Some scientists have argued that living things must not have arisen in shallow, warm seas, as proposed by Oparin and Haldane, because the earth's surface at the time life arose was a very unstable environment.
Meteorites and comets hit this surface all too often, and early life could not stay in such conditions.
Early in the Earth's formation, meteorites collided strongly with the earth's surface, and the energy of these collisions was spent on melting or even vaporizing the rocky surface. Meteorites fragmented and melted, contributing their substance to the growing earth. An especially violent impact may have generated the moon, which still bears on its surface the marks of this meteorite bombardment. On the Earth's surface most of these marks have been erased over time by erosion.
Most meteorites burn until they disappear when they enter the current Earth's atmosphere and shine in the sky like shooting stars. In the early days, meteorites were larger, more numerous, and hit the earth more often.
Some scientists speculate that early living beings could not have survived this cosmic bombardment, and propose that life arose in more protected places, such as the floor of the early seas.
In 1977, so-called deep-sea underwater hot springs, where hot and sulfurous gases emanate from openings in the sea floor. In these places life is abundant. Many bacteria that live there are autotrophic, but perform a process very different from photosynthesis. Where these bacteria live there is no light, and they are the basis of a peculiar food chain. They serve as food for animals or are kept inside their tissues. In this case, both animals and bacteria benefit: they have protection within the animals' bodies, and they receive food produced by the bacteria.
The discovery of hot springs raised the possibility that life would have arisen in this kind of protected environment and that the energy for the metabolism of early living beings would come from an autotrophic mechanism called chemosynthesis. Some scientists believe that the first living things were bacteria, which obtained energy for metabolism from the reaction between inorganic substances, as do bacteria currently found in underwater hot springs and other very hot environments (at about 60 to 105ºC) and sulphurous. According to this hypothesis, it seems that all life we know descends from this type of bacteria, which should be autotrophic.
Those who argue for this hypothesis are based on evidence suggesting abundance of hydrogen sulfide (hydrogen gas, H2S, which smells like rotten eggs) and iron compounds in early Earth. The first bacteria must have obtained energy from reactions involving these compounds for the synthesis of their organic components.
Some bacteria currently living in hot and sulphurous sources may perform the following chemical reaction, which, according to the autotrophic hypothesis, may have been the fundamental energy-supplying reaction for early living beings:
|Ferrous Sulfide + Hydrogen Gas ---> Ferric Sulfide + Hydrogen Gas + Energy (pyrite, a common mineral)|
The energy released from these reactions can be used by bacteria to produce life-critical organic compounds from CO2 and H2O.
Thus, according to this hypothesis, the chemosynthesis - an autotrophic process - would have come first. Then there would have been fermentation, photosynthesis, and finally respiration.
Debates about the origin of life will still have much to talk about. The most accepted hypothesis about the evolution of metabolism is still heterotrophic, although the autotrophic hypothesis has been gaining more and more strength.
How did multicellular beings come about? Evidence obtained from geological studies suggests that The first simple multicellular cells appeared on Earth about 750 million years ago! Prior to this there was a predominance of unicellular life as simple eukaryotic forms. From that date onwards, the first multicellular cells originated from the existing eukaryotic unicellular cells.
Since then, evolution has never stopped!