The Diversity of Life The classification system helps us make sense of and order living things. The largest classification level is a kingdom. However, the kingdom category has been highly unstable. We used to think there were five recognized kingdoms—Bacteria, Plants, Fungi, Protists, and Animals. Then, scientists discovered that archaea were very different from Bacteria, so they pulled them out of Bacteria and placed them in their own sixth kingdom. Then, it was recognized that Archaea and Bacteria are on a similar level as Eukaryotes, so that made for a logical division of all life into three domains: Archaea, Bacteria, and Eukaryota. But now—how many kingdoms? One of the latest ideas for grouping of kingdoms is: Archaea Eubacteria Protista Fungi Plantae Animalia An image of the tree of life is complicated and complex.The tree of life can be depicted in many ways. This image describes it in a circle shape with Bacteria, Archaea, and Eukarya. However, there are two fundamental problems with this system. To visualize these problems, consider the tree of life figure again. Ask yourself: If Fungi are a kingdom (or Animals, or Plants . . . ), then how is it that the vast diversity of bacteria aren't subdivided into multiple kingdoms? Shouldn't at least the Proteobacteria be in their own kingdom? Maybe, but remember, the classification of higher taxa is only meant for human convenience, and it can be inconvenient to memorize 15 to 20 kingdoms. Microbiologists don't lump all bacteria into a single kingdom, but for this level of study, it isn't against the rules. As long as a group is monophyletic, you can give it a name. There lies the second problem with the six-kingdom system, and that one is fatal. All named groups must be monophyletic. Looking at the tree of life figure, which one is not monophyletic? Protists. If we insist that animals and fungi are in their own kingdoms, then there's a minimum of seven eukaryotic kingdoms. Unfortunately, there's nothing about the group "Protista" that explains what they are. There are defining characteristics that separate the other kingdoms, and the next sections will provide more detail on what these kingdoms encompass. You'll have a section on protists with the understanding that they're no longer considered a kingdom. Among the many characteristics that are commonly used to distinguish one kingdom from another are type of cell, food source, and type of reproduction. There are prokaryotic and eukaryotic cells and plant and animal cells. Prokaryotic cells don't have a nucleus, and eukaryotic cells do. Plant cells have a cell wall, and animal cells don't. Some organisms produce their own food and are known as producers, while others have to rely on other organisms for food and are known as consumers. For reproduction, organisms rely on either sexual or asexual reproduction. Sexual reproduction leads to increased diversity among organisms, while asexual reproduction relies on other mechanisms of gene transfer to provide the necessary diversity for survival. Archaea As a domain, Archaea has only been known about since 1977. The scientist team that discovered this kingdom was led by Carl Woese (1928–2012). Developments in DNA sequencing allowed for the identification of Archaea as a separate domain. This allowed for recognition that within prokaryotic organisms, there were actually two completely separate types of organisms. Archaea are phylogenetically distinct from bacteria. Like bacteria, however, they're all single-celled organisms. Archaea can live in extremely inhospitable habitats, such as hot springs, deep in oceans, or in extremely salty environments. They can live where no other organism could, such as in petroleum. This doesn't mean that archaea have to live in extreme environments. They also can live in normal environments where many other organisms are. When they live in the digestive tracts of organisms, they can produce methane, which may be associated with gastrointestinal disorders. Archaea can take on many different types of shapes. They're all microscopic. Under the microscope, bacteria cells and archaea cells look different and are made of different things. Their DNA can distinguish them. There's a common misconception that all microscopic organisms are harmful to us. However, many species of archaea and bacteria are beneficial to humans and other organisms. Archaea don't produce their own food, but they can survive on food sources such as hydrogen or ammonia, which no other organism can do. There's a lot more that has yet to be discovered about archaea. Eubacteria Eubacteria, or "true bacteria," is the other domain of prokaryotic life. Eubacteria exist everywhere but are microscopic. These bacteria are more commonly known as simply bacteria. However, before 1977, archaea were thought to be bacteria, so to distinguish the two types of prokaryotes, the name kingdom name of Eubacteria was proposed. We often think of bacteria as harmful organisms that make us sick. While there are many types of bacteria that do this, there are also beneficial and even essential ones. We carry bacteria on our skin, in our mouths, and in our digestive systems. Prokaryotic cells lack many of the structures found in eukaryotic cells. They don't