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20.1: Organizing Life on Earth - Biology


Skills to Develop

  • Discuss the need for a comprehensive classification system
  • List the different levels of the taxonomic classification system
  • Describe how systematics and taxonomy relate to phylogeny
  • Discuss the components and purpose of a phylogenetic tree

In scientific terms, the evolutionary history and relationship of an organism or group of organisms is called phylogeny. Phylogeny describes the relationships of an organism, such as from which organisms it is thought to have evolved, to which species it is most closely related, and so forth. Phylogenetic relationships provide information on shared ancestry but not necessarily on how organisms are similar or different.

Phylogenetic Trees

Scientists use a tool called a phylogenetic tree to show the evolutionary pathways and connections among organisms. A phylogenetic tree is a diagram used to reflect evolutionary relationships among organisms or groups of organisms. Scientists consider phylogenetic trees to be a hypothesis of the evolutionary past since one cannot go back to confirm the proposed relationships. In other words, a “tree of life” can be constructed to illustrate when different organisms evolved and to show the relationships among different organisms (Figure (PageIndex{1})).

Unlike a taxonomic classification diagram, a phylogenetic tree can be read like a map of evolutionary history. Many phylogenetic trees have a single lineage at the base representing a common ancestor. Scientists call such trees rooted, which means there is a single ancestral lineage (typically drawn from the bottom or left) to which all organisms represented in the diagram relate. Notice in the rooted phylogenetic tree that the three domains— Bacteria, Archaea, and Eukarya—diverge from a single point and branch off. The small branch that plants and animals (including humans) occupy in this diagram shows how recent and miniscule these groups are compared with other organisms. Unrooted trees don’t show a common ancestor but do show relationships among species.

In a rooted tree, the branching indicates evolutionary relationships (Figure (PageIndex{2})). The point where a split occurs, called a branch point, represents where a single lineage evolved into a distinct new one. A lineage that evolved early from the root and remains unbranched is called basal taxon. When two lineages stem from the same branch point, they are called sister taxa. A branch with more than two lineages is called a polytomy and serves to illustrate where scientists have not definitively determined all of the relationships. It is important to note that although sister taxa and polytomy do share an ancestor, it does not mean that the groups of organisms split or evolved from each other. Organisms in two taxa may have split apart at a specific branch point, but neither taxa gave rise to the other.

The diagrams above can serve as a pathway to understanding evolutionary history. The pathway can be traced from the origin of life to any individual species by navigating through the evolutionary branches between the two points. Also, by starting with a single species and tracing back towards the "trunk" of the tree, one can discover that species' ancestors, as well as where lineages share a common ancestry. In addition, the tree can be used to study entire groups of organisms.

Another point to mention on phylogenetic tree structure is that rotation at branch points does not change the information. For example, if a branch point was rotated and the taxon order changed, this would not alter the information because the evolution of each taxon from the branch point was independent of the other.

Many disciplines within the study of biology contribute to understanding how past and present life evolved over time; these disciplines together contribute to building, updating, and maintaining the “tree of life.” Information is used to organize and classify organisms based on evolutionary relationships in a scientific field called systematics. Data may be collected from fossils, from studying the structure of body parts or molecules used by an organism, and by DNA analysis. By combining data from many sources, scientists can put together the phylogeny of an organism; since phylogenetic trees are hypotheses, they will continue to change as new types of life are discovered and new information is learned.

Limitations of Phylogenetic Trees

It may be easy to assume that more closely related organisms look more alike, and while this is often the case, it is not always true. If two closely related lineages evolved under significantly varied surroundings or after the evolution of a major new adaptation, it is possible for the two groups to appear more different than other groups that are not as closely related. For example, the phylogenetic tree in Figure (PageIndex{3}) shows that lizards and rabbits both have amniotic eggs, whereas frogs do not; yet lizards and frogs appear more similar than lizards and rabbits.

Another aspect of phylogenetic trees is that, unless otherwise indicated, the branches do not account for length of time, only the evolutionary order. In other words, the length of a branch does not typically mean more time passed, nor does a short branch mean less time passed— unless specified on the diagram. For example, in Figure (PageIndex{3}), the tree does not indicate how much time passed between the evolution of amniotic eggs and hair. What the tree does show is the order in which things took place. Again using Figure (PageIndex{3}), the tree shows that the oldest trait is the vertebral column, followed by hinged jaws, and so forth. Remember that any phylogenetic tree is a part of the greater whole, and like a real tree, it does not grow in only one direction after a new branch develops. So, for the organisms in Figure (PageIndex{3}), just because a vertebral column evolved does not mean that invertebrate evolution ceased, it only means that a new branch formed. Also, groups that are not closely related, but evolve under similar conditions, may appear more phenotypically similar to each other than to a close relative.

