Chapter 13, Problem 1AQ

Compare and contrast the physical and chemical conditions on Earth at the time life first arose with conditions today. From a physiological standpoint, discuss at least two reasons why animals could not have existed on early Earth. In what ways has microbial metabolism altered Earth's biosphere? How might life on Earth be different if oxygenic photosynthesis had not evolved?

Summary Introduction

Big bang theory states that the entire universe and its subjects exapnded from high temperature and density state. Big bang theory explains that all the planets in the solar system arise from the sun by means of cosmic explosion. Earth formed 4.5 billion years ago by the cosmic explosion. Primitive earth is full of gas in the form of a molten liquid. Gradual cool down process of earth and chemical reaction gives rise to water and living organisms.

Explanation of Solution

The earth surface has changed over the period of time due to several factors. Huge flows of molten rock within the earth surface produced a series of gigantic solid plates known as tectonic plates. These plates have drifted on to the mantle of the planet slowly. The drift of tectonic plates had shown two important effects on the evolution and distribution of the life on the earth. The climate of the earth had shown drastic changes over millions of the years. Sometimes the earth covered with glacial ice and sometimes the ice melted raising the sea levels covering the earth with the oceans and with little land area.

The early earth could not have the magnetic field and gravitation force to hold the gases. Later about 4 billion years ago the earth cooled to form solid crust with dense elements like hydrogen and nitrogen formed in the first atmosphere. The first formed earth was very different from today’s earth as it contained gases like hydrogen cyanide, carbon dioxide, carbon monoxide, nitrogen, hydrogen sulfide and water in the form of water vapor. The earth has undergone several collisions with the comets and asteroids and the surface of the earth was cooled by the formation of oceans that drenched the planet with thunder storms. These conditions made the earth inhospitable to the formation of life 4.5 billion years ago.

The evolution of photosynthesis changed the environment and life on the earth. Earlier photosynthetic forms existed on the earth carry the process of photosynthesis under anaerobic conditions. Therefore, the process was known as anoxygenic photosynthesis. This process involved the electron donors like H₂S and produced elemental sulfur (S^o) as a waste product. Later on phototrophs had diverged from the anoxygenic phototrophs (Photosynthetic bacteria) which had the ability to trap the solar radiation and utilized it as energy source. This was occurred about 2.5 to 3.5 billion years ago with the evolvement of cyanobacterial lineage that were capable of oxygenic photosynthesis. In this process the H₂S was replaced with H₂O reducing the CO₂ producing the O₂ as waste product. The origin of oxygenic photosynthesis filled the earth’s atmosphere with O₂ that caused the evolution of newer forms of life that were able to utilize the O₂ for respiration.

In absence of oxygen the iron in the earth would have been existed in the reduced form and most of this might be dissolved in the ocean. Oxygen produced by the cyanobacteria might react with the reduced iron in the oceans and produce iron oxides. The O₂ produced by cyanobacteria reduced to the iron oxides that are poorly soluble in water and precipitated the ocean floors forming sedimentary structures called banded iron formations or rocks containing the deposits of iron and silica materials. The banded iron formations contribute to the source of iron ore at present times. Thus, the oxygen produced by cyanobacteria rather accumulate in the atmosphere reduced the iron and then accumulated to reach the 21% in the atmosphere at present. Presence of photosynthesis and oxygen makes the life form to exist on the earth.