Haeckel's protists included all microscopic organisms known at that time Amoebozoa, Myxomycetes etc. Hence, Haeckel's Monera included microorganisms that are today known as Bacteria a term not yet coined in Six years later, the German botanist Ferdinand Cohn published a monograph on the occurrence, cultivation, morphology, behaviour, reproduction, nutrition, and systematics of a group of microorganisms he called Bacteria.
Despite Cohn's introduction of the term Bacteria in , Haeckel's term Monera survived in the biomedical literature until , when Robert H. Whittaker introduced his five-kingdom-system of classification. In this scheme, Monera refers to prokaryotic microorganisms bacteria, cyanobacteria , that are juxtaposed to four eukaryotic kingdoms Protista, Fungi, Animalia, Plantae 7.
This taxonomic interpretation of extant biodiversity was replaced by the three-domains-system of Woese et al. The four remaining kingdoms were summarized under the domain Eukarya, which includes all eukaryotic micro- and macroorganisms from amoebae to humans. Hence, according to this Haeckelian interpretation of the phylogeny of life, the Monera that is, Bacteria and Archaea 2 are the progenitors of all more complex living beings on Earth.
This year-old idea is consistent with recent discoveries and genomic analyses that support an archaeal origin of eukaryotes 1 , 8. Hug, L. Nature Microbiol.
Woese, C. Natl Acad. USA 87 , — Haeckel, E. Kutschera, U. Outreach 2 , — Article Google Scholar. Direct 6 , 33 Cohn, F. Pflanzen 1 , II , — In , Linnaeus published Systema Naturae , an page booklet in which he proposed the Linnaean taxonomy , a system of categorizing and naming organisms using a standard format so scientists could discuss organisms using consistent terminology.
He continued to revise and add to the book, which grew into multiple volumes Figure 1. In his taxonomy, Linnaeus divided the natural world into three kingdoms: animal, plant, and mineral the mineral kingdom was later abandoned.
Within the animal and plant kingdoms, he grouped organisms using a hierarchy of increasingly specific levels and sublevels based on their similarities. Species was, and continues to be, the most specific and basic taxonomic unit.
With advances in technology, other scientists gradually made refinements to the Linnaean system and eventually created new systems for classifying organisms. In the s, there was a growing interest in developing taxonomies that took into account the evolutionary relationships, or phylogenies , of all different species of organisms on earth.
One way to depict these relationships is via a diagram called a phylogenetic tree or tree of life. In these diagrams, groups of organisms are arranged by how closely related they are thought to be.
In early phylogenetic trees, the relatedness of organisms was inferred by their visible similarities, such as the presence or absence of hair or the number of limbs. Now, the analysis is more complicated. Today, phylogenic analyses include genetic, biochemical, and embryological comparisons, as will be discussed later in this chapter. In , Ernst Haeckel, a German biologist, philosopher, and physician, proposed another kingdom, Protista, for unicellular organisms Figure 2.
He later proposed a fourth kingdom, Monera, for unicellular organisms whose cells lack nuclei, like bacteria. Figure 2. He later added a fourth kingdom, Monera, for unicellular organisms lacking a nucleus. Nearly years later, in , American ecologist Robert Whittaker — proposed adding another kingdom—Fungi—in his tree of life. Empire Prokaryota contained just the Kingdom Monera. Figure 3 shows how the tree of life has changed over time.
Note that viruses are not found in any of these trees. That is because they are not made up of cells and thus it is difficult to determine where they would fit into a tree of life. Figure 3. This timeline shows how the shape of the tree of life has changed over the centuries. Even today, the taxonomy of living organisms is continually being reevaluated and refined with advances in technology.
Antibiotic drugs are specifically designed to kill or inhibit the growth of bacteria. But after a couple of days on antibiotics, Cora shows no signs of improvement. Also, her CSF cultures came back from the lab negative. Viral meningitis is still a possibility. However, Cora now reports some troubling new symptoms. She is starting to have difficulty walking.
Her muscle stiffness has spread from her neck to the rest of her body, and her limbs sometimes jerk involuntarily. But the advent of molecular genetics in the late 20th century revealed other ways to organize phylogenetic trees.
Genetic methods allow for a standardized way to compare all living organisms without relying on observable characteristics that can often be subjective.
