Chapter 15, Problem 1TC

To determine

The time during which the first true animals evolved, the atmospheric changes that had to happen before animal life was possible, whether the descendants of most of the early animal life forms have marine representatives today.

Answer to Problem 1TC

It was during the oxygen revolution between nearly 2 billion and 400 billion years ago that the primary animal-like creatures developed. The atmosphere that had only less than 1 percent free oxygen was altered by the photosynthetic autotrophs into an atmosphere with greater than 20 percent free oxygen, as seen today, during the oxygen revolution. Most of the primitive animal forms had marine representatives; however, at present, much of them do not exist.

Explanation of Solution

The single-celled organisms formed the primary animal-like creatures on the Earth. During the oxygen revolution, they started to flourish in the ocean. Oxygen revolution was the period of an essential transformation in the Earth’s atmosphere. The photosynthetic autotrophs release oxygen into the atmosphere as a byproduct of the process of their food preparation. This process is called photosynthesis. This process added more oxygen into the Earth’s atmosphere.

Due to the action of such photosynthetic autotrophs, the atmospheric composition of the Earth changed from a less than 1 percent oxygen-containing atmosphere to a new atmosphere having greater than 20 percent free oxygen during the oxygen revolution between approximately 2 billion and 400 million years ago.

Aerobic respiration became possible with the increasing abundance of free oxygen. This accelerated disassembly of food particles that primitive animals attained by consuming the autotrophs. The ozone obtained from this oxygen hindered most of the harmful ultraviolet radiations of the sun from reaching the surface of the Earth. This made the survival of life at the ocean surface and later on the land possible.

All groups of primitive marine animals were not able to survive for longer time periods. Certainly, most of those animals would not have made it to the present time. For instance, the trilobites that were abundant during the past have all disappeared. Perhaps, most of the animals in the ancient fossil beds have vanished and may characterize the failed branches in animal evolution that represents exceptional designs that were not appropriate to later environmental settings.