have mitochondria, chloroplasts, or a nucleus. Prokaryotic cells do have cell walls, though, and they reproduce asexually via binary fission. In binary fission, the parent cell copies all of its DNA and then splits into two identical cells. There are two main types of bacteria: gram-negative and gram-positive. Each is named one of these based on its ability to be stained by a specific dye. There are three main shapes bacteria can take—round, spiral, or rod. Some bacteria need oxygen to live, while other bacteria can't tolerate oxygen. Some bacteria can make their own food (such as cyanobacteria), while others are dependent on external organisms to provide them with food. Almost all bacteria exhibit movement. Many use flagella to move. There are three main groups of bacteria, and they're separated primarily based on how they move and the parts they have to move. Protista The organisms found within the formerly named group "Protista" are very diverse, as you would expect from a group that doesn't share a common ancestor and is defined simply by what it isn't—a plant, fungus, or animal. Protists are all eukaryotic and have a nucleus. Because of their diversity, some protists resemble plants, while others resemble animals or fungi. Protista encompass all of the living organisms that don't fit into another kingdom. Just like bacteria and archaea, some protists are beneficial, while others are harmful. Some protists are visible to the naked eye, while others are only visible with a microscope. Most of these organisms require oxygen to survive. There are a few that live only in conditions without oxygen. Protists vary in how they produce food. Some can make their own food through photosynthesis, while others need to either ingest or absorb nutrients. They primarily reproduce asexually. Prior to the use of DNA and other internal characteristics, scientists relied on what organisms looked like to classify them. Because of this, the Protista kingdom was broadly split into organisms called protozoans and protists. Protozoans resembled animals, and protists resembled plants. Our understanding of these organisms is constantly evolving. As scientists learn more about them, their classifications will certainly change. Looking at the previous tree of life figure, test your understanding of only naming monophyletic groups by considering how you might divide eukaryotes into kingdoms. Moving up the tree, you can see that Excavata and Rhyzaria are separate and can't be combined into a monophyletic group. So, that would be two kingdoms. Then, you could group Alveolata and Chromista together into a kingdom (that's three). Green Algae and Plants could be a kingdom (four), then Amoebozoa (five), and then Animals and Fungi (six and seven, respectively). That gives you seven kingdoms of eukaryotes, and that's just one way of grouping them. If you wanted plants separate from green algae, you would need eight kingdoms. Classifications may vary, as they're used for our convenience. Thus, whether there are seven, eight, or more kingdoms of eukaryotes doesn't matter as long as every named group is monophyletic. Fungi About 100,000 species of fungi have been identified. Scientists estimate that there are at least one million more species yet to be discovered.2 There's extreme diversity in the appearance of fungi. These organisms occupy every part of our ecosystem. Many fungi live in a symbiotic relationship with other organisms. The relationships fungi have with other organisms can be positive for both organisms (mutualistic), positive for one and neutral for the other (commensalistic), or positive for one and negative for the other (parasitic). Some fungi cause disease. All fungi must have certain characteristics to be classified in this kingdom: They must be eukaryotic. They must be nonvascular. They must reproduce sexually or asexually. They must not move. They must have a cell wall composed of chitin. They must be heterotrophic. There are four different groups of fungi: chytrids, sac fungi, club fungi, and zygomycota (bread molds). Chytrids live primarily in water. They can reproduce sexually or asexually and get their food through decomposing matter or parasitism. Sac fungi reproduce both sexually and asexually. The majority of fungi are multicellular sac fungi. Club fungi reproduce sexually. They're called "club fungi" because, during reproduction, they form a structure that looks like a club. The mushrooms we eat are a type of club fungi. Zygomycota can reproduce sexually or asexually. Many of these four groups of fungi have similar sporangia, but that isn't a hard and fast rule. Again, these categories may change as scientists learn more about their phylogeny. Even though fungi are heterotrophic organisms that can't produce their own food, they don't have to excrete waste. The nutrients they ingest are passed from their cell wall to their internal hyphae. An image of each type of fungi. The types of fungi are, from left, first row: threadlike and sac; second row: club and imperfect. (Images by Jason Hollinger and Henry Mühlpfordt [Club Fungi and mold on bread]. [CC BY 2.0], via Wikimedia Commons.) Beneficial fungi are essential in the breakdown of dead matter and aid in decomposition. Without fungi, many