The question of what percentage of life is prokaryotic or eukaryotic requires examining the two fundamental cellular architectures that form the basis of all organisms on Earth. All living things are categorized into these two domains based on their internal structure, but determining the “percentage” is complex. Life can be measured in multiple ways, such as counting individual organisms, cataloging species, or weighing total biomass. Each of these three metrics yields a dramatically different answer, illustrating how perspective shapes the quantitative reality of life on the planet.
Defining the Categories of Life
The division between prokaryotes and eukaryotes is based on cellular complexity. Prokaryotic cells are the simplest and most ancient form of life, encompassing the domains Bacteria and Archaea. They lack internal compartmentalization and do not possess a membrane-bound nucleus to house their genetic material. Instead, their DNA is concentrated in an area called the nucleoid, floating freely within the cytoplasm.
Prokaryotes also lack complex, membrane-bound organelles, such as mitochondria or the endoplasmic reticulum. Their overall size is small, typically measuring between 0.1 to 5 micrometers (μm) in diameter. This simple organization allows them to reproduce quickly and thrive in nearly every environment on Earth.
Eukaryotic cells are structurally more complex, having evolved a true nucleus encased within a membrane. This group includes all animals, plants, fungi, and protists. A distinguishing feature is the presence of numerous membrane-bound organelles that perform specialized functions, organizing the cell into efficient compartments. For example, mitochondria handle energy production, and the Golgi apparatus processes proteins. Eukaryotic cells are also significantly larger than prokaryotes, typically ranging from 10 to 100 μm in diameter.
The Challenge of Measuring Life’s Percentage
Asking for a simple percentage is difficult because the metric of measurement fundamentally changes the answer. Scientists use three main ways to quantify life on Earth, each providing a different sense of the proportion of prokaryotic versus eukaryotic life. The most straightforward metric is the total number of individual organisms, which heavily favors the microscopic world. A count of every living cell will be overwhelmingly dominated by trillions of tiny, single-celled prokaryotes.
A second metric is the total number of distinct species, which introduces a classification challenge. Eukaryotic species, such as mammals, insects, and plants, are relatively easy to observe, collect, and formally name. This results in a massive bias in the current registry of life, where the vast majority of described species are eukaryotes. Prokaryotes are much harder to distinguish and classify using traditional methods. The true number of bacterial and archaeal species is largely unknown and likely orders of magnitude higher than what has been officially cataloged.
The third metric is total biomass, defined as the total mass of living organic carbon. This measure attempts to balance the massive numbers of small organisms against the immense size of individual large organisms. For example, a single massive tree (a eukaryote) contains more carbon mass than billions of individual bacteria combined. Calculating the percentage of life by biomass requires comprehensive surveys and sophisticated estimation techniques across all biospheres, from the surface to the deep subsurface.
The Quantitative Reality: Estimates of Prokaryotic and Eukaryotic Abundance
When measuring the total number of individual living organisms, prokaryotes represent virtually the entirety of life on Earth. Scientists estimate the total number of prokaryotic cells (Bacteria and Archaea) is on the order of \(4 \times 10^{30}\) to \(6 \times 10^{30}\) cells globally. Since eukaryotic cells are a tiny fraction of this figure, the percentage of individual organisms that are prokaryotic is well over 99.9%.
The perspective shifts significantly when considering total biomass, estimated to be approximately 550 gigatons of carbon (Gt C) across all life. In this measure, eukaryotic life, specifically plants, holds the majority. Plants alone account for roughly 450 Gt C, or about 80% of all global biomass, due to the large size and longevity of trees and other vegetation.
Prokaryotes (Bacteria and Archaea) form the second-largest biomass component, contributing approximately 78 Gt C, about 14% of the total. Bacteria contribute about 70 Gt C, while Archaea contribute around 8 Gt C. A significant portion of this biomass resides deep underground in the terrestrial subsurface and deep-sea sediments, where life is slow-growing but abundant. All other eukaryotes, including animals, fungi, and protists, make up the remaining small percentage of total biomass.
By the metric of species count, the percentage is skewed toward eukaryotes simply because of classification bias. Of the approximately 1.9 million species formally described and named, the vast majority are eukaryotes, primarily animals and plants. While the true number of eukaryotic species is estimated to be around 8.7 million, the estimated number of prokaryotic species is intensely debated, ranging from a few million to potentially billions. This uncertainty means that by the metric of true biological diversity, prokaryotes are likely the dominant life form, even if they remain largely uncataloged. The percentage of life that is prokaryotic or eukaryotic thus depends entirely on whether one is counting individual organisms (prokaryotes dominate), total biomass (eukaryotes, specifically plants, dominate), or true biological diversity (prokaryotes are likely dominant but uncounted).