The Tree of Life by Gustav Klimt. Photo by thewholegardenwillbow, Wikimedia Commons
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The Tree of Life is a symbol found throughout a myriad of mythologies and religions. From the ancient Middle East to the shared books of Judaism and Christianity, to Buddhism, Islam, Hinduism, and to a diverse array of beliefs of many indigenous civilizations around the world, some form of the Tree of Life stands at the origin point of life on Earth or at some other critical moment in existence. The Tree, in its many forms, touches both the Earth (and its deep, hidden Underworld recesses) and Heaven. Its leaves, its fruit, or its sap create or preserve life, grant immortality and convey knowledge sometimes for good and sometimes for evil.
This incredibly powerful, archetypal symbol was used by a number of 19th Century naturalists to represent the interconnected structure of Nature. Augier in 1801 drew a Tree to illustrate the unity of a wide range of living plants, and Lamarck in 1809 used a similar (although inverted) model to illustrate the interconnections of an array of the types of animals. Charles Darwin, in his book “On the Origin of Species” in 1859, incorporated a description and also a drawing of the Tree of Life into his narrative not only to provide a visual representation of evolution and his Theory of Natural Selection but also to give it an emotional foundation. As Darwin described his Tree:
“The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great battle for life. The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was small, budding twigs; and this connection of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups.”
Komensky’s Centrum Securitas Tree of Life. Public Domain
The ”green” terminal twigs of this tree, then, represent the living species that have arisen from the now extinct ancestral species(the “dead branches and limbs”) just below them. The green terminal twigs are all that can be actually seen: they are the living organisms of our biotic communities. The lower, “dead branches” may have fossilized relics from which their existence can be inferred or they may have to be logically constructed from the shared features and unique characteristics of the observable living species.
There are many implications inherent to this structure of the Tree of Life: 1. Living species are separated from each other. There are barriers that keep the DNA of each species isolated from the DNA of the other contemporary living species. Each living species is a unique, independent entity traveling along its own time path of evolution! And 2, change (i.e. evolution of new species) occurs only in a “vertical” direction! Genetic variation accumulates within every living population, and that variation interacts with the species’ environment to select for the “most fit” characteristics. These more fit individuals will come to dominate the living spaces of our “green twigs” and eventually, sprout up to form new species.
DNA Strand. Human Genome Research Institute. Public Domain
One of the driving engines for the growth of the Tree of Life is the inevitable change that occurs in an organism’s DNA during its replication when an organism makes new cells. A human cell, for example, contains about 3 billion base pairs whose pattern along the DNA molecule forms the information content of the genome. There are four types of bases in DNA (adenine, cytosine, thymine and guanine) and their precise match-ups with their complimentary bases (adenine and thymine always pair together, and cytosine and guanine always pair together) insures the great precision in DNA replication and also the efficiency and fidelity of RNA synthesis (“transcription”) and protein synthesis (“translation”).
The size of the task to replicate all of the base pairs of a DNA molecule, though, and the speed at which this has to occur inevitably leads to mismatched base pairs in the new DNA strand and potentially altered information in the DNA of the forming daughter cell. It is estimated that when DNA replicates an error in base match-up occurs approximately once in every 10,000 base pairs. This would mean that in a replicated human cell’s DNA 300,000 error points would occur! This would significantly change the genetic information in the newly formed cell and represents a unacceptably high rate of error. To prevent this from happening a robust system of enzymes and RNA molecules swarms over the new DNA strand both during and after replication and repairs most of these errors.
Exactly how many errors persist in the daughter cell DNA varies among different types of organisms and also between different sections of a given organism’s genome. A range of 1 to 6 persisting errors, though, in each and every new strand of replicated DNA is a relatively conservative estimate of the overall change. These replication errors are a type of mutation that inevitably increases the variability of a species’ DNA.
Most of these mutations do not alter the functional information of the new DNA, but, occasionally a significant change will occur! That altered DNA, then, will make a different kind of RNA which may have altered functions in the cell (and maybe even lead to a new type of protein being made in the daughter cell). These new proteins often are non-functional and may lead to the disability and/or death of the daughter cell. In a few instances, though, these altered proteins may lead to some new structure or function that increases the “fitness” (efficiency and survivability) of the daughter cell (and organism)!
So, built into DNA because of an only slightly less than perfect method of replication, is an inevitable process of genetic change and evolution!
Spectrum of light. By Gringer. Wikimedia Commons
Other things can also cause mutations in a cell’s DNA. Exposure to energy sources in an organism’s environment (like UV or ionizing radiation) can directly and randomly change the base pattern of a DNA strand as can exposure of DNA to some of the powerful oxidizing by-products of cellular metabolism (the molecules called the “free radicals”). Toxins in our environment (like PCB’s or the dozens of poisons in cigarette smoke) can also alter the base patterns in, and, therefore, the information content of, a cell’s DNA. These mutations, like the mutations seen in DNA replication errors, may have no effect on the metabolic functioning of a cell, or they may be harmful and lead to cells with altered, often cripplingly inadequate structures or metabolisms. Very occasionally, though, these mutations can lead to proteins that increase a cell’s or organism’s fitness and possibly serve as the foundation for a newly evolving species. Some mutations can also cause cells to break free of their homeostatic control systems and start to replicate wildly. These types of cells will form cancers. Mutagens that cause cancers are called carcinogens.
It is estimated that 300 to 30,000 base pairs are changed via mutation in each generation of a species. Over a thousand generations, then, a considerable number of genes could be altered! Most of these mutations only affect the cells of an organism that form its own tissues and organs. They would not, then, be passed along to offspring and thus have an influence on the growth of their Tree of Life branch unless germ cells (sperm and ova) were altered. These accumulating genetic alterations, though, make each living organism a patchwork cellular quilt of slightly different genomes that becomes increasingly heterogeneous over time.
Simple diagram of a virus. By domdomegg, Wikimedia Commons
Viruses can also influence an organism’s movement up the Tree of Life. A virus is a small piece of either DNA or RNA that is wrapped in a capsule of protein. They are not considered to be true living organisms (they are called “life forms”) and, so, have no specified place of their own on the Tree of Life.
The purpose of a virus is to make more viruses, but sometimes this is accomplished in a roundabout manner by the virus inserting its genetic information into the genome of the cell it has infected. Residing in this host cell’s DNA, the viral genetic information may or may not synthesize entire viruses, but it will be replicated and preserved each time the host cell and then each subsequent daughter cell divides! The viral genes may also participate in vital functions within the host cell organism! It is estimated that 8% of the human genome is of viral origin. This represents 240 million base pairs some of which have been important in our movement up our Tree of Life twig.
So, it is not easy to move up one’s branch on the Tree of Life, but it is inevitable that every species will do so!
In a couple of weeks I will return to these ideas and talk about a major flaw in the Tree of Life metaphor! I will use some new insights and research on the evolutionary origin of land plants to show you that the branches of our tree sometimes fuse at least partially together! Genes, apparently, are able to flow not only vertically up the Tree, but also horizontally between living organisms!
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