Complex life forms

Simple life forms, such as bacteria, consist of single cells, whereas, at the other extreme, complex life forms such as animals, contain many types of cell, each having a specific function (cells in eyes, limbs, stomach, etc.). 

Many investigations, confirmed by theoretical studies (Maher and Stevenson, 1988), indicate that the heavy bombardment of the primeval Earth about 3.8 billion years ago led to its sterilisation, so it was practically uninhabitable . The time stamp for the first stromatolith-forming prokaryotes, about 3.5 billion years ago (Schopf, 1993), does not in fact mark the beginning of the biogenesis process, but the appearance of quite complex life forms. Thus, the point at which the first primitive living systems appeared must have been considerably earlier. 

In the course of phylogeny an efficient control system evolved that enabled the functions of individual organs to be orchestrated in increasingly complex life forms and permitted rapid adaptation to changing environmental conditions. 

An RNA that exhibits enzyme-Hke activity is called a ribozyme. The discovery of ribozymes had a great impact on research into the origins of life. Identifying catalytic capabilities in RNA, an information molecule, led to a new theory the RNA world hypothesis. This suggests that RNA was the first life form on Earth, and when it first evolved it performed both catalytic and enzymatic functions. The natural selection process associated with evolution eventually caused the RNA to evolve into the highly sophisticated supramolecular systems observed in the complex life forms present today. 

A companion element to carbon, directly below it in the periodic table, is silicon. Silicon (Si) can also form long chains of linked silicon nuclei, and it also assumes a tetrahedral shape like carbon. But silicon does not readily form the double and triple bonds of carbon, so it does not appear to possess the versatility necessary for a complex life form, though some science fiction writers would have it otherwise. Still, silicon forms the basis for quartz, gemstones, and glass, which contribute to the beauty of life. 

The basic unit of life is the cell and the simplest living sytems are single cells that possess all the above capabilities, drawing their building materials from simple chemical substances in their, generally aqueous, environment. More complex life forms, for example. Homo sapiens, are multicellular organisms in which every cell does not possess all of the seven pillars of life and life is possible only because the cells form a society whose health is dependent on the integrated activities of the different cell types within the system. In the human body there are some 10 such cells. 

These high-temperature and high-pressure conditions are similar to those found in deep-sea volcanoes and hydrothermal vents, a favorite hypothesized venue for the origin of life. It is plausible that some of the simplest biochemical building blocks could have produced complex life forms over eons of natural selection and evolution. The challenge, however, is to explain how sufficiently complex proteins, or ribozymes, could have been produced in the lipid membranes necessary for the metabolism of their own catalysis. 

The specific improbable event that interested Hoyle throughout much of his life was the appearance of life and its subsequent evolution into the diversity of life that we know today. He believed that both the initial appearance and the subsequent evolution were too improbable to have occurred in such a small place as the earth, and that the whole idea of evolution of complex life forms from simpler ones on earth was absurd. He believed we needed the vastly larger space of the entire universe to explain it. The problem with this argument is that the universe as it is described by most astronomers is not nearly large enough to be of much help. We are so conscious of how much bigger it is than ourselves that we easily forget how small it really is. 

Cells are necessary for more complex life-forms. 

Respiration Animals and plants in the dark use the chermical energy of respiration to create more diverse and complex life forms. 

These properties suggest a versatility and stability of carbon-containing compounds that cannot be matched by any other element. One highly speculative consequence of this is that, in considering the possibility of life on distant planets, it is difficult to imagine complex life forms based on any element other than carbon. 

Class II habitats includes bodies on which life may evolve but due to stellar and geophysical conditions that are different from the class I habitats, the planets rather evolve toward Venus- or Mars-type worlds where complex life-forms may not develop. 

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