Looking at the Diversity of Life
It is a fact that biology, as a science, has such a broad scope. Basically, this is because of the tremendous diversity of life on earth. The source of this diversity is evolution. In general, evolution is the process of gradual change during which new species arise from older species. As a matter of fact, evolutionary biologists study the evolution of living things in everything from the microscopic world to ecosystems.
We can summarize the evolution of various life forms on Earth in a phylogenetic tree (see image below). A phylogenetic tree is a diagram showing the evolutionary relationships among biological species based on similarities and differences in genetic or physical traits or both.
A phylogenetic tree consists of nodes and branches. The internal nodes represent ancestors. In addition, these nodes are points in evolution when, based on scientific evidence, an ancestor is thought to have diverged to form two new species. The length of each branch is proportional to the time elapsed since the split.
Carl Woese and the Phylogenetic Tree
In the past, biologists grouped living organisms into five kingdoms: animals, plants, fungi, protists, and bacteria. The organizational scheme was based mainly on physical features. This is as opposed to physiology, biochemistry, or molecular biology, all of which are used by modern systematics.
The pioneering work of American microbiologist Carl Woese in the early 1970s has shown that life on Earth has evolved along three lineages, now called domains—Bacteria, Archaea, and Eukarya. The first two are prokaryotic cells with microbes that lack membrane-enclosed nuclei and organelles.
On the other hand, the third domain contains the eukaryotes. In fact, this domain includes unicellular microorganisms together with the four original kingdoms (excluding bacteria). Woese defined Archaea as a new domain, and this resulted in a new taxonomic tree (see image above).
Many organisms belonging to the Archaea domain live under extreme conditions and we call them extremophiles. An extremophile is an organism that thrives in physically or geochemically extreme conditions that are detrimental to most life on Earth. In contrast, organisms that live in more moderate environments may be termed mesophiles or neutrophiles. To construct his tree, Woese used genetic relationships rather than similarities based on morphology (shape).
Using Genetic Relationships to Differentiate Organisms
Woese constructed his tree from comparative sequencing of genes. So, he used universally distributed genes present in every organism. Also, he made sure to use conserved genes. Generally, this means that these genes have remained essentially unchanged throughout evolution.
Woese’s approach was revolutionary. This is because comparisons of physical features are insufficient to differentiate between the prokaryotes that appear fairly similar in spite of their tremendous biochemical diversity and genetic variability (see image above). The comparison of homologous DNA and RNA sequences provided Woese with a sensitive device that revealed the extensive variability of prokaryotes, and which justified the separation of the prokaryotes into two domains: bacteria and archaea.