What is the evidence that organisms may have evolved from a common ancestor?

Answer 1

Perhaps the most important outcomes of biochemical research has been the realization that all organisms have much in common. Organisms are remarkably uniform at the molecular level. This observation is frequently referred to as the unity of biochemistry, but, in reality, it illustrates the unity of life.

This uniformity reveals that all organisms on the Earth have arisen from a common ancestor. A core of essential biochemical processes appeared early in the evolution of life. The diversity of life in the modern world has been generated by evolutionary processes acting on these core processes through millions or even billions of years.

Only three naturally occurring elements ---oxygen, hydrogen and carbon--- make up 98% of the atoms in a living organism. Moreover, the abundance of these three elements in life is vastly different from their abundance in Earth's crust. One reason that oxygen and hydrogen are so common is the ubiquity of water and there is a universal consensus among biologists that water has been the life enabler on earth.

After oxygen and hydrogen, the next most common element in living organisms is carbon. Most large molecules in living systems are made up predominantly of carbon. Under a certain point of view, if we divide biological molecules in structural molecules, composing the structure of living beings, and fuel molecules, that are the molecules transformed in the processes providing energy to life, fuel molecules are dominated by oxygen and hydrogen, even if carbon has an important role in them, while structural molecules are dominated by carbon.

As a means of seeing why carbon is uniquely suited for life, let us compare it with silicon, its nearest elemental relative. Silicon is much more plentiful than carbon in Earth's crust, and, like carbon, can form four covalent bonds, a property crucial to the construction of large molecules. However, carbon-to-carbon bonds are stronger than silicon-to-silicon bonds.

This difference in bond strength has an important consequence: after Carbon has undergone combustion, carbon dioxide is readily soluble in water and can exist as a gas; thus, it remains in biochemical circulation, to be used by another organisms. In contrast, silicon is essentially insoluble in reactions with oxygen, silicon oxide in amorphous form is glass while in crystalline form it is quartz. After silicon has combined with oxygen, it is permanently out of circulation.

Thus, carbon based molecules are stronger construction materials and are better fuels than silicon based molecules.

Other elements have essential roles in living systems, for example nitrogen, phosphorus and sulfur. Moreover, some of the trace elements, although present in tiny amounts compared with oxygen, hydrogen and carbon, are absolutely vital to a number of life processes, like iron and potassium.

Answer 2

The uniformity in the chemistry at molecular level in all living organisms and our incapability of repeating the origin of life in the laboratory are the strongest evidence to suggest that life once arose on this planet has evolved and ramified in different forms. Therefore, we can say that all organisms present on this planet have a common ancestry.