Home	The Power of 2 (and other small numbers)

The number 2, 2's complement and paired entities play a unique role in Nature:

• Vertebrates, insects and many other animals have mirror symmetry, not symmetry based on polygons like a starfish or a jellyfish (which are not fish). 
   

• Duplicated organs like paired eyes, nose, scent sensors give extra value and backup. So, 1+1=3, with estimation of distance and direction being the bonus. With our primitive noses, humans do not have stereo scent like some butterflies have.
   

• The two sexes are complementary, 2's complements if you want.
   

• Recent organisms have two sets of chromosomes and thus two slightly different copies of each gene, used for variation and mutual backup.
   

• The DNA molecule has two helical strands, perfectly complementary.
   

• The number of nucleotide bases ATCG is 4, that is 2 to the power of 2 !
  Base A complements with base T, and base C complements with G.
   

• When a cell of a plant or an animal splits, it always splits into two (because of the DNA structure and Nature's method for secure duplication of digital data). Actually, single-cell organisms like bacteria split the same way, only each of the new cells becomes a new organism.
   

• In plants and animals, there are two modes in which a cell can split into two cells - mitosis and meiosis. Mitosis is the regular duplication responsible for growth of an individual from the single zygote, the fertilized egg. (Animal terminology is chosen here.) The zygote is diploid, that is it carries two sets of chromosomes as inherited via an egg plus a sperm. Meiosis is the process producing eggs and sperms. Meiosis only happens in specialized organs, the ovaries and testes, respectively. Meiosis is by far the world's greatest lottery, where single genes are chosen from the two ancestral sets available, seemingly at random. (Supposing that the choice of mating partner is only partly a matter of chance.)
   

• The Evolution's Tree of Species is a binary tree, with two branches at all levels. When a species splits, it splits into two. Either or both of the new species may each split into two, soon after by geological standards.
   

• Vertebrates have two sets of complementary limbs, a front pair and a hind pair. Let us call them arms and legs just here.
   

• The topology of skeleton bones in both arms and legs form a tree. At some point in time an early vertebrate limbs probably had a skeleton topology 1<2<4<8. With this assumption, 8-5=3 fingers and toes (digits!) were suppressed on mammals. An extra rule in the genes of our predecessors truncated the "perfect" binary tree. (Among others, many digits waste energy and are costly in terms of control logic.)
    

• Most vertebrates [not fish] have a double circulatory system, in engineering terms the heart is a two-circuit pump. One of the two circuits is dedicated to oxygenation in the lungs. This has been necessary for endothermic animals, even more so for animals with a large brain.

Other small numbers in Life:

• A triplet of DNA base pairs is the "byte" of DNA that address 20 amino acids. Each triplet encodes 64 numbers (equivalent to 0..63). Some of the unused byte values encode control characters, much like text codes in computers do. (Early computers used 64-character codes, now extended to 256.) Three, twenty, 64 (and 256) are all small number by biological and cosmic standards.
   

• Each cell  of plants and animals carries a number of chromosomes. The number of chromosomes is always even and very significant for the species. If anybody believed that Humans are "the ultimate species" in Nature, the following examples may be a bit surprising. Comparing the number of chromosomes only, among those examples the Potato is our closest relative:
   

  Species	Chromosomes
  Cat (Felix cattus)	38
  Human (Homo sapiens)	46
  Potato (Solanum tuberosum)	48
  Horse (Equus caballus)	64
  Dog (Canis familiaris)	78

   

• Our 2x5=10 fingers have played an important part in western culture. The decimal system is taken for granted, although from a mathematical point of view it is not an ideal numeric base. (Base 12 would have been a better general-purpose base, like 12 inches in a foot.)
   

• The depth of the chain of links in an arm and leg is about 7. The last four levels have no branching - look at your hand.
   

• Somewhat surprisingly, during the lifetime of a complex organism, the number of generations of cells is a small number. This is true even for large animals like a grown-up person, an elephant or a blue whale. Through binary division, a fertilized egg can "ideally" produce the 100 000 000 000 000 cells of a human body in 47 generations. The special organ structure calls for a much slower growth, but even closer to 1000 generations would be a small number compared to 10^14.
    

• The knuckle profile of your hand matches long and short months of the year.