The Tree of Life by Dana Visalli/www.methownaturalist.com It is not known how life on earth began. Genetic and chemical evidence indicates that life was initiat- ed only once, as one species, and that all succeeding organisms are descendants of this first life, which is known as LUCA--the last universal common ancestor. The Earth is thought to be 4.5 billion years old, and life appeared approximately 3.8 billion years ago, almost as soon as the planet had stabilized ade- quately for life to be theoretically possible. Estimates for the total number of species alive today vary wildly, from around 5 million to more than 100 million. 1.8 million species have been named and catego- rized by scientists. The immediate evolutionary relationships among organisms are not universally agreed upon and so the branching of the Tree of Life has different interpretations. The one used here is one of the more straightforward versions. Life began as relatively simple, single cells resembling Bacteria, and diversified from there into 6 distinct Kingdoms: Bacteria, Archea, Protists, Plants, Fungi and Animals. A currently popular alternate tree divides life first into 3 domains--Bacteria, Archea, and Eukaryotes (see below), and then the Eukaryotes radiated into 4 kingdoms. An important evolutionary division is between prokaryotes and eukaryotes. The word prokaryote translates from the Greek as ‘before the seed,’ or ‘before the nucleus.’ Prokaryotes--Bacteria and Archea- -do not have a nucleus in their cells--a separate area with its own membrane that contains the DNA of the cell. The word eukaryote means ‘true seed’ or ‘true nucleus,’ and obviously a eukaryote does have a nu- cleus. This group includes the other 4 kingdoms of life besides Bacteria and Archea: Protists, Plants, Fun- gi and Animals. Primary Life Groups: 6 Kingdoms: Kingdom Bacteria (Greek, ‘cane,’ because the first Bacteria discovered were rod-shaped)- About 2000 species known in 11 phyla. The total number is hard to assess, guesses range from the tens of thousands to billions. Bacteria are prokaryotes; their cells have no nucleus. They are found in almost all environments, from deep underground to the stratosphere. The biomass of bacteria thought to exceed that of all animals and plants combined. Bacterial cells are about 1/10th the size of eukaryotic cells and Bacteria, magnified 1000x are typically 0.5-5.0 u (micrometers; a micrometer is 1 millionth of a meter) in length. Dana Visalli/[email protected]/www.methownaturalist.com, Kingdom Archea (Greek, ‘ancient’)- About 1000 species known in 2 phyla; the actual total existing is unknown. Like Bacteria, Archea are prokaryotes, but they have unique proper- ties that separate them from both eukaryotes and Bacteria. Most Archea are an- aerobes--they obtain their energy without using oxygen, and in fact oxygen is toxic to them. Many Archea are ‘extremeophiles,’ which means ‘lover of extreme conditions.’ They can be found thriving in extremely hot water (up to and above An Archea, Sulfolobus 212°F), in ice, in super-saturated salt water. and deep below ground. Size is 0.1-15 u. Kingdom Protista (Greek, ‘the very first’) (aka Protoctista)- 100,000-200,000 species in 36 phyla. Pro- tists have eukaryotic cells and are mostly unicellular; some are multicellular but have no specialized tissue, algae for example. They are typically divided into 3 groups, animal-like, plant-like and fungi-like protists. Size is variable; while aver- aging 0.1 mm, they vary from 0.01 mm to 65 meters (giant kelps). Paramecium, a Protist Kingdom Plantae (Latin, ‘to sprout’)- About 300,000 species in 12 phyla (often called divisions in the Plant Kingdom). Plants are thought to have evolved from certain green algae. They began to colonize land 450 million years ago, which required some means of dealing the problem desiccation (drying out). Size ranges from the duckweed-like Wolffia which is 0.5 mm long, to the largest redwood tree at 115 meters (380 feet) tall. Kingdom Fungi Flower anthers & pollen (Greek ‘sponge’)- 75,000 species in 6 phyla have been identified; total may num- ber more than a million. One of the 6 phyla of Fungi is lichens, which are mostly fungal with an algae or cyanobacteria symbiont (lives inside the fungus). Fungi are now thought to have diverged from other life 1.5 billion years ago, and colo- nized land about 500 million years ago. Most plant species have a symbiotic (beneficial) relationship with one or more fungal species. The majority of the fun- gi that we see--mushrooms--have most of their organic structure underground and can grow to enormous size. A honey mushroom (Armillaria ostoyae) in Ore- Honey mushroom gon grows over 2200 acres and is a contender for the largest organism on the plan- et. Yeast cells can be as small as 2 micrometers (2 millionths of a meter). Kingdom Animalia (Latin, ‘breath’ or ‘soul’)- The total number of animals on earth is unknown; a mil- lion have been identified and the total is probably well over 5 million (including insects and various worm phyla). 