materials wouldn't be able to decompose. Humans and other animals can eat some types of fungi. One such example of this is a fungus that breaks down leaves into a form that leafcutter ants can ingest. Sometimes, fungi can be beneficial to humans but harmful to another organism. Fungi are the source of many antibiotics that help prevent the spread of pathogenic bacteria. Fungi can be pathogenic parasites in both animals and plants. Plants may become moldy or develop ergot (rot). Animal diseases can be mild or severe. Mild diseases include ringworm or athlete's foot. More severe fungal infections are systemic and impact our organs and tissues. There are countless ways that fungi can exist and thrive. This makes them hard to categorize or fit into neat boxes. Some types of fungi need oxygen, and others can only survive where there's no oxygen. Unlike plants, fungi are incapable of producing their own food. Fungi differ from animals in that they have to digest food before they can absorb it. Fungi are incredibly complex, diverse, and unique. People who train to become fungi scientists are known as mycologists. They have a very important job to describe and classify the multiple species of fungi that have yet to be discovered. These scientists can work in a variety of fields and are vital to the development of new medicine. To become a mycologist, you would go to graduate school and study under a professor who is an expert in mycology. Plantae Plantae is a kingdom that's very familiar to us. Many of us have plants or trees either inside or just outside our homes. Plants are the second most diverse set of organisms and have existed for hundreds of millions of years. An underlying characteristic unique to plants is a process known as the alternation of generations. This refers to the two distinct lifecycles every land plant experiences. Plants reproduce sexually and asexually and produce their own food. By using carbon to make sugar and releasing oxygen as a waste product, they're vital for helping to keep a balance of air on Earth. A select subset of plants is carnivorous and captures insects in addition to producing its own food. An example of this type of plant is a Venus flytrap. This plant lives in nitrogen-poor environments and only uses the insects it captures as nitrogen fertilizer. For plants to get nutrients to their parts, they have to transport them somehow. Plants can be vascular or nonvascular. Nonvascular plants don't have a way to transport nutrients from the ground to the parts of the plant. This means that they have to live low to the ground, close to moisture or water. Vascular plants have the internal structures needed to distribute nutrients and water throughout the plant. Plants can reproduce either sexually or asexually. Sexual reproduction involves the merging of egg and sperm cells (gametes). In asexual reproduction, the genes from parent to offspring are identical. There are multiple ways that asexual reproduction can happen, such as cutting or budding. Sexual reproduction in plants can occur through pollination, in which plants depend on other organisms or the wind to carry their genetic material to another plant. Animalia Animalia is a kingdom that will be most familiar to you since humans (Homo sapiens) are animals. There are broad characteristics that separate the animal kingdom from any other kingdom: Animals are multicellular. Animals are heterotrophs. Animals share the same kinds of extracellular molecules, such as collagen. Animal cells look different than plant cells in that they don't have a hard cell wall on the outside. The outside of animal cells is cellular membrane. Animals also have an increasing complexity to their bodies. They start with cells, and then these cells are organized into tissues, organs, and organ systems. There are a few exceptions to this general order of life because some animals don't have tissues, organs, or organ systems. Animals can move quickly in response to their environments. For most animals, reproduction is sexual, with one copy of each gene coming from each parent. When an organism has two copies of its autosomal genetic material, this is known as a diploid. The sex chromosomes are a bit different because males have only one copy of the Y chromosome. The chromosomes in the female are XX, and the chromosomes in the male are XY. Animals have similar embryonic development. They almost all start with a few cells clumped together, known as a zygote. Then, a small ball of cells known as a blastula forms, followed by a gastrula. The image here tracks this development. An image of the process of embryonic development from zygote to gastrula for animals. All animals have some kind of embryonic development, starting from a zygote and evolving into a gastrula (Image by CNX OpenStax [Embryonic Development]. [CC BY 4.0], via Wikimedia Commons). There are multiple classes of organisms in the animal kingdom. The most populated class is insects. We actually don't know how many species of animals are on Earth, and it's likely that millions of animals have yet to be discovered! 2. OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016. Download for free at http://cnx.org/contents/b3c1e1d2-839c-42b0-a314-e119a8aafbdd@14.1. The Diversity of Life The classification system helps us make sense of and order living things. The largest classification level is a kingdom. However, the kingdom category has been highly unstable. We used to think there were five recognized kingdoms—Bacteria, Plants, Fungi, Protists, and Animals. Then, scientists discovered that archaea were very different from Bacteria, so they pulled them out of Bacteria and placed them in their own sixth kingdom. Then, it was recognized that Archaea and Bacteria are on a similar level as Eukaryotes, so that made for a logical division of all life into three domains: Archaea, Bacteria, and Eukaryota. But now—how many kingdoms? One of the latest ideas for grouping of kingdoms is: Archaea Eubacteria Protista Fungi Plantae Animalia An image of the tree of life is complicated and complex.The tree of life can be depicted in many ways. This image describes it in a circle shape with Bacteria, Archaea, and Eukarya. However, there are two fundamental problems with this system. To visualize these problems, consider the tree of life figure again. Ask yourself: If Fungi are a kingdom (or Animals, or Plants . . . ), then how is it that the vast diversity of bacteria aren't subdivided into multiple kingdoms? Shouldn't at least the Proteobacteria be in their own kingdom? Maybe, but remember, the classification of higher taxa is only meant for human convenience, and it can be inconvenient to memorize 15 to 20 kingdoms. Microbiologists don't lump all bacteria into a single kingdom, but for this level of study, it isn't against the rules. As long as a group is monophyletic, you can give it a name. There lies the second problem with the six-kingdom system, and that one is fatal. All named groups must be monophyletic. Looking at the tree of life figure, which one is not monophyletic? Protists. If we insist that animals and fungi are in their own kingdoms, then there's a minimum of seven eukaryotic kingdoms. Unfortunately, there's nothing about the group "Protista" that explains what they are. There are defining characteristics that separate the other kingdoms, and the next sections will provide more detail on what these kingdoms encompass. You'll have a section on protists with the understanding that they're no longer considered a kingdom. Among the many characteristics that are commonly used to distinguish one kingdom from another are type of cell, food source, and type of reproduction. There are prokaryotic and eukaryotic cells and plant and animal cells. Prokaryotic cells don't have a nucleus, and eukaryotic cells do. Plant cells have a cell wall, and animal cells don't. Some organisms produce their own food and are known as producers, while others have to rely on other organisms for food and are known as consumers. For reproduction, organisms rely on either sexual or asexual reproduction. Sexual reproduction leads to increased diversity among organisms, while asexual reproduction relies on other mechanisms of gene transfer to provide the necessary diversity for survival. Archaea As a domain, Archaea has only been known about since 1977. The scientist team that discovered this kingdom was led by Carl Woese (1928–2012). Developments in DNA sequencing allowed for the identification of Archaea as a separate domain. This allowed for recognition that within prokaryotic organisms, there were actually two completely separate types of organisms. Archaea are phylogenetically distinct from bacteria. Like bacteria, however, they're all single-celled organisms. Archaea can live in extremely inhospitable habitats, such as hot springs, deep in oceans, or in extremely salty environments. They can live where no other organism could, such as in petroleum. This doesn't mean that archaea have to live in extreme environments. They also can live in normal environments where many other organisms are. When they live in the digestive tracts of organisms, they can produce methane, which may be associated with gastrointestinal disorders. Archaea can take on many different types of shapes. They're all microscopic. Under the microscope, bacteria cells and archaea cells look different and are made of different things. Their DNA can distinguish them. There's a common misconception that all microscopic organisms are harmful to us. However, many species of archaea and bacteria are beneficial to humans and other organisms. Archaea don't produce their own food, but they can survive on food sources such as hydrogen or ammonia, which no other organism can do. There's a lot more that has yet to be discovered about archaea. Eubacteria Eubacteria, or "true bacteria," is the other domain of prokaryotic life. Eubacteria exist everywhere but are microscopic. These bacteria are more commonly known as simply bacteria. However, before 1977, archaea were thought to be bacteria, so to distinguish the two types of prokaryotes, the name kingdom name of Eubacteria was proposed. We often think of bacteria as harmful organisms that make us sick. While there are many types of bacteria that do this, there are also beneficial and even essential ones. We carry bacteria on our skin, in our mouths, and in our digestive systems. Prokaryotic cells lack many of the structures found in eukaryotic cells. They don't