Life’s Diversity And Unity
Life’s diversity and unity
All living things are composed of cells, and virtually all cells perform similar functions necessary to maintain life. How could the cell, this tiny basic unit of life, give rise to the incredible number of organisms or species that live on earth? This question will be addressed later. Regardless of how they arose, there are about 1.7 million species identified by biologists, and more are identified every year. There is no denying the diversity of life. How can we make sense of the bewildering array of life forms? Taxonomy is the branch of biology that names and classifies species in a hierarchical order. Carolus Linnaeus, considered the father of taxonomy (a brief biography about “Carl Linnaeus” is available at http://www.ucmp.berkeley.edu/history/linnaeus.html), devised a scheme for classifying organisms in which each species was assigned a two-part name. This binomial nomenclature consists first of the genus (plural, genera), which is a grouping of species based on a number of observable traits unique to that group. The second part of the name refers to a specific species within the genus. For example, humans are named Genus Homo and species sapiens (Homo sapiens). All names are in Latin, the language of the educated community at the time of Linnaeus. Also, the choice of Latin (a ‘dead’ language) avoided showing favoritism to a particular scientific community by the choice of, for example, French over English language.
Recent advances have enabled biologists to determine similarities and differences in the DNA found within organisms. The degree of relatedness in DNA has led to new classification schemes based DNA similarities as opposed to similarities in observable traits. In general, scientists agree that at the broadest level of classification life can be assigned to three domains called BACTERIA, ARCHAEA, and EUKARYA.
Domains Bacteria and Archaea consist of single-cell (PROKARYOTIC) organisms. All prokaryotes share the characteristics of being very small (around 1 micrometer in length, or 1/1,000,000 of a meter), and have very simple internal features (DNA not contained within a nucleus, a membrane-bound intracellular structure). Domain Bacteria (also called Eubacteria) are the “true bacteria” we are most familiar with – very common organisms found in diverse habitats. Archaea (also called Archaebacteria) are prokaryotes that exist in extreme environments, such as boiling ocean water around volcanic vents, freezing rocks, sulfur-rich lakes, and other harsh habitats. The Archaea are thought to be similar to the earliest life forms on earth, living in harsh environments that are thought to have prevailed when life originated on Earth.
Domain Eukarya (also called EUKARYOTES) is characterized by organisms consisting of a cell or cells that have a nucleus. Eukaryotes can be single-celled or multicellular. This domain consists of a much larger and more complex group of organisms than the prokaryotes. This number of organisms assigned to this domain is very large, so four subcategories (called KINGDOMS) were devised. These kingdoms are Protista, Plantae, Fungi, and Animalia. The very large and diverse Kingdom Protista consists of single-celled and multicellular eukaryotic species, ranging from microscopic single-celled protozoa (like the amoebas) to giant seaweeds. The remaining three kingdoms consist of multicellular eukaryotes and are primarily distinguished by their mode of nutrient acquisition. Kingdom Fungi are mostly decomposers, acquiring nutrients by breaking down organic wastes and dead organisms. Members of Kingdom Plantae are the plants that produce their own food by the sunlight-driven process of photosynthesis. The animals of Kingdom Animalia obtain food by ingestion (eating other organisms).
In the midst of all this diversity, where is the unity in life? The information contained in DNA, the material of the cellular ‘blueprint’ common to all life, is the great unifier. The common blueprint material and the shared language of DNA common to all life lead to common cellular structures, common chemical processes, and common functions shared by all cells, from the simplest to most complex. From this commonality, however, arose diversity. The scientific explanation for this diversity is the theory of evolution.