Every fact in the three classification lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the facts are simplified or less certain: classification is a human system that changes as new evidence arrives; evolutionary trees are hypotheses [1]. Different sources use different numbers of kingdoms: Whittaker's five-kingdom system of 1969 is still common in schools [5], while biologists today place the kingdoms inside three domains [1]. The lessons for young children only use 'animals, plants and fungi' and leave out microbes. The share of animals that are invertebrates (99%) is a rounded figure from a children's museum card [6]; other sources give slightly different values.
All sources
[1] Biology 2e, 20.1 Organizing life on Earth
OpenStax (Rice University) · Reliability: High
Type: Peer-reviewed open university textbook
Taxonomy is the science of classifying organisms. Scientists divide organisms into three domains (Bacteria, Archaea and Eukarya), then kingdoms, then phylum, class, order, family, genus and species. This is the Linnaean system, after Carl Linnaeus; organisms are referred to by a two-word scientific name (binomial nomenclature). At each sublevel organisms become more similar because they are more closely related, although closely related organisms do not always look more alike. The dog, Canis lupus familiaris, is a subspecies of Canis lupus. A phylogenetic tree is a diagram of evolutionary relationships; trees are hypotheses that change as new data emerge.
[2] Concepts of Biology, 12.1 Organizing life on Earth
OpenStax (Rice University) · Reliability: High
Type: Peer-reviewed open textbook
The dog is classified as domain Eukarya, kingdom Animalia, phylum Chordata, class Mammalia, order Carnivora, family Canidae, genus Canis, species Canis lupus, subspecies Canis lupus familiaris. Each taxon name is capitalised except for species, and the genus and species names are italicised. Members of a taxon should share a common ancestor.
Harvard Museum of Natural History · Reliability: High
Type: University natural history museum
Carl von Linné (Linnaeus, 1707–1778) established the system of tiered classification and latinised his own name. The first part of a scientific name is the genus, linking an organism to its close relatives; the second part marks the species. Names were formed from Latin words. The genus name begins with a capital letter and the species name with a lower-case letter. A species may have several common names but only one scientific name. Every scientifically named species has a type specimen. Names often describe a feature, habitat or place, or honour a person.
Type: National botanic garden and research institute
Neither plants nor animals, fungi belong to their own separate kingdom, and they are more closely related to animals than to plants. Unlike plants, fungi cannot get their energy from the sun by photosynthesis. About 93% of the world's fungal species are yet to be discovered or described.
American Society for Microbiology · Reliability: High
Type: Professional scientific society
Fungi lack chloroplasts; they secrete digestive enzymes and absorb nutrients from their surroundings; and genetic (molecular) evidence shows fungi are more closely related to animals than to plants. Early taxonomists grouped fungi with plants partly because of their rigid cell walls. Robert Whittaker proposed five kingdoms in a 1969 article in Science, with fungi and plants in separate kingdoms, based largely on nutrition and ecological role.
American Museum of Natural History · Reliability: High
Type: Natural history museum (for children)
Invertebrates have no backbone; vertebrates are animals with backbones. Ninety-nine percent of all animals on Earth are invertebrates, for example insects, crabs, octopuses, worms and jellyfish.
[7] Classification: making a dichotomous key (lesson plan)
US National Park Service · Reliability: High
Type: Government agency (teaching resource)
A dichotomous key is a scientific tool used to identify organisms: a series of statements with two choices at each step, based on observable traits. In the activity, students build a branching key and a written key from ten pictures of whole organisms and test a classmate's key to see whether it works. Scientists have catalogued about 1.5 million species.