have mitochondria, chloroplasts, or a nucleus. Prokaryotic cells do have cell walls, though, and they reproduce asexually via binary fission. In binary fission, the parent cell copies all of its DNA and then splits into two identical cells. There are two main types of bacteria: gram-negative and gram-positive. Each is named one of these based on its ability to be stained by a specific dye. There are three main shapes bacteria can take—round, spiral, or rod. Some bacteria need oxygen to live, while other bacteria can't tolerate oxygen. Some bacteria can make their own food (such as cyanobacteria), while others are dependent on external organisms to provide them with food. Almost all bacteria exhibit movement. Many use flagella to move. There are three main groups of bacteria, and they're separated primarily based on how they move and the parts they have to move. Protista The organisms found within the formerly named group "Protista" are very diverse, as you would expect from a group that doesn't share a common ancestor and is defined simply by what it isn't—a plant, fungus, or animal. Protists are all eukaryotic and have a nucleus. Because of their diversity, some protists resemble plants, while others resemble animals or fungi. Protista encompass all of the living organisms that don't fit into another kingdom. Just like bacteria and archaea, some protists are beneficial, while others are harmful. Some protists are visible to the naked eye, while others are only visible with a microscope. Most of these organisms require oxygen to survive. There are a few that live only in conditions without oxygen. Protists vary in how they produce food. Some can make their own food through photosynthesis, while others need to either ingest or absorb nutrients. They primarily reproduce asexually. Prior to the use of DNA and other internal characteristics, scientists relied on what organisms looked like to classify them. Because of this, the Protista kingdom was broadly split into organisms called protozoans and protists. Protozoans resembled animals, and protists resembled plants. Our understanding of these organisms is constantly evolving. As scientists learn more about them, their classifications will certainly change. Looking at the previous tree of life figure, test your understanding of only naming monophyletic groups by considering how you might divide eukaryotes into kingdoms. Moving up the tree, you can see that Excavata and Rhyzaria are separate and can't be combined into a monophyletic group. So, that would be two kingdoms. Then, you could group Alveolata and Chromista together into a kingdom (that's three). Green Algae and Plants could be a kingdom (four), then Amoebozoa (five), and then Animals and Fungi (six and seven, respectively). That gives you seven kingdoms of eukaryotes, and that's just one way of grouping them. If you wanted plants separate from green algae, you would need eight kingdoms. Classifications may vary, as they're used for our convenience. Thus, whether there are seven, eight, or more kingdoms of eukaryotes doesn't matter as long as every named group is monophyletic. Fungi About 100,000 species of fungi have been identified. Scientists estimate that there are at least one million more species yet to be discovered.2 There's extreme diversity in the appearance of fungi. These organisms occupy every part of our ecosystem. Many fungi live in a symbiotic relationship with other organisms. The relationships fungi have with other organisms can be positive for both organisms (mutualistic), positive for one and neutral for the other (commensalistic), or positive for one and negative for the other (parasitic). Some fungi cause disease. All fungi must have certain characteristics to be classified in this kingdom: They must be eukaryotic. They must be nonvascular. They must reproduce sexually or asexually. They must not move. They must have a cell wall composed of chitin. They must be heterotrophic. There are four different groups of fungi: chytrids, sac fungi, club fungi, and zygomycota (bread molds). Chytrids live primarily in water. They can reproduce sexually or asexually and get their food through decomposing matter or parasitism. Sac fungi reproduce both sexually and asexually. The majority of fungi are multicellular sac fungi. Club fungi reproduce sexually. They're called "club fungi" because, during reproduction, they form a structure that looks like a club. The mushrooms we eat are a type of club fungi. Zygomycota can reproduce sexually or asexually. Many of these four groups of fungi have similar sporangia, but that isn't a hard and fast rule. Again, these categories may change as scientists learn more about their phylogeny. Even though fungi are heterotrophic organisms that can't produce their own food, they don't have to excrete waste. The nutrients they ingest are passed from their cell wall to their internal hyphae. An image of each type of fungi. The types of fungi are, from left, first row: threadlike and sac; second row: club and imperfect. (Images by Jason Hollinger and Henry Mühlpfordt [Club Fungi and mold on bread]. [CC BY 2.0], via Wikimedia Commons.) Beneficial fungi are essential in the breakdown of dead matter and aid in decomposition. Without fungi, many