Single Primordial Ancestor
The living organisms are composed of cells and despite the many differences in appearance and function, all cells have three elements in common: plasma membrane, cytoplasm, and DNA. Similar biochemical reactions that constitute the metabolism (development, energy production, waste disposal) take place inside the cells. And they contain the hereditary information encoded in molecules of deoxyribonucleic acid, that direct the cellular activities and reproduction. These shared characteristics and the presence of identical or almost identical molecules, demonstrate that every living being is related and they descend from a common ancestor.
Multiple Primordial Ancestors
Nobody can be positive that various types of life didn’t originate on our planet, billions of years ago. But, even if that happened, these hypothetical forms of life —different from the one we know— must have gone extinct very “quickly”, without leaving any trace. The fossil record, instead, tell us that the first cells go back at least 3.5 billion years, they belong to a cyanobacteria-like organism and they are very similar to the modern ones.
Current Primordial Ancestors Formation
Okay, but why aren’t new “ancestors” forming on present-day Earth? There are two problems. The chemical composition of the oceans and the competition. In the Precambrian, there was no oxygen in the atmosphere/hydrosphere and there was a lot of CO2. Although it may seem absurd, this reducing environment was more conducive to the development of life as we know it. Then, those organisms have evolved, have diversified themselves, have occupied all the possible space, and have become more efficient in exploiting resources, denying the possibility to any other —at this point ineffective— pre-biotic system to develop.
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