Classification of Life

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Last Updated: Saturday, 05 October 2024 00:01
Published: Saturday, 25 February 2023 01:30
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Classification of Life:

The very first thing that we should know when studying biology is what is a living being. This is what Biology is all about. Since there are so may different kinds of living things, scientists have come up with grouping all of these living things into different buckets. Cell is the fundamental thing that is common to most of the living things.

Wikipedia article on Life: https://en.wikipedia.org/wiki/Life

Origin of Life:

We talked about Starting of the Universe and subsequent formation of Earth in "Astronomical Science" section. Here we study about when life started forming on Earth.

Timeline of Evolution : https://en.wikipedia.org/wiki/Timeline_of_the_evolutionary_history_of_life

The formation of planet where life could form started around 4 Gya. Some fossils indicate life formed between 4Gya and 3.5Gya. 

The last universal common ancestor (LUCA) is the most recent population from which all organisms now living on Earth share common descent. It is believed to have lived around 4Gya in high temperature water near ocean floor, was single celled and had genes or genetic code. There has never been any fossil evidence confirming it's presence. However based on the tree of life, there had to be some common ancestor, and that last common ancestor is named LUCA. There may have ancestors to LUCA, but we are limiting ourselves to the last common one since that is what matters for our future evolution.

Photosynthetic organisms appeared between 3.2 and 2.4 Gya and began enriching the atmosphere with oxygen. More complex unicellular cells called Eukaryotes appeared capable of sexual reproduction appeared around 2 Gya. Earliest multicellular cells appeared around 1.5Gya. Life remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose, developed over time, and culminated in the Cambrian Explosion about 538.8 million years ago. It's called the "Biological Big Bang". This sudden diversification of life forms produced most of the major phyla known today. It is estimated that 99 percent of all species that ever lived on Earth, over five billion years ago have gone extinct. Estimates on the number of Earth's current species range from 10 million to 14 million of which about 1.2 million are documented, but over 86 percent have not been described. However, it was recently claimed that 1 trillion species currently live on Earth, with only one-thousandth of one percent described.

First life must have arose from non living material. Biology, abiogenesis (from a- 'not' + Greek bios 'life' + genesis 'origin') or the origin of life is the natural process by which life has arisen from non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to living entities on Earth was not a single event, but a process of increasing complexity involving the formation of a habitable planet, the prebiotic synthesis of organic molecules, molecular self replication, self-assembly, autocatalysis, and the emergence of cell membranes.

 

Cells:

Cell is the structural and functional unit of life. Every living thing is made up of cells, with simplest life made of 1 cell, and more complex ones made of trillions of cells. Look in "Human cell" section for details.

 

Classification of life:

Earliest classification of life was conducted by the Greek philosopher Aristotle (384–322 BC), who classified all living organisms known at that time as either a plant or an animal, based mainly on their ability to move. However as more complex plats and animals were found, new classification was needed. In 1740s, Carl Linnaeus introduced his system of binomial nomenclature for the classification of species. Linnaeus attempted to improve the composition and reduce the length of the previously used many-worded names. The Linnaean classification has eight levels: domains, kingdoms, phyla, class, order, family, genus, and species. Only cellular life are included in life's classification. Virus being non cellular are not considered in this classification.

Below is the classification:

