As can be seen from what has already happened, the evolution of the cell is a relatively advanced stage of organic evolution. As the abundant components of the biotic soup became exhausted, it became necessary to evolve water-soluble organic materials from the atmosphere. From fermentation, the simpler but less efficient form of metabolism, photosynthesis was the next step. The special chlorophyll molecule had evolved. This allowed living organisms to capture solar energy for the synthesis of organic molecules. The first photosynthesizers removed themselves from the competition for dwindling natural energy-rich molecules and set themselves up as primary producers. Once the photosynthetic process was achieved, the future of life was assured. As soon as it emerges and produces enough oxygen, respiration becomes possible. In accordance with the laws of natural selection, once photosynthesis started it made its mark on all subsequent living things, and was undoubtedly so successful that it wiped out earlier forms of life.
This development represents a qualitative leap. The subsequent evolution to more complex forms is a drawn out process eventually leading to a new branch of life, the nucleated cell. At the top of the eukaryotic tree, several branches appear simultaneously, such as plants, animals and fungi. According to the American molecular biologist Mitchell Sogin the amount of oxygen affected the pace of evolution. The chemical composition of ancient rocks suggests that atmospheric oxygen increased in relatively distinct steps separated by long periods of stability. Some biologists believe that the explosion of life could have been triggered by oxygen reaching a certain level.
The nucleated cell – the eukaryotes – completely adapted to oxygen and showed little variation. The emergence of this revolutionary new life form allowed the existence of advanced sexual reproduction, which in turn, accelerated the pace of evolution. Whereas the prokaryotes consisted of only two groups of organisms, the bacteria and the blue-green algae (the latter produced oxygen through photosynthesis), the eukaryotes consist of all green plants, all animals and fungi. Sexual reproduction represents another qualitative leap forward. This requires the genetic material to be packaged inside the nucleus. Sexual reproduction allows the mixing of genes between two cells, the chances of variation being far greater. In reproduction, the chromosomes of the eukaryotic cells fuse to produce new cells. Natural selection serves to preserve favourable genetic variants in the gene pool.
One of the key aspects of life is reproduction. All animal and plant cells have the same basic internal structures. Reproduction and the passing on of parental characteristics (heredity) take place through the union of sex cells, the egg and sperm. The genetic material DNA through which the characteristics of life forms are transmitted from one generation to the next is contained in the nucleus of all cells. The cell structure, which is made up of cytoplasm, also contains a number of miniature organs called organelles. The internal structure of the organelles is identical to different types of bacteria, which seems to indicate that the composition of the animal and plant cell is the result of these once independent organs, with their own DNA, combining to form a co-operative whole. In the 1970s microtubules were discovered. These are protein rods, which fill every cell in the body like an internal scaffolding. This internal “skeleton” gives shape to the cell and appears to play a role in the circulation of protein and plasma products. The advent of the eukaryotic or nucleated cell constituted a biological revolution some 1,500 million years ago.
From asexual budding and fission emerged sexual reproduction. Such an advance served to mix up the hereditary material of two individuals, so that the offspring would differ from the parents. This provided the variation on which natural selection could work. In every animal and plant cell the DNA is arranged in pairs of chromosomes in the nucleus. These chromosomes carry the genes that determine individual characteristics. The new offspring, while combining the characteristics of its parents, is nevertheless different from them. It appears that the origin of sexual reproduction is connected with primitive organisms ingesting one another. The genetic material of two individuals was fused producing an organism with two sets of chromosomes. The larger organism then split into two parts with the correct amount of chromosomes. Single and paired chromosomes existed, but through time the paired condition became the normal mode of existence of plants and animals. This laid the basis for the evolution of multicellular organisms.
By about 700-680 million years ago, the first metazoa appeared. These were complex multicellular organisms that require oxygen for their growth. During that period the oxygen content of the atmosphere increased constantly, reaching its present level only 140 million years ago. The processes at work in evolution have a markedly dialectical character in which long periods of gradual quantitative change are interrupted by sudden explosions. Such a period occurred about 570 million years ago.