The task of accounting for all life on Earth is one of the most ambitious and long-running scientific projects. A species is defined as a group of organisms that can interbreed and produce fertile offspring, though this definition can vary depending on the life form being studied. This vast array of life is organized and named using the Linnaean system of binomial nomenclature, which assigns every organism a unique two-part Latinized name, such as Homo sapiens. Tracking every unique life form presents a monumental challenge, and the number of species known to science is constantly evolving through continuous discovery and classification.
The Officially Described Species Count
The most straightforward answer to how many species have been discovered is the count of those formally named, described, and published in the scientific literature. This official global inventory currently registers between 1.5 million and 2 million unique species. This figure represents only species that have been collected, examined by a taxonomist, and given their two-part scientific name according to international codes of nomenclature. This count is tracked by global databases and serves as a baseline, distinguishing confirmed scientific knowledge from larger, theoretical estimates of total life.
Distribution Across Major Life Forms
The described species count reveals a highly disproportionate distribution across the major groups of life. The animal kingdom, specifically invertebrates, accounts for the overwhelming majority of known species. Insects alone represent more than half of all described life forms, including over 350,000 species of beetles.
In comparison, well-studied groups like mammals (6,000 to 7,000 species) and birds (roughly 11,000 species) represent a small fraction of the animal count. The plant kingdom contributes nearly 400,000 described species globally. Fungi are also numerous, with hundreds of thousands of species formally described, though this is far smaller than what is estimated to exist. This imbalance highlights the bias in past discovery efforts toward larger, more visible organisms and certain terrestrial habitats.
How Species Are Identified and Catalogued
The process of officially adding a new species to the world’s register is governed by the field of taxonomy and follows a multi-step scientific protocol. The work typically begins with collecting a specimen from the field, which is then brought to a laboratory or museum collection for detailed study. The taxonomist performs a meticulous comparative analysis involving both morphological features and modern genetic sequencing.
The morphological analysis involves examining physical characteristics, such as size, shape, and internal anatomy, to distinguish the organism from its closest known relatives. This classic method is supported by molecular phylogenetic analysis, where DNA sequencing confirms the specimen is genetically distinct from all previously identified species. Once confirmed as new, the species is given its unique binomial name following the rules of the International Code of Zoological Nomenclature (ICZN) or the International Code of Nomenclature for Algae, Fungi, and Plants (ICN).
A single physical example, known as the holotype or type specimen, is then designated as the permanent reference for the species and deposited in a recognized collection, such as a natural history museum or herbarium. This specimen acts as the official reference point for all future scientific study. Finally, the full species description, including the diagnostic characteristics, the new name, and details of the holotype deposition, must be published in a peer-reviewed scientific journal. Only after this formal publication is the species considered officially described and added to the global count.
The Vastness of Undiscovered Life
Despite the millions of species already named, scientists agree that the known count represents only a fraction of life on Earth. The estimated total number of species, particularly for eukaryotes (organisms with complex cells, like animals, plants, and fungi), is widely placed at around 8.7 million, though some projections suggest this figure could be much higher. The difference between the 1.5 to 2 million described species and these estimates is known as the Linnaean shortfall.
These estimates are often generated using sophisticated statistical methods, such as extrapolating from the rate of discovery within higher taxonomic ranks or by analyzing species-area relationships. Molecular methods, like environmental DNA sequencing, have recently revealed millions of putative species, especially among smaller and less accessible organisms. For instance, some studies suggest the number of fungal species alone could exceed six million, vastly more than the few hundred thousand currently described.
The vast majority of undiscovered life is thought to reside in specific, under-sampled habitats. These include the deep ocean floor, tropical rainforest canopies, and the intricate ecosystems within soil, which are considered major reservoirs of unknown species. Furthermore, microbial life, including bacteria and archaea, represents an entirely different scale of diversity, with some estimates suggesting their numbers could reach into the trillions of species. The ongoing work of taxonomy, accelerating with new genetic tools, continues charting the complete map of global biodiversity.