Link to Learning: Head to this website to see interactive exercises that allow you to explore the evolutionary relationships among species.

The Levels of Classification

Taxonomy (which literally means “arrangement law”) is the science of classifying organisms to construct internationally shared classification systems with each organism placed into more and more inclusive groupings. Think about how a grocery store is organized. One large space is divided into departments, such as produce, dairy, and meats. Then each department further divides into aisles, then each aisle into categories and brands, and then finally a single product. This organization from larger to smaller, more specific categories is called a hierarchical system.

The taxonomic classification system (also called the Linnaean system after its inventor, Carl Linnaeus, a Swedish botanist, zoologist, and physician) uses a hierarchical model. Moving from the point of origin, the groups become more specific, until one branch ends as a single species. For example, after the common beginning of all life, scientists divide organisms into three large categories called a domain: Bacteria, Archaea, and Eukarya. Within each domain is a second category called a kingdom. After kingdoms, the subsequent categories of increasing specificity are: phylum, class, order, family, genus, and species (Figure (PageIndex{4})).

The kingdom Animalia stems from the Eukarya domain. For the common dog, the classification levels would be as shown in Figure (PageIndex{4}). Therefore, the full name of an organism technically has eight terms. For the dog, it is: Eukarya, Animalia, Chordata, Mammalia, Carnivora, Canidae, Canis, and lupus. Notice that each name is capitalized except for species, and the genus and species names are italicized. Scientists generally refer to an organism only by its genus and species, which is its two-word scientific name, in what is called binomial nomenclature. Therefore, the scientific name of the dog is Canis lupus. The name at each level is also called a taxon. In other words, dogs are in order Carnivora. Carnivora is the name of the taxon at the order level; Canidae is the taxon at the family level, and so forth. Organisms also have a common name that people typically use, in this case, dog. Note that the dog is additionally a subspecies: the “familiaris” in Canis lupus familiaris. Subspecies are members of the same species that are capable of mating and reproducing viable offspring, but they are considered separate subspecies due to geographic or behavioral isolation or other factors.

Figure (PageIndex{5}) shows how the levels move toward specificity with other organisms. Notice how the dog shares a domain with the widest diversity of organisms, including plants and butterflies. At each sublevel, the organisms become more similar because they are more closely related. Historically, scientists classified organisms using characteristics, but as DNA technology developed, more precise phylogenies have been determined.

Art Connection

At what levels are cats and dogs considered to be part of the same group?

Link to Learning: Visit this website to classify three organisms—bear, orchid, and sea cucumber—from kingdom to species. To launch the game, under Classifying Life, click the picture of the bear or the Launch Interactive button.

Recent genetic analysis and other advancements have found that some earlier phylogenetic classifications do not align with the evolutionary past; therefore, changes and updates must be made as new discoveries occur. Recall that phylogenetic trees are hypotheses and are modified as data becomes available. In addition, classification historically has focused on grouping organisms mainly by shared characteristics and does not necessarily illustrate how the various groups relate to each other from an evolutionary perspective. For example, despite the fact that a hippopotamus resembles a pig more than a whale, the hippopotamus may be the closest living relative of the whale.

Summary

Scientists continually gain new information that helps understand the evolutionary history of life on Earth. Each group of organisms went through its own evolutionary journey, called its phylogeny. Each organism shares relatedness with others, and based on morphologic and genetic evidence, scientists attempt to map the evolutionary pathways of all life on Earth. Historically, organisms were organized into a taxonomic classification system. However, today many scientists build phylogenetic trees to illustrate evolutionary relationships.

Art Connections

[link] At what levels are cats and dogs considered to be part of the same group?

[link] Cats and dogs are part of the same group at five levels: both are in the domain Eukarya, the kingdom Animalia, the phylum Chordata, the class Mammalia, and the order Carnivora.

Review Questions

What is used to determine phylogeny?

  1. mutations
  2. DNA
  3. evolutionary history
  4. organisms on earth

C

What do scientists in the field of systematics accomplish?

  1. discover new fossil sites
  2. organize and classify organisms
  3. name new species
  4. communicate among field biologists

B

Which statement about the taxonomic classification system is correct?

  1. There are more domains than kingdoms.
  2. Kingdoms are the top category of classification.
  3. Classes are divisions of orders.
  4. Subspecies are the most specific category of classification.

D

On a phylogenetic tree, which term refers to lineages that diverged from the same place?