materials wouldn't be able to decompose. Humans and other animals can eat some types of fungi. One such example of this is a fungus that breaks down leaves into a form that leafcutter ants can ingest. Sometimes, fungi can be beneficial to humans but harmful to another organism. Fungi are the source of many antibiotics that help prevent the spread of pathogenic bacteria. Fungi can be pathogenic parasites in both animals and plants. Plants may become moldy or develop ergot (rot). Animal diseases can be mild or severe. Mild diseases include ringworm or athlete's foot. More severe fungal infections are systemic and impact our organs and tissues. There are countless ways that fungi can exist and thrive. This makes them hard to categorize or fit into neat boxes. Some types of fungi need oxygen, and others can only survive where there's no oxygen. Unlike plants, fungi are incapable of producing their own food. Fungi differ from animals in that they have to digest food before they can absorb it. Fungi are incredibly complex, diverse, and unique. People who train to become fungi scientists are known as mycologists. They have a very important job to describe and classify the multiple species of fungi that have yet to be discovered. These scientists can work in a variety of fields and are vital to the development of new medicine. To become a mycologist, you would go to graduate school and study under a professor who is an expert in mycology. Plantae Plantae is a kingdom that's very familiar to us. Many of us have plants or trees either inside or just outside our homes. Plants are the second most diverse set of organisms and have existed for hundreds of millions of years. An underlying characteristic unique to plants is a process known as the alternation of generations. This refers to the two distinct lifecycles every land plant experiences. Plants reproduce sexually and asexually and produce their own food. By using carbon to make sugar and releasing oxygen as a waste product, they're vital for helping to keep a balance of air on Earth. A select subset of plants is carnivorous and captures insects in addition to producing its own food. An example of this type of plant is a Venus flytrap. This plant lives in nitrogen-poor environments and only uses the insects it captures as nitrogen fertilizer. For plants to get nutrients to their parts, they have to transport them somehow. Plants can be vascular or nonvascular. Nonvascular plants don't have a way to transport nutrients from the ground to the parts of the plant. This means that they have to live low to the ground, close to moisture or water. Vascular plants have the internal structures needed to distribute nutrients and water throughout the plant. Plants can reproduce either sexually or asexually. Sexual reproduction involves the merging of egg and sperm cells (gametes). In asexual reproduction, the genes from parent to offspring are identical. There are multiple ways that asexual reproduction can happen, such as cutting or budding. Sexual reproduction in plants can occur through pollination, in which plants depend on other organisms or the wind to carry their genetic material to another plant. Animalia Animalia is a kingdom that will be most familiar to you since humans (Homo sapiens) are animals. There are broad characteristics that separate the animal kingdom from any other kingdom: Animals are multicellular. Animals are heterotrophs. Animals share the same kinds of extracellular molecules, such as collagen. Animal cells look different than plant cells in that they don't have a hard cell wall on the outside. The outside of animal cells is cellular membrane. Animals also have an increasing complexity to their bodies. They start with cells, and then these cells are organized into tissues, organs, and organ systems. There are a few exceptions to this general order of life because some animals don't have tissues, organs, or organ systems. Animals can move quickly in response to their environments. For most animals, reproduction is sexual, with one copy of each gene coming from each parent. When an organism has two copies of its autosomal genetic material, this is known as a diploid. The sex chromosomes are a bit different because males have only one copy of the Y chromosome. The chromosomes in the female are XX, and the chromosomes in the male are XY. Animals have similar embryonic development. They almost all start with a few cells clumped together, known as a zygote. Then, a small ball of cells known as a blastula forms, followed by a gastrula. The image here tracks this development. An image of the process of embryonic development from zygote to gastrula for animals. All animals have some kind of embryonic development, starting from a zygote and evolving into a gastrula (Image by CNX OpenStax [Embryonic Development]. [CC BY 4.0], via Wikimedia Commons). There are multiple classes of organisms in the animal kingdom. The most populated class is insects. We actually don't know how many species of animals are on Earth, and it's likely that millions of animals have yet to be discovered! 2. OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016. Download for free at http://cnx.org/contents/b3c1e1d2-839c-42b0-a314-e119a8aafbdd@14.1.