Modern taxonomy relies heavily on comparing the nucleic acids deoxyribonucleic acid [DNA] or ribonucleic acid [RNA] or proteins from different organisms. The more similar the nucleic acids and proteins are between two organisms, the more closely related they are considered to be. He and his collaborator George Fox created a genetics-based tree of life based on similarities and differences they observed in the small subunit ribosomal RNA rRNA of different organisms.
In the process, they discovered that a certain type of bacteria, called archaebacteria now known simply as archaea , were significantly different from other bacteria and eukaryotes in terms of the sequence of small subunit rRNA.
To accommodate this difference, they created a tree with three Domains above the level of Kingdom: Archaea, Bacteria, and Eukarya Figure 4. As a result, he theorized that early stages of development must be faster in higher organisms than in lower ones. Haeckel also used the concept of truncation to explain inconsistencies between the stages of animals from different taxa.
For instance, pigs and humans may look similar to each other as early embryos, but as ontogeny progresses, the embryos start to look different from one another. If embryos pass through the linear stages of their evolutionary ancestors, as Haeckel claimed, then the two embryos should go through the same stages until the pig reaches full development and the human continues through the subsequent stages of its evolutionary ancestry.
However, in many cases, scientists found no such progressions. Haeckel hypothesized that truncation of ontogeny caused these inconsistencies.
Haeckel supported his biogenetic law with his drawings of embryos during different stages of development. In , his work Anthropogenie included drawings of embryonic fish , salamanders, tortoises, chicks, pigs, cows, rabbits, and humans at different stages of development placed next to one another for comparison.
Haeckel's drawings made the embryos of the different groups look almost identical in their earliest stages of development. He argued that they only become recognizable as species later in their respective developments. These similarities, according to Haeckel, demonstrated the linear progression from what he called lower forms to higher forms of animals, and he concluded that the stages recapitulated the evolutionary history of the organisms' ancestors.
He argued that embryologists shouldn't aim to construct phylogenetic trees and argued that embryologists should instead aim to explain development. He agreed with Haeckel that one should use causal theories to explain development, but he argued that Haeckel's theory was flawed in positing the stages of development as representations of adult ancestors.
He argued the Haeckel's biogenetic law overemphasized evolution as the cause of development and exaggerated the similarities between embryos of different species. He said that there were obvious differences between the early stages of embryos of different species, and that those differences, not the similarities, were important to explain development. In the decades after Haeckel's publication of the biogenetic law , other biologists struggled to recreate Haeckel's results.
Franz Keibel , a student of Wihelm His and a professor of anatomy at the University of Strasbourg in Alsace, France, tried to recreate Haeckel's drawings from his own specimens and concluded that Haeckel had exaggerated the similarity between embryos in his drawings. Keibel therefore rejected the biogenetic law and labeled it an exaggeration of the truth.
In , Keibel published this conclusion in the first volume of Normentafeln zur Entwicklungsgeschichte der Wirbelthiere Standard Panels to the Developmental History of the Verterbra. Furthermore, many scientists adopted a competing theory in the beginning of the twentieth century. Von Baer formulated these laws to discredit conception of recapitulation theory published in by Johann Friedrich Meckel.
In his laws, von Baer stated that the more general characters of a taxonomic group appear earlier in an animal embryo than the specialized characters do. He argued that rather than animals passing through successive stages of other adult animals, they diverge from one another as development progresses.
Therefore, he concluded, the stages embryos pass through during ontogeny never represent adult forms of other animals; they only represent embryonic stages of other animals. This conception was part of Darwin's account of ontogeny in The Origin of Species. Although von Baer's theory was overshadowed by recapitulation theory for most of the nineteenth century, scientists in the twentieth century began to adopt von Baer's view as the more accurate representation of development.
Haeckel's biogenetic law was further discredited by the results of experimental embryologists in the early twentieth century. Researchers abandoned Haeckel's theory when they couldn't confirm his observations. Embryologists showed that cases of recapitulation were less prevalent than were the inconsistencies between the developmental stages of normal organisms from different species.
Elizabeth Barnes. Keywords: biogenetic law , recapitulation. Ernst Haeckel 's Biogenetic Law The biogenetic law is a theory of development and evolution proposed by Ernst Haeckel in Germany in the s.
Sources von Baer, Karl Ernst.
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