  1. sister taxa
  2. basal taxa
  3. rooted taxa
  4. dichotomous taxa

A

Free Response

How does a phylogenetic tree relate to the passing of time?

The phylogenetic tree shows the order in which evolutionary events took place and in what order certain characteristics and organisms evolved in relation to others. It does not relate to time.

Some organisms that appear very closely related on a phylogenetic tree may not actually be closely related. Why is this?

In most cases, organisms that appear closely related actually are; however, there are cases where organisms evolved through convergence and appear closely related but are not.

List the different levels of the taxonomic classification system.

domain, kingdom, phylum, class, order, family, genus, species

Glossary

basal taxon
branch on a phylogenetic tree that has not diverged significantly from the root ancestor
binomial nomenclature
system of two-part scientific names for an organism, which includes genus and species names
branch point
node on a phylogenetic tree where a single lineage splits into distinct new ones
class
division of phylum in the taxonomic classification system
family
division of order in the taxonomic classification system
genus
division of family in the taxonomic classification system; the first part of the binomial scientific name
kingdom
division of domain in the taxonomic classification system
order
division of class in the taxonomic classification system
phylogenetic tree
diagram used to reflect the evolutionary relationships among organisms or groups of organisms
phylogeny
evolutionary history and relationship of an organism or group of organisms
phylum
(plural: phyla) division of kingdom in the taxonomic classification system
polytomy
branch on a phylogenetic tree with more than two groups or taxa
rooted
single ancestral lineage on a phylogenetic tree to which all organisms represented in the diagram relate
sister taxa
two lineages that diverged from the same branch point
systematics
field of organizing and classifying organisms based on evolutionary relationships
taxon
(plural: taxa) single level in the taxonomic classification system
taxonomy
science of classifying organisms

31 Organizing Life on Earth

By the end of this section, you will be able to do the following:

  • Discuss the need for a comprehensive classification system
  • List the different levels of the taxonomic classification system
  • Describe how systematics and taxonomy relate to phylogeny
  • Discuss a phylogenetic tree’s components and purpose

In scientific terms, phylogeny is the evolutionary history and relationship of an organism or group of organisms. A phylogeny describes the organisim’s relationships, such as from which organisms it may have evolved, or to which species it is most closely related. Phylogenetic relationships provide information on shared ancestry but not necessarily on how organisms are similar or different.


Art Connection

At each sublevel in the taxonomic classification system, organisms become more similar. Dogs and wolves are the same species because they can breed and produce viable offspring, but they are different enough to be classified as different subspecies. (credit “plant”: modification of work by "berduchwal"/Flickr credit “insect”: modification of work by Jon Sullivan credit “fish”: modification of work by Christian Mehlführer credit “rabbit”: modification of work by Aidan Wojtas credit “cat”: modification of work by Jonathan Lidbeck credit “fox”: modification of work by Kevin Bacher, NPS credit “jackal”: modification of work by Thomas A. Hermann, NBII, USGS credit “wolf”: modification of work by Robert Dewar credit “dog”: modification of work by "digital_image_fan"/Flickr)

At what levels are cats and dogs considered to be part of the same group?


Taxonomy (which literally means “arrangement law”) is the science of classifying organisms to construct internationally shared classification systems with each organism placed into more and more inclusive groupings. Think about how a grocery store is organized. One large space is divided into departments, such as produce, dairy, and meats. Then each department further divides into aisles, then each aisle into categories and brands, and then finally a single product. This organization from larger to smaller, more specific categories is called a hierarchical system.

The taxonomic classification system (also called the Linnaean system after its inventor, Carl Linnaeus, a Swedish botanist, zoologist, and physician) uses a hierarchical model. Moving from the point of origin, the groups become more specific, until one branch ends as a single species. For example, after the common beginning of all life, scientists divide organisms into three large categories called a domain: Bacteria, Archaea, and Eukarya. Within each domain is a second category called a kingdom . After kingdoms, the subsequent categories of increasing specificity are: phylum , class , order , family , genus , and species (Figure).

The taxonomic classification system uses a hierarchical model to organize living organisms into increasingly specific categories. The common dog, Canis lupus familiaris, is a subspecies of Canis lupus, which also includes the wolf and dingo. (credit “dog”: modification of work by Janneke Vreugdenhil)

Figure shows how the levels move toward specificity with other organisms. Notice how the dog shares a domain with the widest diversity of organisms, including plants and butterflies. At each sublevel, the organisms become more similar because they are more closely related. Historically, scientists classified organisms using characteristics, but as DNA technology developed, more precise phylogenies have been determined.


