The table shows the total number of known species on earth. The data are divided into two major categories on rows, namely prokaryotes and eukaryotes; and two major categories on columns two and three, namely the environment. Note that 1k means 1000 species.
1.7 Approaches for Compartmentalization
Compartmentalization is not a condition for life; however, a closed environment for biochemical containment might be. Most likely, unicellular species that evolved in water had fewer survival issues related to mechanics and gravity. To obtain a specialization, some of these species evolved a cooperation between internal biochemical processes rather than between individual cells. Thus, their volume and shape increased to the point where a single cell began to resemble a multicellular organism. Such unicellular organisms contain multiple nuclei, and for historical reasons are called coenocytes (Greek – coeno, “common”; cyte, “box”). Nevertheless, the virtual “cells” of these unicellular organisms are not defined by a physical barrier. This virtualization may be achieved only through controlled biochemical interactions between concentration gradients of different types of molecules (i.e. from gradients of simple nucleic acids, amino acids, fatty acids and sugars, up to RNA and proteins). Spatially spaced point sources of such chemical and biochemical gradients can form a well-organized virtual structure in these unicellular organisms. Moreover, the biochemical versatility can continue up to the point of inclusion of smaller unicellular species to form a biochemical symbiosis.
1.7.1 Two Main Approaches for Organism Formation
Life shows two main approaches for organism formation. The first approach forced a cooperation between biochemical processes (virtual cells in one physical boundary). The second approach forced a cooperation for gradual specialization among individual cells of the same species (or even between species). Interestingly, the second approach shows a lower entropy than the first. However, cooperation between individual cells did not rule out further specialization between biochemical processes inside individual cells.
1.7.2 Size and Metabolism
Competition and gravity preclude the emergence of unicellular organisms over a certain size. Moreover, gradient-based biochemical signaling and interactions would be inefficient on long distances inside large unicellular organisms. Multicellular organisms seem to have found a balance between the speed of response and the size of the cells. Small cells have a larger surface area relative to their volume. Each unit of volume can exchange gases and nutrients at a higher rate compared to larger cells. Note that the principle is equivalent to smaller salt granules that dissolve faster in water than large ones. Cooperation for development of cell specialization in the direction of a circulatory system formation ensured an optimal exchange with the outside environment and a fast response for the entire organism. In the case of very large unicellular organisms, the response time for any stimulus may be dictated by distances inside the cell and the metabolic rate. For instance, a biochemical interaction between two points in the cytoplasm of such an organism would require time and high amounts of messenger molecules to diffuse in a large volume until the target is stochastically encountered. In other words, “time contracts” for giant unicellular organisms. It is likely that giant single-celled organisms have existed in the distant past. However, competition with smaller unicellular organisms with higher response times may have eliminated them from the evolutionary chain.
1.8 Sizes in Eukaryotes
Physical sizes of different organisms provide an intuitive assessment over their complexity (Table 1.2.). The comparison between the extremes of eukaryotes can be particularly important for a good view of nonlinearity that exists between entropy and size, of which, we will discuss about later. This subchapter briefly discusses the physical sizes of different unicellular and multicellular eukaryotic organisms.
Table 1.2 Extreme sizes of unicellular organisms.
| Unicellular organisms | Eukaryotes (μm) | Prokaryotes (μm) |
|---|---|---|
| Min | 0.8 | 0.15 |
| Max | 300 000 | 1400 |
The table shows the minimum and maximum physical dimensions of unicellular organisms in both eukaryotes and prokaryotes. The values represent averages of the measurements published in the scientific literature and are presented in micrometers.
1.8.1 Sizes in Unicellular Eukaryotes
Eukaryotic unicellular organisms provide important clues to the schism that led to multicellularity. Also, single-celled eukaryotic organisms are particularly important biological models that can lead to a deeper understanding of the fundamental mechanisms behind the evolutionary process.
Marine life shows both the maximum and minimum sizes for unicellular organisms. For instance, a member of the green algae, Caulerpa taxifolia, is a unicellular organism of 30 centimeters in length, or more [78]. The Syringammina fragilissima is another example of a unicellular organism, which reaches ∼20–25 cm in diameter or Ventricaria ventricosa, which is a cell of 2–4 cm in diameter [79, 80]. On the other hand, the smallest unicellular eukaryote appears to be Ostreococcus tauri, a marine green alga with a diameter of about 0.8 μm [81, 82].
1.8.2 Sizes in Multicellular Eukaryotes
Through cooperation, eukaryotic multicellular organisms have been able to evolve large dimensions. In water, buoyancy counterbalances gravity and it allowed for evolution of the largest organisms on the planet. For instance, Balaenoptera musculus (the blue whale) is a marine mammal of 27–30 m and around 170–200 tones [83]. It may be the largest contemporary organism on the planet. On land, Loxodonta africana (the African savanna elephant) is the largest living land animal [84]. Among birds, Struthio camelus (the common ostrich) can reach 2.8 m in height and weigh over 150 kg [85].
1.9 Sizes in Prokaryotes
On the other hand, prokaryotes from Mycoplasma species show some of the smallest possible dimensions for life (∼100 species). For instance, bacteria Mycoplasma gallicepticum and Mycoplasma genitalium are likely two of the smallest self-replicating forms of life, with a diameter of ∼0.0002 mm (0.2 μm or 200 nm) [86, 87]. This small size is 2 up to four times smaller than the wavelength of a photon of light from the visible spectrum (700–400 nm). However, the largest species of bacterium found among prokaryotes are Thiomargarita namibiensis and Epulopiscium fishelsoni (between 0.5 and 0.7 mm), which are comparable in size to some unicellular eukaryotes [88, 89].
1.10 Virus Sizes
Small viruses are predominant and may be round, or rod shaped, or a combination of the two in the case of prokaryotes. A capsid is the protein shell of a virus, which can remind us of the idea of Platonic solids. For some small viruses, usually the self-assembly process is dictated by their nucleic acids sequence (motif seeds) [90]. This protein shell seals their genetic material from the environment. However, many capsid proteins can self-assemble with no additional help. Viral proteins have structural properties that allow regular and repetitive interactions among them. Identical protein subunits are distributed with helical