  1. Domain: A domain, superkingdom, or empire, is the highest rank of all Organisms taken together. There used to be 2 empire system: Prokaryota and Eukaryota. However, Carl Woese made a revolutionary breakthrough in 1977, and realized that Prokaryota domain that grouped Archaea and Bacteria into one were actually genetically different. They arose separately from an ancestor with poorly developed genetic machinery. This gave rise to 3 domain system whose taxonomy was devised by Carl, Otto and Mark in 1990.
    1. Archaea: These are prokaryotes. The first observed archaea were extremophlies, living in extreme environments such as hot springs and salt lakes with no other organisms. Improved molecular detection tools led to the discovery of archaea in almost every habitat. Archaea are particularly numerous in the oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the planet. Archaea were initially classified as bacteria, because of numerous similarity b/w them. Archaea have membrane lipids that are branched hydrocarbon chains attached to glycerol by ether linkages. The presence of these ether linkages in Archaea adds to their ability to withstand extreme temperatures and highly acidic conditions,
    2. Bacteria: Bacteria are prokaryotic cells just like Archaea, but their cell membranes are instead made of phosopholipid bilayers. They characteristically have none of the ether linkages that Archaea have. Internally, bacteria have different RNA structures in their ribosomes making it a domain by themselves. They constitute a large domain of prokaryotic microorganisms.  Most bacterial species exist as single cells; although they can sometimes group to form larger multicellular structures. These multicellular structures are often only seen in certain conditions. Humans and most other animals carry millions of bacteria. Most of the bacteria in and on the body are harmless. However, several species of bacteria are pathogenic and cause infectitious disease. Antibiotics are used to treat those bacterial infections.
    3. Eukaryota: This includes eukaryotes explained above. Archaea and Bacteria are Prokaryotes.
  2. Kingdom: A kingdom is the second highest taxonomic rank, just below domain. There are 6 kingdoms as per US books, while some countries only refer to 5 kingdoms.
    1. Empire Prokaryota is divided into 2 kingdoms: Eubacteria (Bacteria domain) and Archaebacteria (Archaea domain). This was originally just 1 kingdom called Monera, but Carl Woese discovery caused it to separate out into 2 kingdoms.
    2. Empire Eukaryota is divided into 4 kingdoms: Plantae (plants), Animalia (animals), Fungi and Protista. Protista is kingdom of primitive forms, which doesn't fall in other kingdoms. Protista consists mostly unicellular and simple multicellular organisms. Fungi don't photosynthesize like plants, and don't move like animals (growth is their way of mobility). They had to be classified separately mainly because of difference in nutrition. These are 3 modes of nutrition for organisms:
      1. Autotroph: An autotroph is an organism that produces complex organic compunds using carbon from simple substances such as CO2 generally using energy from light (photosynthesis) or inorganic chemical reactions (chemosynthesis) They convert an abiotic source of energy (e.g. light) into energy stored in organic compounds, which can be used by other organisms (e.g. heterotrophs). Autotrophs do not need a living source of carbon or energy and are the producers ina food chain (plant or algae)
      2. Heterotroph: A heterotroph is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but not producers.
      3. Saprotroph: A saprotroph is an organism involved in the processing of decayed (dead or waste) organic matter. Sapro means "decayed matter". Saprotrophic nutrition is a process of chemoheterotrophic extracellular digestion, mostly seen in fungi.
  3. Phylum: A Phylum is below Kingdom. Phyle means tribe or clan. Phyla can be thought of as groupings of organisms based on general specialization of body plan. At its most basic, a phylum can be defined in two ways: as a group of organisms with a certain degree of morphological or developmental similarity, or a group of organisms with a certain degree of evolutionary relatedness. The minimal requirement is that all organisms in a phylum should be clearly more closely related to one another than to any other group. Link: https://en.wikipedia.org/wiki/Phylum. Following are current Phyla classification:
    1. Archaea Kingdom = contains 2 phyla
    2. Bacteria Kingdom = contains 40 phyla
    3. Animalia Kingdom = contains 40 phyla. Arthropods (vertebrates with segmented body and jointed limbs) phyla account for over 80 percent of all known living animal species. Humans belong to Chordata phylum, which includes birds, amphibians, mammals, etc.
    4. Plantae Kingdom = contains 14 phyla
    5. Fungi Kingdom = contains 8 phyla
    6. Protista Kingdom = contains 19 phyla
  4. Class: A class has historically been conceived as embracing taxa that combine a distinct grade of organization—i.e. a 'level of complexity', measured in terms of how differentiated their organ systems are into distinct regions or sub-organs—with a distinct type of construction, which is to say a particular layout of organ systems. Not too many details available. Not important. Link: https://en.wikipedia.org/wiki/Class_(biology)
  5. Order: Order is one level below class. Not too many details available. Not important. Link: https://en.wikipedia.org/wiki/Order_(biology)
  6. Family: Family is one level below Order. Not too many details available. Not important. Link: https://en.wikipedia.org/wiki/Family_(biology)
  7. Genus: Genus is an important level just below Family and above Species. There are several criteria to group organisms into one genus. Link: https://en.wikipedia.org/wiki/Genus
  8. Species: Species is the basic unit of classification. It is often defined as the largest group of organisms in which any two individuals of the appropriate sexes can produce offspring. The most recent rigorous estimate for the total number of species of Eurokytes is between 8 and 8.7 million, with many more discovered every year. "Animalia" kingdom has the largest number of species at around 300K, with "Plantae" kingdom a distant second with 30K species. Link: https://en.wikipedia.org/wiki/Species

Binomial Nomenclature: All species are given 2 part name in biology. The first part of a binomial is the Genus to which the species belongs. The second part is the specific name or specific epithet to which they belong within that species.

  • Ex: "Boa Constrictor" is one of the species of the genus "Boa", with "constrictor" being the species' epithet.
  • Ex: "Gray wolf's" scientific name is Canis lupus, with Canis (Latin for 'dog') being the generic name shared by the wolf's close relatives and lupus (Latin for 'wolf') being the specific name particular to the wolf.