Section Summary

Scientists continually gain new information that helps understand the evolutionary history of life on Earth. Each group of organisms went through its own evolutionary journey, called its phylogeny. Each organism shares relatedness with others, and based on morphologic and genetic evidence, scientists attempt to map the evolutionary pathways of all life on Earth. Historically, organisms were organized into a taxonomic classification system. However, today many scientists build phylogenetic trees to illustrate evolutionary relationships.


Organizing Life on Earth

All life on Earth evolved from a common ancestor. Biologists map how organisms are related by constructing phylogenetic trees. In other words, a “tree of life” can be constructed to illustrate when different organisms evolved and to show the relationships among different organisms, as shown in [link]. Notice that from a single point, the three domains of Archaea, Bacteria, and Eukarya diverge and then branch repeatedly. The small branch that plants and animals (including humans) occupy in this diagram shows how recently these groups had their origin compared with other groups.

The phylogenetic tree in [link] illustrates the pathway of evolutionary history. The pathway can be traced from the origin of life to any individual species by navigating through the evolutionary branches between the two points. Also, by starting with a single species and tracing backward to any branch point, the organisms related to it by various degrees of closeness can be identified.

A phylogeny is the evolutionary history and the relationships among a species or group of species. The study of organisms with the purpose of deriving their relationships is called systematics.

Many disciplines within the study of biology contribute to understanding how past and present life evolved over time, and together they contribute to building, updating, and maintaining the “tree of life.” Information gathered may include data collected from fossils, from studying morphology, from the structure of body parts, or from molecular structure, such as the sequence of amino acids in proteins or DNA nucleotides. By considering the trees generated by different sets of data scientists can put together the phylogeny of a species.

Scientists continue to discover new species of life on Earth as well as new character information, thus trees change as new data arrive.

The Levels of Classification

Taxonomy (which literally means “arrangement law”) is the science of naming and grouping species to construct an internationally shared classification system. The taxonomic classification system (also called the Linnaean system after its inventor, Carl Linnaeus, a Swedish naturalist) uses a hierarchical model. A hierarchical system has levels and each group at one of the levels includes groups at the next lowest level, so that at the lowest level each member belongs to a series of nested groups. An analogy is the nested series of directories on the main disk drive of a computer. For example, in the most inclusive grouping, scientists divide organisms into three domains: Bacteria, Archaea, and Eukarya. Within each domain is a second level called a kingdom. Each domain contains several kingdoms. Within kingdoms, the subsequent categories of increasing specificity are: phylum, class, order, family, genus, and species.

As an example, the classification levels for the domestic dog are shown in [link]. The group at each level is called a taxon (plural: taxa). In other words, for the dog, Carnivora is the taxon at the order level, Canidae is the taxon at the family level, and so forth. Organisms also have a common name that people typically use, such as domestic dog, or wolf. Each taxon name is capitalized except for species, and the genus and species names are italicized. Scientists refer to an organism by its genus and species names together, commonly called a scientific name, or Latin name. This two-name system is called binomial nomenclature. The scientific name of the wolf is therefore Canis lupus. Recent study of the DNA of domestic dogs and wolves suggest that the domestic dog is a subspecies of the wolf, not its own species, thus it is given an extra name to indicate its subspecies status, Canis lupus familiaris.

[link] also shows how taxonomic levels move toward specificity. Notice how within the domain we find the dog grouped with the widest diversity of organisms. These include plants and other organisms not pictured, such as fungi and protists. At each sublevel, the organisms become more similar because they are more closely related. Before Darwin’s theory of evolution was developed, naturalists sometimes classified organisms using arbitrary similarities, but since the theory of evolution was proposed in the 19 th century, biologists work to make the classification system reflect evolutionary relationships. This means that all of the members of a taxon should have a common ancestor and be more closely related to each other than to members of other taxa.

Recent genetic analysis and other advancements have found that some earlier taxonomic classifications do not reflect actual evolutionary relationships, and therefore, changes and updates must be made as new discoveries take place. One dramatic and recent example was the breaking apart of prokaryotic species, which until the 1970s were all classified as bacteria. Their division into Archaea and Bacteria came about after the recognition that their large genetic differences warranted their separation into two of three fundamental branches of life.

In what levels are cats and dogs considered to be part of the same group?

Visit this PBS site to learn more about taxonomy. Under Classifying Life, click Launch Interactive.

Classification and Phylogeny

Scientists use a tool called a phylogenetic tree to show the evolutionary pathways and relationships between organisms. A phylogenetic tree is a diagram used to reflect evolutionary relationships among organisms or groups of organisms. The hierarchical classification of groups nested within more inclusive groups is reflected in diagrams. Scientists consider phylogenetic trees to be a hypothesis of the evolutionary past because one cannot go back through time to confirm the proposed relationships.

