The techniques of modern molecular biology have recast our view of life on earth and its evolution. The traditional view of life you might have found in a textbook of biology 50 years ago or more, would have shown a tree of life. At the base was a common ancestor some billions of years ago on the Earth and life progress through this tree which is both a metaphor but often taken literally towards higher organisms. Almost inevitably in this tree of life, primates, apes, and humans are at the pinnacle of the tree. It's a seductive metaphor because it gives the impression that life was leading to us, and that life builds towards greater complexity and intelligence, that we are indeed the pinnacle of life on Earth. The modern view of our place in the spectrum of living things is less reassuring to our egos. The modern tree of life is based on molecular techniques and essentially tracks the slow variation of genetic material from the common ancestor using RNA or DNA. This doesn't require us to identify species because it's being done with the genetic material itself. The tree of life has the same form, where time moves upwards from the bottom and the base is a common ancestor that was the branching point for all subsequent life on Earth. In this tree of life, most of the real estate is unfamiliar to us because it's microbial. In fact, most of the genetic diversity of life on Earth is microbial. In this tree of life, the entire plant and animal kingdoms are small twigs. We are the tiniest little twiglet in this tree of life, most of which is composed of microbial species. The three major branches of the tree of life consists of bacteria, archaea, and eukarya. Eukarya are the cells with nuclei, bacteria are the cells without nuclei, and archaea was a domain of life only found a few decades ago, which probably relates to the earliest forms of life on Earth that formed near volcanic vents and in quite warm conditions. The advantage of using genetic material to trace the evolution of life on Earth is that we can do it precisely and accurately using the gradual deviation in the base pair sequence of either RNA or DNA. When the tree of life was drawn in terms of species or organisms we were using morphology or the way the organisms appear, which is often quite a crude way to look for similarities or differences between forms of life. The tree of life does show time evolving in terms of gradual separation of the genetic material, but the time resolution of the tree is relatively poor, perhaps only a few 100 million years. The root of the tree, the last common ancestor, also has a time uncertainty because we're unsure exactly when life on Earth started, but it's at least 3.7 and maybe as much as 4.1 billion years ago. Another interesting experiment that can be done using deviations in genetic material between present-day species, is to look at the degree of genetic overlap. This is usually done in one particular part of the base pair sequence of RNA. By looking at the number of deviant nucleic acids which is essentially the overlap of the genetic material, we can see how much we are similar to other species. As is well know, we are similar to rhesus monkeys or chimpanzees by 99 or 98 percent. We are similar to dogs by about 87 percent and chickens by about 82 percent. More strikingly, two-thirds of our genetic material overlaps with moths and we have more than half genetic overlap with simple microbes like yeast. Next time you look into your beer glass, acknowledge there is kinship there, that yeast is half you. The standard model of biology is based on something that biologists ironically refer to as the central dogma. The fundamental genetic material is housed in RNA, a less stable form of the DNA that evolved somewhat later in the history of life on Earth. This RNA creates expression in the phenotype which is the basis for an organism, and then propagates through replication. The gradual divergence of the genetic material is caused by genetic mutations often stemming from the environment and occasionally from poor transcription as the DNA or RNA is copied. A slightly more complex form of this diagram includes the work of proteins. The RNA containing the genetic code, codes for sequences of proteins that carry out the functions within a cell, this, in turn, leads to the structure and function of the organism. The loop is closed because the organism lives in an external environment which molds evolution and decides which species or organisms will survive and why. This is the role of natural selection which applies just as well to microbes as it does to larger creatures on Earth. One of the interesting questions astrobiology will ask, "Is our form of life unique and is our genetic code unique?" To biologists thinking theoretically, it's not obvious that life had to be the way it is. Other possible variants can be imagined. Some can even be computed. The genetic code of terrestrial biology consists of four different base pairs with rules on how they couple across the DNA double helix. Essentially, biology has a four letter alphabet of base pairs and it's this alphabet that codes proteins and then the functions in a cell. The minimal unit of the genetic code is a three base pair sequence called the codon, and we can see how the genetic code works in these terms. There's redundancy in the genetic code because it has to be tolerant of errors. This is the way the genetic code checks itself and makes sure transcription and replication happens with great fidelity. It's useful to make an analogy for the genetic code with books and words, because the analogy is quite literal. A base pair is like a letter in the genetic code, a codon is like a word, and a gene coding for a set of proteins is like a sentence. Roughly 10,000 atoms are involved in a gene. The simplest living organism may have hundreds or thousands of genes. The most complex organisms like humans have 30 or 40,0000 genes. In terms of word analog, this corresponds to the information in an encyclopedia. The relationship between genes and complexity is not literal because there are organisms with more genes than humans that are relatively simple in their functions. If we made a scale model of DNA where we took the cell down to about the size of an egg and imagine what the DNA would look like if it were unwound from a human cell, we can see the enormous information storage in the genetic code. Reduced to this scale, the distance between base pairs is a 30th of a millimeter and a codon is about a 10th of a millimeter long. A gene is about half a meter of this DNA and the entire amount spooled into that cell is 2,000 kilometers long. A single gene would be 500 meters long. Imagine the tiny letters describing one aspect of a person and then the time it would take to read 2,000 kilometers of the tape, The Book of Life. Information storage in biology is incredibly efficient because an encyclopedia is worth of information is stored in each and every cell nucleus. Part of the reason that biology is only maturing as a subject, is the sheer complexity of biochemistry and the fact that computers are needed to model and understand it. There are several complexity in biology. One is the metabolic complexity involved in biochemical networks involving hundreds of different chemicals regulated in myriad and subtle ways. The other is morphological or structural complexity with the different elements operate on different scales in concert and coordination. If we could imagine ourselves reduced by six orders of magnitude, shrunk to fit within a cell, biologists have told us what we might see. This visualization is based on actual research and is not pure imagination. This is how the structures look and behave inside our cells. All life on Earth from fungal spores to elephants are based on one thing. That one thing is the genetic code stored the base pair sequence of RNA or DNA within the cell. Life as it evolved gradually deviated in its genetic material, and the modern tree of life is based on tracing that genetic evolution. In the modern tree of life-based on the analysis of the genetic material, humans or animals occupy a tiny portion of the genetic real estate. This is by far a microbial planet. Large organisms are a small portion of the biological and metabolic diversity of the planet. Study of the genetic material has also shown that the genetic code operates identically in every living creature on Earth.
