Looking for a place to get started with EOL? Start with these introductory articles to learn more about life on Earth. Read articles online, or download a printable PDF.
Algae are morphologically simple, chlorophyll-containing organisms that range from microscopic and unicellular (single-celled) to very large and multicellular.
The term amphibian comes from the Greek amphibios meaning 'both lives'. This is an apt description because most adult amphibians are better adapted to life on land than in water, while their larval phases are entirely aquatic.
All animals are members of the Kingdom Animalia, also called Metazoa. This Kingdom does not contain prokaryotes (Kingdom Monera, which includes bacteria, blue-green algae) or protists (Kingdom Protista, which includes unicellular eukaryotic organisms).
Archaea are a domain of single-celled microorganisms. They have no cell nucleus or any other organelles inside their cells.
Bacteria are any of a very large group of single-celled microorganisms that display a wide range of metabolic types, geometric shapes and environmental habitats and niches of occurrence.
Taxonomy is the field of biology dealing with identifying, naming, and classifying species. Every species has a unique two-part name situating it within a genus, and is further assigned to a series of higher-order taxonomic rankings.
Birds form a class of animals that includes over 10,000 species worldwide and range in size from the small bee hummingbird (Mellisuga helenae) to the large flightless ostrich (Struthio camelus).
No matter your passion - butterflies, mushrooms, birds, seashells. or all life on Earth - there is a citizen science project that needs your help to thrive.
The Latin root, crustaceus, 'having a crust or shell,' really doesn’t entirely narrow it down to crustaceans. They belong to the phylum Arthropoda, as do insects, arachnids, and many other groups; all arthropods have hard exoskeletons or shells, segmented bodies, and jointed limbs.
Biological evolution refers to genetically based changes in populations of organisms across generations.
While undiscovered biodiversity exists everywhere in the world, certain locations on Earth are particularly species rich. Some of the spots with the most biological diversity are also the ones most threatened with habitat degradation and species extinction.
There are more than 32,000 described species of fishes, more than all the amphibians, reptiles, birds, and mammals combined. Fishes are an important food resource worldwide, and fishing pressure has caused many fish stocks to crash or be at risk.
Flowering plants, or angiosperms—described taxonomically as the division Magnoliophyta—are the largest grouping within the plant kingdom (Kingdom Plantae or Viridiplantae) in terms of the numbers of described species.
The word fungus usually evokes images of mushrooms and toadstools. Although mushrooms are fungi, the forms that a fungus may take are varied. There are over 100,000 species of described fungi and probably over 200,000 undescribed.
With around one million named species and perhaps several times that number unnamed, insects account for a great majority of the species of animals on earth.
An indicator species is an organism whose presence, absence or abundance reflects a specific environmental condition. Indicator species can signal a change in the biological condition of a particular ecosystem, and thus may be used as a proxy to diagnose the health of an ecosystem.
An invasive species is defined legally in the USA as 'An alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.'
When most people think of animals, they think of things like mammals or birds, or maybe lizards, frogs, or fishes. All these familiar animals, along with humans, are part of a group called vertebrates or Vertebrata.
The Class Mammalia includes about 5000 species placed in 26 orders. Systematists do not yet agree on the exact number or on how some orders and families are related to others.
A model organism is a species that has been widely studied, usually because it is easy to maintain and breed in a laboratory setting and has particular experimental advantages.
Within fungi, 'mold' applies to a wide variety of species including members of the Zygomycota, Ascomycota, and Basidiomycota phylla.
Mollusca, from the Latin root for 'soft,' might seem an odd descriptor for these animals if you’re mostly familiar with mollusks in a culinary, shell-collecting, or decorative context.
All mushrooms are fungi, but not all fungi are mushrooms. For those fungi that produce them, the mushroom plays a similar role to a flower or a fruit in plants.
People naturally give names to the organisms they come across, but due to differences in language, region, and knowledge, the same species may go by many different common names, or the same name may be used to refer to several different species.
A plant is any one of the vast number of organisms within the biological kingdom Plantae; in general, these species are considered of limited motility and generally manufacture their own food.
The taxonomic kingdom Protista is a collection of single-celled organisms that do not fit into any other category. Protists are a group made up of protozoa, unicellular algae, and slime molds.
Reptiles do not form a distinct evolutionary group as birds and mammals do. Rather, the Class Reptilia consists of four orders which are very different from each other.
Spiders belong to the phylum Arthropoda, along with insects and crustaceans. The order of spiders, Araneae—together with scorpions, harvestmen, and the large order of mites and ticks—make up the class of Arachnida.
What is a tree? How are trees different from grasses, sunflowers, dandelions, and other types of plants? And what do trees have in common with shrubs and woody vines?
A virus is a microscopic organism that can replicate only inside the cells of a host organism. Most viruses are so tiny they are only observable with at least a conventional optical microscope.
Biodiversity can be measured in a number of ways, depending on what aspect is being examined. Most of the time when people talk about biodiversity, though, they’re referring to the number of different species (or 'types') of organisms.
The term 'biodiversity' refers to the variety of life forms in a habitat, whether it’s a local environment or an entire planet.
What is a worm? Of the thirty-odd phyla in the animal kingdom, at least a third are generally referred to as worms. If you include the more exotic, lesser-known phyla described as 'worm-like,' it’s well over half.