Unlike with a taxonomic classification, a phylogenetic tree can be read like a map of evolutionary history, as shown in [link]. Shared characteristics are used to construct phylogenetic trees. The point where a split occurs in a tree, called a branch point, represents where a single lineage evolved into distinct new ones. Many phylogenetic trees have a single branch point at the base representing a common ancestor of all the branches in the tree. Scientists call such trees rooted, which means there is a single ancestral taxon at the base of a phylogenetic tree to which all organisms represented in the diagram descend from. When two lineages stem from the same branch point, they are called sister taxa, for example the two species of orangutans. A branch point with more than two groups illustrates a situation for which scientists have not definitively determined relationships. An example is illustrated by the three branches leading to the gorilla subspecies their exact relationships are not yet understood. It is important to note that sister taxa share an ancestor, which does not mean that one taxon evolved from the other. The branch point, or split, represents a common ancestor that existed in the past, but that no longer exists. Humans did not evolve from chimpanzees (nor did chimpanzees evolve from humans) although they are our closest living relatives. Both humans and chimpanzees evolved from a common ancestor that lived, scientists believe, six million years ago and looked different from both modern chimpanzees and modern humans.

The branch points and the branches in phylogenetic tree structure also imply evolutionary change. Sometimes the significant character changes are identified on a branch or branch point. For example, in [link], the branch point that gives rise to the mammal and reptile lineage from the frog lineage shows the origin of the amniotic egg character. Also the branch point that gives rise to organisms with legs is indicated at the common ancestor of mammals, reptiles, amphibians, and jawed fishes.

This interactive exercise allows you to explore the evolutionary relationships among species.

Limitations of Phylogenetic Trees

It is easy to assume that more closely related organisms look more alike, and while this is often the case, it is not always true. If two closely related lineages evolved under significantly different surroundings or after the evolution of a major new adaptation, they may look quite different from each other, even more so than other groups that are not as closely related. For example, the phylogenetic tree in [link] shows that lizards and rabbits both have amniotic eggs, whereas salamanders (within the frog lineage) do not yet on the surface, lizards and salamanders appear more similar than the lizards and rabbits.

Another aspect of phylogenetic trees is that, unless otherwise indicated, the branches do not show length of time, they show only the order in time of evolutionary events. In other words, a long branch does not necessarily mean more time passed, nor does a short branch mean less time passed— unless specified on the diagram. For example, in [link], the tree does not indicate how much time passed between the evolution of amniotic eggs and hair. What the tree does show is the order in which things took place. Again using [link], the tree shows that the oldest trait is the vertebral column, followed by hinged jaws, and so forth. Remember that any phylogenetic tree is a part of the greater whole, and similar to a real tree, it does not grow in only one direction after a new branch develops. So, for the organisms in [link], just because a vertebral column evolved does not mean that invertebrate evolution ceased, it only means that a new branch formed. Also, groups that are not closely related, but evolve under similar conditions, may appear more similar to each other than to a close relative.

Section Summary

Scientists continually obtain new information that helps to understand the evolutionary history of life on Earth. Each group of organisms went through its own evolutionary journey, called its phylogeny. Each organism shares relatedness with others, and based on morphologic and genetic evidence scientists attempt to map the evolutionary pathways of all life on Earth. Historically, organisms were organized into a taxonomic classification system. However, today many scientists build phylogenetic trees to illustrate evolutionary relationships and the taxonomic classification system is expected to reflect evolutionary relationships.

Art Connections

[link] In what levels are cats and dogs considered to be part of the same group?

[link] Cats and dogs are part of the same group at five levels: both are in the domain Eukarya, the kingdom Animalia, the phylum Chordata, the class Mammalia, and the order Carnivora.

Multiple Choice

What is a phylogeny a description of?

What do scientists in the field of systematics accomplish?

  1. discover new fossil sites
  2. organize and classify organisms
  3. name new species
  4. communicate between field biologists

Which statement about the taxonomic classification system is correct?

  1. There are more domains than kingdoms.
  2. Kingdoms are the top category of classification.
  3. A phylum may be represented in more than one kingdom.
  4. Species are the most specific category of classification.

Which best describes the relationship between chimpanzees and humans?

  1. chimpanzees evolved from humans
  2. humans evolved from chimpanzees
  3. chimpanzees and humans evolved from a common ancestor
  4. chimpanzees and humans belong to the same species

Which best describes a branch point in a phylogenetic tree?

Free Response

How does a phylogenetic tree indicate major evolutionary events within a lineage?

