

To infer:

The effects of increase in oxygen due to the first photosynthetic organisms.

Introduction:

Scientists did not know what the first cells were like as the first cells left no fossils. Many scientists think that modern prokaryotes called archaebacteria are the closest relatives of Earth’s first cells. The early prokaryotes were anaerobic and produced energy from inorganic molecules.

Answer to Problem 31A

The primitive photosynthetic organisms on Earth during the Precambrian period released oxygen due to photosynthesis. Aerobic prokaryotes evolved which further increased the diversity of prokaryotes. More autotrophs produced enough oxygen to support the formation of ozone layer. Ozone layer protected the organisms from UV radiation which led to the evolution of eukaryotes.

Explanation of Solution

Dome shaped structures called stromatolites are formed from cyanobacteria. These are evidence of existence of primitive photosynthetic organisms on Earth during the Precambrian period. The cyanobacteria are primitive prokaryotes that are found in rocks as old as 3.5 billion years.

The photosynthesizing prokaryotes released oxygen from water. As their number increased, the concentration of oxygen in Earth’s atmosphere also increased. Then organisms that could respire aerobically evolved and survived. There was an increase in diversity of prokaryotic organisms.

They produced enough oxygen to support the formation of ozone layer. The ozone layer shielded the organisms from the harmful effects of ultraviolet radiation and further enabled evolution of more complex organisms called the eukaryotes.