99% of all animals are invertebrates--they lack a backbone. Of the 36 phyla of animals, most are aquatic worms of one sort or an- other. Truly land-dwelling forms are found only in two phyla, Arthropoda A moss piglet, in moss (insects and their kin) and Cordata (animals with backbones). One contender for the smallest animal in the world is the water bear or moss piglet (tardigrade); di- minutive ones are 0.1 mm long. The largest animal in the world and the largest that has ever lived is the blue whale. It can grow to 100 feet in length and 400,000 pounds. Before whaling there were probably a million blue whales in the oceans; today there are about 8000. Kingdoms break down as follows: Subkingdom, Phylum, Class, Order, Family, Ge- A blue whale nus, Species. For example humans are in Phylum Chordata, Class Mammalia, Order Primates, Family Hominidae, Genus Homo, Species H. sapiens. 2 Branches on the Tree of Life Prions (The name derived from ‘protein-infection’)-A prion is an infectious agent, a pro- tein in a misfolded form. All known prion diseases in mammals affect the struc- ture of the brain or other neural tissue and all are currently untreatable and fatal. Diseases include bovine spongiform encephalapathy (mad cow disease) and Creutzfeldt-Jacob Disease. Prions are not considered living organisms but are mis- folded protein molecules which may propagate by transmitting a misfolded pro- tein state. If a prion enters a healthy organism, it induces existing, properly White spots are microscopic holes folded proteins to convert into the disease-associated, misfolded prion form. This in neural tissue caused by prions triggers a chain reaction that produces large amounts of the prion form. Prion rep- lication is subject to natural selection just like in living organisms. Viruses (Latin, ‘poison’)- About 5000 viruses are known, with another 4000 unclassified, and probably over million extant. A virus is a minute infectious agent that can reproduce only by entering a host cell in a living organism and using its cell ma- chinery. Some can crystallize like minerals and persist for years in that state. Vi- ruses can infect all types of life forms, from animals and plants to Bacteria and Archea. The smallest ones are only 20 nm (nanometers, 20 billionths of a meter, 20 millionths of a millimeter). Viruses attacking a bacteria The virus particle, or virion, in its simplest form is just nucleic acid enclosed in a protein shell. Its genome may consist of double- or single-stranded DNA or RNA. the smallest viruses have as few as four genes, while the largest have several hundred. Much smaller still are viroids, tiny molecules of naked RNA, which foul up the metabolism of plants. Prokaryotes (Greek, ‘before the seed’) Prokaryotes are organisms that have what could be considered primitive cells which have no nucleus--no membrane-bound area inside the cell containing the cell’s DNA. A second type of larger cells, the eukaryotes (‘true seed’), do have a nucleus. Two kingdoms make up the prokaryotes, Bacteria and Archea. They are the earliest life form we know of, 3.8 billion years old, and the only life-form on Earth for 2 billion years. Prokaryotes outnumber all eukaryotes combined: there are more bacteria in one human mouth than all the people that have ever lived; the human body contains more bacterial cells than human cells, and it is thought that the biomass of Bacteria and Archea outweigh the combined biomass of the other 4 kingdoms. They are pervasive, being found in salty, acidic, alkaline, and very hot and cold environments as well as what life typically considers comfortable. Bacteria are essential to the other kingdoms, as decomposers of organic material, fixers of nitrogen, and pro- ducers of atmospheric oxygen and nitrogen. If Bacteria disappeared, the other four kingdoms would soon fol- low. On the other hand half of all human disease is caused by Bacteria. Some are opportunistic- they are always present, for example Streptoccus pneumonia, which is always in the throat, and will multiply when body defenses are down. Prokaryotes are 1/10 to 1/100 the size of eukaryotic cells, have 1/1000 the DNA, and are haploid, which means they only have one strand of DNA, rather than two (diploid). Bacterial motility (movement) is by flagella or slime secretion. Reproduce occurs by binary fission--splitting in half--which progresses geometri- cally. Some cells can reproduce every 20 minutes, which would equal 1 million kg in 24 hrs. Like sex in eu- karyotes, prokaryotes do indulge in bacterial conjugation, when two organisms come into contact, a thin bridge can form, through which one of the bacteria donates a copy of some of its DNA.