The phylogenetic tree shows the order in which evolutionary events took place and in what order certain characteristics and organisms evolved in relation to others. It does not generally indicate time durations.

List the different levels of the taxonomic classification system.

Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species.

Glossary


Glossary

Basal taxon

branch on a phylogenetic tree that has not diverged significantly from the root ancestor

Binomial nomenclature

system of two-part scientific names for an organism, which includes genus and species names

Branch point

node on a phylogenetic tree where a single lineage splits into distinct new ones

Class

division of phylum in the taxonomic classification system

Family

division of order in the taxonomic classification system

Genus

division of family in the taxonomic classification system the first part of the binomial scientific name

Kingdom

division of domain in the taxonomic classification system

Order

division of class in the taxonomic classification system

Phylogenetic tree

diagram used to reflect the evolutionary relationships among organisms or groups of organisms

Phylogeny

evolutionary history and relationship of an organism or group of organisms

Phylum

(plural: phyla) division of kingdom in the taxonomic classification system

Polytomy

branch on a phylogenetic tree with more than two groups or taxa

Rooted

single ancestral lineage on a phylogenetic tree to which all organisms represented in the diagram relate

Sister taxa

two lineages that diverged from the same branch point

Systematics

field of organizing and classifying organisms based on evolutionary relationships


Art Connection

At each sublevel in the taxonomic classification system, organisms become more similar. Dogs and wolves are the same species because they can breed and produce viable offspring, but they are different enough to be classified as different subspecies. (credit “plant”: modification of work by "berduchwal"/Flickr credit “insect”: modification of work by Jon Sullivan credit “fish”: modification of work by Christian Mehlführer credit “rabbit”: modification of work by Aidan Wojtas credit “cat”: modification of work by Jonathan Lidbeck credit “fox”: modification of work by Kevin Bacher, NPS credit “jackal”: modification of work by Thomas A. Hermann, NBII, USGS credit “wolf”: modification of work by Robert Dewar credit “dog”: modification of work by "digital_image_fan"/Flickr)

At what levels are cats and dogs part of the same group?


How is life on earth organized?

Assuming you are referring to taxonomy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

Explanation:

Taxonomy, the study of organizing life on Earth, classifies organisms by their similar traits. The broadest category, the Domains, can each contain millions of species. There are 3: Archea, Bacteria, and Eukarya.

Archea is made up of prokaryotic (a more primitive type of cell) cells without nuclei, that are extremophiles, or organisms that survive in extreme environments like volcanic vents, or under sheets of ice.

Bacteria is also made up of prokaryotic cells without nuclei, only more conventional/common forms.

Eukarya, however, is made up of eukaryotic (more modern) cells, which have nuclei. This includes multicellular organisms (organisms with more than one cell) and unicellular organisms (organisms that are only one cell).

The second-broadest category is Kingdom. For example, in Eukarya, there are four kingdoms: Protista, Fungi, Plantae, and Animalia, which contain protists, fungi, plants, and animal respectively.

Each kingdom is further divided into phyla (singular: phylum), which are divided into classes, which are divided into orders, which are divided into families, which are divided into genera (singular: genus), which are divided into single species.

A species name is the name of its genus, followed by the name of its individual species.

An example: the domestic cat. This example includes why it is in a particular category, as well as the names of said categories.

Domain - Eukarya (It is made of more than one eukaryotic cell)
Kingdom - Animalia (It is an animal)
Phylum - Chordata (It has a backbone)
Class - Mammalia (It is a mammal)
Order - Carnivora (It is a carnivore [eats only meat])
Family - Felidae (It is a type of feline)
Genus - Felis (It is a small feline)
Species - Catus


Explainer: Earth — layer by layer

Scientists understand much about Earth’s structural layers — the inner core, core, mantle and crust. Yet there are still great mysteries to solve about our planet’s inner workings.

Yuri_Arcurs/iStock/Getty Images Plus

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November 11, 2019 at 6:45 am

Mountain ranges tower to the sky. Oceans plummet to impossible depths. Earth’s surface is an amazing place to behold. Yet even the deepest canyon is but a tiny scratch on the planet. To really understand Earth, you need to travel 6,400 kilometers (3,977 miles) beneath our feet.

Starting at the center, Earth is composed of four distinct layers. They are, from deepest to shallowest, the inner core, the outer core, the mantle and the crust. Except for the crust, no one has ever explored these layers in person. In fact, the deepest humans have ever drilled is just over 12 kilometers (7.6 miles). And even that took 20 years!

Still, scientists know a great deal about Earth’s inner structure. They’ve plumbed it by studying how earthquake waves travel through the planet. The speed and behavior of these waves change as they encounter layers of different densities. Scientists — including Isaac Newton, three centuries ago — have also learned about the core and mantle from calculations of Earth’s total density, gravitational pull and magnetic field.

Here’s a primer on Earth’s layers, starting with a journey to the center of the planet.

A cut-away of Earth’s layers reveals how thin the crust is when compared to the lower layers. USGS

The inner core

This solid metal ball has a radius of 1,220 kilometers (758 miles), or about three-quarters that of the moon. It’s located some 6,400 to 5,180 kilometers (4,000 to 3,220 miles) beneath Earth’s surface. Extremely dense, it’s made mostly of iron and nickel. The inner core spins a bit faster than the rest of the planet. It’s also intensely hot: Temperatures sizzle at 5,400° Celsius (9,800° Fahrenheit). That’s almost as hot as the surface of the sun. Pressures here are immense: well over 3 million times greater than on Earth’s surface. Some research suggests there may also be an inner, inner core. It would likely consist almost entirely of iron.

The outer core

This part of the core is also made from iron and nickel, just in liquid form. It sits some 5,180 to 2,880 kilometers (3,220 to 1,790 miles) below the surface. Heated largely by the radioactive decay of the elements uranium and thorium, this liquid churns in huge, turbulent currents. That motion generates electrical currents. They, in turn, generate Earth’s magnetic field. For reasons somehow related to the outer core, Earth’s magnetic field reverses about every 200,000 to 300,000 years. Scientists are still working to understand how that happens.

The mantle

At close to 3,000 kilometers (1,865 miles) thick, this is Earth’s thickest layer. It starts a mere 30 kilometers (18.6 miles) beneath the surface. Made mostly of iron, magnesium and silicon, it is dense, hot and semi-solid (think caramel candy). Like the layer below it, this one also circulates. It just does so far more slowly.

Explainer: How heat moves

Near its upper edges, somewhere between about 100 and 200 kilometers (62 to 124 miles) underground, the mantle’s temperature reaches the melting point of rock. Indeed, it forms a layer of partially melted rock known as the asthenosphere (As-THEEN-oh-sfeer). Geologists believe this weak, hot, slippery part of the mantle is what Earth’s tectonic plates ride upon and slide across.

Diamonds are tiny pieces of the mantle we can actually touch. Most form at depths above 200 kilometers (124 miles). But rare “super-deep” diamonds may have formed as far down as 700 kilometers (435 miles) below the surface. These crystals are then brought to the surface in volcanic rock known as kimberlite.

The mantle’s outermost zone is relatively cool and rigid. It behaves more like the crust above it. Together, this uppermost part of the mantle layer and the crust are known as the lithosphere.

The thickest part of Earth’s crust is about 70 kilometers (43 miles) thick and lies under the Himalayan Mountains, seen here. den-belitsky/iStock/Getty Images Plus

The crust

Earth’s crust is like the shell of a hard-boiled egg. It is extremely thin, cold and brittle compared to what lies below it. The crust is made of relatively light elements, especially silica, aluminum and oxygen. It’s also highly variable in its thickness. Under the oceans (and Hawaiian Islands), it may be as little as 5 kilometers (3.1 miles) thick. Beneath the continents, the crust may be 30 to 70 kilometers (18.6 to 43.5 miles) thick.

Along with the upper zone of the mantle, the crust is broken into big pieces, like a gigantic jigsaw puzzle. These are known as tectonic plates. These move slowly — at just 3 to 5 centimeters (1.2 to 2 inches) per year. What drives the motion of tectonic plates is still not fully understood. It may be related to heat-driven convection currents in the mantle below. Some scientists think it’s caused by the tug from slabs of crust of different densities, something called “slab pull.” In time, these plates will converge, pull apart or slide past each other. Those actions cause most earthquakes and volcanoes. It’s a slow ride, but it makes for exciting times here on Earth’s surface.

Power Words

aluminum A metallic element, the third most abundant in Earth’s crust. It is light and soft, and used in many items from bicycles to spacecraft.

behavior The way something, often a person or other organism, acts towards others, or conducts itself.

continent (in geology) The huge land masses that sit upon tectonic plates. In modern times, there are six established geologic continents: North America, South America, Eurasia, Africa, Australia and Antarctica. In 2017, scientists also made the case for yet another: Zealandia.

convection The rising and falling of material in a fluid or gas due to uneven temperatures. This process occurs in the outer layers of some stars.

core Something — usually round-shaped — in the center of an object. (in geology) Earth’s innermost layer. Or, a long, tube-like sample drilled down into ice, soil or rock. Cores allow scientists to examine layers of sediment, dissolved chemicals, rock and fossils to see how the environment at one location changed through hundreds to thousands of years or more.

crust (in geology) Earth’s outermost surface, usually made from dense, solid rock.

crystal (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.

current A fluid — such as of water or air — that moves in a recognizable direction. (in electricity) The flow of electricity or the amount of charge moving through some material over a particular period of time.

decay (for radioactive materials) The process whereby a radioactive isotope — which means a physically unstable form of some element — sheds energy and subatomic particles. In time, this shedding will transform the unstable element into a slightly different but stable element. For instance, uranium-238 (which is a radioactive, or unstable, isotope) decays to radium-222 (also a radioactive isotope), which decays to radon-222 (also radioactive), which decays to polonium-210 (also radioactive), which decays to lead-206 — which is stable. No further decay occurs. The rates of decay from one isotope to another can range from timeframes of less than a second to billions of years.

density The measure of how condensed some object is, found by dividing its mass by its volume.

diamond One of the hardest known substances and rarest gems on Earth. Diamonds form deep within the planet when carbon is compressed under incredibly strong pressure.

earthquake A sudden and sometimes violent shaking of the ground, sometimes causing great destruction, as a result of movements within Earth’s crust or of volcanic action.

Earth’s crust The outermost layer of Earth. It is relatively cold and brittle.

element A building block of some larger structure. (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

field An area of study, as in: Her field of research was biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory. (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).

iron A metallic element that is common within minerals in Earth’s crust and in its hot core. This metal also is found in cosmic dust and in many meteorites.

Isaac Newton This English physicist and mathematician became most famous for describing his law of gravity. Born in 1642, he developed into a scientist with wide-ranging interests. Among some of his discoveries: that white light is made from a combination of all the colors in the rainbow, which can be split apart again using a prism the mathematics that describe the orbital motions of things around a center of force that the speed of sound waves can be calculated from the density of air early elements of the mathematics now known as calculus and an explanation for why things “fall:” the gravitational pull of one object towards another, which would be proportional to the mass of each. Newton died in 1727.

lithosphere The upper layer of Earth, which includes its thin brittle crust and upper mantle. The lithosphere is relatively rigid and is broken into slowly moving tectonic plates.

magnesium A metallic element that is number 12 on the periodic table. It burns with a white light and is the eighth most abundant element in Earth’s crust.

magnetic field An area of influence created by certain materials, called magnets, or by the movement of electric charges.

mantle (in geology) The thick layer of the Earth beneath its outer crust. The mantle is semi-solid and generally divided into an upper and lower mantle.

metal Something that conducts electricity well, tends to be shiny (reflective) and malleable (meaning it can be reshaped with heat and not too much force or pressure).

moon The natural satellite of any planet.

nickel Number 28 on the periodic table of elements, this hard, silvery element resists oxidation and corrosion. That makes it a good coating for many other elements or for use in multi-metal alloys.

oxygen A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

pressure Force applied uniformly over a surface, measured as force per unit of area.

radioactive decay A process by which an element is converted into a lighter element through the shedding of subatomic particles (and energy).

radius A straight line from the center to the circumference of a circle or sphere.

range The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists.

semi An adjective meaning “somewhat.”

shell The protective, hard outer covering of mollusk or crustacean, such as a mussel or crab.

silica A mineral, also known as silicon dioxide, containing silicon and oxygen atoms. It is a basic building block of much of the rocky material on Earth and of some construction materials, including glass.

silicon A nonmetal, semiconducting element used in making electronic circuits. Pure silicon exists in a shiny, dark-gray crystalline form and as a shapeless powder.

slide In microscopy, the piece of glass onto which something will be attached for viewing under the device’s magnifying lens.

solid Firm and stable in shape not liquid or gaseous.

sun The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.

tectonic plates The gigantic slabs — some spanning thousands of kilometers (or miles) across — that make up Earth’s outer layer.

thorium A naturally radioactive element which appears as a silvery metal when it is pure. It reacts chemically with air, turning black on its surface. It is found in some minerals, and can be used to trace the source of some mineral grains that are carried long distances by water or wind. Its scientific symbol is Th.

turbulent (n. turbulence) An adjective for the unpredictable fluctuation of a fluid (including air) in which its velocity varies irregularly instead of maintaining a steady or calm flow.

uranium The heaviest naturally occurring element known. It’s called element 92, which refers to the number of protons in its nucleus. Uranium atoms are radioactive, which means they decay into different atomic nuclei.

wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.


Watch the video: Chapter 25 The History of Life on Earth (December 2021).